Patent Publication Number: US-6707463-B1

Title: Data normalization technique

Description:
This application is a division of application Ser. No. 09/025,614, filed on Feb. 18, 1998 now abandoned. 
    
    
     MICROFICHE APPENDIX 
     There are 2 microfiche in total, and 103 frames in total. 
     FIELD OF THE INVENTION 
     The present invention relates to the field of creation of graphical images on a computer device such as a computer graphics coprocessor unit connected to an overall computer system, the graphics coprocessor assisting in the creation of images. 
     BACKGROUND OF THE INVENTION 
     Computer graphical images tend to come in many different forms. For example, in the past, only black and white bitmap displays were available and hence bitmaps having one bit per pixel were utilised. Subsequently, colored displays have become more significant and, as a result, a format comprising 8 bits per color channel of red, green and blue pixel data has become significant. 
     Over time, a technique of combining multiple images, each image having a transparency component, has become popular, resulting in a further opacity channel being added to pixel data. Further, output color display devices often utilise different color space mapping techniques, and hence other forms of color space representations (YUV or CYMK, for example) are also popular. 
     As a result of these and other developments, graphical objects utilized in the creation of computer graphic images may be presented in any of a large number of formats, making it relatively difficult for a processor or co-processor to deal with arbitrary graphical objects at relatively high speeds. 
     It is an object of the present invention to overcome or at least ameliorate one or more of the disadvantages of the prior art. 
     SUMMARY OF THE INVENTION 
     Accordingly, the invention provides a graphics processor for performing graphical operations on graphical objects, each of the graphical objects being represented in an external data format selected from a set thereof , the graphics processor including: 
     first mapping means to map each of the external data formats to a corresponding internal data format selected from a set of internal data formats; 
     calculation means to perform graphical operations on the graphical objects when in the 
     second mapping means to map each of the data formats in the set of internal data formats to a data format selected from the set of external data formats after the graphical operations have been performed. 
     Preferably, the set of external data formats include a contiguous stream of data of up to four channels per data quantum. Preferably also, each channel consists of 1-, 2-, 4-, 8-, or 16-bit samples. 
     Desirably, the external data format set includes an unpacked bit stream format consisting of a sequence of words, each word containing a predetermined number of valid bits. 
     In one preferred form, the internal data format set includes a 32-bit word format, each 32-bit word comprising four active-byte channels. Preferably, the internal data format set also includes an unpacked byte, 32-bit word format, each 32-bit word containing one active-byte channel. 
     A a particularly preferred embodiment, the first and second mapping means are each configured to perform one or more of at least the following mapping operations: 
     byte substitution; 
     byte lane-swapping; and 
     data replication. 
     In the following detailed description, the reader&#39;s attention is directed, in particular, to FIG.  2  and any one or more of FIGS. 22 to  48 , and their associated description, without intending to detract from the disclosure of the remainder of the description. 
     TABLE OF CONTENTS 
     1.0 Brief Description of the Drawings 
     2.0 List of Tables 
     3.0 Description of the Preferred and Other Embodiments 
     3.1 General Arrangement of Plural Stream Architecture 
     3.2 Host/Co-processor Queuing 
     3.3 Register Description of Co-processor 
     3.4 Format of Plural Streams 
     3.5 Determine Current Active Stream 
     3.6 Fetch Instruction of Current Active Stream 
     3.7 Decode and Execute Instruction 
     3.8 Update Registers of Instruction Controller 
     3.9 Semantics of the Register Access Semaphore 
     3.10 Instruction Controller 
     3.11 Description of a Modules Local Register File 
     3.12 Register Read/Write Handling 
     13.13 Memory Area Read/Write Handling 
     3.14 CBus Structure 
     3.15 Co-processor Data Types and Data Manipulation 
     3.16 Data Normalization Circuit 
     3.17 Image Processing Operations of Accelator Card 
     3.17.1 Compositing 
     3.17.2 Color Space Conversion Instructions 
     a. Single Output General Color Space (SOGCS) Conversion Mode 
     b. Multiple Output General Color Space Mode 
     3.17.3 JPEG Coding/Decoding 
     a. Encoding 
     b. Decoding 
     3.17.4 Table Indexing 
     3.17.5 Data Coding Instructions 
     3.17.6 A Fast DCT Apparatus 
     3.17.7 Huffman Decoder 
     3.17.8 Image Transformation Instructions 
     3.17.9 Convolution Instructions 
     3.17.10 Matrix Multiplication 
     3.17.11 Halftoning 
     3.17.12 Hierarchial Image Format Decompression 
     3.17.13 Memory Copy Instructions 
     a. General purpose data movement instructions 
     b. Local DMA instructions 
     3.17.14 Flow Control Instructions 
     3.18 Modules of the Accelerator Card 
     3.18.1 Pixel Organizer 
     3.18.2 MUV Buffer 
     3.18.3 Result Organizer 
     3.18.4 Operand Organizers B and C 
     3.18.5 Main Data Path Unit 
     3.18.6 Data Cache Controller and Cache 
     a. Normal Cache Mode 
     b. The Single Output General Color Space Conversion Mode 
     c. Multiple Output General Color Space Conversion Mode 
     d. JPEG Encoding Mode 
     e. Slow JPEG Decoding Mode 
     f. Matrix Multiplication Mode 
     g. Disabled Mode 
     h. Invalidate Mode 
     3.18.7 Input Interface Switch 
     3.18.8 Local Memory Controller 
     3.18.9 Miscellaneous Module 
     3.18.10 External Interface Controller 
     3.18.11 Peripheral Interface Controller 
     APPENDIX A—Microprogramming 
     APPENDIX B—Register tables 
    
    
     1.0. BRIEF DESCRIPTION OF THE DRAWINGS 
     Notwithstanding any other forms which may fall within the scope of the present invention, preferred forms of the invention will now be described, by way of example only, with reference to the accompanying drawings: 
     FIG. 1 illustrates the operation of a raster image co-processor within a host computer environment; 
     FIG. 2 illustrates the raster image co-processor of FIG. 1 in further detail; 
     FIG. 3 illustrates the memory map of the raster image co-processor; 
     FIG. 4 shows the relationship between a CPU, instruction queue, instruction operands and results in shared memory, and a co-processor; 
     FIG. 5 shows the relationship between an instruction generator, memory manager, queue manager and co-processor; 
     FIG. 6 shows the operation of the graphics co-processor reading instructions for execution from the pending instruction queue and placing them on the completed instruction queue; 
     FIG. 7 shows a fixed length circular buffer implementation of the instruction queue, indicating the need to wait when the buffer fills; 
     FIG. 8 illustrates to instruction execution streams as utilized by the co-processor; 
     FIG. 9 illustrates an instruction execution flow chart; 
     FIG. 10 illustrates the standard instruction word format utilized by the co-processor; 
     FIG. 11 illustrates the instruction word fields of a standard instruction; 
     FIG. 12 illustrates the data word fields of a standard instruction; 
     FIG. 13 illustrates schematically the instruction controller of FIG. 2; 
     FIG. 14 illustrates the execution controller of FIG. 13 in more detail; 
     FIG. 15 illustrates a state transition diagram of the instruction controller; 
     FIG. 16 illustrates the instruction decoder of FIG. 13; 
     FIG. 17 illustrates the instruction sequencer of FIG. 16 in more detail; 
     FIG. 18 illustrates a transition diagram for the ID sequencer of FIG. 16; 
     FIG. 19 illustrates schematically the prefetch buffer controller of FIG. 13 in more detail; 
     FIG. 20 illustrates the standard form of register storage and module interaction as utilized in the co-processor; 
     FIG. 21 illustrates the format of control bus transactions as utilized in the co-processor; 
     FIG. 22 illustrates the data flow through a portion of the co-processor; 
     FIGS. 23-29 illustrate various examples of data reformatting as utilized in the co-processor; 
     FIGS. 30 and 31 illustrate the format conversions carried out by the co-processor; 
     FIG. 32 illustrates the process of input data transformation as carried out in the co-processor; 
     FIGS. 33-41 illustrate various further data transformations as carried out by the co-processor; 
     FIG. 42 illustrates various internal to output data transformations carried out by the co-processor; 
     FIGS. 43-47 illustrate various further example data transformations carried out by the co-processor; 
     FIG. 48 illustrates various fields utilized by internal registers to determine what data transformations should be carried out; 
     FIG. 49 depicts a block diagram of a graphics subsystem that uses data normalization; 
     FIG. 50 illustrates a circuit diagram of a data normalization apparatus; 
     FIG. 51 illustrates the pixel processing carried out for compositing operations; 
     FIG. 52 illustrates the instruction word format for compositing operations; 
     FIG. 53 illustrates the data word format for compositing operations; 
     FIG. 54 illustrates the instruction word format for tiling operations; 
     FIG. 55 illustrates the operation of a tiling instruction on an image; 
     FIG. 56 illustrates the process of utilization of interval and fractional tables to re-map color gamuts; 
     FIG. 57 illustrates the form of storage of interval and fractional tables within the MUV buffer of the co-processor; 
     FIG. 58 illustrates the process of color conversion utilising interpolation as carried out in the co-processor; 
     FIG. 59 illustrates the refinements to the rest of the color conversion process at gamut edges as carried out by the co-processor; 
     FIG. 60 illustrates the process of color space conversion for one output color as implemented in the co-processor; 
     FIG. 61 illustrates the memory storage within a cache of the co-processor when utilising single color output color space conversion; 
     FIG. 62 illustrates the methodology utilized for multiple color space conversion; 
     FIG. 63 illustrates the process of address re-mapping for the cache when utilized during the process of multiple color space conversion; 
     FIG. 64 illustrates the instruction word format for color space conversion instructions; 
     FIG. 65 illustrates a method of multiple color conversion; 
     FIGS. 66 and 67 illustrate the formation of MCU&#39;s during the process of JPEG conversion as carried out in the co-processor; 
     FIG. 68 illustrates the structure of the JPEG coder of the co-processor; 
     FIG. 69 illustrates the quantizer portion of FIG. 68 in more detail; 
     FIG. 70 illustrates the Huffman coder of FIG. 68 in more detail; 
     FIGS. 71 and 72 illustrate the Huffman coder and decoder in more detail; 
     FIGS. 73-75 illustrate the process of cutting and limiting of JPEG data as utilized in the co-processor; 
     FIG. 76 illustrates the instruction word format for JPEG instructions; 
     FIG. 77 shows a block diagram of a typical discrete cosine transform apparatus (prior art); 
     FIG. 78 illustrates an arithmetic data path of a prior art DCT apparatus; 
     FIG. 79 shows a block diagram of a DCT apparatus utilized in the co-processor; 
     FIG. 80 depicts a block diagram of the arithmetic circuit of FIG. 79 in more detail; 
     FIG. 81 illustrates an arithmetic data path of the DCT apparatus of FIG. 79; 
     FIG. 82 presents a representational stream of Huffman-encoded data units interleaved with not encoded bit fields, both byte aligned and not, as in JPEG format; 
     FIG. 83 illustrates the overall architecture of a Huffman decoder of JPEG data of FIG. 84 in more detail; 
     FIG. 84 illustrates the overall architecture of the Huffman decoder of JPEG data; 
     FIG. 85 illustrates data processing in the stripper block which removes byte aligned not encoded bit fields from the input data. Examples of the coding of tags corresponding to the data outputted by the stripper are also shown; 
     FIG. 86 shows the organization and the data flow in the data preshifter; 
     FIG. 87 shows control logic for the decoder of FIG. 81; 
     FIG. 88 shows the organization and the data flow in the marker preshifter; 
     FIG. 89 shows a block diagram of a combinatorial unit decoding Huffman encoded values in JPEG context; 
     FIG. 90 illustrates the concept of a padding zone and a block diagram of the decoder of padding bits; 
     FIG. 91 shows an example of a format of data outputted by the decoder, the format being used in the co-processor; 
     FIG. 92 illustrates methodology utilized in image transformation instructions; 
     FIG. 93 illustrates the instruction word format for image transformation instructions; 
     FIGS. 94 and 95 illustrate the format of an image transformation kernal as utilized in the co-processor; 
     FIG. 96 illustrates the process of utilising an index table for image transformations as utilized in the co-processor; 
     FIG. 97 illustrates the data field format for instructions utilising transformations and convolutions; 
     FIG. 98 illustrates the process of interpretation of the bp field of instruction words; 
     FIG. 99 illustrates the process of convolution as utilized in the co-processor; 
     FIG. 100 illustrates the instruction word format for convolution instructions as utilized in the co-processor; 
     FIG. 101 illustrates the instruction word format for matrix multiplication as utilized in the co-processor; 
     FIGS. 102-105 illustrates the process utilized for hierarchial image manipulation as utilized in the co-processor; 
     FIG. 106 illustrates the instruction word coding for hierarchial image instructions; 
     FIG. 107 illustrates the instruction word coding for flow control instructions as illustrated in the co-processor; 
     FIG. 108 illustrates the pixel organizer in more detail; 
     FIG. 109 illustrates the operand fetch unit of the pixel organizer in more detail; 
     FIGS. 110-114 illustrate various storage formats as utilized by the co-processor; 
     FIG. 115 illustrates the MUV address generator of the pixel organizer of the co-processor in more detail; 
     FIG. 116 is a block diagram of a multiple value (MUV) buffer utilized in the co-processor; 
     FIG. 117 illustrates a structure of the encoder of FIG. 116; 
     FIG. 118 illustrates a structure of the decoder of FIG.  116 : 
     FIG. 119 illustrates a structure of an address generator of FIG. 116 for generating read addresses when in JPEG mode (pixel decomposition); 
     FIG. 120 illustrates a structure of an address generator of FIG. 116 for generating read addresses when in JPEG mode (pixel reconstruction); 
     FIG. 121 illustrates an organization of memory modules comprising the storage device of FIG. 116; 
     FIG. 122 illustrates a structure of a circuit that multiplexes read addresses to memory modules; 
     FIG. 123 illustrates a representation of how lookup table entries are stored in the buffer operating in a single lookup table mode; 
     FIG. 124 illustrates a representation of how lookup table entries are stored in the buffer operating in a multiple lookup table mode; 
     FIG. 125 illustrates a representation of how pixels are stored in the buffer operating in JPEG mode (pixel decomposition); 
     FIG. 126 illustrate a representation of how single color data blocks are retrieved from the buffer operating in JPEG mode (pixel reconstruction); 
     FIG. 127 illustrates the structure of the result organizer of the co-processor in more detail; 
     FIG. 128 illustrates the structure of the operand organizers of the co-processor in more detail; 
     FIG. 129 is a block diagram of a computer architecture for the main data path unit utilized in the co-processor; 
     FIG. 130 is a block diagram of a input interface for accepting, storing and rearranging input data objects for further processing; 
     FIG. 131 is a block diagram of a image data processor for performing arithmetic operations on incoming data objects; 
     FIG. 132 is a block diagram of a color channel processor for performing arithmetic operations on one channel of the incoming data objects; 
     FIG. 133 is a block diagram of a multifunction block in a color channel processor; 
     FIG. 134 illustrates a block diagram for compositing operations; 
     FIG. 135 shows an inverse transform of the scanline; 
     FIG. 136 shows a block diagram of the steps required to calculate the value for a designation pixel; 
     FIG. 137 illustrates a block diagram of the image transformation engine; 
     FIG. 138 illustrates the two formats of kernel descriptions; 
     FIG. 139 shows the definition and interpretation of a bp field; 
     FIG. 140 shows a block diagram of multiplier-adders that perform matrix multiplication; 
     FIG. 141 illustrates the control, address and data flow of the cache and cache controller of the co-processor; 
     FIG. 142 illustrates the memory organization of the cache; 
     FIG. 143 illustrates the address format for the cache controller of the co-processor; 
     FIG. 144 is a block diagram of a multifunction block in a color channel processor; 
     FIG. 145 illustrates the input interface switch of the co-processor in more FIG. 144 illustrates, a block diagram of the cache and cache controller; 
     FIG. 146 illustrates a four-port dynamic local memory controller of the co-processor showing the main address and data paths; 
     FIG. 147 illustrates a state machine diagram for the controller of FIG. 146; 
     FIG. 148 is a pseudo code listing detailing the function of the arbitrator of FIG. 146; 
     FIG. 149 depicts the structure of the requester priority bits and the terminology used in FIG. 146; 
     FIG. 150 illustrates the external interface controller of the co-processor in more detail; 
     FIGS. 151-154 illustrate the process of virtual to/from physical address mapping as utilized by the co-processor; 
     FIG. 155 illustrates the IBus receiver unit of FIG. 150 in more detail; 
     FIG. 156 illustrates the RBus receiver unit of FIG. 2 in more detail; 
     FIG. 157 illustrates the memory management unit of FIG. 150 in more detail; 
     FIG. 158 illustrates the peripheral interface controller of FIG. 2 in more detail. 
    
    
     2.0 LIST OF TABLES 
     Table 1: Register Description 
     Table 2: Opcode Description 
     Table 3: Operand Types 
     Table 4: Operand Descriptors 
     Table 5: Module Setup Order 
     Table 6: CBus Signal Definition 
     Table 7: CBus Transaction Types 
     Table 8: Data Manipulation Register Format 
     Table 9: Expected Data Types 
     Table 10: Symbol Explanation 
     Table 11: Compositing Operations 
     Table 12: Address Composition for SOGCS Mode 
     Table 12A: Instruction Encoding for Color Space Conversion 
     Table 13: Minor Opcode Encoding for Color Conversion Instructions 
     Table 14: Huffman and Quantization Tables as stored in Data Cache 
     Table 15: Fetch Address 
     Table 16: Tables Used by the Huffman Encoder 
     Table 17: Bank Address for Huffman and Quantization Tables 
     Table 18: Instruction Word—Minor Opcode Fields 
     Table 19: Instruction Word—Minor Opcode Fields 
     Table 20: Instruction Operand and Results Word 
     Table 21: Instruction Word 
     Table 22: Instruction Operand and Results Word 
     Table 23: Instruction Word 
     Table 24: Instruction Operand and Results Word 
     Table 25: Instruction Word—Minor Opcode Fields 
     Table 26: Instruction Word—Minor Opcode Fields 
     Table 27: Fraction Table 
     3.0 DESCRIPTION OF THE PREFERRED AND OTHER EMBODIMENTS 
     In the preferred embodiment, a substantial advantage is gained in hardware rasterization by means of utilization of two independent instruction streams by a hardware accelerator. Hence, while the first instruction stream can be preparing a current page for printing, a subsequent instruction stream can be preparing the next page for printing. A high utilization of hardware resources is available especially where the hardware accelerator is able to work at a speed substantially faster than the speed of the output device. 
     The preferred embodiment describes an arrangement utilising two instruction streams. However, arrangements having further instruction streams can be provided where the hardware trade-offs dictate that substantial advantages can be obtained through the utilization of further streams. 
     The utilization of two streams allows the hardware resources of the raster image co-processor to be kept fully engaged in preparing subsequent pages or bands, strips, etc., depending on the output printing device while a present page, band, etc is being forwarded to a print device. 
     3.1 General Arrangement of Plural Stream Architecture 
     In FIG. 1 there is schematically illustrated a computer hardware arrangement  201  which constitutes the preferred embodiment. The arrangement  201  includes a standard host computer system which takes the form of a host CPU  202  interconnected to its own memory store (RAM)  203  via a bridge  204 . The host computer system provides all the normal facilities of a computer system including operating systems programs, applications, display of information, etc. The host computer system is connected to a standard PCI bus  206  via a PCI bus interface  207 . The PCI standard is a well known industry standard and most computer systems sold today, particularly those running Microsoft Windows (trade mark) operating systems, normally come equipped with a PCI bus  206 . The PCI bus  206  allows the arrangement  201  to be expanded by means of the addition of one or more PCI cards, eg.  209 , each of which contain a further PCI bus interface  210  and other devices  211  and local memory  212  for utilization in the arrangement  201 . 
     In the preferred embodiment, there is provided a raster image accelerator card  220  to assist in the speeding up of graphical operations expressed in a page description language. The raster image accelerator card  220  (also having a PCI bus interface  221 ) is designed to operate in a loosely coupled, shared memory manner with the host CPU  202  in the same manner as other PCI cards  209 . It is possible to add further image accelerator cards  220  to the host computer system as required. The raster image accelerator card is designed to accelerate those operations that form the bulk of the execution complexity in raster image processing operations. These can include: 
     (a) Composition 
     (b) Generalized Color Space Conversion 
     (c) JPEG compression and decompression 
     (d) Huffman, run length and predictive coding and decoding 
     (e) Hierarchial image (Trade Mark) decompression 
     (f) Generalized affine image transformations 
     (g) Small kernel convolutions 
     (h) Matrix multiplication 
     (i) Halftoning 
     (j) Bulk arithmetic and memory copy operations 
     The raster image accelerator card  220  further includes its own local memory  223  connected to a raster image co-processor  224  which operates the raster image accelerator card  220  generally under instruction from the host CPU  202 . The co-processor  224  is preferably constructed as an Application Specific Integrated Circuit (ASIC) chip. The raster image co-processor  224  includes the ability to control at least one printer device  226  as required via a peripheral interface  225 . The image accelerator card  220  may also control any input/output device, including scanners. Additionally, there is provided on the accelerator card  220  a generic external interface  227  connected with the raster image co-processor  224  for its monitoring and testing. 
     In operation, the host CPU  202  sends, via PCI bus  206 , a series of instructions and data for the creation of images by the raster image co-processor  224 . The data can be stored in the local memory  223  in addition to a cache  230  in the raster image co-processor  224  or in registers  229  also located in the co-processor  224 . 
     Turning now to FIG. 2, there is illustrated, in more detail, the raster image co-processor  224 . The co-processor  224  is responsible for the acceleration of the aforementioned operations and consists of a number of components generally under the control of an instruction controller  235 . Turning first to the co-processor&#39;s communication with the outside world, there is provided a local memory controller  236  for communications with the local memory  223  of FIG. 1. A peripheral interface controller  237  is also provided for the communication with printer devices utilising standard formats such as the Centronics interface standard format or other video interface formats. The peripheral interface controller  237  is interconnected with the local memory controller  236 . Both the local memory controller  236  and the external interface controller  238  are connected with an input interface switch  252  which is in turn connected to the instruction controller  235 . The input interface switch  252  is also connected to a pixel organizer  246  and a data cache controller  240 . The input interface switch  252  is provided for switching data from the external interface controller  238  and local memory controller  236  to the instruction controller  235 , the data cache controller  240  and the pixel organizer  246  as required. 
     For communications with the PCI bus  206  of FIG. 1 the external interface controller  238  is provided in the raster image co-processor  224  and is connected to the instruction controller  235 . There is also provided a miscellaneous module  239  which is also connected to the instruction controller  235  and which deals with interactions with the co-processor  224  for purposes of test diagnostics and the provision of clocking and global signals. 
     The data cache  230  operates under the control of the data cache controller  240  with which it is interconnected. The data cache  230  is utilized in various ways, primarily to store recently used values that are likely to be subsequently utilized by the co-processor  224 . The aforementioned acceleration operations are carried out on plural streams of data primarily by a JPEG coder/decoder  241  and a main data path unit  242 . The units  241 ,  242  are connected in parallel arrangement to all of the pixel organizer  246  and two operand organizers  247 ,  248 . The processed streams from units  241 ,  242  are forwarded to a results organizer  249  for processing and reformatting where required. Often, it is desirable to store intermediate results close at hand. To this end, in addition to the data cache  230 , a multi-used value buffer  250  is provided, interconnected between the pixel organizer  246  and the result organizer  249 , for the storage of intermediate data. The result organizer  249  outputs to the external interface controller  238 , the local memory controller  236  and the peripheral interface controller  237  as required. 
     As indicated by broken lines in FIG. 2, a further (third) data path unit  243  can, if required be connected “in parallel” with the two other data paths in the form of JPEG coder/decoder  241  and the main data path unit  242 . The extension to 4 or more data paths is achieved in the same way. Although the paths are “parallel” connected, they do not operate in parallel. Instead only one path at a time operates. 
     The overall ASIC design of FIG. 2 has been developed in the following manner. Firstly, in printing pages it is necessary that there not be even small or transient artefacts. This is because whilst in video signal creation for example, such small errors if present may not be apparent to the human eye (and hence be unobservable), in printing any small artefact appears permanently on the printed page and can sometimes be glaringly obvious. Further, any delay in the signal reaching the printer can be equally disastrous resulting in white, unprinted areas on a page as the page continues to move through the printer. It is therefore necessary to provide results of very high quality, very quickly and this is best achieved by a hardware rather than a software solution. 
     Secondly, if one lists all the various operational steps (algorithms) required to be carried out for the printing process and provides an equivalent item of hardware for each step, the total amount of hardware becomes enormous and prohibitively expensive. Also the speed at which the hardware can operate is substantially limited by the rate at which the data necessary for, and produced by, the calculations can be fetched and despatched respectively. That is, there is a speed limitation produced by the limited bandwidth of the interfaces. 
     However, overall ASIC design is based upon a surprising realization that if the enormous amount of hardware is represented schematically then various parts of the total hardware required can be identified as being (a) duplicated and (b) not operating all the time. This is particularly the case in respect of the overhead involved in presenting the data prior to its calculation. 
     Therefore various steps were taken to reach the desired state of reducing the amount of hardware whilst keeping all parts of the hardware as active as possible. The first step was the realization that in image manipulation often repetitive calculations of the same basic type were required to be carried out. Thus if the data were streamed in some way, a calculating unit could be configured to carry out a specific type of calculation, a long stream of data processed and then the calculating unit could be reconfigured for the next type of calculation step required. If the data streams were reasonably long, then the time required for reconfiguration would be negligible compared to the total calculation time and thus throughput would be enhanced. 
     In addition, the provision of plural data processing paths means that in the event that one path is being reconfigured whilst the other path is being used, then there is substantially no loss of calculating time due to the necessary reconfiguration. This applies where the main data path unit  242  carries out a more general calculation and the other data path(s) carry out more specialized calculation such as JPEC coding and decoding as in unit  241  or, if additional unit  243  is provided, it can provide entropy and/or Huffman coding/decoding. 
     Further, whilst the calculations were proceeding, the fetching and presenting of data to the calculating unit can be proceeding. This process can be further speeded up, and hardware resources better utilized, if the various types of data are standardized or normalized in some way. Thus the total overhead involved in fetching and despatching data can be reduced. 
     Importantly, as noted previously, the co-processor  224  operates under the control of host CPU  202  (FIG.  1 ). In this respect, the instruction controller  235  is responsible for the overall control of the co-processor  224 . The instruction controller  235  operates the co-processor  224  by means of utilising a control bus  231 , hereinafter known as the CBus. The CBus  231  is connected to each of the modules  236 - 250  inclusive to set registers ( 231  of FIG. 1) within each module so as to achieve overall operation of the co-processor  224 . In order not to overly complicate FIG. 2, the interconnection of the control bus  231  to each of the modules  236 - 250  is omitted from FIG.  2 . 
     Turning now to FIG. 3, there is illustrated a schematic layout  260  of the available module registers. The layout  260  includes register  261  dedicated to the overall control of the co-processor  224  and its instruction controller  235 . The co-processor modules  236 - 250  include similar registers  262 . 
     3.2 Host/Co-processor Queuing 
     With the above architecture in mind, it is clear that there is a need to adequately provide for cooperation between the host processor  202  and the image co-processor  224 . However, the solution to this problem is general and not restricted to the specific above described architecture and therefore will be described hereafter with reference to a more general computing hardware environment. 
     Modern computer systems typically require some method of memory management to provide for dynamic memory allocation. In the case of a system with one or more co-processors, some method is necessary to synchronize between the dynamic allocation of memory and the use of that memory by a co-processor. 
     Typically a computer hardware configuration has both a CPU and a specialized co-processor, each sharing a bank of memory. In such a system, the CPU is the only entity in the system capable of allocating memory dynamically. Once allocated by the CPU for use by the co-processor, this memory can be used freely by the co-processor until it is no longer required, at which point it is available to be freed by the CPU. This implies that some form of synchronization is necessary between the CPU and the co-processor in order to ensure that the memory is released only after the co-processor is finished using it. There are several possible solutions to this problem but each has undesirable performance implications. 
     The use of statically allocated memory avoids the need for synchronization, but prevents the system from adjusting its memory resource usage dynamically. Similarly, having the CPU block and wait until the co-processor has finished performing each operation is possible, but this substantially reduces parallelism and hence reduces overall system performance. The use of interrupts to indicate completion of operations by the co-processor is also possible but imposes significant processing overhead if co-processor throughput is very high. 
     In addition to the need for high performance, such a system also has to deal with dynamic memory shortages gracefully. Most computer systems allow a wide range of memory size configurations. It is important that those systems with large amounts of memory available make full use of their available resources to maximize performance. Similarly those systems with minimal memory size configurations should still perform adequately to be useable and, at the very least, should degrade gracefully in the face of a memory shortage. 
     To overcome these problems, a synchronization mechanism is necessary which will maximize system performance while also allowing co-processor memory usage to adjust dynamically to both the capacity of the system and the complexity of the operation being performed. 
     In general, the preferred arrangement for synchronising the (host) CPU and the co-processor is illustrated in FIG. 4 where the reference numerals used are those already utilized in the previous description of FIG.  1 . 
     Thus in FIG. 108, the CPU  202  is responsible for all memory management in the system. It allocates memory  203  both for its own uses, and for use by the co-processor  224 . The co-processor  224  has its own graphics-specific instruction set, and is capable of executing instructions  1022  from the memory  203  which is shared with the host processor  202 . Each of these instructions can also write results  1024  back to the shared memory  203 , and can read operands  1023  from the memory  203  as well. The amount of memory  203  required to store operands  1023  and results  1024  of co-processor instructions varies according to the complexity and type of the particular operation. 
     The CPU  202  is also responsible for generating the instructions  1022  executed by the co-processor  224 . To maximize the degree of parallelism between the CPU  202  and the co-processor  224 , instructions generated by the CPU  202  are queued as indicated at  1022  for execution by the co-processor  224 . Each instruction in the queue  1022  can reference operands  1023  and results  1024  in the shared memory  203 , which has been allocated by the host CPU  202  for use by the co-processor  224 . 
     The method utilizes an interconnected instruction generator  1030 , memory manager  1031  and queue manager  1032 , as shown in FIG.  5 . All these modules execute in a single process on the host CPU  202 . 
     Instructions for execution by the co-processor  224  are generated by the instruction generator  1030 , which uses the services of the memory manager  1031  to allocate space for the operands  1023  and results  1024  of the instructions being generated. The instruction generator  1030  also uses the services of the queue manager  1032  to queue the instructions for execution by the co-processor  224 . 
     Once each instruction has been executed by the co-processor  224 , the CPU  202  can free the memory which was allocated by the memory manager  1031  for use by the operands of that instruction. The result of one instruction can also become an operand for a subsequent instruction, after which its memory can also be freed by the CPU. Rather than fielding an interrupt, and freeing such memory as soon as the co-processor  224  has finished with it, the system frees the resources needed by each instruction via a cleanup function which runs at some stage after the co-processor  224  has completed the instruction. The exact time at which these cleanups occur depends on the interaction between the memory manager  1031  and the queue manager  1032 , and allows the system to adapt dynamically according to the amount of system memory available and the amount of memory required by each co-processor instruction. 
     FIG. 6 schematically illustrates the implementation of the co-processor instruction queue  1022  instructions are inserted into a pending instruction queue  1040  by the host CPU  202 , and are read by the co-processor  224  for execution. After execution by the co-processor  224 , the instructions remain on a cleanup queue  1041 , so that the CPU  202  can release the resources that the instructions required after the co-processor  224  has finished executing them. 
     The instruction queue  1022  itself can be implemented as a fixed or dynamically sized circular buffer. The instruction queue  1022  decouples the generation of instructions by the CPU  202  from their execution by the co-processor  224 . 
     Operand and result memory for each instruction is allocated by the memory manager  1031  (FIG. 5) in response to requests from the instruction generator  1030  during instruction generation. It is the allocation of this memory for newly generated instructions which triggers the interaction between the memory manager  1031  and the queue manager  1032  described below, and allows the system to adapt automatically to the amount of memory available and the complexity of the instructions involved. 
     The instruction queue manager  1032  is capable of waiting for the co-processor  224  to complete the execution of any given instruction which has been generated by the instruction generator  1030 . However, by providing a sufficiently large instruction queue  1022  and sufficient memory  203  for allocation by the memory manager  1031 , it becomes possible to avoid having to wait for the co-processor  224  at all, or at least until the very end of the entire instruction sequence, which can be several minutes on a very large job. However, peak memory usage can easily exceed the memory available, and at this point the interaction between the queue manager  1032  and the memory manager  1031  comes into play. 
     The instruction queue manager  1032  can be instructed at any time to “cleanup” the completed instructions by releasing the memory that was dynamically allocated for them. If the memory manager  1031  detects that available memory is either running low or is exhausted, its first recourse is to instruct the queue manager  1032  to perform such a cleanup in an attempt to release some memory which is no longer in use by the co-processor  224 . This can allow the memory manager  1031  to satisfy a request from the instruction generator  1030  for memory required by a newly generated instruction, without the CPU  202  needing to wait for, or synchronize with, the co-processor  224 . 
     If such a request made by the memory manager  1031  for the queue manager  1032  to cleanup completed instructions does not release adequate memory to satisfy the instruction generator&#39;s new request, the memory manager  1031  can request that the queue manager  1032  wait for a fraction, say half, of the outstanding instructions on the pending instruction queue  1040  to complete. This will cause the CPU  202  processing to block until some of the co-processor  224  instructions have been completed, at which point their operands can be freed, which can release sufficient memory to satisfy the request. Waiting for cleanup of a fraction of the outstanding instructions ensures that the co-processor  224  is kept busy by maintaining at least some instructions in its pending instruction queue  1040 . In many cases the cleanup from the fraction of the pending instruction queue  1040  that the CPU  202  waits for, releases sufficient memory for the memory manager  1031  to satisfy the request from the instruction generator  1030 . 
     In the unlikely event that waiting for the co-processor  224  to complete execution of, say, half of the pending instructions does not release sufficient memory to satisfy the request, then the final recourse of the memory manager  1031  is to wait until all pending co-processor instructions have completed. This should release sufficient resources to satisfy the request of the instruction generator  1030 , except in the case of extremely large and complex jobs which exceed the system&#39;s present memory capacity altogether. 
     By the above described interaction between the memory manager  1031  and the queue manager  1032 , the system effectively tunes itself to maximize throughput for the given amount of memory  203  available to the system. More memory results in less need for synchronization and hence greater throughput. Less memory requires the CPU  202  to wait more often for the co-processor  224  to finish using the scarce memory  203 , thereby yielding a system which still functions with minimal memory available, but at a lower performance. 
     The steps taken by the memory manager  1031  when attempting to satisfy a request from the instruction generator  1030  are summarized below. Each step is tried in sequence, after which the memory manager  1031  checks to see if sufficient memory  203  has been made available to satisfy the request. If so, it stops because the request can be satisfied, otherwize it proceeds to the next step in a more aggressive attempt to satisfy the request: 
     1. Attempt to satisfy the request with the memory  203  already available. 
     2. Cleanup all completed instructions. 
     3. Wait for a fraction of the pending instructions. 
     4. Wait for all the remaining pending instructions. 
     Other options can also be used in the attempt to satisfy the request, such as waiting for different fractions (such as one-third or two-thirds) of the pending instructions, or waiting for specific instructions which are known to be using large amounts of memory. 
     Turning now to FIG. 7, in addition to the interaction between, the memory manager  1031  and the queue manager  1032 , the queue manager  1032  can also initiate a synchronization with the co-processor  224  in the case where space in a fixed-length instruction queue buffer  1050  is exhausted. Such a situation is depicted in FIG.  7 . In FIG. 7 the pending instructions queue  1040  is ten instructions in length. The latest instruction to be added to the queue  1040  has the highest occupied number. Thus where space is exhausted the latest instruction is located at position  9 . The next instruction to be input to the co-processor  224  is waiting at position zero. 
     In such a case of exhausted space, the queue manager  1032  will also wait for, say, half the pending instructions to be completed by the co-processor  224 . This delay normally allows sufficient space in the instruction queue  1040  to be freed for new instructions to be inserted by the queue manager  1032 . 
     The method used by the queue manager  1032  when scheduling new instructions is as follows: 
     1. Test to see if sufficient space is available in the instruction queue  1040 . 
     2. If sufficient space is not available, wait for the co-processor to complete some predetermined number or fraction of instructions. 
     3. Add the new instructions to the queue. 
     The method used by the queue manager  1032  when asked to wait for a given instruction is as follows: 
     1. Wait until the co-processor  224  indicates that the instruction is complete. 
     2. While there are instructions completed which are not yet cleaned up, clean up the next completed instruction in the queue. 
     The method used by the instruction generator  1030  when issuing new instructions is as follows: 
     1. Request sufficient memory for the instruction operands  1023  from the memory manger  1031 . 
     2. Generate the instructions to be submitted. 
     3. Submit the co-processor instructions to the queue manager  1032  for execution. 
     The following is an example of pseudo code of the above decision making processes. 
     MEMORY MANAGER 
     ALLOCATE_MEMORY 
     BEGIN 
     IF sufficient memory is NOT available to satisfy request 
     THEN 
     Clean up all completed instructions. 
     ENDIF 
     IF sufficient memory is still NOT available to satisfy request 
     THEN 
     CALL WAIT_FOR_INSTRUCTION for half the pending instructions. 
     ENDIF 
     Insufficient memory is still NOT available to satisfy request 
     THEN 
     RETURN with an error. 
     ENDIF 
     RETURN the allocated memory 
     END 
     QUEUE MANAGER 
     SCHEDULE_INSTRUCTION 
     BEGIN 
     IF sufficient space is NOT available in the instruction queue 
     THEN 
     WAIT for the co-processor to complete some predetermined 
     number of instructions. 
     ENDIF 
     Add the new instructions to the queue. 
     END 
     WAIT_FOR_INSTRUCTION(i) 
     BEGIN 
     WAIT until the co-processor indicates that instruction i is complete. 
     WHILE there are instructions completed which are not yet cleaned up 
     DO 
     IF the next completed instruction has a cleanup function 
     THEN 
     CALL the cleanup function 
     ENDIF 
     REMOVE the completed instruction from the queue 
     DONE 
     END 
     INSTRUCTION GENERATOR 
     GENERATE_INSTRUCTIONS 
     BEGIN 
     CALL ALLOCATE_MEMORY to allocate sufficient memory for the instructions operands from the memory manager. 
     GENERATE the instructions to be submitted. 
     CALL SCHEDULE_INSTRUCTION submit the co-processor instructions to the queue manager for execution. 
     END 
     3.3 Register Description of Co-processor 
     As explained above in relation to FIGS. 1 and 3, the co-processor  224  maintains various registers  261  for the execution of each instruction stream. 
     Referring to each of the modules of FIG. 2, Table 1 sets out the name, type and description of each of the registers utilized by the co-processor  224  while Appendix B sets out the structure of each field of each register. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Register Description 
               
            
           
           
               
               
               
            
               
                 NAME 
                 TYPE 
                 DESCRIPTION 
               
               
                   
               
            
           
           
               
            
               
                 External Interface Controller Registers 
               
            
           
           
               
               
               
            
               
                 eic_cfg 
                 Config2 
                 Configuration 
               
               
                 eic_stat 
                 Status 
                 Status 
               
               
                 eic_err_int 
                 Interrupt 
                 Error and Interrupt Status 
               
               
                 eic_err_int_en 
                 Config2 
                 Error and Interrupt Enable 
               
               
                 eic_test 
                 Config2 
                 Test modes 
               
               
                 eic_gen_pob 
                 Config2 
                 Generic bus programmable output bits 
               
               
                 eic_high_addr 
                 Config1 
                 Dual address cycle offset 
               
               
                 eic_wtlb_v 
                 Control2 
                 Virtual address and operation bits for TLB 
               
               
                   
                   
                 Invalidate/Write 
               
               
                 eic_wtlb_p 
                 Config2 
                 Physical address and control bits for TLB 
               
               
                   
                   
                 Write 
               
               
                 eic_mmu_v 
                 Status 
                 Most recent MMU virtual address 
               
               
                   
                   
                 translated, and current LRU location. 
               
               
                 eic_mmu_v 
                 Status 
                 Most recent page table physical address 
               
               
                   
                   
                 fetched by MMU. 
               
               
                 eic_ip_addr 
                 Status 
                 Physical address for most recent 
               
               
                   
                   
                 IBus access to the PCI Bus. 
               
               
                 eic_rp_addr 
                 Status 
                 Physical address for most recent RBus 
               
               
                   
                   
                 access to the PCI Bus. 
               
               
                 eic_ig_addr 
                 Status 
                 Address for most recent IBus access to the 
               
               
                   
                   
                 Generic Bus. 
               
               
                 eic_rg_data 
                 Status 
                 Address for most recent RBus 
               
               
                   
                   
                 access to the Generic Bus. 
               
            
           
           
               
            
               
                 Local Memory Controller Registers 
               
            
           
           
               
               
               
            
               
                 lmi_cfg 
                 Control2 
                 General configuration register 
               
               
                 lmi_sts 
                 Status 
                 General status register 
               
               
                 lmi_err_int 
                 Interrupt 
                 Error and interrupt status register 
               
               
                 lmi_err_int_en 
                 Control2 
                 Error and interrupt enable register 
               
               
                 lmi_dcfg 
                 Control2 
                 DRAM configuration register 
               
               
                 lmi_mode 
                 Control2 
                 SDRAM mode register 
               
            
           
           
               
            
               
                 Peripheral Interface Controller Registers 
               
            
           
           
               
               
               
            
               
                 pic_cfg 
                 Config2 
                 Configuration 
               
               
                 pic_stat 
                 Status 
                 Status 
               
               
                 pic_err_int 
                 Interrupt 
                 Interrupt/Error Status 
               
               
                 pic_err_int_en 
                 Config2 
                 Interrupt/Error Enable 
               
               
                 pic_abus_cfg 
                 Control2 
                 Configuration and control for ABus 
               
               
                 pic_abus_addr 
                 Config1 
                 Start address for ABus transfer 
               
               
                 pic_cent_cfg 
                 Control2 
                 Configuration and control for Centronics 
               
               
                 pic_cent_dir 
                 Config2 
                 Centronics pin direct control register 
               
               
                 pic_reverse_cfg 
                 Control2 
                 Configuration and control for reverse 
               
               
                   
                   
                 (input) data transfers 
               
               
                 pic_timer0 
                 Config1 
                 Initial data timer value 
               
               
                 pic_timer1 
                 Config1 
                 Subsequent data timer value 
               
            
           
           
               
            
               
                 Miscellaneous Module Registers 
               
            
           
           
               
               
               
            
               
                 mm_cfg 
                 Config2 
                 Configuration Register 
               
               
                 mm_stat 
                 Status 
                 Status Register 
               
               
                 mm_err_int 
                 Interrupt 
                 Error and Interrupt Register 
               
               
                 mm_err_int_en 
                 Config2 
                 Error and Interrupt Masks 
               
               
                 mm_gefg 
                 Config2 
                 Global Configuration Register 
               
               
                 mm_diag 
                 Config 
                 Diagnostic Configuration Register 
               
               
                 mm_grst 
                 Config 
                 Global Reset Register 
               
               
                 mm_gerr 
                 Config2 
                 Global Error Register 
               
               
                 mm_gexp 
                 Config2 
                 Global Exception Register 
               
               
                 mm_gint 
                 Config2 
                 Global Interrupt Register 
               
               
                 mm_active 
                 Status 
                 Global Active signals 
               
            
           
           
               
            
               
                 Instruction Controller Registers 
               
            
           
           
               
               
               
            
               
                 ic_cfg 
                 Config2 
                 Configuration Register 
               
               
                 ic_stat 
                 Status/ 
                 Status Register 
               
               
                   
                 Interrupt 
               
               
                 ic_err_int 
                 Interrupt 
                 Error and Interrupt Register (write to clear 
               
               
                   
                   
                 error and interrupt) 
               
               
                 ic_err_int_en 
                 Config2 
                 Error and Interrupt Enable Register 
               
               
                 ic_ipa 
                 Control1 
                 A stream Instruction Pointer 
               
               
                 ic_tda 
                 Config1 
                 A stream Todo Register 
               
               
                 ic_fna 
                 Control1 
                 A stream Finished Register 
               
               
                 ic_inta 
                 Config1 
                 A stream Interrupt Register 
               
               
                 ic_loa 
                 Status 
                 A stream Last Overlapped Instruction 
               
               
                   
                   
                 Sequence number 
               
               
                 ic_ipb 
                 Control1 
                 B stream Instruction Pointer 
               
               
                 ic_tdb 
                 Config1 
                 B stream Todo Register 
               
               
                 ic_fnb 
                 Control1 
                 B stream Finished Register 
               
               
                 ic_intb 
                 Config1 
                 B stream Interrupt Register 
               
               
                 ic_lob 
                 Status 
                 B stream Last Overlapped Instruction 
               
               
                   
                   
                 Sequence number 
               
               
                 ic_sema 
                 Status 
                 A stream Semaphore 
               
               
                 ic_semb 
                 Status 
                 B stream Semaphore 
               
            
           
           
               
            
               
                 Data Cache Controller Registers 
               
            
           
           
               
               
               
            
               
                 dcc_cfg1 
                 Config2 
                 DCC configuration 1 register 
               
               
                 dcc_stat 
                 Status 
                 state machine status bits 
               
               
                 dcc_err_int 
                 Status 
                 DCC error status register 
               
               
                 dcc_err_int_en 
                 Control1 
                 DCC error interrupt enable bits 
               
               
                 dcc_cfg2 
                 Control2 
                 DCC configuration 2 register 
               
               
                 dcc_addr 
                 Config1 
                 Base address register for special address 
               
               
                   
                   
                 modes. 
               
               
                 dcc_lv0 
                 Control1 
                 “valid” bit status for lines 0 to 31 
               
               
                 dcc_lv1 
                 Control1 
                 “valid” bit status for lines 32 to 63 
               
               
                 dcc_lv2 
                 Control1 
                 “valid” bit status for lines 64 to 95 
               
               
                 dcc_lv3 
                 Control1 
                 “valid” bit status for lines 96 to 127 
               
               
                 dcc_raddrb 
                 Status 
                 Operand Organizer B request address 
               
               
                 dcc_raddrc 
                 Status 
                 Operand Organizer C request address 
               
               
                 dcc_test 
                 Control1 
                 DCC test register 
               
            
           
           
               
            
               
                 Pixel Organizer Registers 
               
            
           
           
               
               
               
            
               
                 po_cfg 
                 Config2 
                 Configuration Register 
               
               
                 po_stat 
                 Status 
                 Status Register 
               
               
                 po_err_int 
                 Interrupt 
                 Error/Interrupt Status Register 
               
               
                 po_err_int_en 
                 Config2 
                 Error/Interrupt Enable Register 
               
               
                 po_dmr 
                 Config2 
                 Data Manipulation Register 
               
               
                 po_subst 
                 Config2 
                 Substitution Value Register 
               
               
                 po_cdp 
                 Status 
                 Current Data Pointer 
               
               
                 po_len 
                 Control1 
                 Length Register 
               
               
                 po_said 
                 Control1 
                 Start Address or Immediate Data 
               
               
                 po_idr 
                 Control2 
                 Image Dimensions Register 
               
               
                 po_muv_valid 
                 Control2 
                 MUV valid bits 
               
               
                 po_muv 
                 Config1 
                 Base address of MUV RAM 
               
            
           
           
               
            
               
                 Operand Organizer B Registers 
               
            
           
           
               
               
               
            
               
                 oob_cfg 
                 Config2 
                 Configuration Register 
               
               
                 oob_stat 
                 Status 
                 Status Register 
               
               
                 oob_err_int 
                 Interrupt 
                 Error/Interrupt Register 
               
               
                 oob_err_int_en 
                 Config2 
                 Error/Interrupt Enable Register 
               
               
                 oob_dmr 
                 Config2 
                 Data Manipulation Register 
               
               
                 oob_subst 
                 Config2 
                 Substitution Value Register 
               
               
                 oob_cdp 
                 Status 
                 Current Data Pointer 
               
               
                 oob_len 
                 Control1 
                 Input Length Register 
               
               
                 oob_said 
                 Control1 
                 Operand Start Address 
               
               
                 oob_tile 
                 Control1 
                 Tiling length/offset Register 
               
            
           
           
               
            
               
                 Operand Organizer C Registers 
               
            
           
           
               
               
               
            
               
                 ooc_cf 
                 Config2 
                 Configuration Register 
               
               
                 ooc_stat 
                 Status 
                 Status Register 
               
               
                 ooc_err_int 
                 Interrupt 
                 Error/Interrupt Register 
               
               
                 ooc_err_int_en 
                 Config2 
                 Error/Interrupt Enable Register 
               
               
                 ooc_dmr 
                 Config2 
                 Data Manipulation Register 
               
               
                 ooc_subst 
                 Config2 
                 Substitution Value Register 
               
               
                 ooc_cdp 
                 Status 
                 Current Data Pointer 
               
               
                 ooc_len 
                 Control1 
                 Input Length Register 
               
               
                 ooc_said 
                 Control1 
                 Operand Start Address 
               
               
                 ooc_tile 
                 Control1 
                 Tiling length offset Register 
               
            
           
           
               
            
               
                 JPEG Coder Register 
               
            
           
           
               
               
               
            
               
                 jc_cfg 
                 Config2 
                 configuration 
               
               
                 jc_stat 
                 Status 
                 status 
               
               
                 jc_err_int 
                 Interrupt 
                 error and interrupt status register 
               
               
                 jc_err_int_en 
                 Config2 
                 error and interrupt enable register 
               
               
                 jc_rsi 
                 Config1 
                 restart interval 
               
               
                 jc_decode 
                 Control2 
                 decode of current instruction 
               
               
                 jc_res 
                 Control1 
                 residual value 
               
               
                 jc_table_sel 
                 Control2 
                 table selection from decoded instruction 
               
            
           
           
               
            
               
                 Main Data Path Register 
               
            
           
           
               
               
               
            
               
                 mdp_cfg 
                 Config2 
                 configuration 
               
               
                 mdp_stat 
                 Status 
                 status 
               
               
                 mdp_err_int 
                 Interrupt 
                 error/interrupt 
               
               
                 mdp_err_int_en 
                 Config2 
                 error/interrupt enable 
               
               
                 mdp_test 
                 Config2 
                 test modes 
               
               
                 mdp_op1 
                 Control2 
                 current operation 1 
               
               
                 mdp_op2 
                 Control2 
                 current operation 2 
               
               
                 mdp_por 
                 Control1 
                 offset for plus operator 
               
               
                 mdp_bi 
                 Control1 
                 blend start/offset to index table entry 
               
               
                 mdp_bm 
                 Control1 
                 blend end or number of rows and columns 
               
               
                   
                   
                 in matrix, binary places, and number of 
               
               
                   
                   
                 levels in halftoning 
               
               
                 mdp_len 
                 Control1 
                 Length of blend to produce 
               
            
           
           
               
            
               
                 Result Organizer Register 
               
            
           
           
               
               
               
            
               
                 ro_cfg 
                 Config2 
                 Configuration Register 
               
               
                 ro_stat 
                 Status 
                 Status Register 
               
               
                 ro_err_int 
                 Interrupt 
                 Error/Interrupt Register 
               
               
                 ro_err_int_en 
                 Config2 
                 Error/Interrupt Enable Register 
               
               
                 ro_dmr 
                 Config2 
                 Data Manipulation Register 
               
               
                 ro_subst 
                 Config1 
                 Substitution Value Register 
               
               
                 ro_cdp 
                 Status 
                 Current Data Pointer 
               
               
                 ro_len 
                 Status 
                 Output Length Register 
               
               
                 ro_sa 
                 Config1 
                 Start Address 
               
               
                 ro_idr 
                 Config1 
                 Image Dimensions Register 
               
               
                 ro_vbase 
                 Config1 
                 co-processor Virtual Base Address 
               
               
                 ro_cut 
                 Config1 
                 Output Cut Register 
               
               
                 ro_lmt 
                 Config1 
                 Output Length Limit 
               
            
           
           
               
            
               
                 PCIBus Configuration Space alias 
               
            
           
           
               
               
               
            
               
                   
                   
                 A read only copy of PCI configuration 
               
               
                   
                   
                 space registers 0 × 0 to 0 × D and 0 × F. 
               
               
                 pci_external_cfg 
                 Status 
                 32-bit field downloaded at reset from an 
               
               
                   
                   
                 external serial ROM. Has no influence on 
               
               
                   
                   
                 coprocessor operation. 
               
            
           
           
               
            
               
                 Input Interface Switch Registers 
               
            
           
           
               
               
               
            
               
                 iis_cfg 
                 Config2 
                 Configuration Register 
               
               
                 iis_stat 
                 Status 
                 Status Register 
               
               
                 iis_err_int 
                 Interrupt 
                 Interrupt/Error Status Register 
               
               
                 iis_err_int_en 
                 Config2 
                 Interrupt/Error Enable Register 
               
               
                 iis_ic_addr 
                 Status 
                 Input address from IC 
               
               
                 iis_doc_addr 
                 Status 
                 Input address from DCC 
               
               
                 iis_po_addr 
                 Status 
                 Input address from PO 
               
               
                 iis_burst 
                 Status 
                 Burst Length from PO, DCC &amp; IC 
               
               
                 iis_base_addr 
                 Config1 
                 Base address of co-processor 
               
               
                   
                   
                 memory object in host memory map. 
               
               
                 iis_test 
                 Config1 
                 Test mode register 
               
               
                   
               
            
           
         
       
     
     The more notable ones of these registers include: 
     (a) Instruction Pointer Registers (ic_ipa and ic_ipb). This pair of registers each contains the virtual address of the currently executing instruction. Instructions are fetched from ascending virtual addresses and executed. Jump instruction can be used to transfer control across non-contiguous virtual addresses. Associated with each instruction is a 32 bit sequence number which increments by one per instruction. The sequence numbers are used by both the co-processor  224  and by the host CPU  202  to synchronize instruction generation and execution. 
     (b) Finished Registers (ic_fna and ic_fnb). This pair of registers each contains a sequence number counting completed instructions. 
     (c) Todo Register (ic_tda and ic_tdb). This pair of registers each contains a sequence number counting queued instructions. 
     (d) Interrupt Register (ic_inta and ic_intb). This pair of registers each contains a sequence number at which to interrupt. 
     (e) Interrupt Status Registers (ic_stat.a_primed and ic_stat.b_primed). This pair of registers each contains a primed bit which is a bit enabling the interrupt following a match of the Interrupt and Finished Registers. This bit appears alongside other interrupt enable bits and other status/configuration information in the Interrupt Status (ic_stat) register. 
     (f) Register Access Semaphores (ic_sema and ic_semb). The host CPU  202  must obtain this semaphore before attempting resister accesses to the co-processor  224  that requires atomicity, ie. more than one register write. Any register accesses not requiring atomicity can be performed at any time. A side effect of the host CPU  202  obtaining this semaphore is that co-processor execution pauses once the currently executing instruction has completed. The Register Access Semaphore is implemented as one bit of the configuration/status register of the co-processor  224 . These registers are stored in the Instruction Controllers own register area. As noted previously, each sub-module of the co-processor has its own set of configuration and status registers. These registers are set in the course of regular instruction execution. All of these registers appear in the resister map and many are modified implicitly as part of instruction execution. These are all visible to the host via the register map. 
     3.4 Format of Plural Streams 
     As noted previously, the co-processor  224 , in order to maximize the utilization of its resources and to provide for rapid output on any external peripheral device, executes one of two independent instruction streams. Typically, one instruction stream is associated with a current output page required by an output device in a timely manner, while the second instruction stream utilizes the modules of the co-processor  224  when the other instruction stream is dormant. Clearly, the overriding imperatives are to provide the required output data in a timely manner whilst simultaneously attempting to maximize the use of resources for the preparation of subsequent pages, bands, etc. The co-processor  224  is therefore designed to execute two completely independent but identically implemented instruction streams (hereafter termed A and B). The instructions are preferably generated by software running on the host CPU  202  (FIG. 1) and forwarded to the raster image acceleration card  220  for execution by the co-processor  224 . One of the instruction streams (stream A) operates at a higher priority than the other instruction stream (stream B) during normal operation. The stream or queue of instructions is written into a buffer or list of buffers within the host RAM  203  (FIG. 1) by the host CPU  202 . The buffers are allocated at start-up time and locked into the physical memory of the host  203  for the duration of the application. Each instruction is preferably stored in the virtual memory environment of the host RAM  203  and the raster image co-processor  224  utilizes a virtual to physical address translation scheme to determine a corresponding physical address with the in-host RAM  203  for the location of the next instruction. These instructions may alternatively be stored in the co-processors  224  local memory. 
     Turning now to FIG. 8, there is illustrated the format of two instruction streams A and B  270 ,  271  which are stored within the host RAM  203 . The format of each of the streams A and B is substantially identical. 
     Briefly, the execution model for the co-processor  224  consists of: 
     Two virtual streams of instructions, the A stream and the B stream. 
     In general only one instruction is executed at a time. 
     Either stream can have priority, or priority can be by way of “round robin”. 
     Either stream can be “locked” in, ie. guaranteed to be executed regardless of stream priorities or availability of instructions on the other stream. 
     Either stream can be empty. 
     Either stream can be disabled. 
     Either stream can contain instructions that can be “overlapped”. ie. execution of the instruction can be overlapped with that of the following instruction if the following instruction is not also “overlapped”. 
     Each instruction has a “unique” 32 bit incrementing sequence number. 
     Each instruction can be coded to cause an interrupt, and/or a pause in instruction execution. 
     Instructions can be speculatively prefetched to minimize the impact of external interface latency. 
     The instruction controller  235  is responsible for implementing the co-processor&#39;s instruction execution model maintaining overall executive control of the co-processor  224  and fetching instructions from the host RAM  203  when required. On a per instruction basis, the instruction controller  235  carries out the instruction decoding and configures the various registers within the modules via CBus  231  to force the corresponding modules to carry-out that instruction. 
     Turning now to FIG. 9, there is illustrated a simplified form of the instruction execution cycle carried out by the instructions controller  235 . The instruction execution cycle consists of four main stages  276 - 279 . The first stage  276  is to determine if an instruction is pending on any instruction stream. If this is the case, an instruction is fetched  277 , decoded and executed  278  by means of updating registers  279 . 
     3.5 Determine Current Active Stream 
     In implementing the first stage  276 , there are two steps which must be taken: 
     1. Determine whether an instruction is pending; and 
     2. Decide which stream of instructions should be fetched next. 
     In determining whether instructions are pending, the following possible conditions must be explained: 
     1. whether the instruction controller is enabled: 
     2. whether the instruction controller is paused due to an internal error or interrupt; 
     3. whether there is any external error condition pending; 
     4. whether either of the A or B streams are locked; 
     5. whether either stream sequence numbering is enabled; and 
     6. whether either stream contains a pending instruction. 
     The following pseudo code describes the algorithm for determining whether an instruction is pending in accordance with the above rules. This algorithm can be hardware implemented via a state transition machine within the instruction controller  235  in known manner: 
     if not error and enabled and not bypassed and not self test mode 
     if A stream locked and not paused 
     if A stream enabled and (A stream sequencing disabled or instruction on A stream) 
     instruction pending 
      else 
     no instruction pending 
      end if 
     else 
     if B stream locked and not paused 
     if B stream enabled and (B stream sequencing disabled or instruction on B stream) 
     instruction pending 
      else 
     no instruction pending 
      end if 
     else /* no stream is locked */ 
     if (A stream enabled and not paused and (A stream sequencing disabled or instruction on A stream)) 
     or (B stream enabled and not paused and (B stream sequencing disabled or instruction on B stream)) 
     instruction pending 
      else 
     no instruction pending 
      end if 
     end 
     else /* interface controller not enabled */ 
     no instruction pending 
     end if 
     If no instruction is found pending, then the instruction controller  235  will “spin” or idle until a pending instruction is found. 
     To determine which stream is “active”, and which stream is executed next, the following possible conditions are examined: 
     1. whether either stream is locked; 
     2. what priority is given to the A and B streams and what the last instruction stream was; 
     3. whether either stream is enabled; and 
     4. whether either stream contains a pending instruction. 
     The following pseudo code implemented by the instruction controller describes how to determine the next active instruction stream: 
     if A stream locked 
     next stream is A 
     else if B stream locked 
     next stream is B 
     else /* no stream is locked */ 
     if (A stream enabled and (A stream sequencing disabled or instruction on A stream)) and not (B stream enabled and (B stream sequencing disabled or instruction on B stream)) 
     next stream is A 
     else if (B stream enabled and (B stream sequencing disabled or instruction on B stream)) and not (A stream enabled and (A stream sequencing disabled or instruction on A stream)) 
     next stream is B 
     else /* both stream have instruction */ 
     if pri= 0  /* A high, B low */ 
      next stream is A 
     else if pri= 1  /* A low, B high *, 
     next stream is B 
      else if pri= 2  or  3  /* round robin */ 
      if last stream is A 
     next stream is B 
      else 
      next stream is A 
     end if 
     end if 
     end if 
     end if 
     As the conditions can be constantly changing, all conditions must be determined together atomically. 
     3.6 Fetch Instruction of Current Active Stream 
     After the next active instruction stream is determined, the Instruction Controller  235  fetches the instruction using the address in the corresponding instruction pointer register (ic_ipa or ic_ipb). However, the Instruction Controller  235  does not fetch an instruction if a valid instruction already exists in a prefetch buffer stored within the instruction controller  235 . 
     A valid instruction is in the prefetch buffer if: 
     1. the prefetch buffer is valid; and 
     2. the instruction in the prefetch buffer is from the same stream as the currently active stream. 
     The validity of the contents of the prefetch buffer is indicated by a prefetch bit in the ic_stat register, which is set on a successful instruction prefetch. Any external write to any of the registers of the instruction controller  235  causes the contents of the prefetch buffer to be invalidated. 
     3.7 Decode and Execute Instruction 
     Once an instruction has been fetched and accepted the instruction controller  235  decodes it and configures the registers  229  of the co-processor  224  to execute the instruction. 
     The instruction format utilized by the raster image co-processor  224  differs from traditional processor instruction sets in that the instruction generation must be carried out instruction by instruction by the host CPU  202  and as such is a direct overhead for the host. Further, the instructions should be as small as possible as they must be stored in host RAM  203  and transferred over the PCI bus  206  of FIG. 1 to the co-processor  224 . Preferably, the co-processor  224  can be set up for operation with only one instruction. As much flexibility as possible should be maintained by the instruction set to maximize the scope of any future changes. Further, preferably any instruction executed by the co-processor  224  applies to a long stream of operand data to thereby achieve best performance. The co-processor  224  employs an instruction decoding philosophy designed to facilitate simple and fast decoding for “typical instructions” yet still enable the host system to apply a finer control over the operation of the co-processor  224  for “atypical” operations. 
     Turning now to FIG. 10, there is illustrated the form of a single instruction  280  which comprizes eight words each of 32 bits. Each instruction includes an instruction word or opcode  281 , and an operand or result type data word  282  setting out the format of the operands. The addresses  283 - 285  of three operands A, B and C are also provided, in addition to a result address  286 . Further, an area  287  is provided for use by the host CPU  202  for storing information relevant to the instruction. 
     The structure  290  of an instruction opcode  281  of an instruction is illustrated in FIG.  11 . The instruction opcode is 32 bits long and includes a major opcode  291 , a minor opcode  292 , an interrupt (I) bit  293 , a partial decode (Pd) bit  294 , a register length (R) bit  295 , a lock (L) bit  296  and a length  297 . A description of the fields in the instruction word  290  is as provided by the following table. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Opcode Description 
               
            
           
           
               
               
            
               
                 Field 
                 Description 
               
               
                   
               
               
                 major opcode [3..0] 
                 Instruction category 
               
               
                   
                 0: Reserved 
               
               
                   
                 1: General Colour Space Conversion 
               
               
                   
                 2: JPEG Compression and Decompression 
               
               
                   
                 3: Matrix Multiplication 
               
               
                   
                 4: Image Convolutions 
               
               
                   
                 5: Image Transformations 
               
               
                   
                 6: Data Coding 
               
               
                   
                 7: Halftone 
               
               
                   
                 8: Hierarchial image decompression 
               
               
                   
                 9: Memory Copy 
               
               
                   
                 10: Internal Register and Memory Access 
               
               
                   
                 11: Instruction Flow Control 
               
               
                   
                 12: Compositing 
               
               
                   
                 13: Compositing 
               
               
                   
                 14: Reserved 
               
               
                   
                 15: Reserved 
               
               
                 minor opcode 
                 Instruction detail. The coding of this field is 
               
               
                 [7..0] 
                 dependent on the major opcode. 
               
               
                 I 
                 1 = Interrupt and pause when competed, 
               
               
                   
                 0 = Don&#39;t interrupt and pause when completed 
               
               
                 pd 
                 Partial Decode 
               
               
                   
                 1 = use the “partial decode” mechanism. 
               
               
                   
                 0 = Don&#39;t use the “partial decode” mechanism 
               
               
                 R 
                 1 = length of instruction is specified by the Pixel 
               
               
                   
                 Organizer&#39;s input length register (po_len) 
               
               
                   
                 0 = length of instruction is specified by the opcode 
               
               
                   
                 length field. 
               
               
                 L 
                 1 = this instruction stream (A or B) is “locked” 
               
               
                   
                 for the next instruction. 
               
               
                   
                 0 = this instruction stream (A or B) is not 
               
               
                   
                 “locked” in for the next instruction. 
               
               
                 length [15..0] 
                 number of data items to read or generate 
               
               
                   
               
            
           
         
       
     
     By way of discussion of the various fields of an opcode, by setting the I-bit field  293 , the instruction can be coded such that instruction execution sets an interrupt and pause on completion of that instruction. This interrupt is called an “instruction completed interrupt”. The partial decode bit  294  provides for a partial decode mechanism such that when the bit is set and also enabled in the ic_cfg register, the various modules can be micro coded prior to the execution of the instruction in a manner which will be explained in more detail hereinafter. The lock bit  296  can be utilized for operations which require more than one instruction to set up. This can involve setting various registers prior to an instruction and provides the ability to “lock” in the current instruction stream for the next instruction. When the L-bit  296  is set, once an instruction is completed, the next instruction is fetched from the same stream. The length field  297  has a natural definition for each instruction and is defined in terms of the number of “input data items” or the number of “output data items” as required. The length field  297  is only 16 bits long. For instructions operating on a stream of input data items greater than 64,000 items the R-bit  295  can be set, in which case the input length is taken from a pollen register within the pixel organizer  246  of FIG.  2 . This register is set immediately before such an instruction. 
     Returning to FIG. 10, the number of operands  283 - 286  required for a given instruction varies somewhat depending on the type of instruction utilized. The following table sets out the number of operands and length definition for each instruction type: 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Operand Types 
               
            
           
           
               
               
               
            
               
                 Instruction 
                   
                 # of 
               
               
                 Class 
                 Length defined by 
                 operands 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 Compositing 
                 input 
                 pixels 
                 3 
               
               
                 General Color Space Conversion 
                 input 
                 pixels 
                 2 
               
               
                 JPEG decompression/compression 
                 input 
                 bytes 
                 2 
               
               
                 other decompression/compression 
                 input 
                 bytes 
                 2 
               
               
                 Image Transformations and 
                 output 
                 bytes 
                 2 
               
               
                 Convolutions 
               
               
                 Matrix Multiplication 
                 input 
                 pixels 
                 2 
               
               
                 Halftoning 
                 input 
                 pixels, bytes 
                 2 
               
               
                 Memory Copying 
                 input 
                 pixels, bytes 
                 1 
               
               
                 Hierarchial Image Decompression 
                 input 
                 pixels, bytes 
                 1 or 2 
               
               
                 Flow Control 
                 fixed 
                 fixed 
                 2 
               
               
                 Internal Access Instructions 
                 fixed 
                 fixed 
                 4 
               
               
                   
               
            
           
         
       
     
     Turning now to FIG. 12, there is illustrated, firstly, the data word format  300  of the data word or operand descriptor  282  of FIG. 10 for three operand instructions and, secondly, the data word format  301  for two operand instructions. The details of the encoding of the operand descriptors are provided in the following table: 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Operand Descriptors 
               
            
           
           
               
               
            
               
                 Field 
                 Description 
               
               
                   
               
               
                 what 
                 0 = instruction specific mode: 
               
               
                   
                 This indicates that the remaining fields of the descriptor will be 
               
               
                   
                 interpreted in line with the major opcode. Instruction specific 
               
               
                   
                 modes supported are: 
               
               
                   
                 major opcode = 0-11: Reserved 
               
               
                   
                 major opcode = 12-13: (Compositing): Implies that Operand C 
               
               
                   
                 is a bitmap attenuation. The occ_dmr register will be set 
               
               
                   
                 appropriately, with the cc=1 and normalize=0 
               
               
                   
                 major opcode = 14-15: Reserved 
               
               
                   
                 1 = sequential addressing 
               
               
                   
                 2 = tile addressing 
               
               
                   
                 3 = constant data 
               
               
                 L 
                 0 = not long: immediate data 
               
               
                   
                 1 = long: pointer to data 
               
               
                 if 
                 internal format: 
               
               
                   
                 0 = pixels 
               
               
                   
                 1 = unpacked bytes 
               
               
                   
                 2 = packed bytes 
               
               
                   
                 3 = other 
               
               
                 S 
                 0 = set up Data Manipulation Register as appropriate for this 
               
               
                   
                 operand 
               
               
                   
                 1 = use the Data Manipulation Register as is 
               
               
                 C 
                 0 = not cacheable 
               
               
                   
                 1 = cacheable 
               
               
                   
                 Note: In general a performance gain will be achieved if an 
               
               
                   
                 operand is specified as cacheable. Even operands displaying low 
               
               
                   
                 levels of referencing locality (such as sequential data) still 
               
               
                   
                 benefit from being cached - as it allows data to be burst trans- 
               
               
                   
                 ferred to the host processor and is more efficient. 
               
               
                 P 
                 external format: 
               
               
                   
                 0 = unpacked bytes 
               
               
                   
                 1 = packed stream 
               
               
                 bo[2:0] 
                 bit offset. Specifies the offset within a byte of the start of bitwize 
               
               
                   
                 data. 
               
               
                 R 
                 0 = Operand C does not describe a register to set. 
               
               
                   
                 1 = Operand C describes a register to set. 
               
               
                   
                 This bit is only relevant for instructions with less than three 
               
               
                   
                 operands. 
               
               
                   
               
            
           
         
       
     
     With reference to the above table, it should be noted that, firstly, in respect of the constant data addressing mode, the co-processor  224  is set up to fetch, or otherwize calculate, one internal data item, and use this item for the length of the instruction for that operand. In the tile addressing mode, the co-processor  224  is set up to cycle through a small set of data producing a “tiling effect”. When the L-bit of an operand descriptor is zero then the data is immediate, ie. the data items appear literally in the operand word. 
     Returning again to FIG. 10, each of the operand at result words  283 - 286  contains either the value of the operand itself or a 32-bit virtual address to the start of the operand or result where data is to be found or stored. 
     The instruction controller  235  of FIG. 2 proceeds to decode the instruction in two stages. It first checks to see whether the major opcode of the instruction is valid, raising an error if the major opcode  291  (FIG. 11) is invalid. Next, the instruction is executed by the instruction controller  235  by means of setting the various registers via CBus  231  to reflect the operation specified by the instruction. Some instructions can require no registers to be set. 
     The registers for each module can be classified into types based on their behavior. Firstly, there is the status register type which is “read only” by other modules and “read/write” by the module including the register. Next, a first type of configuration register, hereinafter called “config1”, is “read/write” externally by the modules and “read only” by the module including the register. These registers are normally used for holding larger type configuration information, such as address values. A second type of configuration register, herein known as “config2”, is readable and writable by any module but is read only by the module including the register. This type of register is utilized where bit by bit addressing of the register is required. 
     A number of control type registers are provided. A first type, hereinafter known as “control1” registers, is readable and irritable by all modules (including the module which includes the register). The controll registers are utilized for holding large control information such as address values. Analogously, there is further provided a second type of control register, hereinafter known as “control2”, which can be set on a bit by bit basis. 
     A final type of register known as an interrupt register has bits within the register which are settable to 1 by the module including the register and resettable to zero externally by writing a “1” to the bit that has been set. This type of register is utilized for dealing with the interrupts/errors flagged by each of the modules. 
     Each of the modules of the co-processor  224  sets a c_active line on the CBus  231  when it is busy executing an instruction. The instruction controller  235  can then determine when instructions have been completed by “OR-ing” the c_active lines coming from each of the modules over the CBus  231 . The local memory controller module  236  and the peripheral interface controller module  237  are able to execute overlapped instructions and include a c_background line which is activated when they are executing an overlapped instruction. The overlapped instructions are “local DMA” instructions transferring data between the local memory interface and the peripheral interface. 
     The execution cycle for an overlapped local DMA instruction is slightly different from the execution cycle of other instructions. If an overlapped instruction is encountered for execution, the instruction controller  235  picks whether there is already an overlapped instruction executing. If there is, or overlapping is disabled, the instruction controller  235  waits for that instruction to finish before proceeding with execution of that instruction. If there is not, and overlapping is enabled, the instruction controller  235  immediately decodes the overlapped instruction and configures the peripheral interface controller  237  and local memory controller  236  to carry out the instruction. After the register configuration is completed, the instruction controller  235  then goes on to update its registers (including finished register, status register, instruction pointer, etc.) without waiting for the instruction to “complete” in the conventional sense. At this moment, if the finished sequence number equals the interrupt sequence number, ‘the overlapped instruction completed’ interrupt is primed rather than raising the interrupt immediately. The ‘overlapped instruction completed’ interrupt is raized when the overlapped instruction has fully completed. 
     Once the instruction has been decoded, the instruction controller attempts to prefetch the next instruction while the current instruction is executing. Most instructions take considerably longer to execute than they will to fetch and decode. The instruction controller  235  prefetches an instruction if all of the following conditions are met: 
     1. the currently executing instruction is not set to interrupt and pause; 
     2. the currently executing instruction is not a jump instruction; 
     3. the next instruction stream is prefetch-enabled; and 
     4. there is another instruction pending. 
     If the instruction controller  235  determines that prefetching is possible it requests the next instruction, places it in a prefetch buffer and then validates he buffer. At this point there is nothing more for the instruction controller  235  to do until the currently executing instruction has completed. The instruction controller  235  determines the completion of an instruction by examining the c_active and c_background lines associated with the CBus  231 . 
     3.8 Update Registers of Instruction Controller 
     Upon completion of an instruction, the instruction controller  235  updates its registers to reflect the new state. This must be done atomically to avoid problems with synchronising with possible external accesses. This atomic update process involves: 
     1. Obtaining the appropriate Register Access Semaphore. If the semaphore is taken by an agent external to the Instruction Controller  235 , the instruction execution cycle waits at this point for the semaphore to be released before proceeding. 
     2. Updating the appropriate registers. The instruction pointer (ic_ipa or ic_ipb) is incremented by the size of an instruction, unless the instruction was a successful jump, which case the target value of the jump is loaded into the instruction pointer. 
     The finished register (ic_fna or ic_fnb), is then incremented if sequence numbering is enabled. 
     The status register (ic_stat) is also updated appropriately to reflect the new state. This includes setting the pause bits if necessary. The Instruction Controller  235  pauses if an interrupt has occurred and pausing is enabled for that interrupt or if any error has occurred. Pausing is implemented by setting the instruction stream pause bits in the status register (a_pause or b_pause bits in ic_stat). To resume instruction execution, these bits should be reset to 0. 
     3. Asserting a c_end signal on the CBus  231  for one clock cycle, which indicates to other modules in the co-processor  224  that an instruction has been completed. 
     4. Raising an interrupt if required. An interrupt is raized if: 
     a. “Sequence number completed” interrupt occurs. That is, if the finished register (ic_fna or ic_fnb) sequence number is the same as interrupt sequence number. Then this interrupt is primed, sequence numbering is enabled, and the interrupt occurs; or 
     b. the just completed instruction was coded to interrupt on completion, then this mechanism is enabled. 
     3.9 Semantics of the Register Access Semaphore 
     The Register Access Semaphore is a mechanism that provides atomic accesses to multiple instruction controller registers. The registers that can require atomic access are as follows: 
     1. Instruction pointer register (ic_ipa and ic_ipb) 
     2. Todo registers (ic_tda and ic_tdb) 
     3. Finished registers (ic_fna and ic_fnb) 
     4. Interrupt registers (ic_inta and ic_intb) 
     5. The pause bits in the configuration register (ic_cfp) 
     External agents can read all registers safely at any time. External agents are able to write any registers at any time, however to ensure that the Instruction Controller  235  does not update values in these registers, the external agent must first obtain the Register Access Semaphore. The Instruction Controller does not attempt to update any values in the abovementioned registers if the Register Access Semaphore is claimed externally. The instruction controller  235  updates all of the above mentioned registers in one clock cycle to ensure atomicity. 
     As mentioned above, unless the mechanism is disabled, each instruction has associated with it a 32 bit “sequence number”. Instruction sequence numbers increment wrapping through from 0xFFFFFFFF to 0x00000000. 
     When an external write is made into one of the Interrupt Registers (ic_inta or ic_intb), the instruction controller  235  immediately makes the following comparisons and updates: 
     1. If the interrupt sequence number (ie. the value in the Interrupt Register) is “greater” (in a modulo sense) than the finished sequence number (ie. the value in the Finished Register) of the same stream, the instruction controller primes the “sequence number completed” interrupt mechanism by setting the “sequence number completed” primed bit (a_primed or b_primed bit in ic_stat) in the status register. 
     2. If the interrupt sequence number is not “greater” than the finished sequence number, but there is an overlapped instruction in progress in that stream and the interrupt sequence number equals the last overlapped instruction sequence number (ie. the value in the ic_loa or ic_lob register), then the instruction controller primes the “overlapped instruction sequence number completed” interrupt mechanism by setting the a_ol_primed or b_ol_primed bits in the icstat register. 
     3. If the interrupt sequence number is not “greater” than the finished sequence number, and there is an overlapped instruction in progress in that stream, but the interrupt sequence number does not equal the last overlapped instruction sequence number, then the interrupt sequence number represents a finished instruction, and no interrupt mechanism is primed. 
     4. If the interrupt sequence number is not “greater” than the finished sequence number, and there is no overlapped instruction in progress in that stream, then the interrupt sequence number must represent a finished instruction, and no interrupt mechanism is primed. 
     External agents can set any of the interrupt primed bits (bits a_primed, a_ol_primed. b_primed or b_ol_primed) in the status register to activate or de-activate this interrupt mechanism independently. 
     3.10 Instruction Controller 
     Turning now to FIG. 13, there is illustrated the instruction controller  235  in more detail. The instruction controller  235  includes an execution controller  305  which implements the instruction execution cycle as well as maintaining overall executive control of the co-processor  224 . The functions of the execution controller  305  include maintaining overall executive control of the instruction controller  235 , determining instructing sequencing, instigating instruction fetching and prefetching, initiating instructing decoding and updating the instruction controller registers. The instruction controller further includes an instruction decoder  306 . The instruction decoder  306  accepts instructions from a prefetch buffer controller  307  and decodes them according the aforementioned description. The instruction decoder  306  is responsible for configuring registers in the other co-processor modules to execute the instruction. The prefetch buffer controller  307  manages the reading and writing to a prefetch buffer within the prefetch buffer controller and manages the interfacing between the instruction decoder  306  and the input interface switch  252  (FIG.  2 ). The prefetch buffer controller  307  is also responsible for managing the updating of the two instruction pointer registers (ic_ipa and ic_ipb). Access to the CBus  231  (FIG. 2) by the instruction controller  235 , the miscellaneous module  239  (FIG. 2) and the external interface controller  238  (FIG. 2) is controlled by a “CBus” arbitrator  308  which arbitrates between the three modules&#39; request for access. The requests are transferred by means of a control bus (CBus)  231  to the register units of the various modules. 
     Turning now to FIG. 14, there is illustrated the execution controller  305  of FIG. 13 in more detail. As noted previously, the execution controller is responsible for implementing the instruction execution cycle  275  of FIG. 9 and, in particular, is responsible for: 
     1. Determining which instruction stream the next instruction is to come from; 
     2. Initiating fetching of that instruction; 
     3. Signalling the instruction decoder to decode the instruction as residing in the prefetch buffer; 
     4. Determining and initiating any prefetching of the next instruction; 
     5. Determining instruction completion; and 
     6. Updating the registers after the instruction has completed. 
     The execution controller includes a large core state machine  310  hereinafter known as “the central brain” which implements the overall instruction execution cycle. Turning to FIG. 15, there is illustrated the state machine diagram for the central brain  310  implementing the instruction execution cycle as aforementioned. Returning to FIG. 14, the execution controller includes an instruction prefetch logic unit  311 . This unit is responsible for determining whether there is an outstanding instruction to be executed and which instruction stream the instruction belongs to. The start  312  and prefetch  313  states of the transition diagram of FIG. 15 utilize this information in obtaining instructions. A register management unit  317  of FIG. 14 is responsible for monitoring the register access semaphores on both instruction streams and updating all necessary registers in each module. The register management unit  317  is also responsible for comparing the finished register (ic_fna or ic_fnb) With the interrupt register (ic_inta or ic_intb) to determine if a “sequence number completed” interrupt is due. The register management unit  317  is also responsible for interrupt priming. An overlapped instructions unit  318  is responsible for managing the finishing off of an overlapped instruction through management of the appropriate status bits in the ic_stat register. The execution controller also include a decoder interface unit  319  for interfacing between the central brain  310  and the instruction decoder  306  of FIG.  13 . 
     Turning now to FIG. 16, there is illustrated the instruction decoder  306  in more detail. The instruction decoder is responsible for configuring the co-processor to execute the instructions residing in the prefetch buffer. The instruction decoder  306  includes an instruction decoder sequencer  321  which comprizes one large state machines broken down into many smaller state machines. The instruction sequencer  321  communicates with a CBus dispatcher  312  which is responsible for setting the registers within each module. The instruction decoder sequencer  321  also communicates relevant information to the execution controller such as instruction validity and instruction overlap conditions. The instruction validity check being to (a check that the instruction opcode is not one of the reserved opcodes. 
     Turning now to FIG. 17, there is illustrated, in more detail, the instruction dispatch sequencer  321  of FIG.  16 . The instruction dispatch sequencer  321  includes a overall sequencing control state machine  324  and a series of per module configuration sequencer state machines, eg.  325 ,  326 . One per module configuration sequencer state machine is provided for each module to be configured. Collectively the state machines implement the co-processor&#39;s microprogramming of the modules. The state machines, ea.  325 , instruct the CBus dispatcher to utilize the global CBus to set various registers so as to configure the various modules for processing. A side effect of writing to particular registers is that the instruction execution commences. Instruction execution typically takes much longer than the time it takes for the sequencer  321  to configure the co-processor registers for execution. In appendix A, attached to the present specification, there is disclosed the microprogramming operations performed by the instruction sequencer of the co-processor in addition to the form of set up by the instruction sequencer  321 . 
     In practice, the Instruction Decode Sequencer  321  does not configure all of the modules within the co-processor for every instruction. The table below shows the ordering of module configuration for each class of instruction with the module configured including the pixel organizer  246  (PO), the data cache controller  240  (DCC), the operand organizer B  247  (OOB), the operand organizer C  248  (OOC), main data path  242  (MDP), results organizer  249  (RO), and JPEG encoder  241  (JC). Some of the modules are never configured during the course of instruction decoding. These modules are the External Interface Controller  238  (EIC), the Local Memory Controller  236  (LMC), the Instruction Controller  235  itself (IC), the Input Interface Switch  252  (IIS) and the Miscellaneous Module (MM). 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Module Setup Order 
               
            
           
           
               
               
               
            
               
                 Instruction 
                 Module Configuration 
                 Sequence 
               
               
                 Class 
                 Sequence 
                 ID 
               
               
                   
               
               
                 Compositing 
                 PO. DCC. OOB. OOC. MDP. RO 
                 1 
               
               
                 CSC 
                 PO. DCC. OOB. OOC. MDP, RO 
                 2 
               
               
                 JPEG coding 
                 PO. DCC. OOB. OOC, JC. RO 
                 3 
               
               
                 Data coding 
                 PO. DCC. OOB. OOC, JC. RO 
                 3 
               
               
                 Transformations and 
                 PO. DCC. OOB. OOC, MDP, RO 
                 2 
               
               
                 Convolutions 
               
               
                 Matrix Multiplication 
                 PO. DCC. OOB. OOC. MDP, RO 
                 2 
               
               
                 Halftoning 
                 PO. DCC. OOB. MDP, RO 
                 4 
               
               
                 General memory copy 
                 PO. JC. RO 
                 8 
               
               
                 Peripheral DMA 
                 PIC 
                 5 
               
               
                 Hierarchial Image - 
                 PO. DCC. OOB. OOC, MDP. RO 
                 6 
               
               
                 Horizontal Interpolation 
               
               
                 Hierarchial Image - 
                 PO. DCC. OOB. OOC. MDP. RO 
                 4 
               
               
                 others 
               
               
                 Internal access 
                 RO. RO. RO. RO 
                 7 
               
               
                 others 
                 — 
                 — 
               
               
                   
               
            
           
         
       
     
     Turning now to FIG. 17, each of the module configuration sequencers, eg.  325  is responsible for carrying out the required register access operations to configure the particular module. The overall sequencing control state machine  324  is responsible for overall operation of the module configuration sequencer in the aforementioned order. 
     Referring now to FIG. 18, there is illustrated  330  the state transition diagram for the overall sequencing control unit which basically activates the relevant module configuration sequencer in accordance with the above table. Each of the modules configuration sequencers is responsible for controlling the CBus dispatcher to alter register details in order to set the various registers in operation of the modules. 
     Turning now to FIG. 19, there is illustrated the prefetch buffer controller  307  of FIG. 13 in more detail. The prefetch buffer controller consists of a prefetch buffer  335  for the storage of a single co-processor instruction (six times 32 bit words). The prefetch buffer includes one write port controlled by a IBus sequencer  336  and one read port which provides data to the instruction decoder, execution controller and the instruction controller CBus interface. The IBus sequencer  336  is responsible for observing bus protocols in the connection of the prefetch buffer  335  to the input interface switch. An address manager unit  337  is also provided which deals with address generation for instruction fetching. The address manager unit  337  performs the functions of selecting one of ic_ipa or ic_ipb to place on the IBus to the input interface switch, incrementing one of ic_ipa or ic_ipb based on which stream the last instructions was fetched from and channelling jump target addresses back to the ic_ipa and ic_ipb register. A PBC controller  339  maintains overall control of the prefetched buffer controller  307 . 
     3.11 Description of a Modules Local Register File 
     As illustrated in FIG. 13, each module, including the instruction controller module itself, has an internal set of registers  304  as previously defined in addition to a CBus interface controller  303  as illustrated in FIG.  20  and which is responsible for receiving CBus requests and updating internal registers in light of those requests. The module is controlled by writing registers  304  within the module via a CBus interface  302 . A CBus arbitrator  308  (FIG. 13) is responsible for determining which module of the instruction controller  235 , the external interface controller or the miscellaneous module is able to control the CBus  309  for acting as a master of the CBus and for the writing or reading of registers. 
     FIG. 20, illustrates, in more detail, the standard structure of a CBus interface  303  as utilized by each of the modules. The standard CBus interface  303  accepts read and write requests from the CBus  302  and includes a register file  304  which is utilized  341  and updated on  341  by the various submodules within a module. Further, control lines  344  are provided for the updating of any submodule memory areas including reading of the memory areas. The standard CBus interface  303  acts as a destination on the CBus, accepting read and write requests for the register  304  and memory objects inside other submodules. 
     A “c_reset” signal  345  sets every register inside the Standard CBus interface of  103  to their default states. However, “c_reset” will not reset the state machine that controls the handshaking of signals between itself and the CBus Master, so even if “c_reset” is asserted in the middle of a CBus transaction, the transaction will still finish, with undefined effects. The “c_int”  347 . “c_exp”  348  and “c_err”  349  signals are generated from the content of a modules err_int and err_int_en registers by the following equations:              c_err   =       ∑       error        [   i   ]                     not                 reserved                         error        [   i   ]                     AND                   err_mask        [   i   ]                   (   1   )               c_int   =       ∑     int                   errupt        [   i   ]                     not                 reserved                       int                   errput        [   i   ]                     AND                   int_mask        [   i   ]                   (   2   )                         
     c_exp=exception[i] AND exp_mask[i] (3) 
     The signals “c_sdata_in”  345  and “c_svalid_in” are data and valid signals from the previous module in a daisy chain of modules. The signals “c_sdata_out” and “c_svalid_out”  350  are data and valid signals going to the next module in the daisy chain. 
     The functionality of the Standard CBus interface  303  includes: 
     1. register read/write handling 
     2. memory area read write handling 
     3. test mode read/write handling 
     4. submodule observe/update handling 
     3.12 Register Read/WNrite Handling 
     The Standard CBus Interface  303  accepts register read/write and bit set requests that appears on the CBus. There are two types of CBus instructions that Standard CBus Interface handles: 
     1. Type A 
     Type A operations allow other modules to read or write 1, 2, 3, or 4 bytes into any register inside Standard CBus Interface  303 . For write operations, the data cycle occurs in the clock cycle immediately after the instruction cycle. Note that the type field for register write and read are “1000” and “1001” respectively. The Standard CBus Interface  303  decodes the instruction to check whether the instruction is addressed to the module, and whether it is a read or write operation. For read operation, the Standard CBus Interface  303  uses the “reg” field of the CBus transaction to select which register output is to put into the “c_sdata” bus  350 . For write operations, the Standard CBus Interface  303  uses the “reg” and “byte” fields to write the data into the selected register. After read operation is completed, the Standard CBus Interface returns the data and asserts “c_svalid”  350  at the same time. After write operations are completed, the Standard CBus Interface  303  asserts “c_svalid”  350  to acknowledge. 
     2. Type C 
     Type C operations allow other modules to write one or more bits in one of the bytes in one of the registers. Instruction and data are packed into one word. 
     The Standard CBus Interface  303  decodes the instruction to check whether the instruction is addressed to the module. It also decodes “reg”, “byte” and “enable” fields to generate the required enable signals. It also latches the data field of the instruction, and distributes it to all four bytes of a word so the required bit(s) are written in every enabled bit(s) in every enabled byte(s). No acknowledgment is required for this operation. 
     3.13 Memory Area Read/Write Handling 
     The Standard CBus Interface  303  accepts memory read memory write requests that appears on the CBus. While accepting a memory read/write request, the Standard CBus Interface  303  checks whether the request is addressed to the module. Then, by decoding the address field in the instruction, the Standard CBus Interface generates the appropriate address and address strobe signals  344  to the submodule which a memory read/write operation is addressed to. For write operations the Standard CBus Interface also passes on the byte enable signals from the instruction to the submodules. 
     The operation of the standard CBus interface  303  is controlled by a read/write controller  352  which decodes the type field of a CBus instruction from the CBus  302  and generates the appropriate enable signals to the register file  304  and output selector  353  so that the data is latched on the next cycle into the register file  304  or forwarded to other submodules  344 . If the CBus instruction is a register read operation, the read/write controller  352  enables the output selector  353  to select the correct register output going onto the “c_sdata bus”  345 . If the instruction is a register write operation, the read/write controller  352  enables the register file  304  to select the data in the next cycle. If the instruction is a memory area read or write, then the read/write controller  352  generates the appropriate signals  344  to control those memory areas under a modules control. The register file  304  contains four parts, being a register select decoder  355 , an output selector  353 , interrupt  356 , error  357  and exception  358  generators, unmasked error generator  359  and the register components  360  which make up the registers of that particular module. The register select decoder  355  decodes the signal “ref_en” (register file enable), “write” and “reg” from the read/write controller  352  and generates the register enable signals for enabling the particular register of interest. The output selector  353  selects the correct register data to be output on c_sdata_out lines  350  for register read operations according to the signal “reg” output from the read/write controller  352 . 
     The exception generators  356 - 359  generate an output error signal, ea.  347 - 349 ,  362  when an error is detected on their inputs. The formula for calculating each output error is as aforementioned. 
     The register components  360  can be defined to be of a number of types in accordance with requirements as previously discussed when describing the structure of the register set with reference to Table 5. 
     3.14 CBus Structure 
     As noted previously, the CBus (control bus) is responsible for the overall control of each module by way transferring information for the setting of registers within each module&#39;s standard CBus interface. It will be evident from the description of the standard CBus interface that the CBus serves two main purposes: 
     1. It is a control bus that drives each of the modules. 
     2. It is the access bus for RAMs. FIFOs and state information contained within each of the modules. 
     The CBus uses an instruction-address-data protocol to control modules by the setting configuration registers within the modules. In general, registers will be set on a per instruction basis but can be modified at any time. The CBus gathers status and other information, and accesses RAM and FIFO data from the various modules by requesting data. 
     The CBus is driven on a transaction by transaction basis either by: 
     1. the Instruction Controller  235  (FIG. 2) when executing instructions. 
     2. the External Interface Controller  238  (FIG. 2) when performing a target (slave) mode bus operation, or 
     3. an external device if the External CBus Interface is so configured. 
     In each of these cases, the driving module is considered to be the source module of the CBus, and all other modules possible destinations. Arbitration on this bus is carried out by the Instruction Controller. 
     The following table sets out one form of CBus signal definitions suitable for use with the preferred embodiment: 
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 CBus Signal Definition 
               
            
           
           
               
               
               
               
            
               
                   
                 Name 
                 Type 
                 Definition 
               
               
                   
                   
               
               
                   
                 c_iad[31:0] 
                 source 
                 instruction-address-data 
               
               
                   
                 c_valid 
                 source 
                 CBus instruction valid 
               
               
                   
                 c_sdata[31:0] 
                 destination 
                 status/read data 
               
               
                   
                 c_svalid 
                 destination 
                 status/read data valid 
               
               
                   
                 c_reset[15:0] 
                 source 
                 reset lines to each 
               
               
                   
                   
                   
                 module 
               
               
                   
                 c_active[15:0] 
                 destination 
                 active lines from each 
               
               
                   
                   
                   
                 module 
               
               
                   
                 c_background[15:0] 
                 destination 
                 background active lines 
               
               
                   
                   
                   
                 from each module 
               
               
                   
                 c_int[15:0] 
                 destination 
                 interrupt lines from each 
               
               
                   
                   
                   
                 module 
               
               
                   
                 c_error[15:0] 
                 destination 
                 error lines from each 
               
               
                   
                   
                   
                 module 
               
               
                   
                 c_req1.c_req2 
                 EIC.external 
                 bus control request 
               
               
                   
                 c_gnt1.c_gnt2 
                 IC 
                 bus control grant 
               
               
                   
                 c_end 
                 IC 
                 end of instruction 
               
               
                   
                 clk 
                 global 
                 clock 
               
               
                   
                   
               
            
           
         
       
     
     A CBus c_iad signal contains the addressing data and is driven by the controller in two distinct cycles: 
     1. Instruction cycles (c_valid high, where the CBus instruction and an address is driven onto c_iad; and 
     2. Data cycles (c_valid low) where data is driven onto c_iad (write operations) or c_sdata (read operations). 
     In the case of a write operation, the data associated with an instruction is placed on the c_iad bus in the cycle directly following the instruction cycle. In the case of a read operation, the target module of the read operation drives the c_sdata signal until the data cycle completes. 
     Turning now to FIG. 21, the bus includes a 32 bit instruction-address-data field which can be one of three types  370 - 372 . 
     1. Type A operations ( 370 ) are used to read and write registers and the per-module data areas within the co-processor. These operations can be generated by the external interface controller  238  performing target mode PCI cycles, by the instruction controller  231  configuring the co-processor for an instruction, and by the External CBus Interface. 
     For these operations, the data cycle occurs in the clock cycle immediately following the instruction cycle. The data cycle is acknowledged by the designation module using the c_svalid signal. 
     2. Type B operations ( 371 ) are used for diagnostic purposes to access any local memory and to generate cycles on the Generic Interface. These operations will be generated by the External Interface Controller performing target mode PCI cycles and by the External CBus Interface. The data cycle can follow at any time after the instruction cycle. The data cycle is acknowledged by the destination module using the c_svalid signal. 
     3. Type C operations ( 372 ) are used to set individual bits within a module&#39;s registers. These operations will be generated by the instruction controller  231  configuring the co-processor&#39;s for an instruction and by the External CBus Interface. There is no data cycle associated with a Type C operation, data is encoded in the instruction cycle. 
     The type field of each instruction encodes the relevant CBus transaction type in according with the following table: 
     
       
         
           
               
             
               
                 TABLE 7 
               
             
            
               
                   
               
               
                 CBus Transaction Types 
               
            
           
           
               
               
               
            
               
                 c_iad.type 
                   
                 instruction 
               
               
                 value 
                 transaction type 
                 format type 
               
               
                   
               
               
                 0000 
                 no-op 
                 A, B, C 
               
               
                 0001 
                 reserved 
               
               
                 0010 
                 peripheral interface write 
                 B 
               
               
                 0011 
                 peripheral interface read 
                 B 
               
               
                 0100 
                 generic bus write 
                 B 
               
               
                 0101 
                 generic bus read 
                 B 
               
               
                 0110 
                 local memory write 
                 B 
               
               
                 0111 
                 local memory read 
                 B 
               
               
                 1000 
                 register write 
                 A 
               
               
                 1001 
                 register read 
                 A 
               
               
                 1010 
                 module memory write 
                 A 
               
               
                 1011 
                 module memory read 
                 A 
               
               
                 1100 
                 test mode write 
                 A 
               
               
                 1101 
                 test mode read 
                 A 
               
               
                 1110 
                 bit set 
                 C 
               
               
                 1111 
                 reserved 
               
               
                   
               
            
           
         
       
     
     The byte field is utilized for enabling bits within a register to be set. The module field sets out the particular module to which an instruction on the CBus is addressed. The register field sets out which of the registers within a module is to be updated. The address field is utilized for addressing memory portions where an operation is desired on those memory portions and can be utilized for addressing RAMs, FIFOs, etc. The enable field enables selected bits within a selected byte when a bit set instruction is utilized. The data field contains the bit wize data of the bits to be written to the byte selected for update. 
     As noted previously the CBus includes a c_active line for each module, which is asserted when ever a module has outstanding activity pending. The instruction controller utilizes these signals to determine when an instruction has completed. Further, the CBus contains a c_background line for each module that can operate in a background mode in addition to any preset, error and interrupt lines, one for each module, for resetting, detecting errors and interrupts. 
     3.15 Co-processor Data Types and Data Manipulation 
     Returning now to FIG. 2, in order to substantially simplify the operation of the co-processor unit  224 , and in particular the operation of the major computational units within the co-processor being the JPEG coder  241  and the main data path  242 , the co-processor utilizes a data model that differentiates between external formats and internal formats. The external data formats are the formats of data as it appears on the co-processor&#39;s external interfaces such as the local memory interface or the PCI bus. Conversely, the internal data formats are the formats which appear between the main functional modules of the co-processor  224 . This is illustrated schematically in FIG. 22 which shows the various input and output formats. The input external format  381  is the format which is input to the pixel organizer  246 , the operand organizer B  247  and the operand organizer C  248 . These organizers are responsible for reformatting the input external format data into any of a number of input internal formats  382 , which may be inputted to the JPEG coder unit  241  and the main data path unit  242 . These two functional units output data in any of a number of output internal formats  383 , which are converted by the results organizer  249  to any of a number of required output formats  304 . 
     In the embodiment shown, the external data formats can be divided into three types. The first type is a “packed stream” of data which consists of a contiguous stream of data having up to four channels per data quantum, with each channel consisting of one, two, four, eight or sixteen bit samples. This packed stream can typically represent pixels, data to be turned into pixels, or a stream of packed bits. The co-processor is designed to utilize little endian byte addressing and big endian bit addressing within a byte. In FIG. 23, there is illustrated a first example  386  of the packed stream format. It is assumed that each object  387  is made up of three channels being channel  0 , channel  1  and channel  2 , with two bits per channel. The layout of data for this format is as indicated  388 . In a next example  390  of FIG. 24, a four channel object  395  having eight bits per channel is illustrated  396  with each data object taking up a 32 bit word. In a third example  395  of FIG. 25, one channel objects  396  are illustrated which each take up eight bits per channel starting at a bit address  397 . Naturally, the actual width and number of channels of data will vary depending upon the particular application involved. 
     A second type of external data format is the “unpacked byte stream” which consists of a sequence of 32 bit words, exactly one byte within each word being valid. An example of this format is shown in FIG.  26  and designated  399 , in which a single byte  400  is utilized within each word. 
     A further external data format is represented by the objects classified as an “other” format. Typically, these data objects are large table-type data representing information such as colour space conversion tables. Huffman coding tables and the like. 
     The co-processor utilizes four different internal data types. A first type is known as a “packed bytes” format which comprizes 32 bit words, each consisting of four active bytes, except perhaps for a final 32 bit word. In FIG. 27, there is illustrated one particular example  402  of the packed byte format with 4 bytes per word. 
     The next data type, illustrated with reference to FIG. 28, is “pixel” format and comprises 32 bit words  403 , consisting of four active byte channels. This pixel format is interpreted as four channel data. 
     A next internal data type illustrated with reference to FIG. 29 is an “unpacked byte” format, in which each word consists of one active byte channel  405  and three inactive byte channels, the active byte channel being the least significant byte. 
     All other internal data objects are classified by the “other” data format. 
     Input data in a given external format is converted to the appropriate internal format. FIG. 30 illustrates the possible conversions carried out by the various organizers from an external format  410  to an internal format  411 . Similarly, FIG. 31 illustrates the conversions carried out by the results organizer  249  in the conversion from internal formats  412  to external formats  413 . 
     The circuitry to enable the following conversions to take place are described in greater detail below. 
     Turning firstly to the conversion of input data external formats to internal formats, in FIG. 32 there is shown the methodology utilized by the various organizers in the conversion process. Starting initially with the external other format  416 , this is merely passed through the various organizers unchanged. Next, the external unpacked byte format  417  undergoes unpacked normalization  418  to produce a format  419  known as internally unpacked bytes. The process of unpacked normalization  418  involves discarding the three inactive bytes from an externally unpacked byte stream. The process of unpacked normalization is illustrated in FIG. 33 wherein the input data  417  having four byte channels wherein only one byte channel is valid results in the output format  419  which merely comprizes the bytes themselves. 
     Turning again to FIG. 32, the process of packed normalization  421  involves translating each component object in an externally packed stream  422  into a byte stream  423 . If each component of a channel is less than a byte in size then the samples are interpolated up to eight bit values. For example, when translating four bit quantities to byte quantities, the four bit quantity 0xN is translated to the byte value 0xNN. Objects larger than one byte are truncated. The input object sizes supported on the stream  422  are 1, 2, 4, 8 and 16 bit sizes, although again these may be different depending upon the total width of the data objects and words in any particular system to which the invention is applied. 
     Turning now to FIG. 34, there is illustrated one form of packed normalization  421  on input data  422  which is in the form of 3 channel objects with two bits per channel (as per the data format  386  of FIG.  23 ). The output data comprizes a byte channel format  423  with each channel “interpolated up” when necessary to comprize an eight bit sample. 
     Returning to FIG. 32, the pixel streams are then subjected to either a pack operation  425 , an unpacked operation  426  or a component selection operation  427 . 
     In FIG. 35 there is shown an example of the packed operation  425  which simply involves discarding the inactive byte channel and producing a byte stream, packed up with four active bytes per word. Hence, a single valid byte stream  430  is compressed into a format  431  having four active bytes per word. The unpacking operation  426  involves almost the reverse of the packing operation with the unpacked bytes being placed in the least significant byte of a word. This is illustrated in FIG. 36 wherein a packed byte stream  433  is unpacked to produce result  434 . 
     The process of component selection  427  is illustrated in FIG.  37  and involves selecting N components from an input stream, where N is the number of input channels per quantum. The unpacking process can be utilized to produce “prototype pixels” eg.  437 , with the pixel channels filled from the least significant byte. Turning to FIG. 38, there is illustrated an example of component selection  440  wherein input data in the form  436  is transformed by the component selection unit  427  to produce prototype pixel format  437 . 
     After component selection, a process of component substitution  440  (FIG. 32) can be utilized. The component substitution process  440  is illustrated in FIG.  38  and comprizes replacing selected components with a constant data value stored within an internal data register  441  to produce, as an example, output components  242 . 
     Returning again to FIG. 32, the output of stages  425 ,  426  and  440  is subjected to a lane swapping process  444 . The lane swapping process, as illustrated in FIG. 39, involves a byte-wize multiplexing of any lane to any other lane, including the replication of a first lane onto a second lane. The particular example illustrated in FIG. 39 includes the replacement of channel  3  with channel  1  and the replication of channel  3  to channels  2  and channel  1 . 
     Returning again to FIG. 32, after the lane swapping step  444  the data stream can be optionally stored in the multi-used value RAM  250  before being read back and subjected to a replication process  446 . 
     The replication process  446  simply replicates the data object whatever it may be. In FIG. 40, there is illustrated a process of replication  446  as applied to pixel data. In this case, the replication factor is one. 
     In FIG. 41, there is illustrated a similar example of the process of replication applied to packed byte data. 
     In FIG. 42, there is illustrated the process utilized by the result organizer  249  for transferral of data in an output internal format  383  to an output external format  384 . This process includes equivalent steps  424 ,  425 ,  426  and  440  of the conversion process described in FIG.  32 . Additionally, the process  450  includes the steps of component deselection  451 , denormalization  452 , byte addressing  453  and write masking  454 . The component deselection process  451 , as illustrated in FIG. 43, is basically the inverse operation of the component selection process  427  of FIG.  37  and involves the discardino of unwanted data. For example, in FIG. 43, only 3 valid channels of the input are taken and packed into data items  456 . 
     The denormalization process  452  is illustrated with reference to FIG.  44  and is loosely the inverse operation of the packed normalization process  421  of FIG.  34 . The denormalization process involves the translation of each object or data item, previously treated as a byte, to a non-byte value. 
     The byte addressing process  453  of FIG. 42 deals with any byte wize reorganization that is necessary to deal with byte addressing issues. For an externally unpacked byte output stream, the least two significant bits of the stream&#39;s address correspond to the active stream. The byte addressing step  453  is responsible for re-mapping the output stream from one byte channel to another when external unpacked bytes are utilized (FIG.  45 ). Where an externally packed stream is utilized (FIG.  46 ), the byte addressing module  453  remaps the start address of the output stream as illustrated. 
     The write masks process  454  of FIG. 42 is illustrated in FIG.  47  and is used to mask off a particular channel eg.  460  of a packed stream which is not to be written out. 
     The details of the input and output data type conversion to be applied are specified by the contents of the corresponding Data Manipulation Registers: 
     The Pixel Organizer Data Manipulation Register (po_dmr) 
     The Operand Organizer B and Operand Organizer C Data Manipulation Registers (oob_dmr, ooc_dmr); 
     The Result Organizer Data Manipulation Register (ro_dmr); 
     Each of the Data Manipulation Registers can be set up for an instruction in one of two ways: 
     1. They can be explicitly set using any of the standard methods for writing to the co-processor&#39;s registers immediately prior to the execution of the instruction: or 
     2. They can be set up by the co-processor itself to reflect a current instruction. 
     During the instruction decoding process, the co-processor examines the contents of the Instruction Word and the Data Word of the instruction to determine. amongst other things, how to set up the various Data Manipulation Registers. Not all combinations of the instruction and operands make sense. Several instructions have implied formats for some operands. Instructions that are coded with inconsistent operands may complete without error, although any data so operated is “undefined”. If the ‘S’ bit of the corresponding Data Descriptor is 0, the co-processor sets the Data Manipulation Register to reflect the current instruction. 
     The format of the Data Manipulation Registers is illustrated in FIG.  48 . The following table sets out the format of the various bits within the registers as illustrated in FIG.  48 : 
     
       
         
           
               
             
               
                 TABLE 8 
               
             
            
               
                   
               
               
                 Data Manipulation Register Format 
               
            
           
           
               
               
            
               
                 Field 
                 Description 
               
               
                   
               
               
                 1s3 
                 Lane Swap for byte 3 (most significant byte) 
               
               
                 1s2 
                 Lane swap for byte 2 
               
               
                 1s1 
                 Lane swap for byte 1 
               
               
                 1s0 
                 Lane swap for byte 0 
               
               
                 suben 
                 Substitution Enables 
               
               
                   
                 1 = substitute data from Internal Data Register for this byte 
               
               
                   
                 0 = do not substitute data from Internal Data Register for this 
               
               
                   
                 byte 
               
               
                 replicate 
                 Replication Count 
               
               
                   
                 Indicates the number of additional data items to generate. 
               
               
                 wrmask 
                 Write Masks 
               
               
                   
                 0 = write out corresponding byte channel 
               
               
                   
                 1 = do not write out corresponding byte channel 
               
               
                 cmsb 
                 Choose most significant bits 
               
               
                   
                 0 = choose least significant bits of a byte when performing 
               
               
                   
                 denormalization (useful for halftoning operations) 
               
               
                   
                 1 = choose most significant bits of a byte when performing 
               
               
                   
                 denormalization (useful as inverse of input normalization) 
               
               
                 normalize 
                 Normalization factor: represents the number of bits to be 
               
               
                   
                 translated to a byte: 
               
               
                   
                 0 = 1 bit data objects 
               
               
                   
                 1 = 2 bit data objects 
               
               
                   
                 2 = 4 bit data objects 
               
               
                   
                 3 = 8 bit data objects 
               
               
                   
                 4 = 16 bit data objects 
               
               
                 bo 
                 Bit Offset: represents the starting bit address for objects 
               
               
                   
                 smaller than a byte. Bit addressing is big endian. 
               
               
                 P 
                 External Format: 
               
               
                   
                 0 = unpacked bytes 
               
               
                   
                 1 = packed stream 
               
               
                 if 
                 Internal Format: 
               
               
                   
                 0 = pixels 
               
               
                   
                 1 = unpacked bytes 
               
               
                   
                 2 = packed bytes 
               
               
                   
                 3 = other 
               
               
                 cc 
                 Channel count: 
               
               
                   
                 For the Input Organizers this defines the number of normal- 
               
               
                   
                 ized input bytes collected to form each internal data word 
               
               
                   
                 during component selection. For the Output Organizer this 
               
               
                   
                 defines the number of valid bytes from the internal data word 
               
               
                   
                 that will be sued to construct output data. 
               
               
                   
                 0 = 4 active channels 
               
               
                   
                 1 = 1 active channels 
               
               
                   
                 2 = 2 active channels 
               
               
                   
                 3 = 3 active channels 
               
               
                 L 
                 Immediate data: 
               
               
                   
                 0 = not long: immediate data 
               
               
                   
                 1 = long: pointer to data 
               
               
                 what 
                 addressing mode: 
               
               
                   
                 0 = instruction specific mode 
               
               
                   
                 1 = sequential addressing 
               
               
                   
                 2 = tile addressing 
               
               
                   
                 3 = constant data. ie. one item of internal data is produced, 
               
               
                   
                 and this item is used repetitively. 
               
               
                   
               
            
           
         
       
     
     A plurality of internal and external data types may be utilized with each instruction. All operand, results and instruction type combinations are potentially valid, although typically only a subset of those combinations will lead to meaningful results. Particular operand and result data types that are expected for each instruction are detailed below in a first table (Table 9) summarising the expected data types for external and internal formats: 
     
       
         
           
               
             
               
                 TABLE 9 
               
             
            
               
                   
               
               
                 Expected Data Types 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Operand A 
                 Operand B 
                 Operand C 
                 Result 
               
               
                   
                 (Pixel 
                 (Operand 
                 (Operand 
                 (Result 
               
               
                 Instruction 
                 Organizer) 
                 Organizer B) 
                 Organizer C) 
                 Organizer) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Compositing 
                 ps 
                 px 
                 ps 
                 px 
                 ps 
                 ub 
                 px 
                 ps 
               
               
                   
                   
                   
                   
                 (T) 
                 ub 
                   
                 ub 
                 ub 
               
               
                   
                   
                   
                   
                 bl (B) 
                 const 
               
               
                 GCSC 
                 ps 
                 ift 
                 mcsc 
                 mcsc 
                 mcsc 
                 mcsc 
               
               
                   
                 ift 
                   
                 scsc 
                 scsc 
                 scsc 
                 scsc 
               
               
                   
                   
                   
                 (B) 
                 (B) 
                 (B) 
                 (B) 
               
               
                 JPEG comp. 
                 ps 
                 pb 
                 et 
                 et (B) 
                 et (B) 
                 et (B) 
                 ub 
                 ps 
               
               
                   
                 us 
                   
                 (B) 
               
               
                 JPEG decomp 
                 ps 
                 pb 
                 fdt 
                 fdt 
                 fdt 
                 fdt 
                 pb 
                 ps 
               
               
                   
                   
                   
                 sdt 
                 sdt 
                 sdt 
                 sdt 
                   
                 ub 
               
               
                   
                   
                   
                 (B) 
                 (B) 
                 (B) 
                 (B) 
               
               
                 Data coding 
                 ps 
                 px 
                 et 
                 et 
                 et 
                 et 
                 px 
                 ps 
               
               
                   
                 ub 
                 pb 
                 fdt 
                 fdt 
                 fdt 
                 fdt 
                 pb 
                 ub 
               
               
                   
                   
                 ub 
                 sdt 
                 sdt 
                 sdt 
                 sdt 
                 ub 
               
               
                   
                   
                   
                 (B) 
                 (B) 
                 (B) 
                 (B) 
               
               
                 Transformations 
                 skd 
                 skd 
                 it (B) 
                 it (B) 
                 it (B) 
                 it (B) 
                 px 
                 ps 
               
               
                 and Convolutions 
                 lkd 
                 lkd 
                   
                   
                   
                   
                   
                 ub 
               
               
                 Matrix 
                 ps 
                 px 
                 mm 
                 mm 
                 mm 
                 mm 
                 px 
                 ps 
               
               
                 Multiplication 
                 ub 
                   
                 (B) 
                 (B) 
                 (B) 
                 (B) 
                   
                 ub 
               
               
                 Halftoning 
                 ps 
                 px 
                 ps 
                 px 
                 — 
                 — 
                 px 
                 ps 
               
               
                   
                 ub 
                 pb 
                 ub 
                 pb 
                   
                   
                 pb 
                 ub 
               
               
                   
                   
                 ub 
                   
                 ub 
                   
                   
                 ub 
               
               
                 Hierarchial 
                 ps 
                 px 
                 — 
                 — 
                 — 
                 — 
                 px 
                 ps 
               
               
                 Image: 
                 ub 
                 pb 
                   
                   
                   
                   
                 pb 
                 ub 
               
               
                 horizontal 
                   
                 ub 
                   
                   
                   
                   
                 ub 
               
               
                 interpolation 
               
               
                 Hierarchial 
                 ps 
                 px 
                 ps 
                 px 
                 — 
                 — 
                 px 
                 ps 
               
               
                 Image: verti- 
                 ub 
                 pb 
                 ub 
                 pb 
                   
                   
                 pb 
                 ub 
               
               
                 cal interpolation 
                   
                 ub 
                   
                 ub 
                   
                   
                 ub 
               
               
                 and residual 
               
               
                 merging 
               
               
                 General Memory 
                 ps 
                 px 
                 — 
                 — 
                 — 
                 — 
                 px 
                 ps 
               
               
                 Copy 
                 ub 
                 pb 
                   
                   
                   
                   
                 pb 
                 ub 
               
               
                   
                   
                 ub 
                   
                   
                   
                   
                 ub 
               
               
                 Peripheral DMA 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 Internal Access 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 Flow Control 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                   
               
            
           
         
       
     
     The symbols utilized in the above table are as follows: 
     
       
         
           
               
             
               
                 TABLE 10 
               
             
            
               
                   
               
               
                 Symbol Explanation 
               
            
           
           
               
               
               
            
               
                   
                 Symbol 
                 Explanation 
               
               
                   
                   
               
               
                   
                 ps 
                 packed stream 
               
               
                   
                 pb 
                 packed bytes 
               
               
                   
                 ub 
                 unpacked bytes 
               
               
                   
                 px 
                 pixels 
               
               
                   
                 bl 
                 blend 
               
               
                   
                 const 
                 constant 
               
               
                   
                 mcsc 
                 4 output channel 
               
               
                   
                 scsc 
                 1 output channel color conversion table 
               
               
                   
                 ift 
                 Interval and Fraction tables 
               
               
                   
                 et 
                 JPEG encoding table 
               
               
                   
                 fdt 
                 fast JPEG decoding table 
               
               
                   
                 sdt 
                 slow JPEG decoding table 
               
               
                   
                 skd 
                 short kernel descriptor 
               
               
                   
                 lkd 
                 long kernel descriptor 
               
               
                   
                 mm 
                 matrix co-efficient table 
               
               
                   
                 it 
                 image table 
               
               
                   
                 (B) 
                 this organizer in bypass mode for this operation 
               
               
                   
                 (T) 
                 operand may tile 
               
               
                   
                 — 
                 no data flows via this operand 
               
               
                   
                   
               
            
           
         
       
     
     3.16 Data Normalization Circuit 
     Referring to FIG. 49, there is shown a computer graphics processor having three main functional blocks: a data normalizer  1062  which may be implemented in each of the pixel organizer  246  and operand organizers B and C  247 ,  248 , a central graphics engine in the form of the main data path  242  or JPEG units  241  and a programming agent  1064 , in the form of an instruction controller  235 . The operation of the data normalizer  1062  and the central graphics engine  1064  is determined by an instruction stream  1066  that is provided to the programming agent  1064 . For each instruction, the programming agent  1064  performs a decoding function and outputs internal control signals  1067  and  1068  to the other blocks in the system. For each input data word  1069 , the normalizer  1062  will format the data according to the current instruction and pass the result to the central graphics engine  1063 , where further processing is performed. 
     The data normalizer represents, in a simplified form, the pixel organizer and the operand organizers B and C. Each of these organizers implements the data normalization circuitry, thereby enabling appropriate normalization of the input data prior to it passing to the central graphics engine in the form of the JPEG coder or the main data path. 
     The central graphics engine  1063  operates on data that is in a standard format. which in this case is 32-bit pixels. The normalizer is thus responsible for converting its input data to a 32-bit pixel format. The input data words  1069  to the normalizer are also 32 bits wide, but may take the form of either packed components or unpacked bytes. A packed component input stream consists of consecutive data objects within a data word, the data objects being 1, 2, 4, 8 or 16 bits wide. By contrast, an unpacked byte input stream consists of 32-bit words of which only one 8-bit byte is valid. Furthermore, the pixel data  11  produced by the normalizer may consist of 1, 2, 3 or 4 valid channels, where a channel is defined as being 8 bits wide. 
     Turning now to FIG. 50, there is illustrated in greater detail a particular hardware implementation of the data normalizer  1062 . The data normalization unit  1062  is composed of the following circuits: a First-In-First-Out buffer (FIFO)  1073 , a 32-bit input register (REG 1 )  1074 , a 32-bit output register (REG 2 )  1076 , normalization multiplexors  1075  and a control unit  1076 . Each input data word  1069  is stored in the FIFO  1073  and is subsequently latched into REG 1   1074 , where it remains until all its input bits have been converted into the desired output format. The normalization multiplexors  1075  consist of 32 combinatorial switches that produce pixels to be latched into REG 2  by selecting bits from the value in REG 1   1074  and the current output of the FIFO  1073 . Thus the normalization multiplexors  1075  receive two 32bit input words  1077 ,  1078 , denoted as x[ 63  . . .  32 ] and x[ 31  . . .  0 ]. 
     It has been found that such a method improves the overall throughput of the apparatus, especially when the FIFO contains at least two valid data words during the course of an instruction. This is typically due to the way in which data words originally fetched from memory. In some cases, a desired data word or object may be spread across or “wrapped” into a pair of adjacent input data words in the FIFO buffer. By using an additional input register  1074 , the normalization multiplexers can reassemble a complete input data word using components from adjacent data words in the FIFO buffer, thereby avoiding need for additional storage or bit-stripping operations prior to the main data manipulation stages. This arrangement is particularly advantageous where multiple data words of a similar type are inputted to the normalizer. 
     The control unit generates enable signals REG 1 _EN  20  and REG 2 _EN[ 3  . . .  0 ]  1081  for updating REG 1   1074  and REG 2   1076 , respectively, as well as signals to control the FIFO  1073  and normalization multiplexors  1075 . 
     The programming agent  1064  in FIG. 49 provides the following configuration signals for the data normalizer  1062 : a FIFO_WR  4  signal, a normalization factor n[ 2  . . .  0 ], a bit offset b[ 2  . . .  0 ], a channel count c[ 1  . . .  0 ] and an external format (E). Input data is written into the FIFO  1073  by asserting the FIFO_WR sional  1085  for each clock cycle that valid data is present. The FIFO asserts a fifo_full status flag  1086  when there is no space available. Given 32-bit input data, the external format signal is used to determine whether the input is in the format of a packed stream (when E=1) or consists of unpacked bytes (when E=0). For the case when E=1, the normalization factor encodes the size of each component of a packed stream, namely: n=0 denotes 1-bit wide components, n=1 denotes 2 bits per component, n=2 denotes 4 bits per component, n=3 denotes 8-bit wide components and n&gt;3 denotes 16-bit wide components. The channel count encodes the maximum number of consecutive input objects to format per clock cycle in order to produce pixels with the desired number of valid bytes. In particular, c=1 yields pixels with only the least significant byte valid. c=2 denotes least significant 2 bytes valid, c=3 denotes least significant 3 bytes valid and c=0 denotes all 4 bytes valid. 
     When a packed stream consists of components that are less than 8 bits wide, the bit offset determines the position in x[ 31  . . .  0 ], the value stored in REG 1 , from which to begin processing data. Assuming a bit offset relative to the most significant bit of the first input byte, the method for producing an output data byte y[ 7  . . .  0 ] is described by the following set of equations: 
     where n=0: 
     
       
           y[i]=x[ 7− b] , where 0&lt;= i&lt;= 7 
       
     
     where n=1: 
     
       
           y[i]=x[ 7− b] , where  i= 1, 3, 5, 7 
       
     
     
       
           y[i]=x[ 6− b] , where  i= 0, 2, 4, 6 
       
     
     where n=2: 
     
       
           y[ 3]= x[ 7− b]   
       
     
     
       
           y[ 2]= x[ 6− b]   
       
     
     
       
           y[ 1]= x[ 5− b]   
       
     
     
       
           y[ 0]= x[ 4− b]   
       
     
     
       
           y[ 7]= y[ 3] 
       
     
     
       
           y[ 6]= y[ 2] 
       
     
     
       
           y[ 5]= y[ 1] 
       
     
     
       
           y[ 4]= y[ 0] 
       
     
     where n=3: 
     
       
           y[i]=x[i] , where 0&lt;= i&lt;= 7 
       
     
     where n&gt;3: 
     
       
           y [ 7  . . .  0 ]= x [ 15  . . .  8 ] 
       
     
     Corresponding equations may be used to generate output data bytes y[ 15  . . .  8 ], y[ 23  . . .  16 ] and y[ 31  . . .  24 ]. 
     The above method may be generalized to produce an output array of any length by taking each component of the input stream and replicating it as many times as necessary to generate output objects of standard width. In addition, the order of processing each input component may be defined as little-endian or big-endian. The above example deals with big-endian component ordering since processing always begins from the most significant bit of an input byte. Little-endian ordering requires redefinition of the bit offset to be relative to the least significant bit of an input byte. In situations where the input component width exceeds the standard output width, output components are generated by truncating each input component, typically by removing a suitable number of the least significant bits. In the above set of equations, truncation of 16-bit input components to form 8-bit wide standard output is performed by selecting the most significant byte of each 16-bit data object. 
     The control unit of FIG. 50 performs the decoding of n[ 2  . . .  0 ] and c[ 1  . . .  0 ], and uses the result along with b[ 2  . . .  0 ] and E to provide the select signals for the normalization multiplexors and the enable signals for REG 1  and REG 2 . Since the FIFO may become empty during the course of an instruction, the control unit also contains counters that record the current bit position, in_bit[ 4  . . .  0 ], in REG 1  from which to select input data, and the current byte, out_byte[ 1  . . .  0 ], in REG 2  to begin writing output data. The control unit detects when it has completed processing each input word by comparing the value of in_bit[ 4  . . .  0 ] to the position of the final object in REG 1 , and initiates a FIFO read operation by asserting the FIFO_RD signal for one clock cycle when the FIFO is not empty. The signals fifo_empty and fifo_full denote the FIFO status flags, such that fifo_empty=1 when the FIFO contains no valid data, and fifo_full=1 when the FIFO is full. In the same clock cycle that FIFO_RD is asserted, REG 1 _EN is asserted so that new data are captured into REG 1 . There are 4 enable signals for REG 2 , one for each byte in the output register. The control unit calculates REG 2 _EN[ 3  . . .  0 ] by taking the minimum of the following 3 values: the decoded version of c[ 1  . . .  0 ], the number of valid components remaining to be processed in REG 1 , and the number of unused channels in REG 2 . When E=0 there is only one valid component in REG 1 . A complete output word is available when the number of channels that have been filled in REG 2  is equal to the decoded version of c[ 1  . . .  0 ]. 
     In a particularly preferred embodiment of the invention, the circuit area occupied by the apparatus in FIG. 50 can be substantially reduced by applying a truncation function to the bit offset parameter, such that only a restricted set of offsets are used by the control unit and normalization multiplexors. The offset truncation depends upon the normalization factor and operates according to the following equation:                       b_trunc        [     2                 …                 0     ]       =   0     ,       where                 n     ≥   3                   =     b        [     2                 …                 0     ]         ,       where                 n     =   0                   =             b        [     2                 …                 1     ]       &amp;                  ``          0   ``         ,       where                 n     =   1                   =             b        [   2   ]       &amp;                  ``          00   ``         ,       where                 n     =   2                             
     (Note that “&amp;” denotes bitwize concatenation). 
     The above method allows each of the normalization multiplexors, denoted in FIG. 50 by MUX 0 , MUX 1  . . . MUX 31 , to be reduced from 32-to-1 in size when no truncation is applied, to be a maximum size of 20-to-1 with bit offset truncation. The size reduction in turn leads to an improvement in circuit speed. 
     It can be seen from the foregoing that the preferred embodiment provides an efficient circuit for the transformation of data into one of a few normalized forms. 
     3.17 Image Processing Operations of Accelerator Card 
     Returning again to FIG.  2  and Table 2, as noted previously, the instruction controller  235  “executes” instructions which result in actions being performed by the co-processor  224 . The instructions executed include a number of instructions for the performance of useful functions by the main data path unit  242 . A first of these useful instructions is compositing. 
     3.17.1 Compositing 
     Referring now to FIG. 51, there is illustrated the compositing model implemented by the main data path unit  242 . The compositing model  462  generally has three input sources of data and the output data or sink  463 . The input sources can firstly include pixel data  464  from the same destination within the memory as the output  463  is to be written to. The instruction operands  465  can be utilized as a data source which includes the color and opacity information. The color and opacity can be either flat, a blend, pixels or tiled. The flat or blend is generated by the blend generator  467 . as it is quicker to generate them internally than to fetch via input/output. Additionally, the input data can include attenuation data  466  which attenuates the operand data  465 . The attenuation can be flat, bit map or a byte map. 
     As noted previously, pixel data normally consists of four channels with each channel being one byte wide. The opacity channel is considered to be the byte of highest address. For an introduction to the operation and usefulness of compositing operations, reference is made to the standard texts including the seminal paper by Thomas Porter and Tom Duff “Compositing Digital Images” in Computer Graphics. Volume 18. Number 3, July 1984. 
     The co-processor can utilize pre-multiplied data. Pre-multiplication can consist of pre-multiplying each of the colored channels by the opacity channel. Hence, two optional pre-multiplication units  468 ,  469  are provided for pre-multiplying the opacity channel  470 ,  471  by the colored data to form, where required, pre-multiplied outputs  472 ,  473 . A compositing unit  475  implements a composite of its two inputs in accordance with the current instruction data. The compositing operators are illustrated in Table 11 below: 
     
       
         
           
               
             
               
                 TABLE 11 
               
             
            
               
                   
               
               
                 Compositing Operations 
               
            
           
           
               
               
            
               
                 Operator 
                 Definition 
               
               
                   
               
               
                 (a co , a o ) over (b co , b o ) 
                 (a co  + b co (1 − a o ), a 0  + b o (1 − a o )) 
               
               
                 (a co , a o ) in (b co , b o ) 
                 (a co B 0 , a o b o ) 
               
               
                 (a co a o ) out (b co b o ) 
                 (a co (1 − b o ), a o (1 − b o )) 
               
               
                 (a co , a o ) atop (b co , b o ) 
                 (a co b 0  + b co (1 − a o ), b o ) 
               
               
                 (a co , a o ) xor (b co , b o ) 
                 (a co (1 − b 0 ) + b co (1 − a o ), a o (1 − b o ) + 
               
               
                   
                 b o (1 − a o )) 
               
               
                 (a co , a o ) plus (b co , b o ) 
                 (wc(a co  − b co  − r(a 0  + b 0  − 
               
               
                   
                 255)/255) + r(clamp(a 0  + b o ) − 
               
               
                   
                 255)/255,clamp(a 0  + b o )) 
               
               
                 (a co , a o ) loadzero (b co , b o ) 
                 (0,0) 
               
               
                 (a co , a o ) loadc (b co , b o ) 
                 (b co , a o ) 
               
               
                 (a co , a o ) loado (b co , b o ) 
                 (a co , b o ) 
               
               
                 (a co , a o ) loadco (b co , b o ) 
                 (b co , b o ) 
               
               
                   
               
            
           
         
       
     
     The nomenclature (a co , a o ) refers to a pre-multiplied pixel of color a c  and opacity a o . R is an offset value and “wc” is a wrapping/clamping operator whose operation is explained below. It should be noted that the reverse operation of each operator in the above table is also implemented by a composting unit  475 . 
     A clamp/wrapping unit  476  is provided to clamp or wrap data around the limit values 0-255. Further, the data can be subjected to an optional “unpre-multiplication”  477  restoring the original pixel values as required. Finally, output data  463  is produced for return to the memory. 
     In FIG. 52, there is illustrated the form of an instruction word directed to the main data path unit for composting operations. When the X field in the major op-code is 1, this indicates a plus operator is to be applied in accordance with the aforementioned table. When this field is 0, another instruction apart from the plus operator is to be applied. The P a  field determines whether or not to pre-multiply the first data stream  464  (FIG.  51 ). The P b  field determines whether or not to pre-multiply the second data stream  465 . The P r  field determines whether or not to “unpre-multiply” the result utilising unit  477 . The C field determines whether to wrap or clamp, overflow or underflow in the range 0-255. The “com-code” field determines which operator is to be applied. The plus operator optionally utilizes an offset register (mdp_por). This offset is subtracted from the result of the plus operation before wrapping or clamping is applied. For plus operators, the com-code field is interpreted as a per channel enablement of the offset register. 
     The standard instruction word encoding  280  of FIG. 10 previously discussed is altered for composting operands. As the output data destination is the same as the source, operand A will always be the same operand as the result word so operand A can be utilized in conjunction with operand B to describe at greater length the operand B. As with other instructions, the A descriptor within the instructions still describes the format of the input and the R descriptor defines the format of the output. 
     Turning now to FIG. 53, there is illustrated in a first example  470 , the instruction word format of a blend instruction. A blend is defined to have a start  471  and end value  472  for each channel. Similarly, in FIG. 54 there is illustrated  475  the format of a tile instruction which is defined by a tile address  476  a start offset  477 , a length  478 . All tile addresses and dimensions are specified in bytes. Tiling is applied in a modular fashion and, in FIG. 55, there is shown the interpretation of the fields  476 - 478  of FIG.  54 . The tile address  476  denotes the start address in memory of the tile. A tile start offset  477  designates the first byte to be utilized as a start of the tile. The tile length  478  designates the total length of the tile for wrap around. 
     Returning to FIG. 51, every color component and opacity can be attenuated by an attenuation value  466 . The attenuation value can be supplied in one of three ways: 
     1. Software can specify a flat attenuation by placing the attenuation factor in the operand C word of the instruction. 
     2. A bit map attenuation where 1 means fully on and 0 means fully off can be utilized with software specifying the address of the bit map in the operand C word of the instruction. 
     3. Alternatively, a byte map attenuation can be provided again with the address of the byte map in operand C. 
     Since the attenuation is interpreted as an unsigned integer from 0-255, the pre-multiplied color channel is multiplied by the attenuation factor by effectively calculating: 
     
       
           C   oa   =C   oa   ×A/ 255 
       
     
     Where A is the attenuation and C o  is the pre-multiplied color channel. 
     3.17.2 Color Space Conversion Instructions 
     Returning again to FIG.  2  and Table 2, the main data path unit  242  and data cache  230  are also primarily responsible for color conversion. The color space conversion involves the conversion of a pixel stream in a first color space format, for example suitable for RGB color display, to a second color space format, for example suitable for CYM or CYMK printing. The color space conversion is designed to work for all color spaces and can be used for any function from at least one to one or more dimensions. 
     The instruction controller  235  configures, via the Cbus  231 , the main data path unit  242 , the data cache controller  240 , the input interface switch  252 , the pixel organizer  246 , the MUV buffer  250 , the operand organizer B  247 , the operand organizer C  248  and the result organizer  249  to operate in the color conversion mode. In this mode, an input image consisting of a plurality of lines of pixels is supplied, one line of pixels after another, to the main data path unit  242  as a stream of pixels. The main data path unit  242  (FIG. 2) receives the stream of pixels from the input interface switch  252  via the pixel organizer  246  for color space conversion processing one pixel at a time. In addition, interval and fractional tables are pre-loaded into the MUV buffer  250  and color conversion tables are loaded into the data cache  230 . The main data path unit  242  accesses these tables via the operand organizers B and C, and converts these pixels, for example from the RGB color space to the CYM or CYMK color space and supplies the converted pixels to the result organizer  249 . The main data path unit  242 , the data cache  230 , the data controller  240  and the other abovementioned devices are able to operate in either of the following two modes under control of the instruction controller  235 ; a Single Output General Color Space (SOGCS) Conversion mode or a Multiple Output General Color Space (MOGCS) Conversion Mode. For more details on the data cache controller  240  and data cache  230 , reference is made to the section entitled  Data Cache Controller and Cache    240 ,  230  (FIG.  2 ). 
     Accurate color space conversion can be a highly non-linear process. For example, color space conversion of a RGB pixel to a single primary color component (e.g. cyan) of the CYMK color space is theoretically linear, however in practice non-linearities are introduced typically by the output device which is used to display the colour components of the pixel. Similarly for the color space conversion of the RGB pixel to the other primary color components (yellow, magenta or black) of the CYMK color space. Consequently a non-linear colour space conversion is typically used to compensate for the non-linearities introduced on each colour component. The highly non-linear nature of the color conversion process requires either a complex transfer function to be implemented or a look-up table to be utilized. Given an input color space of, for example, 24 bit RGB pixels, a look-up table mapping each of these pixels to a single 8 bit primary color component of the CYMK color space (i.e. cyan) would require over 16 megabytes. Similarly, a look-up table simultaneously mapping the 214 bit RGB pixels to all four 8 bit primary color components of the CYMK color space would require over 64 megabytes, which is obviously excessive. Instead, the main data path  242  (FIG. 2) uses a look-up table stored in the data cache  230  having sparsely located output color values corresponding to points in the input color space and interpolates between the output color values to obtain an intermediate output. 
     a. Single Output General Color Space (SOGCS) Conversion Mode 
     In both the single and multiple output color conversion modes (SOGCS) and (MOGCS), the RGB color space is comprized of 24 bit pixels having 8 bit red, green and blue color components. Each of the RGB dimensions of the RGB color space is divided into 15 intervals with the length of each interval having a substantially inverse proportionality to the non-linear behavior of the transfer function between the RGB to CYMK color space of the printer. That is, where the transfer function has a highly non-linear behavior the interval size is reduced and where the transfer function has a more linear behavior, the size of the interval is increased. Preferably, the color space of each output printer is accurately measured to determine those non-linear portions of its transfer function. However, the transfer function can be approximated or modelled based on know-how or measured characteristics of a type printer (e.g.: ink-jet). For each color channel of an input pixel, the color component value defines a position within one of the 15 intervals. Two tables are used by the main data path unit  242  to determine which interval a particular input color component value lies within and also to determine a fraction along the interval in which a particular input color component value lies. Of course, different tables may be used for output printers having different transfer functions. 
     As noted previously, each of the RGB dimensions is divided into 15 intervals. In this way the RGB color space forms a 3-dimensional lattice of intervals and the input pixels at the ends of the intervals form sparsely located points in the input color space. Further, only the output color values of the output color space corresponding to the endpoints of the intervals are stored in look-up tables. Hence, an output color value of an input color pixel can be calculated by determining the output color values corresponding to the endpoints of the intervals within which the input pixel lies and interpolating such output color values utilising the fractional values. This technique reduces the need for large memory storage. 
     Turning now to FIG. 56, there is illustrated  480  an example of determining for a particular input RGB color pixel, the corresponding interval and fractional values. The conversion process relies upon the utilization of an interval table  482  and a fractional table  483  for each 8 bit input color channel of the 24 bit input pixel. The 8 bit input color component  481 , shown in a binary form in FIG. 56 having the example decimal number 4, is utilized as a look-up to each of the interval and fractional tables. Hence, the number of entries in each table is 256. The interval table  482  provides a 4 bit output defining one of the intervals numbered 0 to 14 into which the input color component value  481  falls. Similarly, the fractional table  483  indicates the fraction within an interval that the input color value component  481  falls. The fractional table stores 8 bit values in the range of 0 to 255 which are interpreted as a fraction of 256. Hence, for an input color value component  481  having a binary equivalent to the decimal value 4, this value is utilized to look-up the interval table  482  to produce an output value of 0. The input value 4 is also utilized to look-up the fractional table  483  to produce an output value of 160 which designates the fraction {fraction (160/256)}. As can be seen from the interval and fractional tables  482  and  483 , the interval lengths are not equal. As noted previously, the length of the intervals are chosen according to the non-linear behavior of the transfer function. 
     As mentioned above, the separate interval and fractional tables are utilized for each of the RGB color components resulting in three interval outputs and three fractional outputs. Each of the interval and fractional tables for each color component are loaded in the MUV buffer  250  (FIG. 2) and accessed by the main data path unit  242  when required. The arrangement of the MUV buffer  250  for the color conversion process is as shown in FIG.  57 . The MUV buffer  250  (FIG. 57) is divided into three areas  488 ,  489  and  490 , one area for each color component. Each area e.g.  488  is further divided into a 4 bit interval table and a 8 bit fractional table. A 12 bit output  492  is retrieved by the main data path unit  242  from the MUV buffer  250  for each input color channel. In the example given above of a single input color component having a decimal value 4, the 12 bit output will be 000001010000. 
     Turning now to FIG. 58, there is illustrated an example of the interpolation process. The interpolation process consists primarily of interpolation from one three dimensional space  500 , for example RGB color space to an alternative color space, for example CMY or CMYK. The pixels P 0  to P 7  form sparsely located points in the RGB input color space and having corresponding output color values CV(P 0 ) to CV(P 7 ) in the output color space. The output color component value corresponding to the input pixel Pi falling between the pixels P 0  to P 7  is determined by; firstly, Is determining the endpoints P 0 , P 1 , . . . , P 7  of the intervals surrounding the input pixel Pi; secondly, determining the fractional components frac_r, frac_g and frac_b; and lastly interpolating between the output color values CV(P 0 ) to CV(P 7 ) corresponding to the endpoints P 0  to P 7  using the fractional components. 
     The interpolation process includes a one dimensional interpolation in the red (R) direction to calculate the values temp  11 , temp  12 , temp  13 , temp  14  in accordance with the following equations: 
     
       
           temp    11 = CV ( P   0 )− frac   —   r  ( CV ( P   1 )− CV ( P   0 )) 
       
     
     
       
           temp    12 = CV ( P   2 )− frac   —   r  ( CV ( P   3 )− CV ( P   2 )) 
       
     
     
       
           temp    13 = CV ( P   4 )− frac   —   r  ( CV ( P   5 )− CV ( P   4 )) 
       
     
     
       
           temp    14 = CV ( P   6 )− frac   —   r  ( CV ( P   7 )− CV ( P   6 )) 
       
     
     Next, the interpolation process includes the calculation of a further one dimensional interpolation in the green (G) direction utilising the following equations to calculate the values temp  21  and temp  22 : 
     
       
           temp    21 = temp    11 − frac   —   g  ( temp    12 − temp    11 ) 
       
     
     
       
           temp    22 = temp    13 − frac   —   g  ( temp    14 − temp    13 ) 
       
     
     Finally, the final dimension interpolation in the blue (B) direction is carried out to calculate a final color output value in accordance with the following equation. 
     
       
         final= temp    21 + frac   —   b  ( temp    22 − temp    21 ) 
       
     
     Unfortunately, it is often the case that the input and output gamut may not match. In this respect, the output gamut may be more restricted that the input gamut and in this case, it is often necessary to clamp the gamut at the extremes. This often produces unwanted artefacts when converting using the boundary gamut colors. An example of how this problem can occur will now be explained with reference to FIG. 59, which represents a one dimensional mapping of input gamut values to output gamut values. It is assumed that output values are defined for the input values at points  510  and  511 . However, if the greatest output value is clamped at the point  512  then the point  511  must have an output value of this magnitude. Hence, when interpolating between the two points  510  and  511 , the line  515  forms the interpolation line and the input point  516  produces a corresponding output value  517 . However, this may not be the best color mapping, especially where, without the gamut limitations, the output value would have been at the point  518 . The interpolation line between  510  and  518  would produce an output value of  519  for the input point  516 . The difference between the two output values  517  and  519  can often lead to unsightly artefacts, particularly when printing edge of gamut colors. To overcome this problem, the main data path unit can optionally calculate in an expanded output color space and then scale and clamp to the appropriate range utilising the following formula: 
     
       
         0 if  x≦ 63 
       
     
     
       
         out=2( x− 64) if (64≦ x≦ 191) 
       
     
     
       
         255 if (192≦ x )  (4) 
       
     
     Returning now to FIG. 58, it will be evident that the interpolation process can either be carried out in the SOCGS conversion mode which converts RGB pixels to a single output color component (for example, cyan) or the MOGCS mode which converts RGB pixels to all the output color components simultaneously. Where color conversion is to be carried out for each pixel in an image, many millions of pixels may have to be independently color converted. Hence, in order for high speed operation, it is desirable to be able to rapidly locate the 8 values (P 0 -P 7 ) around a particular input value. 
     As noted previously with respect to FIG. 57, the main data path unit  242  retrieves for each color input channel, a 12 bit output consisting of a 4 bit interval part and a 8 bit fractional part. The main data path unit  242  concatenates these 4 bit interval parts of the red, green and blue color channels to form a single 12 bit address (I R , I G , I B ), as shown in FIG. 60 as  520 . 
     FIG. 60 shows a data flow diagram illustrating the manner in which a single output color component  563  is obtained in response to the single 12 bit address  520 . The 12 bit address  520  is first fed to an address generator of the data cache controller  240 , such as the generator  1881  (shown in FIG. 141) which generates 8 different 9 bit line and byte addresses  521  for memory banks (B 0 , B 1 , . . . . B 7 ). The data cache  230  (FIG. 2) is divided into 8 independent memory banks  522  which can be independently addressed by the respective 8 line and byte addresses. The 12 bit address  520  is mapped by the address generator into the 8 line and byte addresses in accordance with the following table: 
     
       
         
           
               
             
               
                 TABLE 12 
               
             
            
               
                   
               
               
                 Address Composition for SOGCS Mode 
               
            
           
           
               
               
               
               
            
               
                   
                 Bit [8:6] 
                 Bit [5:3] 
                 Bit [2:0] 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                 Bank 7 
                 R[3:1] 
                 G[3:1] 
                 B[3:1] 
               
               
                 Bank 6 
                 R[3:1] 
                 G[3:1] 
                 B[3:1] + B[0] 
               
               
                 Bank 5 
                 R[3:1] 
                 G[3:1]+G[0] 
                 B[3:1] 
               
               
                 Bank 4 
                 R[3:1] 
                 G[3:1] +G[0] 
                 B[3:1]+B[0] 
               
               
                 Bank 3 
                 R[3:1]+R[0] 
                 G[3:1] 
                 B[3:1] 
               
               
                 Bank 2 
                 R[3:1]+R[0] 
                 G[3:1] 
                 B[3:1]+B[0] 
               
               
                 Bank 1 
                 R[3:1]+R[0] 
                 G[3:1]+G[0] 
                 B[3:1] 
               
               
                 Bank 0 
                 R[3:1]+R[0] 
                 G[3:1]+G[0] 
                 B[3:1] + B[0] 
               
               
                   
               
            
           
         
       
     
     where BIT[ 8 : 6 ], BIT[ 5 : 3 ] and BIT[ 2 : 0 ] represent the sixth to eighth bits, the third to fifth bits and the zero to second bits of the 9 bit bank addresses respectively; and 
     where R[ 3 : 1 ], G[ 3 : 1 ] and B[ 3 : 1 ] represent the first to third bits of the 4 bit intervals I R , I G  and I B  of the 12 bit address  520  respectively. 
     Reference is made to memory bank  5  of Table 12 for a more detailed explanation of the 12 bit to 9 bit mapping. In this particular case, the bits  1  to  3  of the 4 bit red interval I r  of the 12 bit address  520  are mapped to bits  6  to  8  of the 9 bit address B 5 ; bits  1  to  3  and bit  0  of the 4 bit green interval I g  are summed and then mapped to bits  3  to  5  of the 9 bit address B 5 ; and bits  1  to  3  of the 4 bit blue interval I b  are mapped to bits  0  to  2  of the 9 bit address B 5 . 
     Each of the 8 different line and byte addresses  521  is utilized to address a respective memory bank  522  which consists of 512×8 bit entries, and the corresponding 8 bit output color component  523  is latched for each of the memory banks  522 . As a consequence of this addressing method, the output color values of CV(P 0 ) to CV(P 7 ) correseponding to the endpoints P 0  to P 7  may be located at different positions in the memory banks. For example, a 12 bit address of 0000 0000 0000 will result in the same bank address for each bank, ie 000 000 000. However a 12 bit address of 0000 0000 0001 will result in different bank addresses, ie a bank address of 000 000 000 for banks  7 ,  5 ,  3  and  1  and a bank address of 000 000 001 for banks  6 ,  4 ,  2  and  0 . It is in this way the eight single output color values CV(P 0 )-CV(P 7 ) surrounding a particular input pixel value are simultaneously retrieved from respective memory banks and duplication of output color values in the memory banks can be avoided. 
     Turning now to FIG. 61, there is illustrated the structure of a single memory bank of the data cache  230  when utilized in the single color conversion mode. Each memory bank consists of 128 line entries  531  which are 32 bits long and comprize 4×8 bit memories  533 - 536 . The top 7 bits of the memory address  521  are utilized to determine the corresponding row of data within the memory address to latch  542  as the memory bank output. The bottom two bits are a byte address and are utilized as an input to multiplexer  543  to determine which of the 4×8 bit entries should be chosen  544  for output. One data item is output for each of the 8 memory banks per clock cycle for return to the main data path unit  242 . Hence, the data cache controller receives a 12 bit byte address from the operand organizer  248  (FIG. 2) and outputs in return to the operand organizers  247 ,  248 , the 8 output color values for interpolation calculation by the main data path unit  242 . 
     Returning to FIG. 60, the interpolation equations are implemented by the main data path unit  242  (FIG. 2) in three stages. In the main data path unit, a first stage of multiplier and adder units eg.  550  which take as input the relevant color values output by the corresponding memory banks eg.  522  in addition to the red fractional component  551  and calculate the 4 output values in accordance with stage 1 of the abovementioned equations. The outputs eg.  553 ,  554  of this stage are fed to a next stage unit  556  which utilizes the frac_g input  557  to calculate an output  558  in accordance with the aforementioned equation for stage 2 of the interpolation process. Finally, the output  558  in addition to other outputs eg.  559  of this stage are utilized  560  in addition to the frac_b input  562  to calculate a final output color  563  in accordance with the aforementioned equations. 
     The process illustrated in FIG. 60 is implemented in a pipelined manner so as to ensure maximum overall throughput. Further, the method of FIG. 60 is utilized when a single output color component  563  is required. For example, the method of FIG. 60 can be utilized to first produce the cyan color components of an output image followed by the magenta, yellow and black components of an output image reloading the cache tables between passes. This is particularly suitable for a four-pass printing process which requires each of the output colors as part of separate pass. 
     b. Multiple Output General Color Space Mode 
     The co-processor  224  operates in the MOGCS mode in a substantially similar manner to the SOCGS mode, with a number of notable exceptions. In the MOGCS mode, the main data path unit  242 , the data cache controller  240  and data cache of FIG. 2 co-operate to produce multiple color outputs simultaneously with four primary colors components being output simultaneously. This would require the data cache  230  to be four times larger in size. However, in the MOGCS mode of operation, in order to save storage space, the data cache controller  240  stores only one quarter of all the output color values of the output color space. The remaining output color values of the output color space are stored in a low speed external memory and are retrieved as required. This particular apparatus and method is based upon the surprising revelation that the implementation of sparsely located color conversion tables in a cache system have an extremely low miss rate. This is based on the insight there is a low deviation in color values from one pixel to the next in most color images. In addition, there is a high probability the sparsely located output color values will be the same for neighboring pixels. 
     Turning now to FIG. 62 there will now be described the method carried out by the co-processor to implement multi-channel cached color conversion. Each input pixel is broken into its color components and a corresponding interval table value (FIG. 56) is determined as previously described resulting in the three 4 bit intervals Ir, Ig, Ib denoted  570 . The combined 12 bit number  570  is utilized in conjunction with the aforementioned table 12 to again derive eight 9-bit addresses. The addresses eg.  572  are then re-mapped as will be discussed below with reference to FIG. 63, and then are utilized to look up a corresponding memory bank  573  to produce four colour output channels  574 . The memory bank  573  stores 128×32 bit entries out of a total possible 512×32 bit entries. The memory bank  573  forms part of the data cache  230  (FIG. 2) and is utilized as a cache as as will now be described with reference to FIG.  63 . 
     Turning to FIG. 63, the 9 bit bank input  578  is re-mapped as  579  so as to anti-alias memory patterns by re-ordering the bits  580 - 582  as illustrated. This reduces the likelihood of neighboring pixel values aliasing to the same cache elements. 
     The reorganized memory address  579  is then utilized as an address into the corresponding memory bank eg.  585  which comprizes 128 entries each of 32 bits. The 7 bit line address is utilized to access the memory  585  resulting in the corresponding output being latched  586  for each of the memory banks. Each memory bank, eg  585  has an associated tag memory which comprizes 128 entries each of 2 bits. The 7 bit line address is also utilized to access the corresponding tag in tag memory  587 . The two most significant bits of the address  579  are compared with the corresponding tag in tag memory  587  to determine if the relevant output color value is stored in the cache. These two most significant bits of the 9 bit address correspond to the most significant bits of the red and green data intervals (see Table 12). Thus in the MOGCS mode the RGB input color space is effectively divided into quadrants along the red and green dimensions where the two most significant bits of the 9 bit address designates the quadrant of the RGB input color space. Hence the output color values are effectively divided into four quadrants each designated by a two bit tag. Consequently the output color values for each tag value for a particular line are highly spaced apart in the output color space, enabling anti-aligning of memory patterns. 
     Where the two bit tags do not match a cache miss is recorded by the data cache controller and the corresponding required memory read is initiated by the data cache controller with the cache look up process being stalled until all values for that line corresponding to that two bit tag entry are read from an external memory and stored in the cache. This involves the reading of the relevant line of the color conversion table stored in the external memory. The process  575  of FIG. 63 is carried out for each of the memory banks eg.  573  of FIG. 62 resulting, depending on the cache contents, in a time interval elapsing before the results eg.  586  are output from each corresponding memory bank. Each of the eight 32 bit sets of data  586  are then forwarded to the main data path unit ( 242 ) which carries out the aforementioned interpolation process (FIG. 62) in three stages  590 - 592  to each of the colored channels simultaneously and in a pipelined manner so as to produce four color outputs  595  for sending to a printer device. 
     Experiments have shown that the caching mechanism as described with reference to FIGS. 62 and 63 can be advantageously utilized as typical images have a cache miss-rate on average requiring between 0.01 and 0.03 cache line fetches per pixel. The utilization of the caching mechanism therefore leads to substantially reduced requirements, in the typical case, for memory accesses outside of the data cache. 
     The instruction encoding for both color space conversion modes (FIG. 10) utilized by the co-processor has the following structure: 
     
       
         
           
               
             
               
                 TABLE 12A 
               
             
            
               
                   
               
               
                 Instruction Encoding for Color Space Conversion 
               
            
           
           
               
               
               
               
            
               
                 Operand 
                 Description 
                 Internal Format 
                 External Format 
               
               
                   
               
               
                 Operand A 
                 source pixels 
                 pixels 
                 packed stream 
               
               
                 Operand B 
                 multi output channel 
                 other 
                 multi channel csc 
               
               
                   
                 color conversion tables 
                   
                 tables 
               
               
                 Operand C 
                 Interval and Fraction 
                 — 
                 I&amp;F table format 
               
               
                   
                 Tables 
               
               
                 Result 
                 pixels 
                 pixels 
                 packed stream 
               
               
                   
                 bytes 
                 unpacked bytes 
                 unpacked bytes, 
               
               
                   
                   
                   
                 packed stream 
               
               
                   
               
            
           
         
       
     
     The instruction field encoding for color space conversion instruction is illustrated in FIG. 64 with the following minor opcode encoding for the color conversion instructions. 
     
       
         
           
               
             
               
                 TABLE 13 
               
             
            
               
                   
               
               
                 Minor Opcode Encoding for Color Conversion Instructions 
               
            
           
           
               
               
               
            
               
                   
                 Field 
                 Description 
               
               
                   
                   
               
               
                   
                 trans[3:0] 
                 0 = do not apply translation and clamping step to 
               
               
                   
                   
                 corresponding output value on this channel 
               
               
                   
                 M 
                 0 = single channel color table format 
               
               
                   
                   
                 1 = multi channel color table format 
               
               
                   
                   
               
            
           
         
       
     
     FIG. 65 shows a method of converting a stream of RGB pixels into CYMK color values according to the MOGCS mode. In step S 1 , a stream of 24 bit RGB pixels are received by the pixel organiser  246  (FIG.  2 ). In step S 2 , the pixel organiser  246  determines the 4 bit interval values and the 8 bit fractional values of each input pixel from lookup tables, in the manner previously discussed with respect to FIGS. 56 and 57. The interval and fractional values of the input pixel designate which intervals and fractions along the intervals in which the input pixel lies. In step S 3 , the main data path unit  242  concatenates the 4 bit intervals of the red, green and blue color components of the input pixel to form a 12 bit address word and supplies this 12 bit address word to the data cache controller  240  (FIG.  2 ). In step S 4 , the data cache controller  240  converts this 12 bit address word into 8 different 9 bit addresses, in the manner previously discussed with respect to Table 12 and FIG.  62 . These 8 different addresses designate the location of the 8 output color values CV(P 0 )-CV(P 7 ) in the respective memory banks  573  (FIG. 62) of the data cache  230  (FIG.  2 ). In step S 5 , the data cache controller  240  (FIG. 2) remaps the 8 different 9 bit addresses in the manner described previously with respect to FIG.  63 . In this way, the most significant bit of the red and green 4 bit intervals are mapped to the two most significant bits of the 9 bit addresses. 
     In step S 6 , the data cache controller  240  then compares the two most significant bits of the 9 bit addresses with respective 2 bit tags in memory  587  (FIG.  63 ). If the 2 bit tag does not correspond to the two most significant bits of the 9 bit addresses, then the output color values CV(P 0 )-CV(P 7 ) do not exist in the cache memory  230 . Hence, in step S 7 , all the output color values corresponding to the 2 bit tag entry for that line are read from external memory into the data cache  230 . If the 2 bit tag corresponds to these two most significant bits of the 9 bit addresses, then the data cache controller  240  retrieves in step S 8  the eight output color values CV(P 0 )-CV(P 7 ) in the manner discussed previously with respect to FIG.  62 . In this way, the eight output color values CV(P 0 )-CV(P 7 ) surrounding the input pixel are retrieved by the main data path unit  242  from the data cache  230 . In step S 7 , the main data path unit  242  interpolates the output color values CV(P 0 )-CV(P 7 ) utilising the fractional values determined in step S 2  and outputs the interpolated output color values. 
     It will be evident to the man skilled in the art, that the storage space of the data cache storage may be reduced further by dividing the RGB color space and the corresponding output color values into more than four quadrants, for example 32 blocks. In the latter case, the data cache can have the capacity of storing only a {fraction (1/32)} block of output color values. 
     It will also be evident to the man skilled in the art, that the data caching arrangement utilized in the MOGCS mode can also be used in a single output general conversion mode. Hence, in the latter mode the storage space of the data cache can also be reduced. 
     3.17.3 JPEG Coding/Decoding 
     It is well known that a large number of advantages can be obtained from storing images in a compressed format especially in relation to the saving of memory and the speed of transferring images from one place to another. Various popular standards have arizen for image compression. One very popular standard is the JPEG standard and for a full discussion of the implementation of this standard reference is made to the well known text JPEG:  Still Image Data Compression Standard  by Pennebaker and Mitchell published 1993 by Van Nostrand Reinhold. The co-processor  224  utilizes a subset of the JPEG standard in the storage of images. The JPEG standard has the advantage that large factor compression can be gained with the retention of substantial image quality. Of course, other standards for storing compressed images could be utilized. The JPEG standard is well-known to those skilled in the art, and the various JPEG alternative implementations readily available in the marketplace from manufacturers including JPEG core products for incorporation into ASICS. 
     The co-processor  224  implements JPEG compression and decompression of images consisting of 1, 3 or 4 color components. One-color-component images may be meshed or unmeshed. That is, a single-color-component can be extracted from meshed data or extracted from unmeshed data. An example of meshed data is three-color components per pixel datum (i.e., RGB per pixel datum), and an example of unmeshed data is where each color component for an image is stored separately such that each color component can be processed separately. For three color component images the co-processor  224  utilizes one pixel per word, assuming the three color channels to be encoded in the lowest three bytes. 
     The JPEG standard decomposes an image into small two dimensional units called minimum coded units (MCU). Each minimal coded unit is processed separately. The JPEG coder  241  (FIG. 2) is able to deal with MCU&#39;s which are 16 pixels wide and 8 pixels high for down sampled images or MCU&#39;s which are 8 pixels wide and 8 pixels high for images that are not to be down sampled. 
     Turning now to FIG. 66, there is illustrated the method utilized for down sampling three component images. 
     The original pixel data  600  is stored in the MUV buffer  250  (FIG. 2) in a pixel form wherein each pixel  601  comprizes Y, U and V components of the YUV color space. This data is first converted into a MCU unit which comprizes four data blocks  601 - 604 . The data blocks comprize the various color components, with the Y component being directly sampled  601 ,  602  and the U and V components being sub-sampled in the particular example of FIG. 13 to form blocks  603 ,  604 . Two forms of sub-sampling are implemented by the co-processor  224 , including direct sampling where no filtering is applied and odd pixel data is retained while even pixel data is discarded. Alternatively, filtering of the U and V components can occur with averaging of adjacent values taking place. 
     An alternative form of JPEG sub-sampling is four color channel sub-sampling as illustrated in FIG.  67 . In this form of sub-sampling, pixel data blocks of 16×8 pixels  610  each have four components  611  including an opacity component (O) in addition to the usual Y, U, V components. This pixel data  410  is sub-sampled in a similar manner to that depicted in FIG.  66 . 
     However, in this case, the opacity channel is utilized to form data blocks  612 ,  613 . 
     Turning now to FIG. 68, there is illustrated the JPEG coder  241  of FIG. 2 in more detail. The JPEG encoder/decoder  241  is utilized for both JPEG encoding and decoding. The encoding process receives block data via bus  620  from the pixel organizer  246  (FIG.  2 ). The block data is stored within the MUV buffer  250  which is utilized as a block staging area. The JPEG encoding process is broken down into a number of well defined stages. These stages include: 
     1. taking a discrete cosine transform (DCT) via DCT unit  621 ; 
     2. quantising the DCT output  622 ; 
     3. placing the quantized DCT coefficients in a zig zag order, also carried out by quantizer unit  622 ; 
     4. predictively encoding the DC DCT coefficients and run length encoding the AC DCT co-efficients carried out by co-efficient coder  623 ; and 
     5. variable length encoding the output of the coefficients coder stage, carried out by Huffman coder unit  624 . The output is fed via multiplexer  625  and Rbus  626  to the result organizer  629  (FIG.  2 ). 
     The JPEG decoding process is the inverse of JPEG encoding with the order of operations reversed. Hence, the JPEG decoding process comprizes the steps of inputting on Bus  620  a JPEG block of compressed data. The compressed data is transferred via Bus  630  to the Huffman coder unit  624  which Huffman decodes data into DC differences and AC run lengths. Next, the data is forwarded to the co-efficients coder  623  which decodes the AC and DC coefficients and puts them into their natural order. Next, the quantizer unit  622  dequantizes the DC coefficients by multiplying them by a corresponding quantization value. Finally, the DCT unit  621  applies an inverse discrete cosine transform to restore the original data which is then transferred via Bus  631  to the multiplexer  625  for output via Bus  626  to the Result Organizer. The JPEG coder  241  operates in the usual manner via standard CBus interface  632  which contains the registers set by the instructions controller in order to begin operation of the JPEG coder. Further, both the quantizer unit  622  and the Huffman coder  624  require certain tables which are loaded in the data cache  230  as required. The table data is accessed via an OBus interface unit  634  which connects to the operand organizer B unit  247  (FIG. 2) which in turn interacts with the data cache controller  240 . 
     The DCT unit  621  implements forward and inverse discrete cosine transforms on pixel data. Although many different types of DCT transforming implementations are known and discussed in the  Still Image Data Compression Standard  (ibid), the DCT  621  implements a high speed form of transform more fully discussed in the section herein entitled  A Fast DCT Apparatus , which may implement a DCT transform operation in accordance with the article entitled  A Fast DCT - SQ Scheme for Images  by Arai et. al., published in The Transactions of the IEICE, Vol E71, No. 11, November 1988 at page 1095. 
     The quantizer  622  implements quantization and dequantization of DCT components and operates via fetching relevant values from corresponding tables stored in the data cache via the OBus interface unit  634 . During quantization, the incoming data stream is divided by values read from quantization tables stored in the data cache. The division is implemented as a fixed point multiply. During dequantization, the data stream is multiplied by values kept in the dequantization table. 
     Turning to FIG. 69, there is illustrated the dequantizer  622  in more detail. The quantizer  622  includes a DCT interface  640  responsible for passing data to and receiving data from the DCT module  621  via a local Bus. During quantization, the quantizer  622  receives two DCT co-efficients per clock cycle. These values are written to one of the quantizers internal buffers  641 ,  642 . The buffers  641 ,  642  are dual ported buffers used to buffer incoming data. During quantization, co-efficient data from the DCT sub-module  621  is placed into one of the buffers  641 ,  642 . Once the buffer is full, the data is read from the buffer in a zig zag order and multiplied by multiplier  643  with the quantization values received via OBus interface unit  634 . The output is forwarded to the co-efficient coder  623  (FIG. 68) via co-efficient coder interface  645 . While this is happening, the next block of coefficients is being written to the other buffer. During JPEG decompression, the quantizer module dequantizes decoded DCT coefficients by multiplying them by values stored in the table. As the quantization and dequantization operations are mutually exclusive, the multiplier  643  is utilized during quantization and dequantization. The position of the co-efficient within the block of 8×8 values is used as the index into the dequantization table. 
     As with quantization, the two buffers  641 ,  642  are utilized to buffer incoming co-efficient data from the co-efficient coder  623  (FIG.  68 ). The data is multiplied with its quantization value and written into the buffers in reverse zig zag order. Once full, the dequantized coefficients are read out of the utilized buffer in natural order, two at a time, and passed via DCT interface  640  to the DCT sub-module  621  (FIG.  68 ). Hence the coefficients coder interface module  645  is responsible for interfacing to the co-efficients coder and passes data and receives data from the coder via a local Bus. This module also reads data from buffers in zig zag order during compression and writes data to the buffers in reverse zig zag order during decompression. Both the DCT interface module  640  and the CC interface module  645  are able to read and write from buffers  641 ,  642 . Hence, address and control multiplexer  647  is provided to select which buffer each of these interfaces is interacting with under the control of a control module  648 , which comprizes a state machine for controlling all the various modules in the quantizer. The multiplier  643  can be a 16×8, 2&#39;s complement multiplier which multiplies DCT coefficients by quantization table values. 
     Turning again to FIG. 68, the co-efficient coder  623  performs the functions of 
     (a) predictive encoding/decoding of DC coefficients in JPEG mode; and 
     (b) run length encoding/decoding of AC coefficients in JPEG mode. 
     Preferably, the co-efficient coder  623  is also able to be utilized for predictive encoding/decoding of pixels and memory copy operations as required independently of JPEG mode operation. The co-efficient coder  623  implements predictive and run length encoding and decoding of DC and AC coefficients as specified in the Pink Book. A standard implementation of predictive encoding and predictive decoding in addition to JPEG AC coefficients run lengthing encoding and decoding as specified in the JPEG standard is implemented. 
     The Huffman coder  624  is responsible for Huffman encoding and decoding of the JPEG data train. In Huffman encoding mode, the run length encoded data is received from the coefficients coder  623  and utilized to produce a Huffman stream of packed bytes. Alternatively, or in addition, in Huffman decoding, the Huffman stream is read from the PBus interface  620  in the form of packed bytes and the Huffman decoded coefficients are presented to the co-efficient coder module  623 . The Huffman coder  624  utilizes Huffman tables stored in the data cache and accessed via OBus interface  634 . Alternatively, the Huffman table can be hardwired for maximum speed. 
     When utilising the data cache for Huffman coding, the eight banks of the data store data tables as follows with the various tables being described in further hereinafter. 
     
       
         
           
               
             
               
                 TABLE 14 
               
             
            
               
                   
               
               
                 Huffman and Quantization Tables as stored in Data Cache 
               
            
           
           
               
               
            
               
                 Bank 
                 Description 
               
               
                   
               
               
                 0 
                 This bank hold the 256, 16 bit entries of a EHUFCO_DC_1 or 
               
               
                   
                 EHUFCO table. The least significant bit of the index chooses 
               
               
                   
                 between the two 16 bit items in the 32 bit word. All 128 lines 
               
               
                   
                 of this bank of memory are used. 
               
               
                 1 
                 This bank holds the 256, 16 bit entries of a EHUFCO_DC_2 
               
               
                   
                 table. The least significant bit of the index chooses between the 
               
               
                   
                 two 16 bit items in the 32 bit word. All 128 lines of this bank 
               
               
                   
                 of memory are used. 
               
               
                 2 
                 This bank holds the 256, 16 bit entries of a EHUFCO_AC_ 1 
               
               
                   
                 table. The least significant bit of the index chooses between the 
               
               
                   
                 two 16 bit items in the 32 bit word. All 128 lines of this bank of 
               
               
                   
                 memory are used. 
               
               
                 3 
                 This bank holds the 256, 16 bit entries of a EHUFCO_AC_2 
               
               
                   
                 table. The least significant bit of the index chooses between the 
               
               
                   
                 two 16 bit items in the 32 bit word. All 128 lines of this bank 
               
               
                   
                 of memory  are used. 
               
               
                 4 
                 This bank holds the 256, 4 bit entires of a EHUFSI_DC_ 1 or 
               
               
                   
                 EHUFSI table, as well as the 256, 4 bit entires of a 
               
               
                   
                 EHUFSI_DC_2 table. All 128 lines of this bank of memory are 
               
               
                   
                 used. 
               
               
                 5 
                 This bank holds the 256, 4 bit entries of a EHUFSI_AC_ 1 
               
               
                   
                 table, as well as the 256, 4 bit entries of a EHUFSI_AC_2 
               
               
                   
                 table. All 128 lines of this bank of memory are used. 
               
               
                 6 
                 Not used 
               
               
                 7 
                 This banks holds the 128, 24 bit entries of the quantization 
               
               
                   
                 table. It occupies the least significant 3 bytes of all 128 lines of 
               
               
                   
                 this bank of memory. 
               
               
                   
               
            
           
         
       
     
     Turning now to FIG. 70, the Huffman coder  624  consists primarily of two independent blocks being an encoder  660  and a decoder  661 . Both blocks  660 ,  661  the same OBus interface via a multiplexer module  662 . Each block has its own input and output with only one block active at a time, depending on the function performed by the JPEG encoder. 
     a. Encoding 
     During encoding in JPEG mode, Huffman tables are used to assign codes of varying lengths (up to 16 bits per code) to the DC difference values and to the AC run-length values, which are passed to the HC submodule from the CC submodule. These tables have to be preloaded into the data cache before the start of the operation. The variable length code words are then concatenated with the additional bits for DC and AC coefficients (also passed from the CC submodule, then packed into bytes. A X′00 byte is stuffed in if an X′FF byte is obtained as a result of packing. If there is a need for an RST m  marker it is inserted. This may require byte padding with “1” bits of the last Huffman code and X′00 byte stuffing if the padded byte results in X&#39;FF. The need for an RST m  marker is signalled by the CC submodule. The HC submodule inserts the EOI marker at the end of image, signalled by the “final” signal on the PBus-CC slave interface. The insertion procedure of the EOI marker requires similar packing, padding and stuffing operations as for RST m  markers. The output stream is finally passed as packed bytes to the Result Organizer  249  for writing to external memory. 
     In non-JPEG mode data is passed to the encoder from the CC submodule (PBus-CC slave interface) as unpacked bytes. Each byte is separately encoded using tables preloaded into the cache (similarly to JPEG mode), the variable length symbols are then assembled back into packed bytes and passed to the Results Organizer  249 . 
     The very last byte in the output stream is padded with 1&#39;s. 
     b. Decoding 
     Two decoding algorithms are implemented: fast (real time) and slow (versatile). The fast algorithm works only in JPEG mode, the versatile one works both in JPEG and non-JPEG modes. 
     The fast JPEG Huffman decoding algorithm maps Huffman symbols to either DC difference values or AC run-length values. It is specifically tuned for JPEG and assumes that the example Huffman tables (K 3 , K 4 , K 5  and K 6 ) were used during compression. The same tables are hard wired in to the algorithm allowing decompression without references to the cache memory. This decoding style is intended to be used when decompressing images to be printed where certain data rates need to be guaranteed. The data rate for the HC submodule decompressing a band (a block between RST m  markers) is almost one DC/AC co-efficient per clock cycle. One clock cycle delay between the HC submodule and CC sub-module may happen for each X′00 stuff byte being removed from the data stream, however this is strongly data dependent. 
     The Huffman decoder operates in a faster mode for the extraction of one Huffman symbol per clock cycle. The fast Huffman decoder is described in the section herein entitled Decoder of Variable Length Codes. 
     Additionally, the Huffman decoder  661  also implements a heap-based slow decoding algorithm and has a structure  670  as illustrated in FIG.  71 . 
     For a JPEG encoded stream, the STRIPPER  671  removes the X′00 stuff bytes, the X′FF fill bytes and RST m  markers, passing Huffman symbols with concatenated additional bits to the SHIFTER  672 . This stage is bypassed for Huffman-only coded streams. 
     The first step in decoding a Huffman symbol is to look up the 256 entries HUFVAL table stored in the cache addressing it with the first 8 bits of the Huffman data stream. If this yields a value (and the true length of the corresponding Huffman symbol), the value is passed on to the OUTPUT FORMATTER  676 , and the length of the symbol and the number of the additional bits for the decoded value are fed back to the SHIFTER  672  enabling it to pass the relevant additional bits to the OUTPUT FORMATTER  676  and align the new front of the Huffman stream presented to the decoding unit  673 . The number of the additional bits is a function of the decoded value. If the first look up does not result in a decoded value, which means that the Huffman symbol is longer than 8 bits, the heap address is calculated and successive heap (located in the cache, too) accesses are performed following the algorithm until a match is found or an “illegal Huffman symbol” condition met. A match results in identical behavior as in case of the first match and “illegal Huffman symbol” generates an interrupt condition. 
     The algorithm for heap-based decoding algorithm is as follows: 
     loop until end of image 
     set symbol length N to 8 
     get first 8 bits of the input stream into INDEX 
     fetch HUFVAL(INDEX) 
     if HUFVAL(INDEX)==00xx 0000 111--(ILL) 
     signal “illegal Huffman symbol” 
     exit 
     elsif HUFVAL(INDEX)==1nnn eeee eeee--(HIT) 
     pass nnn bits to eeee eeee as the value 
     pass symbol length N=decimal (nnn)/*000 
     as symbol length 8*/ 
     adjust the input stream 
     break 
     else/* HUFVAL (INDEX)==01iii iiii iiii--(MISS)*/ 
     set HEAPINDEX=ii iiii iiii--(we assume heapbase= 0)    
     set N=9 
     if 9th bit of the input stream=0 
     increment HEAPINDEX 
     fi 
     fetch VALUE=HEAP (HEAPINDEX)--(code for 9th bit) 
     loop 
     if VALUE==0001 0000 1111--(ILL) 
     signal “illegal Huffman symbol” 
     exit 
     elsif VALUE==1000 eeee eeee 
     pass eeee eeee as the value 
     pass symbol length N 
     adjust the input stream 
     break 
     else/* VALUE==0liii iiii iiii--(MISS) *1 
     set N=N+1--(HEAPINDEX=ii iiii iiii) 
     if Nth bit of the input stream==0 
      increment HEAPINDEX 
     fi 
     fetch VALUE=HEAP (HEAPINDEX) 
     pool 
     pool 
     The STRIPPER  671  removes any X′00 stuff bytes, X′FF fill bytes and RST m  markers from the incoming JPEG  671  coded stream and passes “clean” Huffman symbols with concatenated additional bits to the shifter  672 . There are no additional bits in Huffman-only encoding, so in this mode the passed stream consists of Huffman symbols only. 
     The shifter  672  block has a 16 bit output register in which it presents the next Huffman symbol to the decoding unit  673  (bitstream running from MSB to LSB). Often the symbol is shorter than 16 bits, but it is up to the decoding unit  673  to decide how many bits are currently being analysed. The shifter  672  receives a feedback  678  from the decoding unit  673 , namely the length of the current symbol and the length of the following additional bits for the current symbol (in JPEG mode), which allows for a shift and proper alignment of the beginning of the next symbol in the shifter  672 . 
     The decoding unit  673  implements the core of the heap based algorithm and interfaces to the data cache via the OBus  674 . It incorporates a Data Cache fetch block, lookup value comparator, symbol length counter, heap index adder and a decoder of the number of the additional bits (the decoding is based on the decoded value). The fetch address is interpreted as follows: 
     
       
         
           
               
             
               
                 TABLE 15 
               
             
            
               
                   
               
               
                 Fetch Address 
               
            
           
           
               
               
            
               
                 Field (bits) 
                 Description 
               
               
                   
               
               
                 [32:25] 
                 Index into dequantization tables. 
               
               
                 [24:19] 
                 Not used. 
               
               
                 [18:9]  
                 Index into the heap. 
               
               
                 [8:0] 
                 Index into Huffman decode table. 
               
               
                   
               
            
           
         
       
     
     The OUTPUT FORMATTER block  676  packs decoded 8-bit values (standalone Huffman mode), or packs 24-bit value+additional bits+RST m  marker information (JPEG mode) into 32-bit words. The additional bits are passed to the OUTPUT FORMATTER  676  by the shifter  672  after the decoding unit  673  decides on the start position of the additional bits for the current symbol. The OUTPUT FORMATTER  673  also implements a 2 deep FIFO buffer using a one word delay for prediction of the final value word. During the decoding process, it may happen that the shifter  672  (either fast or slow) tries to decode the trailing padding bits at the end of the input bitstream. This situation is normally detected by the shifter and instead of asserting the “illegal symbol” interrupt, it asserts a “force final” signal. Active “force final” signal forces the OUTPUT FORMATTER  676  to signal the last but one decoded word as “final” (this word is still present in the FIFO) and discard the very last word which does not belong to the decoded stream. 
     The Huffman encoder  660  of FIG. 70 is illustrated in FIG. 72 in more detail. The Huffman encoder  660  maps byte data into Huffman symbols via look up tables and includes a encoding unit  681 , a shifter  682  and a OUTPUT FORMATTER  683  with the lookup tables being accessed from the cache. 
     Each submitted value  685  is coded by the encoding unit  681  using coding tables stored in the data cache. One access to the cache  230  is needed to encode a symbol, although each value being encoded requires two tables, one that contains the corresponding code and the other that contains the code length. During JPEG compression, a separate set of tables is needed for AC and DC coefficients. If subsampling is performed, separate tables are required for subsampled and non subsampled components. For non-JPEG compression, only two tables (code and size) are needed. The code is then handled by the shifter  682  which assembles the outgoing stream on bit level. The Shifter  682  also performs RST m  and EOI markers insertion which implies byte padding, if necessary. Bytes of data are then passed to the OUTPUT FORMATTER  683  which does stuffing (with X′00 bytes), filling with X′FF bytes, also the FF bytes leading the marker codes and formatting to packed bytes. In the non-JPEG mode, only formatting of packed bytes is required. 
     Insertion of X′FF bytes is handled by the shifter  682 , which means that the output formatter  683  needs to tell which bytes passed from the shifter  682  represent markers, in order to insert an X′FF byte before. This is done by having a register of tags which correspond to bytes in the shifter  682 . Each marker, which must be on byte boundaries anyway, is tagged by the shifter  682  during marker insertion. The packer  683  does not insert stuff bytes after the X″FF″ bytes preceding the markers. The tags are shifted synchronously with the main shift register. 
     The Huffman encoder uses four or eight tables during JPEG compression, and two tables for straight Huffman encoding. The tables utilized are as follows: 
     
       
         
           
               
             
               
                 TABLE 16 
               
             
            
               
                   
               
               
                 Tables Used by the Huffman Encoder 
               
            
           
           
               
               
               
            
               
                 Name 
                 Size 
                 Description 
               
               
                   
               
            
           
           
               
               
               
            
               
                 EHUFSI 
                 256 
                 Huffman code sizes. Used during straight 
               
               
                   
                   
                 Huffman encoding. Uses the coded value as 
               
               
                   
                   
                 an index. 
               
               
                 EHUFCO 
                 256 
                 Huffman code values used during straight 
               
               
                   
                   
                 Huffman encoding. Uses the coded value as 
               
               
                   
                   
                 an index. 
               
               
                 EHUFSI_DC_1 
                 16 
                 Huffman codes sizes used to code DC co- 
               
               
                   
                   
                 efficients during JPEG compression. Uses 
               
               
                   
                   
                 magnitude category as the index. 
               
               
                 EHUFCO_DC_1 
                 16 
                 Huffman code values used to code DC co- 
               
               
                   
                   
                 efficients during JPEG compression. Uses 
               
               
                   
                   
                 magnitude category as an index. Used for 
               
               
                   
                   
                 subsampled blocks. 
               
               
                 EHUFSI_DC_2 
                 16 
                 Huffman code sizes used to code DC co- 
               
               
                   
                   
                 efficients during JPEG compression. Uses 
               
               
                   
                   
                 magnitude category as an index. Used for 
               
               
                   
                   
                 subsampled blocks. 
               
               
                 EHUFCO_DC_2 
                 16 
                 Huffman code sizes used to code DC co- 
               
               
                   
                   
                 efficients during JPEG compression. Uses 
               
               
                   
                   
                 magnitude category as an index. Used for 
               
               
                   
                   
                 subsampled blocks. 
               
               
                 EHUFSI_AC_1 
                 256 
                 Huffman code sizes used to code AC co- 
               
               
                   
                   
                 efficients during JPEG compression. Uses 
               
               
                   
                   
                 magnitude category and run-length as an 
               
               
                   
                   
                 index. 
               
               
                 EHUFCO_AC_1 
                 256 
                 Huffman code sizes used to code AC co- 
               
               
                   
                   
                 efficients during JPEG compression. Uses 
               
               
                   
                   
                 magnitude category and run-length as an 
               
               
                   
                   
                 index. 
               
               
                 EHUFSI_AC_2 
                 256 
                 Huffman code sizes used to code AC co- 
               
               
                   
                   
                 efficients during JPEG compression for 
               
               
                   
                   
                 subsampled components. Uses magnitude 
               
               
                   
                   
                 category and run-length as an index. 
               
               
                 EHUFCO_AC_2 
                 256 
                 Huffman code sizes used to code AC co- 
               
               
                   
                   
                 efficients during JPEG compression for 
               
               
                   
                   
                 subsampled components. Uses magnitude 
               
               
                   
                   
                 category and run-length as an index. 
               
               
                   
               
            
           
         
       
     
     3.17.4 Table Indexing 
     Huffman tables are stored locally by the co-processor data cache  230 . The data cache  230  is organized as a 128 line, direct mapped cache, where each line comprizes 8 words. Each of the words in a cache line are separately addressable, and the Huffman decoder uses this feature to simultaneously access multiple tables. Because the tables are small (&lt;=256 entries), the 32 bit address field of the OBus can carry indexes into multiple tables. 
     As noted previously, in JPEG slow decoding mode, the data cache is utilized for storing various Huffman tables. The format of the data cache is as follows: 
     
       
         
           
               
             
               
                 TABLE 17 
               
             
            
               
                   
               
               
                 Bank Address for Huffman and Quantization Tables 
               
            
           
           
               
               
            
               
                 Bank 
                 Description 
               
               
                   
               
               
                 0 to 3 
                 These banks hold the 1024. 16 bit entries of the heap. The 
               
               
                   
                 least significant index bit selects between the two 16 bit 
               
               
                   
                 words in each bank. All 128 lines of the four banks of 
               
               
                   
                 memory are used. 
               
               
                 4 
                 This bank holds the 512, least significant 8 bits of the 12 bit 
               
               
                   
                 entries of the DC Huffman decode table. The least 
               
               
                   
                 significant two bits of the index chooses between the four. 
               
               
                   
                 byte items in the 32 bit word. All 128 line of this bank of 
               
               
                   
                 memory are used. 
               
               
                 5 
                 This bank holds the 512, least significant 8 bits of the 12 bit 
               
               
                   
                 entires of the AC Huffman decode table. The least 
               
               
                   
                 significant two bits of the index chooses between the four. 
               
               
                   
                 byte items in the 32 bit word. All 128 lines of this bank of 
               
               
                   
                 memory are used. 
               
               
                 6 
                 This bank holds the most significant 4 bits of both the DC 
               
               
                   
                 and AC Huffman decode tables. The least significant 2 bits 
               
               
                   
                 of each index chooses between the 4 respective nibbles within 
               
               
                   
                 each word. 
               
               
                 7 
                 This bank holds the 128, 24 bit entires of the quantization 
               
               
                   
                 table. It occupies the least significant 3 bytes of all 128 lines 
               
               
                   
                 of this bank of memory. 
               
               
                   
               
            
           
         
       
     
     Prior to each JPEG instruction being executed by the JPEG coder  241  (FIG. 2) the appropriate image width value in the image dimensions register (PO_IDR) or (RO_IDR) must be set. As with other instructions, the length of the instruction refers to the number of input data items to be processed. This includes any padding data and accounts for any sub-sampling options utilized and for the number of color channels used. 
     All instructions issued by the co-processor  224  may utilize two facilities for limiting the amount of output data produced. These facilities are most useful for instructions where the input and output data sizes are not the same and in particular where the output data size is unknown, such as for JPEG coding and decoding. The facilities determine whether the output data is written out or merely discarded with everything else being as if the instruction was properly processed. By default, these facilities are normally disabled and can be enabled by enabling the appropriate bits in the RO_CFG register. JPEG instructions however, include specific option for setting these bits. Preferably, when utilising JPEG compression, the co-processor  224  provides facilities for “cutting” and “limiting” of output data. 
     Turning to FIG. 7, there is now described the process of cutting and limiting. An input image  690  may be of a certain height  691  and a certain width  692 . Often. only a portion of the image is of interest with other portions being irrelevant for the purposes of printing out. However, the JPEG encoding system deals with 8×8 blocks of pixels. It may be the case that, firstly, the image width is not an exact multiple of 8 and additionally, the section of interest comprising MCU  695  does not fit across exact boundaries. An output cut register, RO_cut specifies the number of output bytes at  696  at the beginning of the output data stream to discard. Further, an output limit register, RO_LMT specifies the maximum number of output bytes to be produced. This count includes any bytes that do not get written to memory as a result of the cut register. Hence, it is possible to target a final output byte  698  beyond which no data is to be outputted. 
     There are two particular cases where the cut and limited functionality of the JPEG decoder is considered to be extremely useful. The first case, as illustrated in FIG. 74, is the extraction or decompression of a sub-section  700  of one strip  701  of a dedcompressed image. The second useful case is illustrated in FIG. 75 wherein the extraction or decompression of a number of complete strips (eg.  711 .  712  and  713 ) is required from an overall image  714 . 
     The format and field encoding for JPEG instructions is as illustrated in FIG. 76, the minor opcode fields are interpreted as follows: 
     
       
         
           
               
             
               
                 TABLE 18 
               
             
            
               
                   
               
               
                 Instruction Word -  Minor Opcode Fields 
               
            
           
           
               
               
               
            
               
                   
                 Field 
                 Description 
               
               
                   
                   
               
               
                   
                 D 
                 0 =  encode (compress) 
               
               
                   
                   
                 1 =  decode (decompress) 
               
               
                   
                 M 
                 0 =  single color channel 
               
               
                   
                   
                 1 =  multi channel 
               
               
                   
                 4 
                 0 =  three channel 
               
               
                   
                   
                 1 =  four channel 
               
               
                   
                 S 
                 0 =  do not use a sub/up sampling regime 
               
               
                   
                   
                 1 =  use a subsampling regime 
               
               
                   
                 H 
                 0 =  use fast Huffman coding 
               
               
                   
                   
                 1 =  use general purpose Huffman coding 
               
               
                   
                 C 
                 0 =  do not use cut register 
               
               
                   
                   
                 1 =  use cut register 
               
               
                   
                 T 
                 0 =  do not truncate on output 
               
               
                   
                   
                 1 =  truncate on output 
               
               
                   
                 F 
                 0 =  do not low pass filter before subsampling 
               
               
                   
                   
                 1 =  low pass filter before subsampling 
               
               
                   
                   
               
            
           
         
       
     
     3.17.5 Data Coding Instructions 
     Preferably, the co-processor  224  provides for the ability to utilize portions of the JPEG coder  241  of FIG. 2 in other ways. For example, Huffman coding is utilized for both JPEG and many other methods of compression. Preferably, there is provided data coding instructions for manipulating the Huffman coding unit only for hierarchial image decompression. Further, the run length coder and decoder and the predictive coder can also be separately utilized with similar instructions. 
     3.17.6 A Fast DCT Apparatus 
     Conventionally, a discrete cosine transform (DCT) apparatus as shown in FIG. 77 performs a full two-dimensional (2-D) transformation of a block of 8×8 pixels by first performing a 1-D DCT on the rows of the 8×8 pixel block. It then performs another 1-D DCT on the columns of the 8×8 pixel block. Such an apparatus typically consists of an input circuit  1096 , an arithmetic circuit  1104 , a control circuit  1098 , a transpose memory circuit  1090 , and an output circuit  1092 . 
     The input circuit  1096  accepts 8-bit pixels from the 8×8 block. The input circuit  1096  is coupled by intermediate multiplexers  1100 ,  1102  to the arithmetic circuit  1004 . The arithmetic circuit  1104  performs mathematical operations on either a complete row or column of the 8×8 block. The control circuit  1098  controls all the other circuits, and thus implements the DCT algorithm. The output of the arithmetic circuit is Do coupled to the transpose memory  1090 , register  1095  and output circuit  1092 . The transpose memory is in turn connected to multiplexer  1100 , which provides output to the next multiplexer  1102 . The multiplexer  1102  also receives input from the register  1094 . The transpose circuit  1090  accepts 8×8 block data in rows and produces that data in columns. The output circuit  1092  provides the coefficients of the DCT performed on a 8×8 block of pixel data. 
     In a typical DCT apparatus, it is the speed of the arithmetic circuit  1104  that basically determines the overall speed of the apparatus, since the arithmetic circuit  1104  is the most complex. 
     The arithmetic circuit  1104  of FIG. 77 is typically implemented by breaking the arithmetic process down into several stages as described hereinafter with reference to FIG. 78. A single circuit is then built that implements each of these stages  1114 ,  1148 ,  1152 ,  1156  using a pool of common resources, such as adders and multipliers. Such a circuit  1104  is mainly disadvantageous due to it being slower than optimal, because a single, common circuit is used to implement the various stages of circuit  1104 . This includes a storage means used to store intermediate results. Since the time allocated for the clock cycle of such a circuit must be greater or equal to the time of the slowest stage of the circuit, the overall time is potentially longer than the sum of all the stages. 
     FIG. 78 depicts a typical arithmetic data path, in accordance with the apparatus of FIG. 77, as part of a DCT with four stages. The drawing does not reflect the actual implementation, but instead reflects the functionality. Each of the four stages  1144 ,  1148 ,  1152 , and  1156  is implemented using a single, reconfigurable circuit. It is reconfigured on a cycle-by-cycle basis to implement each of the four arithmetic stages  1144 ,  1148 ,  1152 , and  1156  of the 1-D DCT. In this circuit, each of the four stages  1144 ,  1148 ,  1152 , and  1156  uses pool of common resources (e.g. adders and multipliers) and thus minimises hardware. 
     However, the disadvantage of this circuit is that it is slower than optimal. The four stages  1144 ,  1148 ,  1152 , and  1156  are each implemented from the same pool of adders and multipliers. The period of the clock is therefore determined by the speed of the slowest stage, which in this example is 20 ns (for block  1144 ). Adding in the delay (2 ns each) of the input and output multiplexers  1146  and  1154  and the delay (3 ns) of the flip-flop  1150 , the total time is 27 ns. Thus, the fastest this DCT implementation can run at is 27 ns. 
     Pipelined DCT implementations are also well known. The drawback with such implementations is that they require large amounts of hardware to implement. Whilst the present invention does not offer the same performance in terms of throughput, it offers an extremely good performance/size compromise, and good speed advantages over most of the current DCT implementations. 
     FIG. 79 shows a block diagram of the preferred form of discrete cosine transform unit utilized in the JPEG coder  241  (FIG. 2) where pixel data is inputted to an input circuit  1126  which captures an entire row of 8-bit pixel data. The transpose memory  1118  converts row formatted data into column formatted data for the second pass of the two dimensional discrete cosine transform algorithm. Data from the input circuit  1126  and the transpose memory  1118  is multiplexed by multiplexer  1124 , with the output data from multiplexer  1124  presented to the arithmetic circuit  1122 . Results data from the arithmetic circuit  1122  is presented to the output circuit  1120  after the second pass of the process. The control circuit  1116  controls the flow of data through the discrete cosine transform apparatus. 
     During the first pass of the discrete cosine transform process row data from the image to be transformed, or transformed image coefficients to be transformed back to pixel data is presented to the input circuit  1126 . During this first pass, the multiplexer  1124  is configured by the control circuit  1116  to pass data from the input circuit  1126  to the arithmetic circuit  1122 . 
     Turning to FIG. 80, there is shown the structure of the arithmetic circuit  1122  in more detail. In the case of performing a forward discrete cosine transform, the results from the forward circuit  1138  which is utilized to calculate the forward discrete cosine transform is selected via the multiplexer  1142 , which is configured in this way by the control circuit  1116 . When an inverse discrete cosine transform is to he performed, the output from the inverse circuit  1140  is selected via the multiplexer  1142 , as controlled by the control circuit  1126 . During the first pass, after each row vector has been processed by the arithmetic circuit  1122  (configured in the appropriate way by control circuit  1116 ), that vector is written into the transpose memory  1118 . Once all eight row vectors in an 8×8 block have been processed and written into the transpose memory  1118 , the second pass of the discrete cosine transform begins. 
     During the second pass of either the forward or inverse discrete cosine transforms, column ordered vectors are read from the transpose memory  1118  and presented to the arithmetic circuit  1122  via the multiplexer  1124 . During this second pass, the multiplexer  1124  is configured by the control circuit to ignore data from the input circuit  1136  and pass column vector data from the transpose memory  1118  to the arithmetic circuit  1122 . The multiplexer  1142  in the arithmetic circuit  1122  is configured by the control circuit  1116  to pass results data from the inverse circuit  1140  to the output of the arithmetic circuit  1122 . When results from the arithmetic circuit  1122  are available, they are captured by the output circuit  1120  under direction from the control circuit  1116  to be outputted sometime later. 
     The arithmetic circuit  1122  is completely combinatorial, in that is there are no storage elements in the circuit storing intermediate results. The control circuit  1116  knows how long it takes for data to flow from the input circuit  1136 , through the multiplexer  1124  and through the arithmetic circuit  1122 , and so knows exactly when to capture the results vector from the outputs of the arithmetic circuit  1122  into the output circuit  1120 . The advantage of having no intermediate stages in the arithmetic circuit  1122  is that no time is wasted getting data in and out of intermediate storage elements, but also the total time taken for data to flow through the arithmetic circuit  1122  is equal to the sum of all the internal stages and not N times the delay of the longest stage (as with conventional discrete cosine transform implementations), where N is the number of stages in the arithmetic circuit. 
     Referring to FIG. 81, the total time delay is simply the sum of the four stage  1158 ,  1160 ,  1162 ,  1164 , which is 20 ns+10 ns+12 ns+15 ns=57 ns, which is faster that the circuit depicted in FIG.  78 . The advantage of this circuit is that it provides an opportunity to reduce the overall system&#39;s clock period. Assuming that four clock cycles are allocated to getting a result from the circuit depicted in FIG. 81, the fastest run time for the entire DCT system would be 57/4 ns (14.25 ns), which is a significant improvement over the circuit in FIG. 78 which only allows for a DCT clock period of substantially 27 ns. 
     An examplary implementation of the present DCT apparatus might, but not necessarily, use the DCT algorithm proposed in the paper to The Transactions of the IEICE, Vol. E 71. No. 11. November 1988, entitled  A Fast DCT - SQ Scheme for Images  at page 1095 by Yukihiro Arai, Takeshi Agui and Masayuki Nakajiml. By implementing this algorithm in hardware, it can then easily be placed in the current DCT apparatus in the arithmetic circuit  1122 . Likewize, other DCT algorithms may be implemented in hardware in place of arithmetic circuit  1122 . 
     3.17.7 Huffman Decoder 
     The aspects of the following embodiment relate to a method and apparatus for variable-length codes interleaved with variable length bit fields. In particular, the embodiments of the invention provide efficient and fast, single stage (clock cycle) decoding of variable-length coded data in which byte aligned and not variable length encoded data is removed from the encoded data stream in a separate pre-processing block. Further, information about positions of the removed byte-aligned data is passed to the output of the decoder in a way which is synchronous with the data being decoded. In addition, it provides fast detection and removal of not byte-aligned and not variable length encoded bit fields that are still present in the pre-processed input data. 
     The preferred embodiment of the present invention preferably provides for a fast Huffman decoder capable of decoding a JPEG encoded data at a rate of one Huffman symbol per clock cycle between marker codes. This is accomplished by means of separation and removal of byte aligned and not Huffman encoded marker headers, marker codes and stuff bytes from the input data first in a separate pre-processing block. After the byte aligned data is removed, the input data is passed to a combinatorial data-shifting block, which provides continuous and contiguous filling up of the data decode register that consequently presents data to a decoding unit. Positions of markers removed from the original input data stream are passed on to a marker shifting block, which provides shifting of marker position bits synchronously with the input data being shifted in the data shifting block. 
     The decoding unit provides combinatorial decoding of the encoded bit field presented to its input by the data decode register. The bit field is of a fixed length of n bits. The output of the decoding unit provides the decoded value (v) and the actual length (m) of the input code, where m is less than or equal to n. It also provides the length (a) of a variable length bit field, where (a) is greater than or equal to 0. The variable-length bit field is not Huffman encoded and follows immediately the Huffman o code. The n-long bit field presented to the input of the decoding unit may be longer than or equal to the actual code. The decoding unit determines the actual length of the code (m) and passes it together with the length of the additional bits (a) to a control block. The control block calculates a shift value (a+m) driving the data and marker shifting blocks to shift the input data for the next decoding cycle. 
     The apparatus of the invention can comprise any combinatorial decoding unit. including ROM, RAM, PLA or anything else based as long as it provides a decoded value, the actual length of the input code, and the length of the following not Huffman encoded bit field within a given time frame. 
     In the illustrated embodiment, the decoding unit outputs predictively encoded DC difference values and AC run-length values as defined in JPEG standard. The not Huffman encoded bit fields, which are extracted from the input data simultaneously with decoded values, represent additional bits determining the value of the DC and AC coefficients as defined in JPEG standard. Another kind of not Huffman encoded bit fields, which are removed from the data present in the data decode register, are padding bits as defined in JPEG standard that precede byte-aligned markers in the original input data stream. These bits are detected by the control block by checking the contents of a padding zone of the data register. The padding zone comprises up to k most significant bits of the data register and is indicated by the presence of a marker bit within k most significant bits of the marker register, position of said marker bit limiting the length of the padding zone. If all the bits in the padding zone are identical (and equal to 1s in case of JPEG standard), they are considered as padding bits and are removed from the data register accordingly without being decoded. The contents of the data and marker registers are then adjusted for the next decoding cycle. 
     The exemplary apparatus comprises an output block that handles formatting of the outputted data according to the requirements of the preferred embodiment of the invention. It outputs the decoded values together with the corresponding not variable length encoded bit fields, such as additional bits in JPEG, and a signal indicating position of any inputted byte aligned and not encoded bit fields, such as markers in JPEG with respect to the decoded values. 
     Data being decoded by the JPEG coder  241  (FIG. 2) is JPEG compatible and comprizes variable length Huffman encoded codes interleaved with variable length not encoded bit fields called “additional bits”, variable length not encoded bit fields called “ipadding bits” and fixed length, byte aligned and not encoded bit fields called “markers”, “stuff bytes” and “fill bytes”. FIG. 82 shows a representative example of input data. 
     The overall structure and the data flow in the Huffman decoder of the JPEG coder  241  is presented in FIG.  83  and FIG. 84, where FIG. 83 illustrates the architecture of the Huffman decoder of the JPEG data in more detail. The stripper  1171  removes marker codes (code FFXX hex , XX being non zero), fill bytes (code FF hex ) and stuff bytes (code 00 hex  following code FF hex ), that is all byte aligned components of the input data, which are presented to the stripper as 32 bit words. The most significant bit of the first word to be processed is the head of the input bit stream. In the stripper the byte aligned bit fields are removed from each input data word before the actual decoding of Huffman codes takes place in the downstream parts of the decoder. 
     The input data arrives at the stripper&#39;s  1171  input as 32-bit words, one word per clock cycle. Numbering of the input bytes  1211  from 0 to 3 is shown in FIG.  85 . If a byte of a number (i) is removed because it is a fill byte, a stuff byte or belongs to a marker, the remaining bytes of numbers (i−1) down to 0 are shifted to the left on the output of the stripper  1171  and take numbers (i) down to 1. Byte  0  becoming a “don&#39;t care” byte. Validity of bytes outputted by the stripper  1171  is also coded by means of separate output tags  1212  as shown in FIG.  85 . The bytes which are not removed by the stripper  1171  are left aligned on the stripper&#39;s output. Each byte on the output has a corresponding tag indicating if the corresponding byte is valid (i.e. passed on by the stripper  1171 ), or invalid (i.e. removed by the stripper  1171 ) or valid and following a removed marker. The tags  1212  control loading of the data bytes into the data register  1182  through the data shifter and loading of marker positions into the marker register  1183  through the marker shifter. The same scheme applies if more than one byte is removed from the input word: all the remaining valid bytes are shifted to the left and the corresponding output tags indicate validity of the output bytes. FIG. 85 provides examples  1213  of output bytes and output tags for various example combinations of input bytes. 
     Returning to FIG. 83, the role of the preshifter and postshifter blocks  1172 ,  1173 ,  1180 ,  1181  is to assure loading of the data into the corresponding data register  1182  and marker register  1183  in a contiguous way whenever there is enough room in the data register and the marker register. The data shifter and the marker shifter blocks, which consist of the respective pre- and postshifters, are identical and identically controlled. The difference is that while the data shifter handles data passed by the stripper  1171 , the marker shifter handles the tags only and its role is to pass marker positions to the output of the decoder in a way synchronous with the decoded Huffman values. The outputs of the postshifters  1180 ,  1181  feed directly to the respective registers  1182 ,  1183 , as shown in FIG.  83 . 
     In the data preshifter  1172 , as also shown in FIG. 86, data arriving from the stripper  1171  is firstly extended to 64 bits by appending 32 zeroes to the least significant bit  1251 . Then the extended data is shifted in a 64 bit wide barrel shifter  1252  to the right by a number of bits currently present in the data register  1182 . This number is provided by the control logic  1185  which keeps track of how many valid bits are there in the data  1182  and marker  1183  registers. The barrel shifter  1252  then presents 64 bits to the multiplexer block  1253 , which consists of 64 2×1 elementary multiplexers  1254 . Each elementary 2×1 multiplexer  1254  takes as inputs one bit from the barrel shifter  1252  and one bit from the data register  1182 . It passes the data register bit to the output when this bit is still valid in the data register. Otherwize, it passes the barrel shifter&#39;s 1252 bit to the output. The control signals to all the elementary multiplexers  1254  are decoded from a control block&#39;s shift control  1  signals as shown in FIG. 86, which are also shown in FIG. 87 as preshifter control bits  0  . . .  5  of register  1223 . The outputs of the elementary multiplexers  1254  drive a barrel shifter  1255 . It shifts left by the number of bits provided on a 5 bit control signal shift control  2  as shown in FIG.  86 . These bits represent the number of bits consumed from the data register  1182  by the decoding of the current data, which can be either the length of the currently decoded Huffman code plus the number of the following additional bits, or the number of padding bits to be removed if padding bits are currently being detected, or zero if the number of valid data bits in the data register  1182  is less then the number of bits to be removed. In this way, the data appearing on the output of barrel shifter  1255  contains new data to be loaded into the data register  1182  after a single decoding cycle. The contents of the data register  1182  changes in such a way that the leading (most significant) bits are shifted out of the register as being decoded, and 0, 8, 16, 24 or 32 bits from the stripper  1171  are added to the contents of the data register  1182 . If there are not enough bits in the data register  1182  to decode them, data from the stripper  1171 , if available, is still loaded in the current cycle. If there is no data available from the stripper  1171  in the current cycle, the decoded bits from the data register  1182  are still removed if there is a sufficient amount of them, otherwize the content of the data register  1182  does not change. 
     The marker preshifter  1173 , postshifter  1181  and the marker register  1183  are units identical to the data preshifter  1172 , data postshifter  1180  and the data register  1182 , respectively. The data flow inside units  1173 ,  1181  and  1183  and among them is also identical as the data flow among units  1172 ,  1180  and  1182 . The same control signals are provided to both sets of units by the control unit  1185 . The difference is only in the type of data on the inputs of the marker preshifter  1173  and data preshifter  1172 , as well as in how the contents of the marker register  1183  and the data register  1182  are used. As shown in FIG. 88, tags  1261  from the stripper  1171  come as eight bit words, which provide two bits for each corresponding byte of data going to the data register  1182 . According to the coding scheme shown in FIG. 85, an individual two bit tag indicating valid and following a marker byte has 1 on the most significant position. Only this most significant position of each of the four tags delivered by the stripper  1171  simultaneously is driven to the input  1262  of the marker preshifter  1173 . In this way, on the input to the marker preshifter there may be bits set to 1 indicating positions of the first encoded data bits following markers. At the same time, they mark the positions of the first encoded data bits in the data register  1182  which follow a marker. This synchronous behavior of the marker position bits in the marker register  1183  and the data bits in the data register  1182  is used in the control block  1185  for detection and removal of padding bits, as well as for passing marker positions to the output of the decoder in a way synchronous with the decoded data. As mentioned, the two preshifters (data  1172  and marker  1173 ), postshifters (data  1180  and marker  1181 ) and registers (data  1182  and marker  1183 ) get the same control signals which facilitates fully parallel and synchronous operation. 
     The decoding unit  1184 , also shown in FIG. 89 gets the sixteen most significant bits of the data register  1182  which are driven to a combinatorial decoding unit  1184  for extraction of a decoded Huffman value, the length of the present input code being decoded and the length of the additional bits following immediately the input code (which is a function of the decoded value). The length of the additional bits is known after the corresponding preceding Huffman symbol is decoded, so is the starting position of the next Huffman symbol. This effectively requires, if speed of one value decoded per clock cycle is to be maintained, that decoding of a Huffman value is done in a combinatorial block. Preferably, the decoding unit comprizes four PLA style decoding tables hardwired as a combinatorial block taking a 16-bit token on input from the data register  1182  and producing a Huffman value (8 bits), the length of the corresponding Huffman-encoded symbol (4 bits) and the length of the additional bits (4 bits) as illustrated in FIG.  89 . 
     Removal of padding bits takes place during the actual decoding when a sequence of padding bits is detected in the data register  1182  by a decoder of padding bits which is part of the control unit  1185 . The decoder of padding bits operates as shown in FIG.  90 . Eight most significant bits of the marker register  1183 ,  1242  are monitored for presence of a marker position bit. If a marker position bit is detected, all the bits in the data register  1182 ,  1241  which correspond to, that is have the same positions as, the bits preceding the marker bit in the marker register  1242  are recognized as belonging to a current padding zone. The content of the current padding zone is checked by the detector of padding bits  1243  for 1&#39;s. If all the bits in the current padding zone are 1&#39;s, they are recognized as padding bits and are removed from the data register. Removal is done by means of shifting of the contents of the data register  1182 ,  1241  (and at the same time the marker register  1183 ,  1242 ) to the left using the respective shifters  1172 ,  1173 ,  1180 ,  1181  in one clock cycle, as in normal decode mode with the difference that no decoded value is outputted. If not all the bits in the current padding zone are 1&#39;s, a normal decode cycle is performed rather than a padding bits removal cycle. Detection of padding bits takes place each cycle as described, in case there are some padding bits in the data register  1182  to be removed. 
     The control unit  1185  is shown in detail in FIG.  87 . The central part of the control unit is the register  1223  holding the current number of valid bits in the data register  1182 . The number of valid bits in the marker register  1183  is always equal to the number of valid bits in the data register  1182 . The control unit preforms three functions. Firstly, it calculates a new number of bits in the data register  1182  to be stored in the register  1223 . Secondly, it determines control signals for the shifters  1172 ,  1173 ,  1180 ,  1181 ,  1186 ,  1187  decoding unit  1184 , and the output formatter  1188 . Finally, it detects padding bits in the data register  1182 , as described above. 
     The new number of bits in the data register  1182  (new_nob) is calculated as the current number of bits in the data register  1182  (nob) plus the number of bits (nos) available for loading from the stripper  1171  in the current cycle, less the number of bits (nor) removed from the data register  1182  in the current cycle, which is either a decode cycle or a padding bits removal cycle. The new number of bits is calculated as follows: 
     
       
         new_nob=nob÷nos−nor 
       
     
     The respective arithmetic operations are done in adder  1221  and subtractor  1222 . It should be noted that (nos) can be 0 if there is no data available from the stripper  1171  in the current cycle. Also, (nor) can be 0 if there is no decoding done in the current cycle because of shortage of bits in the data register  1182 , which means there are less bits in the data register than the sum of the current code length and the following additional bits length as delivered by the control unit  1185 . The value (new_nob) may exceed 64 and block  1224  checks for this condition. In such a case, the stripper  1171  is stalled and no new data is loaded. Multiplexer  1233  is used for zeroing the number of bits to be loaded from the stripper  1171 . A corresponding signal for stalling the stripper  1171  is not shown. Signal “padding cycle” driven by decoder  1231  controls multiplexer  1234  to select either the number of padding bits or the number of decoded bits (that is the length of code bits plus additional bits) as number of bits to be removed (nor). If the number of the decoded bits is greater than the number (nob) of the bits in the data register, which is checked in comparator  1228 , the effective number of bits to shift as provided for multiplexer  1234  is set to zero by a complex NAND gate  1230 . As a result, (nor) is set to zero and no bits are removed from the data register. The output of multiplexer  1234  is also used to control postshifters  1182  and  1183 . The width of the data register  1182  must be chosen in a way preventing a deadlock situation. This means that at any time either there needs to be room in the data register to accommodate the maximum number of bits available from the stripper  1171  or sufficient number of valid bits to be removed as a result of a decode or a padding of bits removed cycle. 
     Calculation of the number of bits to be removed in a decode cycle is performed by adder  1226 . Its operands come from the combinatorial decoding unit  1184 . As the code length of 16 bits is coded as “0000” by the decoding unit, “or_reduce” logic  1225  provides encoding of “0000” into “10000”, yielding a correct unsigned operand. This operand together with the output of subtractor  1227  provide control signals to the output formatting shifters  1186  and  1187 . 
     Block  1229  is used for detection of EOI (End Of Image) marker position. The EOI marker itself is remoxed by the stripper  1171 , but there can be some padding bits which are the very last bits of the data and which used to precede the EOI marker before its removal in the stripper  1171 . The comparator  1229  checks if the number of bits in the data register  1182 , stored in register  1223  is less than eight. If it is, and there is no more data to come from the stripper  1171  (that is the data register  1182  holds all the remaining bits for of the data unit being decoded), the remaining bits define the size of the padding zone before the removed EOI marker. Further handling of the padding zone and possible removal of padding bits is identical to the procedure applied in case of padding bits before RST markers, which has been described before. 
     Barrel shifters  1186 ,  1187  and output formatter  1188  play a support role and depending on the embodiment may have a different implementation or may not be implemented at all. Control signals to them come from the control unit  1185 , as described above. The ab_preshifter (additional bits preshifter)  1186  takes 32 bits from the data register as input and shifts them to the left by the length of the Huffman code being presently decoded. In this way, all the additional bits following the code being presently decoded appear left aligned on the output of the barrel shifter  1186  which is also the input to the barrel shifter  1187 . The ab_postshifter (additional bits postshifter)  1187  adjusts the position of the additional bits from left aligned to right aligned in an 11 bit field, as used in the output format of the data and shown in FIG.  91 . The additional bits field extends from bit  8  to bit  18  in the output word format  1196  and some of the most significant bits may be invalid, depending on the actual number of the additional bits. This number in encoded on bits  0  to  3  of  1196 , as specified by the JPEG standard. If a different format of the output data is adopted, the barrel shifters  1186  and  1187  and their functionality may change accordingly. 
     The output formatter block  1188  packs the decoded values, which in JPEG standard are DC and AC coefficients, ( 1196 , bits  0  to  7 ) and a DC coefficient indicator ( 1196 , bit  19 ) passed by the control unit  1185  together with the additional bits ( 1196 , bits  8  to  18 ) passed by the ab_postshifter  1187  and the marker position bit ( 1196 , bit  23 ) from the marker register  1183  into words according to the format presented in FIG.  91 . The output formatter  1188  also handles any particular requirements as to the output interface of the decoder. The implementation of the output formatter is normally expected to change if the output interface changes as a result of different requirements. The foregoing described Huffman decoder provides a highly effective form of decoding providing a high speed decoding operation. 
     3.17.8 Image Transformation Instructions 
     These instructions implement general affine transformations of source images. The operation to construct a portion of a transformed image falls generally into two broad areas. These include firstly working out which parts of the source image are relevant to constructing the current output scanline and, if necessary, decompressing them. The second step normally comprizes necessary sub-sampling and/or interpolation to construct the output image on a pixel by pixel basis. 
     Turning to FIG. 92, there is illustrated a flow chart of the steps required  720  to calculate the value of a destination pixel assuming that the appropriate sections of the source image have been decompressed. Firstly, the relevant sub-sampling, if present, must be taken into account  721 . Next, two processes are normally implemented, one involving interpolation  722  and the other being sub-sampling. Normally interpolation and sub-sampling are alternative steps, however in some circumstances interpolation and sub-sampling may be used together. In the interpolation process, the first step is to find the four surrounding pixels  722 , then determine if pre-multiplication is required  723 , before performing bilinear interpolation  724 . The bilinear interpolation step  724  is often computationally intensive and limits the operation of the image transformation process. The final step in calculating a destination pixel value is to add together the possibly bilinear interpolated sub-samples from the source image. The added together pixel values can be accumulated  727  in different possible ways to produce destination image pixels of  728 . 
     The instruction word encoding for image transformation instructions is as illustrated in FIG. 93 with the following interpretation being placed on the minor opcode fields. 
     
       
         
           
               
             
               
                 TABLE 19 
               
             
            
               
                   
               
               
                 Instruction Word - Minor Opcode Fields 
               
            
           
           
               
               
            
               
                 Field 
                 Description 
               
               
                   
               
               
                 S 
                 0 = bi-linear interpolation is used on the four 
               
               
                   
                 surrounding source image pixels to determine the 
               
               
                   
                 actually sampled value 
               
               
                   
                 1 =  sampled value is snapped to the closest source 
               
               
                   
                 image pixel value 
               
               
                 off[3:0] 
                 0 =  do not apply the offset register (mdp_por) to the 
               
               
                   
                 corresponding channel 
               
               
                   
                 1 =  apply  the offset register (mdp_por) to the 
               
               
                   
                 corresponding channel 
               
               
                 P 
                 0 =  do not pre-multiply source image pixels 
               
               
                   
                 1 =  pre-multiply source image pixels 
               
               
                 C 
                 0 =  do not clamp output values 
               
               
                   
                 1 =  clamp output underflows to 0x00 and overflows to 0xFF 
               
               
                 A 
                 0 =  do not take absolute value of output values 
               
               
                   
                 1 =  take absolute value of output values before 
               
               
                   
                 wrapping or clamping 
               
               
                   
               
            
           
         
       
     
     The instruction operand and result fields are interpreted as follows: 
     
       
         
           
               
             
               
                 TABLE 20 
               
             
            
               
                   
               
               
                 Instruction Operand and Results Word 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Internal 
                 External 
               
               
                 Operand 
                 Description 
                 Format 
                 Format 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 Operand A 
                 kernel descriptor 
                 — 
                 short or long kernel 
               
               
                   
                   
                   
                 descriptor table 
               
               
                 Operand B 
                 Source Image 
                 other 
                 image table format 
               
               
                   
                 Pixels 
               
               
                 Operand C 
                 unused 
                 — 
                 — 
               
               
                 Result 
                 pixels 
                 pixles 
                 packed stream, 
               
               
                   
                   
                   
                 unpacked bytes 
               
               
                   
               
            
           
         
       
     
     Operand A points to a data structure known as a “kernel descriptor” that describes all the information required to define the actual transformation. This data structure has one of two formats (as defined by the L bit in the A descriptor). FIG. 94 illustrates the long form of kernel descriptor coding and FIG. 95 illustrates the short form of encoding. The kernel descriptor describes: 
     1. Source image start co-ordinates  730  (unsigned fixed point, 24.24 resolution). Location (0,0) is at the top left of the image. 
     2. Horizontal  731  and vertical  732  (sub-sample) deltas (2&#39;s complement fixed point, 24.24. resolution) 
     3. A 3 bit bp field  733  defining the location of the binary point within the fixed point matrix coefficients as described hereinafter. 
     4. Accumulation matrix coefficients  735  (if present). These are of “variable” point resolution of 20 binary places (2&#39;s complement), with the location of the binary point implicitly specified by the bp field. 
     5. An rl field  736  that indicates the remaining number of words in the kernel descriptor. This value is equal to the number of rows times the number of columns minus 1. 
     The kernel coefficients in the descriptor are listed row by row, with elements of alternate rows listed in reverse direction, thereby forming a zig zag pattern. 
     Turning now to FIG. 96, the operand B consists of a pointer to an index table indexing into scan lines of a source image. The structure of the index table is as illustrated in FIG. 96, with the operand B  740  pointing to an index table  741  which in turn points to scan lines (eg.  742 ) of the required source image pixels. Typically, the index table and the source image pixels are cacheable and possibly located in the local memory. 
     The operand C stores the horizontal and vertical sub-sample rate. The horizontal and vertical sub-sample rates are defined by the dimensions of the sub-sample weight matrix which are specified if the C descriptor is present. The dimensions of the matrix r and c are encoded in the data word of the image transformation instruction as illustrated in FIG.  97 . 
     Channel N of a resultant pixel P[N] is calculated in accordance with the following equation:          p        [   n   ]       =       (         I   .     offset        [   n   ]         ·     mdp   por            :        0000     )     +       ∑   r                       ∑   c                         w     r   ,   c       ·   s                       (       x   +     r                 Δ                 x       ,     y   +     c                 Δ                 y         )          [   n   ]                               
     Internally, the accumulated value is kept to 36 binary places per channel. The location of the binary point within this field is specified by the BP field. The BP field indicates the number of leading bits in the accumulated result to discard. The 36 bit accumulated value is treated as a signed 2&#39;s compliment number and is clamped or wrapped as specified. In FIG. 98, there is illustrated an example of the interpretation of the BP field in co-efficient encoding. 
     3.17.9 Convolution Instructions 
     Convolutions, as applied to rendering images, involves applying a two dimensional convolution kernel to a source image to produce a resultant image. Convolving is normally used for such matters as edge sharpening or indeed any image filter. Convolutions are implemented by the co-processor  224  in a similar manner to image transformations with the difference being that, in the case of transformations the kernel is translated by the width of the kernel for each output pixel, in the case of convolutions, the kernel is moved by one source pixel for each output pixel. 
     If a source image has values S(x,y) and a n×m convolution kernel has values C(x,y), then the nth channel of the convolution H[n] of S and C is given by:          H                     (     x   ,   y     )          [   n   ]         =       (         I   .     offset        [   n   ]         ·     mdp   por            :        0000     )     +       ∑   i                       ∑   j                     S                     (       x   +   i     ,     y   +   j       )     ·   C                       (     i   ,   j     )          [   n   ]                               
     where iε[0,c] and jε[0,r]. 
     The interpretation of the offset value, the resolution of intermediate results and the interpretation of the bp field are the same as for Image Transformation instructions. 
     In FIG. 99, there is illustrated an example of how a convolution kernel  750  is applied to a source image  751  to produce a resultant image  752 . Source image address generation and output pixel calculations are performed in a similar manner to that for image transformation instructions. The instruction operands take a similar form to image transformations. In FIG. 100, there is illustrated the instruction word encoding for convolution instructions with the following interpretation being applied to the various fields. 
     
       
         
           
               
             
               
                 TABLE 21 
               
             
            
               
                   
               
               
                 Instruction Word 
               
            
           
           
               
               
            
               
                 Field 
                 Description 
               
               
                   
               
               
                 S 
                 0 = bi-linear interpolation is used on the four surrounding 
               
               
                   
                 source image pixels to determine the actually sampled value 
               
               
                   
                 1 = sampled value is snapped to the closest source image pixel 
               
               
                   
                 value 
               
               
                 C 
                 0 = do not clamp resultant vector values 
               
               
                   
                 1 = clamp result vector values: underflow to 0 × 00, overflow to 
               
               
                   
                 0 × FF 
               
               
                 P 
                 0 = do not pre-multiply input pixels 
               
               
                   
                 1 = pre multiply input pixels 
               
               
                 A 
                 0 = do not take absolute value of output values 
               
               
                   
                 1 = take absolute value of output values before wrapping or 
               
               
                   
                 clamping 
               
               
                 off[3:0] 
                 0 = do not apply the offset register to this channel 
               
               
                   
                 1 = apply the offset register to this channel 
               
               
                   
               
            
           
         
       
     
     3.17.10 Matrix Multiplication 
     Matrix multiplication is utilized for many things including being utilized for color space conversion where an affine relationship exists between two color spaces. Matrix multiplication is defined by the following equation:          [           r   x               r   y               r   z               r   0           ]     =       [           b   0.0           b   0.1           b   0.2           b   0.3           b   0.4               b   1.0           b   1.1           b   1.2           b   1.3           b   1.4               b   2.0           b   2.1           b   2.2           b   2.3           b   2.4               b   3.0           b   3.0           b   3.2           b   3.3           b   3.4           ]          [           a   x               a   y               a   z               a   0             1         ]                       
     The matrix multiplication instruction operands and results have the following format: 
     
       
         
           
               
             
               
                 TABLE 22 
               
             
            
               
                   
               
               
                 Instruction Operand and Results Word 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Internal 
                 External 
               
               
                 Operand 
                 Description 
                 Format 
                 Format 
               
               
                   
               
               
                 Operand A 
                 source image pixels 
                 pixels 
                 packed stream 
               
               
                 Operand B 
                 matrix co-efficients 
                 other 
                 image table format 
               
               
                 Operand C 
                 unused 
                 — 
                 — 
               
               
                 Result 
                 pixels 
                 pixels 
                 packed stream, 
               
               
                   
                   
                   
                 unpacked bytes 
               
               
                   
               
            
           
         
       
     
     The instruction word encoding for matrix multiplication instructions as illustrated in FIG. 101 with the following table summarising the minor opcode fields. 
     
       
         
           
               
             
               
                 TABLE 23 
               
             
            
               
                   
               
               
                 Instruction Word 
               
            
           
           
               
               
            
               
                 Field 
                 Description 
               
               
                   
               
               
                 C 
                 0 = do not clamp resultant vector values. 
               
               
                   
                 1 = clamp resultant vector values: underflow to 0 × 00, 
               
               
                   
                 overflow to 0 × FF 
               
               
                 P 
                 0 = do not pre-multiply input pixels 
               
               
                   
                 1 = pre-multiply input pixels 
               
               
                 A 
                 0 = do not take absolute value of output values 
               
               
                   
                 1 = take absolute value of output values before wrapping or 
               
               
                   
                 clamping 
               
               
                   
               
            
           
         
       
     
     3.17.11 Halftoning 
     The co-processor  224  implements a multi-level dither for halftoning. Anything from 2 to 255 is a meaningful number of halftone levels. Data to be halftoned can be either bytes (ie. unmeshed or one channel from meshed data) or pixels (ie. meshed) as long as the screen is correspondingly meshed or unmeshed. Up to four output channels (or four bytes from the same channel) can be produced per clock, either packed bits (for bi-level halftoning) or codes (for more than two output levels) which are either packed together in bytes or unpacked in one code per bye. 
     The output half-toned value is calculated using the following formula: 
     
       
         ( p ×( l− 1)+ d )/255 
       
     
     Where p is the pixel value (0≦p≦255), l is the number of levels (2≦l≦255) and d is the dither matrix value (0≦d≦254). The operand encoding is as follows: 
     
       
         
           
               
             
               
                 TABLE 24 
               
             
            
               
                   
               
               
                 Instruction Operand and Results Word 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Internal 
                 External 
               
               
                 Operand 
                 Description 
                 Format 
                 Format 
               
               
                   
               
               
                 Operand A 
                 source image 
                 pixels 
                 packed stream 
               
               
                   
                 pixels 
               
               
                   
                 source image 
                 packed bytes, 
                 packed stream 
               
               
                   
                 bytes 
                 unpacked bytes 
               
               
                 Operand B 
                 dither matrix co- 
                 pixels, packed 
                 packed stream, 
               
               
                   
                 efficients 
                 bytes, unpacked 
                 unpacked bytes 
               
               
                   
                   
                 bytes 
               
               
                 Operand C 
                 unused 
                 — 
                 — 
               
               
                 Result 
                 halftone codes 
                 pixels, packed bytes 
                 packed stream, 
               
               
                   
                   
                 unpacked bytes 
                 unpacked bytes 
               
               
                   
               
            
           
         
       
     
     In the instruction word encoding, the minor op code specifies a number of halftone levels. The operand B encoding is for the halftone screen and is encoded in the same way as a compositing tile. 
     3.17.12 Hierarchial Image Format Decompression 
     Hierarchial image format decompression involves several stages. These stages include horizontal interpolation, vertical interpolation, Huffman decoding and residual merging. Each phase is a separate instruction. In the Huffman decoding step, the residual values to be added to the interpolated values from the interpolation steps are Huffman coded. Hence, the JPEG decoder is utilized for Huffman decoding. 
     In FIG. 102, there is illustrated the process of horizontal interpolation. The output stream  761  consists of twice as much data as the input stream  762  with the last data value  763  being replicated  764 . FIG. 103 illustrates horizontal interpolation by a factor of 4. 
     In the second phase of hierarchial image format decompression, rows of pixels are up sampled by a factor of two or four vertically by linear interpolation. During this phase, one row of pixels is on operand A and the other row is on operand B. 
     When vertically interpolating, either by a factor of two or four, the output data stream contains the same number of pixels as each input stream. In FIG. 104, there is illustrated an example of vertical interpolation wherein two input data streams  770 ,  771  are utilized to produce a first output stream  772  having a factor of two interpolation or a second output stream  773  having a factor of 4 interpolation. In the case of pixel interpolation, interpolation occurs separately on each of the four channels of four channel pixels. 
     The residual merging process involves the bytewize addition of two streams of data. The first stream (operand A) is a stream of base values and the second stream (operand B) is a stream of residual values. 
     In FIG. 105, there is illustrated two input streams  780 ,  781  and a corresponding output stream  782  for utilising the process of residual merging. 
     In FIG. 106 there is illustrated the instruction word encoding for hierarchial image format instructions Keith the following table providing the relevant details of the minor op code fields. 
     
       
         
           
               
             
               
                 TABLE 25 
               
             
            
               
                   
               
               
                 Instruction Word - Minor Opcode Fields 
               
            
           
           
               
               
            
               
                 Field 
                 Description 
               
               
                   
               
               
                 R 
                 0 = interpolation 
               
               
                   
                 1 = residual merging 
               
               
                 V 
                 0 = horizontal interpolation 
               
               
                   
                 1 = vertical interpolation 
               
               
                 F 
                 0 = interpolate by a factor of 2 
               
               
                   
                 1 = interpolate by a factor of 4 
               
               
                 C 
                 0 = do not clamp resultant values 
               
               
                   
                 1 = clamp resultant values: underflow to 0 × 00, overflow 
               
               
                   
                 to 0 × FF 
               
               
                   
               
            
           
         
       
     
     3.17.13 Memory Copy Instructions 
     These instructions are divided into two specifically disjointed groups. 
     a. General Purpose Data Movement Instructions 
     These instructions utilize the normal data flow path through the co-processor  224 , comprising the input interface module, input interface switch  252 , pixel organizer  246 , JPEG coder  241 , result organizer  249  and then the output interface module. In this case, the JPEG coder module sends data straight through without applying any operation. 
     Other instructions include data manipulation operations including: 
     packing and unpacking sub-byte values (such as bits, two bit values and four bit values) to a byte 
     packing and unpacking bytes within a word 
     aligning 
     meshing and unmeshing 
     byte lane swapping and duplicating 
     memory clearing 
     replicating values 
     The data manipulation operation is carried out by a combination of the pixel organizer (on input) and the result organizer (on output). In many cases, these instructions can be combined with other instructions. 
     b. Local DMA Instructions 
     No data manipulation takes place. As seen in FIG. 2 data transfer occurs (in either direction) between the Local Memory  236  and the Peripheral Interface  237 . These instructions are the only ones for which execution can be overlapped with some other instruction. A maximum of one of these instructions can execute simultaneously with a “non overlapped” instruction. 
     In memory copy instructions, operand A represents the data to be copied and the result operand represents the target address of the memory copy instructions. For general purpose memory copy instructions, the particular data manipulation operation is specified by the operand B for input and operand C for output operand words. 
     3.17.14 Flow Control Instructions 
     The flow control instructions are a family of instructions that provide control over various aspect of the instruction execution model as described with reference to FIG.  9 . The flow control instructions include both conditional and unconditional jumps enabling the movement from one virtual address to another when executing a stream of instructions. A conditional jump instruction is determined by taking a co-processor or register, masking off any relevant fields and comparing it to given value. This provides for reasonable generality of instructions. Further, flow control instructions include wait instructions which are typically used to synchronize between overlapped and non-overlapped instructions or as part of micro-programming. 
     In FIG. 107, there is illustrated instruction when encoding for flow control instructions with the minor opcodes being interpreted as follows: 
     
       
         
           
               
             
               
                 TABLE 26 
               
             
            
               
                   
               
               
                 Instruction Word - Minor Opcode Fields 
               
            
           
           
               
               
               
            
               
                   
                 Field 
                 Description 
               
               
                   
                   
               
               
                   
                 type 
                 00 = jump 
               
               
                   
                   
                 01 = wait 
               
               
                   
                 C 
                 0 = unconditional jump 
               
               
                   
                   
                 1 = condition jump 
               
               
                   
                 S 
                 0 = use Operand B as Condition Register and 
               
               
                   
                   
                 Operand C as Condition mask 
               
               
                   
                   
                 1 = any interrupt condition set 
               
               
                   
                 N 
                 0 = jump if condition is true 
               
               
                   
                   
                 1 = dont jump if condition is true 
               
               
                   
                 O 
                 0 = wait on non-overlapped instruction to finish 
               
               
                   
                   
                 1 = wait on overlapped instruction to finish 
               
               
                   
                   
               
            
           
         
       
     
     In respect of Jump Instructions, the operand A word specified the target address of the jump instruction. If the S bit of the Minor Opcode is set to 0, then operand B specified a co-processor register to use as the source of the condition. The value of the operand B descriptor specifies the address of the register, and the value of the operand B word defines a value to compare the contents of the register against. The operand C word specifies a bitwize mask to apply to the result. That is, the Jump Instruction&#39;s condition is true of the bitwize operation: 
     
       
         (((register_value xor Operand  B ) and Operand  C )=0 x 00000000) 
       
     
     Further instructions are also provided for accessing registers for providing full control at the micro programmed level. 
     3.18 Modules of the Accelerator Card 
     Turning again to FIG. 2, there will now be provided further separate description of the various modules. 
     3.18.1 Pixel Organizer 
     The pixel organizer  246  addresses and buffers data streams from the input interface switch  252 . The input data is stored in the pixel organizer&#39;s internal memory or buffered to the MUV buffer  250 . Any necessary data manipulation is performed upon the input stream before it is delivered to the main data path  242  or JPEG coder  241  as required. The operating modes of the pixel organizer are configurable by the usual CBus interface. The pixel organizer  246  operates in one of five modes, as specified by a PO_CFG control register. These modes include: 
     (a) Idle Mode—where the pixel organizer  246  is not performing any operations. 
     (b) Sequential Mode—when input data is stored in an internal FIFO and the pixel organizer  246  sends out requests for data to the input interface switch  252 , generating 32 bit addresses for this data. 
     (c) Color Space Conversion Mode—when the pixel organizer buffers pixels for color space conversion. In addition, requests are made for interval and fractional values stored in the MUV buffer  250 . 
     (d) JPEG Compression Mode—when the pixel organizer  246  utilizes the NIUV buffer to buffer image data in the form of MCU&#39;s. 
     (e) Convolution and Image Transformation Mode—when the pixel organizer  246  stores matrix coefficients in the MUV buffer  250  and passes them, as necessary, to the main data path  242 . 
     The MUV buffer  250  is therefore utilized by the pixel organizer  246  for both main data path  242  and JPEG coder  241  operations. During color space conversion, the MUV RAM  250  stores the interval and fractional tables and they are accessed as 36 bits of data (four color channels)×(4 bit interval values and 8 bit fractional values). For image transformation and convolution, the MUV RAM  250  stores matrix co-efficients and related configuration data. The co-efficient matrix is limited to 16 row×16 columns with each co-efficient being at a maximum 20 bits wide. Only one co-efficient per clock cycle is required from the MUV RAM  250 . In addition to co-efficient data, control information such as binary point, source start coordinates and sub-sample deltas must be passed to the main data path  242 . This control information is fetched by the pixel organizer  246  before any of the matrix coefficients are fetched. 
     During JPEG compression, the MUV buffer  250  is utilized by the pixel organizer  246  to double buffer MCU&#39;s. Preferrably, the technique of double buffering is employed to increase the performance of JPEG compression. One half of the MUV RAM  250  is written to using data from the input interface switch  252  while the other half is read by the pixel organizer to obtain data to send to the JPEG coder  241 . The pixel organizer  246  is also responsible for performing horizontal sub-sampling of color components where required and to pad MCU&#39;s where an input image does not have a size equal to an exact integral number of MCUs. 
     The pixel organizer  246  is also responsible for formatting input data including byte lane swapping, normalization, byte substitution, byte packing and unpacking and replication operations as hereinbefore discussed with reference to FIG. 32 of the accompanying drawings. The operations are carried out as required by setting the pixel organizers registers. 
     Turning now to FIG. 108, there is shown the pixel organizer  246  in more detail. The pixel organizer  246  operates under the control of its own set of registers contained within a CBus interface controller  801  which is interconnected to the instruction controller  235  via the global CBus. The pixel organizer  246  includes an operand fetch unit  802  responsible for generating requests from the input interface switch  252  for operand data needed by the pixel organizer  246 . The start address for operand data is given by the PO_SAID register which must be set immediately before execution. The PO_SAID register may also hold immediate data, as specified by the L bit in the PO_DMR register. The current address pointer in stored in the PO_CDP register and is incremented by the burst length of any input interface switch request. When data is fetched into the MUV RAM  250 , the current offset for data is concatenated with a base address for the MUV RAM  250  as given by the PL_MUV register. 
     A FIFO  803  is utilized to buffer sequential input data fetched by the operand fetch unit  802  . The data manipulation unit  804  is responsible for implementing for implementing the various manipulations as described with reference to FIG.  32 . The output of the data manipulation unit is passed to the MUV address generator  805  which is responsible for passing data to the MUV RAM  250 , main data path  242  or JPEG coder  241  in accordance with configuration registers. A pixel organizer control unit  806  is a state machine that generates the required control signals for all the sub-modules in the pixel organizer  246 . Included in these signals are those for controlling communication on the various Bus interfaces. The pixel organizer control unit outputs diagnostic information as required to the miscellaneous module  239  according to its status register settings. 
     Turning now to FIG. 109, there is illustrated the operand fetch unit  802  of FIG. 108 in more detail. The operand fetch unit  802  includes an Instruction Bus address generator (IAG)  810  which contains a state machine for generating requests to fetch operand data. These requests are sent to a request arbiter  811  which arbitrates between requests from the address generator  810  and those from the MUV address generator  805  (FIG. 108) and sends the winning requests to the input (MAG) interface switch  252 . The request arbiter  811  contains a state machine to handle requests. It monitors the state of the FIFO via FIFO count unit  814  to decide when it should dispatch the next request. A byte enable generator  812  takes information on the IAG  810  and generates byte enable patterns  816  specifying the valid bytes within each operand data word returned by the input interface switch  252 . The byte enabled pattern is stored along with the associated operand data in the FIFO. The request arbiter  811  handles MAG requests before IAG requests when both requests arrive at the same time. 
     Returning to FIG. 108, the MUV address generator  805  operates in a number of different modes. A first of these modes is the JPEG (compression) mode. In this mode, input data for JPEG compression is supplied by the data manipulation units  804  with the MUV buffer  250  being utilized as a double buffer. The MUV RAM  250  address generator  805  is responsible for generating the right addresses to the MUV buffer to store incoming data processed by the data manipulation unit  804 . The MAG  805  is also responsible for generating read addresses to retrieve color component data from the stored pixels to form 8×8 blocks for JPEG compression. The MAG  805  is also responsible for dealing with the situation when a MCU lies partially on the image. In FIG. 110, there is illustrated an example of a padding operation carried out by the MAG  805 . 
     For normal pixel data, the MAG  805  stores the four color components at the same address within the MUV RAM  250  in four 8 bit rams. To facilitate retrieval of data from the same color channel simultaneously, the MCU data is barrel shifted to the left before it is stored in the MUV RAM  250 . The number of bytes the data is shifted to the left is determined by the lowest two bits of the write address. For example, in FIG. 111 there is illustrated the data organization within the MUV RAM  250  for 32 bit pixel data when no sub-sampling is needed. Sub-sampling of input data maybe selected for three or four channel interleaved JPEG mode. In multichannel JPEG compression mode with subsampling operating, the MAG  805  (FIG. 108) performs the sub-sampling before the 32 bit data is stored in the MUV RAM  250  for optimal JPEG coder performance. For the first four incoming pixels, only the first and fourth channels stored in the MUV RAM  250  contains useful data. The data in the second and third channel is sub-sampled and stored in a register inside the pixel organizer  246 . For the next four incoming pixels, the second and third channel are filled with sub-sampled data. In FIG. 112, there is illustrated an example of MCU data organization for multi-channel sub-sampling mode. The MAG treats all single channel unpacked data exactly the same as multi-channel pixel data. An example of single channel packed data as read from the MUV RAM is illustrated in FIG.  113 . 
     While the writing process is storing an incoming MCU into the MUV RAM, the reading process is reading 8×8 blocks out of the MUV RAM. In general, the blocks are generated by the MAG  805  by reading the data for each channel sequentially, four coefficients at the time. For pixel data and unpacked input data, the stored data is organized as illustrated in FIG.  111 . Therefore, to compose one 8×8 block of non-sampled pixel data, the reading process reads data diagonally from the MUV RAM. An example of this process is illustrated in FIG. 114, which shows the reading sequence for four channel data, the form of storage in the MUV RAM  250  assisting to read multiple values for the same channel simultaneously. 
     When operating in color conversion mode, the MUV RAM  250  is used as a cache to hold the interval and fractional values and the MAG  805  operates as a cache controller. The MUV RAM  250  caches values for three color channels with each color channel containing 256 pairs of four bit interval and fractional values. For each pixel output via the DMU, the MAG  805  is utilized to get the values from the MUV RAM  250 . Where the value is not available, the MAG  805  generates a memory read request to fetch the missing interval and fractional values. Instead of fetching one entry in each request, multiple entries are fetched simultaneously for better utilization of bandwidth. 
     For image transformation and convolution, the MUV RAM  250  stores the matrix coefficients for the MDP. The MAG cycles through all the matrix co-efficient stored in the MUV RAM  250 . At the start of an image transformation and convolution instruction, the MAG  805  generates a request to the operand fetch unit to fetch the kernal description “header” (FIG. 94) and the first matrix co-efficient in a burst request. 
     Turning now to FIG. 115, there is illustrated the MUV address generator (MAG)  805  of FIG. 108 in more detail. The MAG  805  includes an IBus request module  820  which multiplexers IBus requests generated by an image transformation controller (ITX)  821  and a color space conversion (CSC) controller  822 . The requests are sent to the operand fetch unit which services the request. The pixel organizer  246  is only operated either in image transformation or color space conversion mode. Hence, there is no arbitration required between the two controllers  821 ,  822 . The IBus request module  820  derives the information for generating a request to the operand fetch unit including the burst address and burst length from the relevant pixel organizer registers. 
     A JPEG controller  824  is utilized when operating in JPEG mode and comprizes two state machines being a JPEG write controller and a JPEG read controller. The two controllers operate simultaneously and synchronize with each other through the use of internal registers. 
     In a JPEG compression operation, the DMU outputs the MCU data which is stored into the MUV RAM. The JPEG Write Controller is responsible for horizontal padding and control of pixel subsampling, while the JPEG Read Controller is responsible for vertical padding. Horizontal padding is achieved by stalling the DMU output, and vertical padding is achieved by reading the previously read 8×8 block line. 
     The JPEG Write Controller keeps track of the position of the current MCU and DMU output pixel on the source image, and uses this information to decide when the DMU has to be stalled for horizontal padding. When a MCU has been written into the MUV RAM  250 , the JPEG Write Controller sets/resets a set of internal registers which indicates the MCU is on the right edge of the image, or is at the bottom edge of i the image. The JPEG Read Controller then uses the content of these resisters to decide if it is required to perform vertical padding, and if it has read the last MCU on the image. 
     The JPEG Write Controller keeps track of DMU output data, and stores the DMU output data into the MUV RAM  250 . 
     The controller uses a set of registers to record the current position of the input pixel. This information is used to perform horizontally padding by stalling the DMU output. 
     When a complete MCU has been written into the MUV RAM  250 , the controller writes the MCU information into JPEG-RW-IPC registers which is later used by the JPEG Read Controller. 
     The controller enters the SLEEP state after the last MCU has been written into the MUV RAM  250 . The controller stays in this state until the current instruction completes. 
     The JPEG Read Controller read the 8×8 blocks from the MCUs stored in the MUV RAM  250 . For multi-channel pixels, the controller reads the MCU several times, each time extracting a different byte from each pixel stored in the MUV RAM. 
     The controller detects if it needs to perform vertical padding using the information provided by the JPEG-RW-IPC. Vertical padding is achieved by re-reading the last 8-bytes read from the MUV RAM  250 . 
     The Image Transformation Controller  821  is responsible for reading the kernel discriptor from the IBus and passes the kernel header to the MDP  242 , and cycles through the matrix coefficients as many times as specified in the po.len register. All data output by the PO  246  in an image transformation and Convolution instruction are fetched directly from the IBus and not passed through the DMU. 
     The top eight bits of the first matrix co-efficient fetched immediately after the kernel header contains the number of remaining matrix coefficients to be fetched. 
     The kernel header is passed to the MDP directly without modifications, whilst the matrix co-efficients are sign extended before they are passed to the MDP. 
     The pixel sub-sampler  825  comprizes two identical channel sub-samplers, each operating on a byte from the input word. When the relevant configuration register is not asserted, the pixel sub-sampler copies its input to its output. When the configuration register is asserted, the sub-sampler sub-samples the input data either by taking the average or by decimation. 
     An MUV multiplexer module  826  selects the MUV read and write signals from the currently active controller. Internal multiplexers are used to select the read addresses output via the various controllers that utilize the MUV RAM  250 . An MUV RAM write address is held in an 8 bit register in an MUV multiplexer module. The controllers utilising the MUN RAM  250 , load the write address register in addition to providing control for determining a next MUV RAM address. 
     A MUV valid access module  827  is utilized by the color space conversion controller to determine if the interval and fractional values for a current pixel output by the data manipulation unit is available in the MUV RAM  250 . When one or more color channels are missing, the MUV valid access module  827  passes the relevant address to the IBus request module  820  for loading in burst mode, interval and fractional values. Upon servicing a cache miss. The MUV valid access module  827  sets internal validity bits which map the set of interval and fractional values fetched so far. 
     A replicate module  829  replicates the incoming data, the number of times as specified by an internal pixel register. The input stream is stalled while the replication module is replicating the current input word. A PBus interface module  630  is utilized to re-time the output signals of the pixel organizer  246  to the main data path  242  and JPEG coder  241  and vice versa. Finally, a MAG controller  831  generates signals for initiating and shutting down the various sub-modules. It also performs multiplexing of incoming PBus signals from the main data path  242  and JPEG coder  241 . 
     3.18.2 MUV Buffer 
     Returning to FIG. 2, it will be evident from the foregoing discussion that the pixel organizer  246  interacts Keith the MUV buffer  250 . 
     The reconfigurable MIUV buffer  250  is able to support a number of operating modes including the single lookup table mode (mode 0 ), multiple lookup table mode (mode 1 ), and JPEG mode (mode 2 ). A different type of data object is stored in the buffer in each mode. For instance, the data objects that are stored in the buffer can be data words, values of a multiplicity of lookup tables, single channel data and multiple channel pixel data. In general, the data objects can have different sizes. Furthermore, the data objects stored in the reconfigurable MUV buffer  250  can be accessed in substantially different ways which is dependent on the operating mode of the buffer. 
     To facilitate the different methods needed to store and retrieve different types of data objects, the data objects are often encoded before they are stored. The coding scheme applied to a data object is determined by the size of the data object, the format that the data objects are to be presented, how the data objects are retrieved from the buffer, and also the organization of the memory modules that comprize the buffer. 
     FIG. 116 is a block diagram of the components used to implement the reconfigurable MUV buffer  250 . The reconfigurable MUV buffer  250  comprizes an encoder  1290 , a storage device  1293 , a decoder  1291 , and a read address and rotate signal generator  1292 . When a data object arrives from an input data stream  1295 , the data object may be encoded into an internal data format and placed on the encoded input data stream  1296  by the encoder  1290 . The encoded data object is stored in the storage device  1293 . 
     When decoding previously stored data objects, an encoded data object is read out of the storage device via encoded output data stream  1297 . The encoded data object in the encoded output data stream  1297  is decoded by a decoder  1291 . The decoded data object is then presented at the output data stream  1298 . 
     The write addresses  1305  to the storage device  1293  are provided by the MAG  805  (FIG.  108 ). The read addresses  1299 ,  1300  and  1301  are also provided by the MAG  805  (FIG.  108 ), and translated and multiplexed to the storage device  1293  by the Read Address and Rotate Signal Generator  1292 , which also generates input and output rotate control signals  1303  and  1304  to the encoder and decoder respectively. The write enable signals  1306  and  1307  are provided by an external source. An operating mode signal  1302 , which is provided by means of the controller  801  (FIG.  108 ), is connected to the encoder  1290 , the decoder  1291 , the Read Address and Rotate Signal Generator  1292 , and the storage device  1293 . An increment signal  1308  increments internal counter(s) in the read address and rotate signal generator and may be utilized in JPEG mode (mode 2 ). 
     Preferably, when the reconfigurable MUV buffer  250  is operating in the single lookup table mode (mode 0 ), the buffer behaves substantially like a single memory module. Data objects may be stored into and retrieved from the buffer in substantially the same way used to access memory modules. 
     When the reconfigurable MUV buffer  250  is operating in the multiple lookup table mode (mode  1 ), the buffer  250  is divided into a plurality of tables with up to three lookup tables may be stored in the storage device  1293 . The lookup tables may be accessed separately and simultaneously. For instance, in one example, interval and fraction values are stored in the storage device  1293  in the multiple lookup table mode, and the tables are indexed utilizing the lower three bytes of the input data stream  1295 . Each of the three bytes are issued to access a separate lookup table stored in the storage device  1293 . 
     When an image undergoes JPEG compression, the image is converted into an encoded data stream. The pixels are retrieved in the form of MCUs from the original image. The MCUs are read from left to right, and top to bottom from the image. Each MCU is decomposed into a number of single component 8×8 blocks. The number of 8×8 blocks that can be extracted from a MCU depends on several factors including: the number of color components in the source pixels, and for a multiple channel JPEG mode, whether subsampling is needed. The 8×8 blocks are then subjected to forward DCT (FDCT), quantization, and entropy encoding. In the case of JPEG decompression, the encoded data are read sequentially from a data stream. The data stream undergoes entropy decoding, dequantization and inverse DCT (IDCT). The output of the IDCT operation are 8×8 blocks. A number of single component 8×8 blocks are combined to reconstruct a MCU. As with JPEG compression, the number of single component 8×8 blocks are dependent on the same factors mentioned above. The reconfigurable MUV buffer  250  may be used in the process to decompose MCUs into a multiplicity of single component 8×8 blocks, to reconstruct MCUs from a multiplicity of single component 8×8 blocks. 
     When the reconfigurable MUV buffer  250  is operating in JPEG mode (mode 2 ), the input data stream  1295  to the buffer  250  comprizes pixels for a JPEG compression operation, or single component data in a JPEG decompression operation. The output data stream  1298  of the buffer  250  comprizes single channel data blocks for a JPEG compression operation, or pixel data in a JPEG decompression operation. In this example, for a JPEG compression operation, an input pixel may comprize up to four channels denoted Y, U. V and O. When the required number of pixels have been accumulated in the buffer to form a complete pixel block, the extraction of single component data blocks can commence. Each single component data block comprizes data from the like channel of each pixel stored in the buffer. Thus in this example, up to four single component data blocks may be extracted from one pixel data block. In this embodiment, when the reconfigurable MUV buffer  250  is operating in the JPEG mode (mode 2 ) for JPEG compression, a multiplicity of Minimum Coded Units (MCUs) each containing 64 single or 64 multiple channel pixels may be stored in the buffer, and a multiplicity of 64-byte lone single channel component data blocks are extracted from each MCU stored in the buffer. In this embodiment, for the buffer  1289  operating in the JPEG mode (mode 2 ) for a JPEG decompression operations, the output data stream contains output pixels that have up to four components Y, U, V and O. When the required number of complete single component data blocks have been written into the buffer, the extraction of pixel data may commence. A byte from up to four single component block corresponding to different color components are retrieved to form an output pixel. 
     FIG. 117 illustrates the encoder  1290  of FIG. 116 in more detail. For the pixel block decomposition mode only, each input data object is encoded using a byte-wize rotation before it is stored into the storage device  1293  (FIG.  129 ). The amount of rotation is specified by the input rotate control signal  1303 . As the pixel data has a maximum of four bytes in this example, a 32-bit 4-to-1 multiplexer  1320  and output  1325  is used to select one of the four possible rotated versions of the input pixel. For example, if the four bytes in a pixel are labelled (3,2,1,0), the four possible rotated versions of this pixel are (3,2,1,0), (0,3,2,1), (1,0,3,2) and (2,1,0,3). The four encoded bytes are output  1296  for storage in the storage device. 
     When the buffer is placed in an operating mode other than the JPEG mode (mode 2 ), for example, single lookup table mode (mode 0 ) and multiple lookup table mode (mode 1 ), byte-wize rotation may not be necessary and may not be performed on is the input data objects. The input data object is prevented from being rotated in the latter cases by overriding the input rotate control signal with a no-operation value. This value  1323  can be zero. A 2-to-1 multiplexer  1321  produces control signals  1326  by selecting between the input rotate control signal  1303  and the no-operation value  1323 . The current operating mode  1302  is compared with the value assigned to the pixel block decomposition mode to produce the multiplexer select signal  1322 . The 4-to-1 multiplexer  1320 , which is controlled by signal  1326  selects one of the four rotated version of the input data object on the input data stream  1325 , and produces an encoded input data object on the encoded input data stream  1326 . 
     FIG. 118 illustrates a schematic of a combinatorial circuit which implements the decoder  1291  for the decoding of the encoded output data stream  1297 . The decoder  1321  operates in a substantially similar manner to the encoder. The decoder only operates on the data when the data buffer is in the JPEG mode (mode 2 ). The lower 32-bit of an encoded output data object in the encoded output data stream  1297  is passed to the decoder. The data is decoded using a byte-wize rotation with an opposite sense of rotation to the rotation performed by the encoder  1290 . A 32-bit 4-to-1 multiplexer  1330  is used to select one of the four possible rotated version of the encoded data. For example, if the four bytes in an input pixel are labelled (3,2,1,0), the four possible rotated version of this pixel are (3,2,1,0), (2,1,0,3), (1,0,3,2) and (0,3,2,1). The output rotate control signal  1304  is utilized only when the buffer is in a pixel block decomposition mode, and when overridden by a no-operation value in other operating modes. The no-operation value utilized  1333  is zero. A 2-to-1 multiplexer  1331  produces signal  1334  by selecting selects between the output rotate control sinal  1304  and the no-operation value  1333 . The current operating mode  1302  is compared with the value assigned to the pixel block decomposition mode to produce the multiplexer select signal  132 . The 4-to-1 multiplexer  1330 , which is controlled by signal  1334 , selects one of the four rotated version of the encoded output data object on the encoded output data stream  1297 , and produces an output data object on the output data stream  1298 . 
     Returning to FIG. 116, the method of internal read address generation used by the circuit is selected by the operating mode  1302  of the reconfigurable MUV buffer  250 . For the single lookup table mode (mode 0 ) and multiple lookup table mode (mode 1 ), the read addresses are provided by the MAG  805  (FIG. 108) in the form of external read addresses  1299 ,  1300 , and  1301 . For the single lookup table mode (mode 0 ), the memory modules  1380 ,  1381 ,  1382 ,  1383 ,  1384  and  1385  (FIG. 121) of the storage device  1293  operate together. The read address and the write address supplied to the memory modules  1380  to  1385  (FIG. 121) are substantially the same. Hence the storage device  1293  only needs the external circuits to supply one read address and one write address, and uses internal logic to multiplex these addresses to the memory modules  1380  to  1385  (FIG.  121 ). For mode 0 , the read address is supplied by the external read address  1299  (FIG. 116) and is multiplexed to the internal read address  1348  (FIG. 121) without substantial changes. The external read addresses  1300  and  1301  (FIG.  116 ), and the internal read addresses  1349 ,  1350  and  1351  (FIG.  121 ), are not used in mode 0 . The write address is supplied by the external write address  1305  (FIG.  116 ), and is connected to the write address of each memory module  1380  to  1385  (FIG. 121) without substantial modification. 
     In this example, a design that provides three lookup tables in the multiple lookup table mode (mode  1 ) is presented. The encoded input data is written simultaneously into all memory modules  1380  to  1385  (FIG.  121 ), while the three tables are accessed independently, and thus require one index to each of the three tables. Three indices, that is, read addresses to the memory modules  1380  to  1385  (FIG.  121 ), are supplied to the storage device  1293 . These read addresses are multiplexed to the appropriate memory modules  1380  to  1385  using internal logic. In substantially the same manner as in the single lookup table mode, the write address supplied externally is connected to the write address of each of the memory modules  1380  to  1385  without substantial modifications. Hence, for the multiple lookup table mode (mode  1 ), the external read addresses  1299 .  1300  and  1311  are multiplexed to internal read addresses  1348 ,  1349  and  1350  respectively. The internal read address  1351  is not used in mode  1 . The method of generating the internal read addresses need in the JPEG mode (mode  2 ) is different to the method described above. 
     FIG. 119 illustrates a schematic of a combinatorial circuit which implements the read address and rotate control signals generation circuit  1292  (FIG.  116 ), for the reconfigurable data buffer operating in the JPEG mode (mode  2 ) for JPEG compression. In the JPEG mode (mode  2 ), the generator  1292  uses the output of a component block counter  1340  and the output of a data byte counter  1341  to compute the internal read addresses to the memory modules comprising the storage device  1293 . The component block counter  1340  gives the number of component blocks extracted from a pixel data block, which is stored in the storage device. The number of like components extracted from the pixel data block is given by multiplying the output of the data byte counter  1341  by four. In this embodiment, an internal read address  1348 ,  1349 ,  1350  or  1351  for the pixel data block decomposition mode is computed as follows. The output of the component block counter is used to generate an offset value  1343 ,  1344 .  1345 ,  1346  or  1347 , and the output of the data byte counter  1341  is used to generate a base read address  1354 . The offset value  1343  is added  1358  to the base read address  1354  and the sum is an internal read address  1348  (or  1349 ,  1350  or  1351 ). The offset values for the memory modules are in general different for simultaneous read operations performed on multiple memory modules, but the offset value to each memory module is in general substantially the same during the extraction to of one component data block. The base addresses  1354  used to compute the four internal read addresses in the pixel data block decomposition mode are substantially the same. The increment signal  1308  is used as the component byte counter increment signal. The counter is incremented after every successful read operation has been performed. A component block counter increment signal  1356  is used to increment the component block counter  1340 , after a complete single component data block has been retrieved from the buffer. 
     The output rotate control signal  1304  (FIG. 116) is derived from the output of the component block counter, and the output of the data byte counter, in substantially similar manner to the generation of an internal read address. The output of the component block counter is used to compute a rotation offset  1347 . The output rotate control signal  1304  is given by the lowest two bits of the sum of the base read address  1354  and the rotation offset  1355 . The input rotate control signal  1303  is simply given by the lowest two bytes of the external write addresses  1305  in this example of the address and rotate control signals generator. 
     FIG. 120 shows another example of the address generator  1292  for reassembling multiple channel pixel data from single component data stored in the reconfigurable MUX buffer  250 . In this case, the buffer is operating in the JPEG (mode 2 ) for JPEG decompression operation. In this case, single component data blocks are stored in the buffer, and pixel data blocks are retrieved from the buffer. In this example, the write address to the memory modules are provided by the external write address  1305  without substantial changes. The single component blocks are stored in contiguous memory locations. The input rotate control signal  1303  in this example is simply set to the lowest two bits of the write address. A pixel counter  1360  is used to keep track of the number of pixels extracted from the single component blocks stored in the buffer. The output of the pixel counter is used to generate the read addresses  1348 ,  1349 ,  1350  and  1351 , and the output rotate control signal  1304 . The read addresses are in general different for each memory module that comprize the storage device  1293 . In this example, a read address comprizes two parts, a single component block index  1362 ,  1363 ,  1364  or  1365 , and a byte index  1361 . An offset is added to bit  3  and  4  of the output of the pixel counter to calculate the single component block index for a particular block. The offsets  1366 ,  1367 ,  1368  and  1369  are in general different for each read address. Bit  2  to bit  0  of the output of the pixel counter are used as the byte index  1361  of a read address. A read address is the result of the concatenation of a single component block index  1362 ,  1363 ,  1364  or  1365  and a byte index  1361 , as illustrated in FIG.  120 . In this example, the output rotate control signal  1304  is generated using bit  4  and bit  3  of the output of the pixel counter without substantial change. The increment signal  1308  is used as the pixel counter increment signal to increment the pixel counter  1360 . The pixel counter  1360  is incremented after a pixel has been successfully retrieved from the buffer. 
     FIG. 121 illustrates an example of a structure of the storage device  1293 . The storage device  1293  can comprize three 4-bit wide memory modules  1383 ,  1384  and  1385 , and three 8-bit Nwide memory modules  1380 ,  1381  and  1382 . The memory modules can be combined together to store  36 -bit words in the single lookup table mode (mode 0 ), 3×12-bit words in the multiple lookup table mode (mode 1 ), and 32-bit pixels or 4×8-bit single component data in JPEG mode (mode 2 ). Typically each memory module is associated with a different part of the encoded input and output data streams ( 1296  and  1297 ). For example, memory module  1380  has its data input port connected to bit  0  to bit  7  of the encoded input data stream  1296 , and its data output port connected to bit  0  to bit  7  of the encoded output data stream  1297 . In this example, the write addresses to all the memory modules are connected together, and share substantially the same value. In contrast, the read addresses  1386 ,  1387 ,  1388 ,  1389 ,  1390  and  1391  to the memory modules of the example illustrated in FIG. 121 are supplied by the read address generator  1292 , and are in general different. In the example, a common write enable signal is used to provide the write enable signals to all three 8-bit memory modules, and a second common write enable signal is used to provide the write enable signals to all three 4-bit memory modules. 
     FIG. 122 illustrates a schematic of a combinatorial circuit used for generating read addresses  1386 ,  1387 .  1388 ,  1389 ,  1390  and  1391  for accessing to the memory modules contained in a storage device  1293 . Each encoded input data object is broken up into parts, and each part is stored into a separate memory module in the storage device. Hence, typically the write addresses to all memory modules for all operating modes are substantially the same and thus substantially no logic is required to compute the write address to the memory modules. The read addresses in this example, on the other hand, are typically different for different operations, and are also different to each memory module within each operating mode. All bytes in the output data stream  1298  of the reconfigurable MU% buffer  250  must contain single component data extracted from the pixel data stored in the buffer in the JPEG mode (mode 2 ) for JPEG compression, or pixel data extracted from the single component data blocks stored in the buffer in the JPEG mode for JPEG decomposition. The requirements on the output data stream are achieved by providing four read addresses  1348 ,  1349 ,  1350  and  1351  to the buffer. In the multiple lookup table mode (mode 1 ), up to three lookup tables are stored in the buffer, and thus only up to three read addresses  1348 ,  1349  and  1350  are needed to index the three lookup tables. The read addresses to all memory modules are substantially the same in the single lookup table mode (mode 0 ), and only read address  248  is used in this mode. The example controller circuit shown in FIG. 122 uses the operating mode signals to the buffer, and up to four read addresses, to compute the read address  1386 - 1391  to each of the six memory modules comprising the storage device  1293 . The read address generator  1292  takes, as its inputs, the external read addresses  1299 , which comprizes external address buses  1348 ,  1349 ,  1350  and  1351 , and generates the internal read addresses  1386 ,  1387 ,  1388 ,  1389 ,  1390  and  1391  to the memory modules that comprize the storage device  1293 . No manipulation on the external write addresses  1305  is required in the operation of this example. 
     FIG. 123 illustrates a representation of an example of how 20-bit matrix coefficients may be stored in the buffer  250  when the buffer  250  is operating in single lookup table mode (mode 0 i In this example, typically no encoding is applied on the data objects stored in the cache when the data objects are written into the reconfigurable MUV buffer. The matrix coefficients are stored in the 8-bit memory modules  1380 ,  1381  and  1382 . Bit  7  to bit  0  of the matrix coefficient are stored in memory module  1380 , bit  15  to bit  8  of the matrix coefficient are stored in memory module  1381 , and bit  19  to bit  16  of the matrix coefficient are stored in the lower 4 bits of memory module  1382 . The data objects stored in the buffer may be retrieved as many times as required for the rest of the instruction. The write and read addresses to all memory modules involved in the single lookup table mode are substantially the same. 
     FIG. 124 illustrates a representation of how the table entries are stored in the buffer in the multiple lookup table mode (mode 1 ). In this example, up to three lookup tables may be stored in the buffer, and each lookup table entry comprises a 4-bit interval value and an 8-bit fraction value. Typically the interval values are stored in the 4-bit memory modules, and the fraction values are stored in the 8-bit memory modules. The three lookup tables  1410 .  1411  and  1412  are stored in the memory banks  1380  and  1383 .  1381  and  1384 ,  1382  and  1385  in the example. The separate write enable control signals  1306  and  1307  (FIG. 121) allow the interval values to be written into the storage device  1293  without affecting the fraction values already stored in the storage device. In substantially the same manner, the fraction values may be written into storage device without affecting the interval values already stored in the storage device. 
     FIG. 125 illustrates a representation of how pixel data is stored in the reconfigurable MUV buffer  250  when the JPEG mode (mode 2 ) for decomposing pixel data blocks into single component data blocks. The storage device  1293  is organized as four 8-bit memory banks, which comprizes the memory modules  1380 ,  1381 ,  1382 ,  1383  and  1384 , with  1383  and  1384  used together to operate substantially in the same manner as an 8-bit memory module. Memory module  1385  is not used in the JPEG mode (mode 2 ). A 32-bit encoded pixel is broken up into four bytes, and each is stored into a different 8-bit memory module. 
     FIG. 126 illustrates a representation of how the single component data blocks are stored in the storage device  1293  in single component mode. The storage device  1293  is organized as four 8-bit memory banks, which comprizes the memory modules  1380 ,  1381 ,  1382 ,  1383  and  1384 , with  1383  and  1384  used together to operate substantially in the same manner as an 8-bit memory module. A single component block in this example comprizes 64 bytes. A different amount of byte rotation can be applied to each single component block when it is written into the buffer. A 32-bit encoded pixel data is retrieved by reading from the different single component data block stored in the buffer. 
     For further details on the organization of the data within the MUV buffer  250  reference is made herein to the section entitled  Pixel Organizer.    
     This preferred embodiment has shown that a reconfigurable data buffer may be used to handle data involved in different instructions. A reconfigurable data buffer that provides three operating modes has been disclosed. Different address generation techniques may be needed in each operating mode of the buffer. The single look-up table mode (mode 0 ) may be used to store matrix coefficients in the buffer for an image transformation operation. The multiple look-up table mode (mode 1 ) may be used to store a multiplicity of interval and fraction lookup tables in the buffer in a multiple channel color space conversion (CSC) operation. The JPEG mode (mode 2 ) may he used either to decompose MCU data into single component 8×8 blocks, or to reconstruct MCU data from single-component 8×8 blocks, in JPEG compression and decompression operation respectively. 
     3.18.3 Result Organizer 
     The MUV buffer  250  is also utilized by the result organizer  249 . The result organizer  249  buffers and formats the data stream from either the main data path  242  or the JPEG coder  241 . The result organizer  249  also is responsible for data packing and unpacking, denormalization, byte lane swapping and realignment of result data as previously discussed with reference to FIG.  42 . Additionally the result organizer  249  transmits its results to the external interface controller  238 , the local memory controller  236 , and the peripheral interface controller  237  as required. 
     When operating in JPEG decompression mode, the results organizer  249  utilizes the MUV RAM  250  to double buffer image data produced by the JPEG coder  241 . Double buffering increases the performance of the JPEG decompression by allowing data from the JPEG coder  241  to be written to one half of the MUV RAM  250  while at the same time image data presently in the other half of the MUV RAM  250  is output to a desired destination. 
     The 1, 3 and 4 channel image data is passed to the result organizer  249  during JPEG decompression in a form of 8×8 blocks with each block consisting of 8 bit components from the same channel. The result organizer stores these blocks in the MUV RAM  250  in the order provided and then, for multi-channel interleaved images, meshing of the channels in performed when reading data from the MUV RAM  250 . For example, in a three channel JPEG compression based on Y, U, V color space, the JPEG coder  241  outputs three 8×8 blocks, the first consisting of Y components, the second made of the U components and the third made up of the V components. Meshing is accomplished by taking one component from each block and constructing the pixel in the form of (YUVX) where X represents an unused channel. Byte swapping may be applied to each output to swap the channels as desired. The result organizer  249  must also do any required sub-sampling to reconstruct chroma-data from decompressed output. This can involve replicating each program channel to produce and an one. 
     Turning to FIG. 127, there is illustrated the result organizer  249  of FIG. 2 in more detail. The result organizer  249  is based around the usual standard CBus interface  840  which includes a register file of registers to be set for operation of the result organizer  249 . The operation of the result organizer  249  is similar to that of the pixel organizer  246 , however the reverse data manipulation operations take place. A data manipulation unit  842  performs byte lane swapping, component substitution, component deselection and denormalization operations on data provided by the MUV address generator (MAG)  805 . The operations carried out are those previously described with reference to FIG.  42  and operate in accordance with various fields set in internal registers. The FIFO queue  843  provides buffering of output data before it is output via RBus control unit  844 . 
     The RBus control unit  844  is composed of an address decoder and state machines for address generation. The address for the destination module is stored in an internal register in addition to data on the number of output bytes required. Further, an internal RO_CUT register specifies how many output bytes to discard before sending a byte stream on the output bus. Additionally, a RO_LMT register specifies the maximum number of data items to be output with subsequent data bytes after the output limit being ignored. The MAG  805  generates addresses for the MUV RAM  250  during JPEG decompression. The MUV RAM  250  is utilized to double buffer output from the JPEG decoder. The MAG  805  performs any appropriate meshing of components in the MUV RAM  250  in accordance with an internal configuration register and outputs single channel, three channel or four channel interleaved pixels. The data obtained from the MUV RAM  250  is then passed through the data, manipulation unit  842 , since byte lane swapping may need to be applied before pixel data is sent to the appropriate destination. When the results organizer  249  is not configured for JPEG mode, the MAG  805  simply forwards data from the PBus receiver  845  straight through to the data manipulation unit  842 . 
     3.18.4 Operand Organizers B and C 
     Returning again to FIG. 2, the two identical operand organizers  247 ,  248  perform the function of buffering data from the data cache control  240  and forwarding the data to the JPEG coder  241  or the main data path  242 . The operand organizers  15   247 ,  248  are operated in a number of modes: 
     (a) Idle mode wherein the operand organizer only responds to CBus requests. 
     (b) Immediate mode when the data of the current instruction is stored in an internal register of the operand organizer. 
     (c) Sequential mode wherein the operator organizer generates sequential addresses and requests data from the data cache controller  240  whenever its input buffer requires filling. 
     A number of modes of operation of the main data path  242  require at least one of the operand organizers  247 ,  248  to operate in sequential mode. These modes include compositing wherein operand organizer B  247  is required to buffer pixels which are to be composited with another image. Operand organizer C  248  is used for compositing operations for attenuation of values for each data channel. In halftoning mode, operand organizer B  247  buffers 8 bit matrix coefficients and in hierarchial image format decompression mode the operand organizer B  247  buffers data for both vertical interpolation and residual merging instructions. 
     (d) In constant mode, an operand organizer B constructs a single internal data word and replicates this word a number of times as given by an internal register. 
     (e) In tiling mode an operand organizer B buffers data that comprizes a pixel tile. 
     (f) In random mode the operand organizer forwards addresses from the MDP  242  or JPEG coder  241  directly to the data cache controller. These addresses are utilized to index the data cache  230 . 
     An internal length register specifies the number of items to be generated by individual operand organizers  247 ,  248  when operated in sequential/titling/constant mode. Each operand organizer  247 ,  248  keeps account of the number of data items processed so far and stops When the count reaches the value specified in its internal register. Each operand organizer is further responsible for formatting input data via byte lane swapping, component substitution, packed/unpacked and normalization functions. The desired operations are configured utilising internal registers. Further, each operand organizer  247 ,  248  may also be configured to constrict data items. 
     Turning now to FIG. 128, there is illustrated the structure of operand organizers ( 247 ,  248 ) in more detail. The operand organizer  247 ,  248  includes the usual standard CBus interface and registers  850  responsible for the overall control of the operand organizer. Further, an OBus control unit  851  is provided for connection to the data cache controller  240  and is responsible for performing address generation for sequential/tile/ constant modes, generating control signals to enable communications on the OBus interface to each operand organizer  247 ,  248  and controlling data manipulation unit operations such as normalization and replication, that require the state to be saved from previous clock cycles of the input stream. When an operand organizer  247 ,  248  is operating in sequential or tiling mode, the OBus control unit  851  sends requests for data to the data cache controller  240 , the addresses being determined by internal registers. 
     Each operand organizer further contains a  36  bit wide FIFO buffer  852  used to buffer data from the data cache controller  240  in various modes of operation. 
     A data manipulation unit  853  performs the same functions as the corresponding data manipulation unit  804  of the pixel organizer  246 . 
     A main data path/JPEG coder interface  854  multiplexer address and data to and from the main data path and JPEG coder modules  242 ,  241  in normal operating mode. The MDP/JC interface  854  passes input data from the data manipulation units  853  to the main data path and in the process may be configured to replicate this data. When operating in color conversion mode, the units  851 ,  854  are bypassed in order to ensure high speed access to the data cache controller  240  and the color conversion tables. 
     3.18.5 Main Data Path Unit 
     The aspects of the following embodiment relate to an image processor providing a low cost computer architecture capable of performing a number of image processing operations at high speed. Still further, the image processor seeks to provide a flexible computer architecture capable of being configured to perform image processing operations that are not originally specified. The image processor also seeks to provide a computer architecture having a large amount of identical logic, which simplifies the design process and lowers the cost of designing such an architecture. 
     The computer architecture comprises a control register block, a decoding block, a data object processor, and flow control logic. The control register block stores all the relevant information about the image processing operation. The decoding block decodes the information into configuration signals, which configure an input data object interface. The input data object interface accepts and stores data objects from outside, and distributes these data objects to the data object processor. For some image processing operations, the input data object interface may also generate addresses for data objects, so that the source of these data objects can provide the correct data objects. The data object processor performs arithmetic operations on the data objects received. The flow control logic controls the flow of data objects within the data object processing logic. 
     More particularly, the data object processor can comprise a number of identical data object sub-processors, each of which processes part of an incoming data object. The data object sub-processor includes a number of identical multifunctional arithmetic units that perform arithmetic operations on these parts of data objects, post processing logic that processes the outgoing data objects, and multiplexer logic that connects the multifunctional arithmetic units and the post-processing unit together. The multifunctional arithmetic units contain storage for parts of the calculated data objects. The storage is enabled or disabled by the flow control logic. The multifunctional arithmetic units and multiplexer logic are configured by the configuration signals generated by the decoding logic. 
     Furthermore, the configuration signals from the decoding logic can be overridden by an external programming agent. Through this mechanism any multifunctional blocks and multiplexer logic can be individually configured by an external programming agent, allowing it to configure the image processor to perform image processing operations that are not specified beforehand. These and other aspects of the embodiments of the invention are described in greater detail hereinafter. 
     Returning to FIG. 2, as noted previously the main data path unit  242  performs all data manipulation operations and instructions other than JPEG data coding. These instructions include compositing, color space conversion, image transformations. convolution, matrix multiplication, halftoning, memory copying and hierarchial image format decompression. The main data path  242  receives pixel and operand data from the pixel organizer  246 , and operand organizers  247 ,  248  and feeds the resultant output to the result organizer  249 . 
     FIG. 129 illustrates a block diagram of the main data path unit  242 . The main data path unit  242  is a general image processor and includes input interface  1460 , image data processor  1462 , instruction word register  1464 , instruction word decoder  1468 , control signal register  1470 , register file  1472 , and a ROM  1475 . 
     The instruction controller  235  transfers instruction words to the instruction word register  1464  via bus  1454 . Each instruction word contains information such as the kind of image processing operation to be executed, and flags to enable or disable various options in that image processing operation. The instruction word is then transferred to the instruction word decoder  1468  via bus  1465 . Instruction controller  235  can then indicate to the instruction word decoder  1468  to decode the instruction word. Upon receiving that indication, the instruction decoder  1468  decodes the instruction word into control signals. These control signals are then transferred via bus  1469  to the control signal register  1470 . The output of the control signal register is then connected to the input interface  1460  and image data processor  1462  via bus  1471 . 
     To add further flexibility to the main data path unit  242 , the instruction controller  235  can also write into the control signal register  1470 . This allows anyone who is familiar with the structure of the main data path unit  242  to micro-configure the main data path unit  242  so that the main data path unit  242  will execute image processing operations that are not be described by any instruction word. 
     In cases when all the necessary information to perform the desired image processing operation does not fit into the instruction word, the instruction controller  235  can write all the other information necessary to perform the desired image processing operation into some of the selected registers in register file  1472 . The information is then transferred to the input interface  1460  and the image data processor  1462  via bus  1473 . For some image processing operations, the input interface  1460  may update the contents of selected registers in the register file  1472  to reflect the current status of the main data path unit  242 . This feature helps the instruction controller  235  to find out what the problem is when there is a problem in executing an image processing operation. 
     Once the decoding of the instruction word is finished, and/or the control signal register is loaded with the desired control signals, the instruction controller  235  can indicate to the main data path unit  242  to start performing the desired image processing operation. Once that indication is received, the input interface  1460  begins to accept data objects coming from bus  1451 . Depending on the kind of image processing operation performed, the input interface  1460  may also begins to accept operand data coming from operand bus  1452  and/or operand bus  1453 , or generates addresses for operand data and receive operand data from operand bus  1452  and/or operand bus  1453 . The input interface  1460  then stores and rearranges the incoming data in accordance with the output of the control signal register  1470 . The input interface  1460  also generates coordinates to be fetched via buses  1452  and  1453  when calculating such functions as affine image transformation operations and convolution. 
     The image data processor  1462  performs the major arithmetic operations on the rearranged data objects from the input interface  1460 . The image processor  1462  can: interpolate between two data objects with a provided interpolation factor, multiply two data objects and divide the product by 255; multiply and add two data objects in general; round off fraction parts of a data object which may have various resolutions; clamp overflow of a data object to some maximum value and underflow of a data object to some minimum value; and perform scaling and clamping on a data object. The control signals on bus  1471  control which of the above arithmetic operations are performed on the data objects, and the order of the operations. 
     A ROM  1475  contains the dividends of 255/x, where x is from 0 to 255, rounded in 8.8 format. The ROM  1475  is connected to the input interface  1460  and the image data processor  1462  via bus  1476 . The ROM  1475  is used to generate blends of short lengths and multiply one data object by 255 and dividing the product by another data object. 
     Preferably, the number of operand buses eg  1452  is limited to 2, which is sufficient for most image processing operations. 
     FIG. 130 illustrates the input interface  1460  in further detail. Input interface  1460  includes data object interface unit  1480 , operand interface units  1482  and  1484 , address generation state machine  1486 , blend generation state machine  1488 , matrix multiplication state machine  1490 , interpolation state machine  1490 , data synchronizer  1500 , arithmetic unit  1496 , miscellaneous register  1498 , and data distribution logic  1505 . 
     Data object interface unit  1480  and operand interface units  1482  and  1484  are responsible to receive data objects and operands from outside. These interface units  1482 ,  1484  are all configured by control signals from control bus  1515 . These interface units  1482 ,  1484  have data registers within them to contain the data objects/operands that they have just received, and they all produce a VALID signal which is asserted when the data within the data register is valid. The outputs of the data registers in these interface units  1482 ,  1484  are connected to data bus  1505 . The VALID signals of these interface units  1482 ,  1484  are connected to flow bus  1510 . When configured to fetch operands, operand interface units  1482  and  1484  accept addresses from arithmetic unit  1496 , matrix multiplication state machine  1490  and/or the output of data register in data object interface unit  1480 , and select amongst them the required address in accordance with the control signals from control bus  1515 . In some cases, the data registers in operand interface units  1482  and  1484  can be configured to store data from the output of data register in data object interface unit  1480  or arithmetic unit  1496 , especially when they are not needed to accept and store data from outside. 
     Address generation state machine  1486  is responsible for controlling arithmetic unit  1496  so that it calculates the next coordinates to be accessed in the source image in affine image transformation operations and convolution operations. 
     The address generation state machine  1486  waits for START signal on control bus  1515  to be set. When the START signal on control bus  1515  is set, address generation state machine  1486  then de-asserts the STALL signal to data object interface unit  1480 , and waits for data objects to arrive. It also sets a counter to be the number of data objects in a kernel descriptor that address generation state machine  1486  needs to fetch. The output of the counter is decoded to become enable signals for data registers in operand interface units  1482  and  1484  and miscellaneous register  1498 . When the VALID signal from data object interface unit  1480  is asserted, address generation state machine  1486  decrements the counter, so the next piece of data object is latched into a different register. 
     When the counter reaches zero, address generation state machine  1486  tells operand interface unit  1482  to start fetching index table values and pixels from operand interface unit  1484 . Also, it loads two counters, one with the number of rows, another with the number of columns. At every clock edge, when it is not paused by STALL signals from the operand interface unit  1482  or others, the counters are decremented to give the remaining rows and columns, and the arithmetic unit  1496  calculates the next coordinates to be fetched from. When both counters have reached zero, the counters reload themselves with the number of rows and columns again, and arithmetic unit  1496  is configured to find the top left hand corner of the next matrix. 
     If interpolation is used to determine the true value of a pixel, address generation state machine  1486  decrements the number of rows and columns after every second clock cycle. This is implemented using a 1-bit counter, with the output used as the enable of the row and column counter. After the matrix is traversed around once. the state machine sends a signal to decrement the count in the length counter. When the counter reaches 1, and the final index table address is sent to the operand interface unit  1482 , the state machine asserts a final signal, and resets the start bit. 
     Blend generation state machine  1488  is responsible for controlling arithmetic unit  1496  to generate a sequence of numbers from 0 to 255 for the length of a blend. This sequence of numbers is then used as the interpolation factor to interpolate between the blend start value and blend end value. 
     Blend generation state machine  1488  determines which mode it should run in (ump mode or step mode). If the blend length is less than or equal to 256, then jump mode is used, otherwize step mode is used. 
     The blend generation state machine  1488  calculates the following and puts them in registers (reg 0 , reg 1 , reg 2 ). If a blend ramp is in step mode for a predetermined length, then latch 511−length in reg 0  (24 bits), 512−2*length in reg  1  (24 bits), and end-start in reg  2  (4×9 bits). If the ramp is in jump mode, then latch 0 into reg 0 , 255/(length−1) into reg 1 , and end-start into reg 2  (4×9 bits). 
     In step mode, the following operations are performed for every cycle: 
     If reg 0 &gt;0, then add reg 0  with reg  1  and store the result in reg 0 . Another incrementor can also be enabled so its output is incremented by 1. If reg 0 &lt;=0, then add reg 0  with 510 and store the result in reg 0 . Incrementor is not incremented. The output of the incrementor is the ramp value. 
     In jump mode, the following is done for every cycle: Add reg 0  with reg 1 . The Adder output is 24 bits, in fixed point format of 16.8. Store the adder output in reg 0 . If the first bit of fraction result is 1, then increment the integer part. 
     The least 8 bits of the integer part of the incrementor is the ramp value. The ramp value, the output of reg 2 , and the blend start value is then fed into the image data processor  1462  to produce the ramp. 
     Matrix multiplication state machine  1490  is responsible for performing linear color space conversion on input data objects using a conversion matrix. The conversion matrix is of the dimension 4×5. The first four columns multiply with the 4 channels in the data object, while the last column contains constant coefficients to be added to the sum of products. When the START signal from control bus  1515  is asserted, matrix multiplication state machine does the following: 
     1) It generates line numbers to fetch constant coefficients of the conversion matrix from buses  1482  and  1484 . It also enables miscellaneous register  1498  to store these constant coefficients. 
     2) It contains a 1-bit flip-flop, which generates a line number which is used as an address to fetch half of matrix from buses  1482  and  1484 . It also generates a “MAT_SEL” signal that selects which half of the data object to be multiplied with that half of matrix. 
     3) It finishes when there is no data objects coming from data object interface unit  1480 . 
     Interpolation state machine  1494  is responsible for performing horizontal interpolation of data objects. During horizontal interpolation, main data path unit  242  accepts a stream of data objects from bus  1451 , and interpolates between adjacent data objects to output a stream of data objects which is twice or 4 times as long as the original stream. Since the data objects can be packed bytes or pixels, interpolation state machine  1494  operates differently in each case to maximize the throughput. Interpolation state machine  1494  does the following: 
     1) It generates INT_SEL signal to data distribution logic  1503  to rearrange the incoming data objects so that the right pair of data objects are interpolated. 
     2) It generates interpolation factors to interpolate between adjacent pairs of data objects. 
     3) It generates a STALL signal to stop data object interface unit  1480  from accepting more data objects. This is necessary as the output stream is longer than the input stream. The STALL signal goes to flow bus  1510 . 
     Arithmetic unit  1496  contains circuitry for performing arithmetic calculations. It is configured by control signals on control bus  1515 . It is used by two instructions only: affine image transformation and convolution, and blend generation in compositing. 
     In affine image transformation and convolution, arithmetic unit  1496  is responsible for: 
     1) Calculating the next x and y coordinates. To calculate x coordinates arithmetic unit  1496  uses an adder/subtractor to add/subtract the x part of horizontal and vertical delta to/from the current x coordinate. To calculate the y coordinates arithmetic unit  1498  uses an adder/subtractor to add/subtract the y part of the horizontal or vertical delta to/from the current y coordinate. 
     2) Adding the y coordinate to the index table offset to calculate the index table address. This sum is also incremented by 4 to find the next index table entry, when interpolation is used to find true value of a pixel. 
     3) Adding the x coordinate to the index table entry to find the address of the pixel. 
     4) Subtract 1 from the length count. 
     In blend generation, arithmetic unit  1496  does the following: 
     1) In step mode, one of the ramp adders is used to calculate an internal variable in the ramp generation algorithm, while the other adder is used to increment the ramp value when the internal variable is greater than 0. 
     2) In jump mode, only one of the adders is required to add the jump value to the current ramp value. 
     3) Round off fractions occur in jump mode. 
     4) Subtract start of blend from end of blend at the beginning of ramp generation. 
     5) Subtract one from the length count. 
     Miscellaneous register  1498  provides extra storage space apart from the data registers in data object interface unit  1480  and operand interface units  1482  and  1484 . It is usually used to store internal variables or as a buffer of past data objects from data object interface unit  1480 . It is configured by control signals on control bus  1515 . 
     Data synchronizer  1500  is configured by control signals on control bus  1515 . It provides STALL signals to data object interface unit  1480  and operand interface units  1482  and  1484  so that if one of the interface units receives a piece of data object others have not, that interface unit is stalled until all the other interface units have received their pieces of data. 
     Data distribution logic  1505  rearranges data objects from data bus  1510  and register file  1472  via bus  1530  in accordance with control signals on control bus  1515 . including a MAT_SEL signal from matrix multiplication state machine  1490  and a INT_SEL signal from interpolation state machine  1494 . The rearranged data is outputed onto bus  1461 . 
     FIG. 131 illustrates image data processor  1462  of FIG. 129 in further detail. Image data processor  1462  includes a pipeline controller  1540 , and a number of color channel processors  1545 ,  1550 ,  1555  and  1560 . All color channel processors accept inputs from bus  1565 , which is driven by the input interface  1460  (FIG.  131 ). All color channel processors and pipeline controller  1540  are configured by control signals from control signal register  1470  via bus  1472 . All the color channel processors also accept inputs from register file  1472  and ROM  1475  of FIG. 129 via bus  1580 . The outputs of all the color channel processors and pipeline controller are grouped together to form bus  1570 , which forms the output  1455  of image data processor  1462 . 
     Pipeline controller  1540  controls the flow of data objects within all the color channel processors by enabling and disabling registers within all the color channel processors. Within pipeline controller  1540  there is a pipeline of registers. The shape and depth of the pipeline is configured by the control signals from bus  1471 , and the pipeline in pipeline controller  1540  has the same shape as the pipeline in the color channel processors. The Pipeline controller accepts VALID signals from bus  1565 . For each pipeline stage within pipeline controller  1540 , if the incoming VALID signal is asserted and the pipeline stage is not stalled, then the pipeline stage asserts the register enable signals to all color channel processors, and latch the incoming VALID signal. The output of the latch then a VALID signal going to the next pipeline stage. In this way the movement of data objects in the pipeline is simulated and controlled, without storage of any data. 
     Color channel processors  1545 ,  1550 ,  1555  and  1560  perform the main arithmetic operations on incoming data objects, with each of them responsible for one of the channels of the output data object. In the preferred embodiment the number of color channel processors is limited to 4, since most pixel data objects have a maximum of 4 channels. 
     One of the color channel processors processes the opacity channel of a pixel. There is additional circuitry (not shown in FIG.  131 ), connected to the control bus  1471 , which transforms the control signals from the control bus  1471  so that the color channel processor processes the opacity channel correctly, as for some image processing operations the operations on the opacity channel is slightly different from the operations on the color channels. 
     FIG. 132 illustrates color channel processor  1545 ,  1550 ,  1555  or  1560  (generally denoted by  1600  in FIG. 132) in further detail. Each color channel processor  1545 ,  1550 ,  1555  or  1560  includes processing block A  1610 , processing block B  1615 , big adder  1620 , fraction rounder  1625 , clamp-or-wrapper  1630 , and output multiplexer  1635 . The color channel processor  1600  accepts control signals from control signal register  1470  via bus  1602 , enable signals from pipeline controller  1540  via bus  1604 , information from register file  1472  via bus  1605 , data objects from other color channel processor via bus  1603 , and data objects from input interface  1460  via bus  1601 . 
     Processing block A  1610  performs some arithmetic operations on the data objects from bus  1601 , and produces partially computed data objects on bus  1611 . The following illustrates what processing block A  1610  does for designated image processing operations. 
     In compositing, processing block A  1610  pre-multiplies data objects from data object bus  1451  with opacity, interpolates between a blend start value and a blend end value with an interpolation factor from input interface  1460  in FIG. 129, pre-multiplies operands from operand bus  1452  in FIG. 129 or multiplies blend color by opacity, and attenuates multiplication on pre-multiplied operand or blend color data. 
     In general color space conversion, the processing block A  1610  interpolates between 4 color table values using two fraction values from bus  1451  in FIG.  129 . 
     In affine image transformation and convolution, the processing block A  1610  pre-multiplies the color of the source pixel by opacity, and interpolates between pixels on the same row using the fraction part of current x-coordinate. 
     In linear color space conversion, the processing block A  1610  pre-multiplies color of the source pixel by opacity, and multiplies pre-multiplied color data with conversion matrix coefficients. 
     In horizontal interpolation and vertical interpolation, the processing block A  1610  interpolates between two data objects. 
     In residual merging, the processing block A  1610  adds two data objects. 
     Processing block A  1610  includes a number of multifunction blocks  1040  and processing block A glue logic  1645 . The multifunction blocks  1640  are configured by control signals, and may perform any one of the following functions: 
     add/subtract two data objects; 
     passing one data object; 
     interpolate between two data objects with a interpolation factor; 
     pre-multiply a color with an opacity; 
     multiply two data objects, and then add a third data object to the product; and 
     add/subtract two data objects, and then pre-multiply the sum/difference with an opacity. 
     The registers within the multifunction blocks  1640  are enabled or disabled by enable signals from bus  1604  generated by pipelined controller  1540  in FIG.  131 . Processing block A glue logic  1645  accepts data objects from bus  1601  and data objects from bus  1603 , and the outputs of some of the multifunction blocks  1640 , and routes them to inputs of other selected multifunction blocks  1640 . Processing block A glue logic  1645  is also configured by control signals from bus  1602 . 
     Processing block B  1615  performs arithmetic operations on the data objects from bus  1601 , and partially computed data objects from bus  1611 , to produce partially computed data objects on bus  1616 . The following description illustrates what processing block B  1615  does for designated image processing operations. 
     In compositing (with non-plus operators), the processing block B  1615  multiplies pre-processed data objects from data object bus  1451  and operands from operand bus  1452  with compositing multiplicands from bus  1603 , and multiplies clamped/wrapped data objects by output of the ROM, which is 255/opacity in 8.8 format. 
     In compositing with plus operator, the processing block B  1615  adds two pre-processed data objects. In the opacity channel, it also subtracts 255 from the sum. multiplies an offset with the difference, and divides the product by 255. 
     In general color space conversion, the processing block B  1615  interpolates between 4 color table values using 2 of the fraction values from bus  1451 , and interpolates between partially interpolated color value from processing block A  1610  and the result of the previous interpolation using the remaining fraction value. 
     In affine image transformation and convolution, the processing block B  1615  interpolates between partially interpolated pixels using the fraction part of current y-coordinate, and multiplies interpolated pixels with coefficients in a sub-sample weight matrix. 
     In linear color space conversion, the processing block B  1615  pre-multiplies the color of the source pixel by opacity, and multiplies pre-multiplied color with conversion matrix coefficients. 
     Processing block B  1615  again includes a number of multifunction blocks and processing block B glue logic  1650 . The multifunction blocks are exactly the same as those in processing block A  1610 , but the processing block B glue logic  1650  accepts data objects from buses  1601 ,  1603 ,  1611 ,  1631  and the outputs of selected multifunction blocks and routes them to the inputs of selected multifunction blocks. Processing block B glue logic  1650  is also configured by control signals from bus  1602 . 
     Big adder  1620  is responsible for combining some of the partial results from processing block A  1610  and processing block B  1615 . It accepts inputs from input interface  1460  via bus  1601 , processing block A  1610  via bus  1611 , processing block B  1615  via bus  1616 , and register file  1472  via bus  1605 , and it produces the combined result on bus  1621 . It is also configured by control signals on bus  1602 . 
     For various image processing operations, big adder  1620  may be configured differently. The following description illustrates its operation during designated image processing operations. 
     In compositing with non-plus operators, the big adder  1620  adds two partial products from processing block B  1615  together. 
     In compositing with plus operator, the big adder  1620  subtracts the sum of pre-processed data objects with offset from the opacity channel, if an offset enable is on. 
     In affine image transformation/convolution, the big adder  1620  accumulates the products from processing block B  1615 . 
     In linear color space conversion, in the first cycle, the big adder adds the two matrix coefficients/data object products and the constant coefficient together. In the second cycle, it adds the sum of last cycle with another two matrix coefficients/data object products together. 
     Fraction rounder  1625  accepts input from the big adder  1620  via bus  1621  and rounds off the fraction part of the output. The number of bits representing the fraction part is described by a BP signal on bus  1605  from register file  1472 . The following table shows how the BP signal is interpreted. The rounded output is provided on bus  1626 . 
     
       
         
           
               
             
               
                 TABLE 27 
               
             
            
               
                   
               
               
                 Fraction Table 
               
            
           
           
               
               
               
            
               
                   
                 bp field 
                 Meaning 
               
               
                   
                   
               
               
                   
                 0 
                 Bottom 26 bits are fractions. 
               
               
                   
                 1 
                 Bottom 24 bits are fractions. 
               
               
                   
                 2 
                 Bottom 22 bits are fractions. 
               
               
                   
                 3 
                 Bottom 20 bits are fractions. 
               
               
                   
                 4 
                 Bottom 18 bits are fractions. 
               
               
                   
                 5 
                 Bottom 16 bits are fractions. 
               
               
                   
                 6 
                 Bottom 14 bits are fractions. 
               
               
                   
                 7 
                 Bottom 12 bits are fractions. 
               
               
                   
                   
               
            
           
         
       
     
     As well as rounding off fraction, fraction rounder  1625  also does two things: 
     1) determines whether the rounded result is negative; and 
     2) determines whether the absolute value of the rounded result is greater than 255. 
     Clamp-or-wrapper  1630  accepts inputs from fraction rounder  1625  via bus  1626  and does the following in the order described: 
     finds the absolute value of the rounded result, if such option is enabled; and 
     clamps any underflow of the data object to the minimum value of the data object, and any overflow of the data object to the maximum value of the data object. 
     Output multiplexer  1635  selects the final output from the output of processing block B on bus  1616  and the output of clamp-or-wrapper on bus  1631 . It also performs some final processing on the data object. The following description illustrates its operation for designated image processing operations. 
     In compositing with non-plus operators and un-pre-multiplication, the multiplexer  1635  combines some of the outputs of processing block B  1615  to form the un-pre-multiplied data object. 
     In compositing with non-plus operator and no un-pre-multiplication, the multiplexer  1635  passes on the output of clamp-or-wrapper  1630 . 
     In compositing with plus operator, the multiplexer  1635  combines some of the outputs of processing block B  1630  to form resultant data object. 
     In general color space conversion, the multiplexer  1635  applies the translate-and-clamp function on the output data object. 
     In other operations, the multiplexer  1635  passes on the output of clamp-or-wrapper  1630 . 
     FIG. 133 illustrates a single multifunction block (e.g.  1640 ) in further detail. Multifunction block  1640  includes mode detector  1710 , two addition operand logic units  1660  and  1670 , 3 multiplexing logic units  1680 ,  1685  and  1690 , a 2-input adder  1675 , a 2-input multiplier with 2 addends  1695 , and register  1705 . 
     Mode detector  1710  accepts one input from control signal register  1470 , in FIG. 129 the MODE signal  1711 , and two inputs from input interface  1460 , in FIG. 129 SUB signal  1712  and SWAP signal  1713 . Mode detector  1710  decodes these signals into control signals going to addition operand logic units  1660  and  1670 , and multiplexing logic units  1680 ,  1685  and  1690 , and these control signals configure multifunction block  1640  to perform various operations. There are 8 modes in multifunction block  1640 : 
     1) Add/sub mode: adds or subtract input  1655  to/from input  1665 , in accordance with the SUB signal  1712 . Also, the inputs can be swapped in accordance with the SWAP signal  693 . 
     2) Bypass mode: bypass input  1655  to output. 
     3) Interpolate mode: interpolates between inputs  1655  and  1665  using input  1675  as the interpolation factor. Inputs  1655  and  1665  can be swapped in accordance with the SWAP signal  1713 . 
     4) Pre-multiply mode: multiplies input  1655  with input  1675  and divide it by 255. The output of the INC register  1708  tells the next stage whether to increment the result of this stage in bus  1707  to obtain the correct result. 
     5) Multiply mode: multiplies input  1655  with  1675 . 
     6) Add/subtract-and-pre-multiply mode: adds/subtracts input  1665  to/from input  1655 , multiplies the sum/difference with input  1675 , and then divide the product by 255. The output of the INC register  1708  tells the next stage whether to increment the result of this stage in bus  1707  to obtain the correct result. 
     Addition operand logic units  1660  and  1670  find one&#39;s complement of the input on demand, so that the adder can do subtraction as well. Adder  1675  adds the outputs of addition operand logic  1660  and  1670  in buses  1662  and  1672  together, and outputs the sum in bus  1677 . 
     Multiplexing logic  1680 ,  1685  and  1690  select suitable multiplicands and addends to implement, a desired function. They are all configured by control signals on bus  1714  from mode detector  1710 . 
     Multiplier with two addends  1695  multiplies input from bus  1677  with input from bus  1682 , then adds the products to the sum of inputs from buses  1687  and  1692 . 
     Adder  1700  adds the least significant 8 bits of the output of multiplier  1695  with the most significant 8 bits of the output of multiplier  1695 . The carry-out of adder  1700  is latched in INC register  1701 . INC register  1701  is enabled by signal  1702 . Register  1705  stores the product from multiplier  1695 . It is also enabled by signal  1702 . 
     FIG. 134 illustrates a block diagram for the compositing operations. The compositing operation accepts three input streams of data: 
     1) The accumulated pixel data, which is derived from the same location as the result is stored to in this accumulator model. 
     2) A compositing operand—which consists of color and opacity. The color and opacity can both be either flat, a blend, pixels or tiled. 
     3) Attenuation—which attenuates the operand data. The attenuation can be flat, a bit map or a byte map. 
     Pixel data typically consists of four channels. Three of these channels make up the color of the pixel. The remaining channel is the opacity of the pixel. Pixel data can be pre-multiplied or normal. When pixel data is pre-multiplied, each of the color channels are multiplied with the opacity. Since equations for compositing operators are simple with pre-multiplied pixels, usually pixel data is pre-multiplied before it is composited with another pixel. 
     The compositing operators implemented in the preferred embodiments are shown in Table 1. Each operator works on pre-multiplied data. (a co , a o ) refers to a pre-multiplied pixel of color a c  and opacity a o , r is the “offset” value and wc( ) is the wrapping/clamping operator the reverse operator of each of the over, in, out, atop operators in Table 1 is also implemented, and the compositing model has the accumulator on the left. 
     Composite block  1760  in FIG. 134 comprizes three color sub-blocks and a opacity sub-block. Each color sub-block operates on one color channel, and opacity channel of the input pixels to obtain the color of the output pixel. The following pseudo code shows how this is done. 
     
       
         
           
               
               
               
             
               
                   
               
             
            
               
                   
                 PIXEL Composite( 
                 IN colorA, colorB: PIXEL; 
               
               
                   
                   
                 IN opacityA, opacityB: PIXEL; 
               
               
                   
                   
                 IN comp_op: COMPOSITE_OPERATOR 
               
               
                   
                   
                 ) 
               
               
                 ( 
               
               
                   
                 PIXEL result; 
               
               
                   
               
            
           
         
       
     
     PIXEL result; 
     IF comp_op is rover, rin, rout, ratop THEN swap colorA and colorB; 
     swap opacityA and opacityB; 
     END IF; 
     IF comp-op is over or rover or loado or plus THEN X=1; 
     ELSE IF comp_op is in or rin or atop or ratop THEN X=opacityB; 
     ELSE IF comp-op is out or rout or xor THEN X=not(opacityB); 
     ELSE IF comp-op is loadzero or loadc or loadco THEN END IF; 
     IF comp-op is over or rover or atop or ratop or xor THEN Y=not(opacitya); 
     ELSE IF comp_op is plus or loadc or loadco THEN Y=not(opacitya); 
     ELSE IF comp_op is plus or loadc or loadco THEN Y=1; 
     ELSE IF comp-op is in or rin or out or rout or loadzero or loado THEN Y=0 END IF; 
     result=colorA * X+colorB * Y. 
     RETURN result; 
     The above pseudo code is different for the opacity sub-block, since the operators loade and loado have different meaning in the opacity channel. 
     Block  1765  in FIG. 134 is responsible for clamping or wrapping the output of block  1760 . When block  1765  is configured to clamp, it forces all values less than the minimum allowable value to the minimum allowed value, and all values more than the maximum allowed value to the maximum allowed value. If block  1765  is configured to wrap, it calculates the follow%ing equation: 
      (( x −min)mod(max−min))+min, 
     whereby min and max are the minimum and maximum allowed value of the color respectively. Preferably the minimum value for a color is 0, and the maximum value is 255. 
     Block  1770  in FIG. 134 is responsible for un-pre-multiplying the result from block  1765 . It un-pre-multiplies a pixel by multiplying the pre-multiplied color value with 255/o, where o is the opacity after composition. The value 255/o is obtained from a ROM inside the compositing engine. The value stored in the ROM is in the format of 8.8 and the rest of the fraction is rounded. The result of multiplication is stored in the format of 16.8. The result would be rounded to 8 bits to produce the un-pre-multiplied pixel. 
     Blend generator  1721  generates a blend of a specified length with specified start and end values. Blend generation is done in two stages: 
     1) ramp generation, and 
     2) interpolation 
     In ramp generation, the compositing engine generates a linearly increasing number sequence from 0 to 255 over the length of the instruction. There are two modes in ramp generation: the “jump” mode, when the length is less than or equal to 255, and the “step” mode when the length is greater than 255. The mode is determined by examining the 24 most significant bits of the length. In the jump mode, the ramp value increases by at least one in every clock period. In the step mode, the ramp value increases by at most one in every clock period. 
     In the jump mode, the compositing engine uses the ROM to find out the step value 255/(length−1), in 8.8 format. This value is then added to a 16-bit accumulator. The output of the accumulator is rounded to  8  bits to form the number sequence. In the step mode, the compositing engine uses an algorithm similar to Bresenham&#39;s line drawing algorithm, as described by the following pseudo code. 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 Void linedraw (   length: INTEGER 
               
            
           
           
               
               
            
               
                   
                 ) 
               
            
           
           
               
               
            
               
                   
                 { 
               
            
           
           
               
               
            
               
                   
                 d = 511 - length; 
               
               
                   
                 incrE = 510; 
               
               
                   
                 incrNE = 512 - 2*length; 
               
               
                   
                 ramp - 0; 
               
               
                   
                 for (i=0; iflength; i++) 
               
               
                   
                 { 
               
            
           
           
               
               
            
               
                   
                 if d(= 0 then 
               
            
           
           
               
               
            
               
                   
                 d += incrE; 
               
            
           
           
               
               
            
               
                   
                 else { 
               
            
           
           
               
               
            
               
                   
                 d += incrNE; 
               
               
                   
                 ramp ++; 
               
            
           
           
               
               
            
               
                   
                 } 
               
            
           
           
               
               
            
               
                   
                 } 
               
            
           
           
               
               
            
               
                   
                 } 
               
               
                   
                   
               
            
           
         
       
     
     After that, the following equation is calculated to generate the blend from the ramp. 
     
       
         Blend=((end-start)×ramp/255)+start 
       
     
     The division by 255 is rounded. The above equation requires 2 adders and a block that “pre-multiplies” (end-start) by ramp for each channel. 
     Another image processing operation that the main data path unit  242  is able to perform is general color space conversion. Generalized Color Space Conversion (GCSC) uses piecewize tri-linear interpolation to find out the output color value. Preferably, conversion is from a three dimensional input space to one or four dimensional output space. 
     In some cases, there is a problem with the accuracy of tri-linear interpolation at the edges of the color gamut. This problem is most noticeable in printing devices that have high sensitivity near an edge of the gamut. To overcome this problem, GCSC can optionally be calculated in an expanded output color space and then scaled and clamped to the appropriate range using the formula in equation:        out   =         0         if                 x                   (   63                 2                   (     x   -   64     )             if                   (     64        (     x        (   191   )                       255         if                   (     192                   (   x   )                                 
     Yet other image processing operations that the preferred embodiment is able to perform are image transformation and convolution. In image transformation, the source image is scaled, rotated, or skewed to form the destination image. In convolution, the source image pixels are sampled with a convolution matrix to provide the destination image. To construct a scanline in the destination image, the following steps are required: 
     1) Perform an inverse transform of the scanline in the destination image back to the source image as illustrated in FIG.  135 . This tells what pixels in the source image are needed to construct that scanline in the destination image. 
     2) Decompress the necessary portions of the source image. 
     3) Inverse-transform the starting x and y coordinates, horizontal and vertical subsampling distances in the destination image back to source image. 
     4) Pass all these information to the processing units which performs the necessary sub-sampling and/or interpolation to construct the output image pixel by pixel. 
     The calculations to work out which parts of the source image are relevant, sub-sampling frequencies to use, etc, are performed by the host application. Sub-sampling, interpolation, and writing the pixels into the destination image memory are done by the preferred embodiments. 
     FIG. 136 shows a block diagram of the steps required to calculate the value for a destination pixel. In general, the computation-intensive part is the bi-linear interpolation. The block diagram in FIG. 136 assumes that all the necessary source image pixels are available. 
     The final step in calculating a destination pixel is to add together all the possibly bi-linearly interpolated sub-samples from the source image. These values are given different weights. 
     FIG. 137 illustrates a block diagram of the image transformation engine that can be derived from suitable settings within the main data path unit  242 . Image transformation engine  1830  includes address generator  1831 , pre-multiplier  1832 , interpolator  1833 , accumulator  1834 , and logic for rounding, clamping and finding absolute value  1835 . 
     Address generator  1831  is responsible for generating x and y coordinates of the source image which are needed to construct a destination pixel. It also generates addresses to obtain index offsets from an input index table  1815  and pixels from image  1810 . Before address generator  1831  begins generating x and y coordinates in the source image, it reads in a kernel descriptor. These are two formats of kernel descriptors. They are shown in FIG.  138 . The kernel descriptor describes: 
     1) Source image start coordinates (unsigned fixed point, 24.24 resolution). Location (0,0) is at the top left of the image. 
     2) Horizontal and vertical sub-sample deltas (2&#39;s complement fixed point, 24.24 resolution). 
     3) a 3 bit bp field defining the location of the binary point within the fixed point matrix coefficients. The definition and interpretation of the bp field is shown in FIG.  150 . 
     4) Accumulation matrix coefficients. These are of “variable” point resolution of 20 binary places (2&#39;s complement), with the location of the binary point implicitly specified by the bp field. 
     5) an rl field that indicates the remaining number of words in the kernel descriptor. This value is equal to the number of rows times the number of columns minus 1. 
     For the short kernel descriptor, apart from the integer part of start x coordinate, the other parameters are assumed to have the following values: 
     starting x coordinate fraction&lt;−0, 
     starting y coordinate&lt;−0, 
     horizontal delta&lt;−1.0, 
     vertical delta&lt;−1.0. 
     After address generator  1831  is configured, it calculates the current coordinates. It does this in two different ways, depending on the dimensions of the subsample matrix. If the dimensions of the subsample matrix are 1×1, address generator  1831  adds the horizontal delta to the current coordinates until it has generated enough coordinates. 
     If the dimensions of the subsample matrix are not 1×1, address generator  1831  adds the horizontal delta to the current coordinates until one row of the matrix is finished. After that, address generator  1831  adds the vertical delta to the current coordinates to find the coordinates on the next row. After that, address generator  1831  subtracts the horizontal delta from the current coordinates to find the next coordinates, until one more row is finished. After that, address generator  1831  adds the vertical delta to the current coordinates and the procedure is repeated again. Top diagram in FIG. 150 illustrates this method of accessing the matrix. Using this scheme, the matrix is traversed in a zig-zag wav, and fewer registers are required since the current x and y coordinates are calculated using the above method, the accumulation matrix coefficients must be listed in the kernel descriptor in the same order. 
     After generating the current coordinates, the address generator  1831  adds the y coordinate to the index table base address to get the address to the index table. (In case when source pixels are interpolated, address generator  1831  needs to obtain the next index table entry as well.) The index table base address should point to the index table entry for y+0. After obtaining the index offset from the index table, the address generator  1831  adds that to the x coordinate. The sum is used to get 1 pixel from the source image (or 2 if source pixels are interpolated). In case when source pixels are interpolated, the address generator  1831  adds the x coordinates to the next index offset, and two more pixels are obtained. 
     Convolution uses a similar method to generate coordinates to image transformation. The only difference is that in convolution, the start coordinates of the matrix for the next output pixel is one horizontal delta away from the starting coordinates of the matrix for the previous pixel. In image transformation, the starting coordinates of the matrix for the next pixel is one horizontal delta away from the coordinates of the top right pixel in the matrix for the previous output pixel. 
     The middle diagrams in FIG. 139 illustrates this difference. 
     Pre-multiplier  1832  multiplies the color channels with the opacity channel of the pixel if required. 
     Interpolator  1832  interpolates between source pixels to find the true color of the pixel required. It gets two pixels from the source image memory at all times. Then it interpolates between those two pixels using the fraction part of the current x coordinate and puts the result in a register. After that, it obtains the two pixels on the next row from the source image memory. Then it interpolates between those two pixels using the same x fraction. After that, interpolator  1833  uses the fraction part of the current y coordinate to interpolate between this interpolated result and the last interpolated result. 
     Accumulator  1834  does two things: 
     1) it multiplies the matrix coefficients with the pixel, and 
     2) it accumulates the product above until the whole matrix is traversed. Then it outputs a value to the next stage. 
     Preferably the accumulator  1834  can be initialized with  0  or a special value on a channel-by-channel basis. 
     Block  1835  rounds the output of accumulator  1834 , then clamps any underflows or overflows to the maximum and minimum values if required, and finds the absolute value of the output if required. The location of the binary point within the output of the accumulator is specified by the bp field in the kernel descriptor. The bp field indicates the number of leading bits in the accumulated result to discard. This is shown in the bottom diagram of FIG.  139 . Note that the accumulated value is treated as a signed two&#39;s complement number. 
     Yet another image processing operation that the main data path unit  242  can perform is matrix multiplication. Matrix Multiplication is used for color space conversion where an affine relationship exists between the two spaces. This is distinct from General Color Space Conversion (based on tri-linear interpolation). 
     The result of Matrix Multiplication is defined by the following equation:          [           r   x               r   y               r   z               r   o           ]     =       [           b     o   .   o             b     o      .1             b     o      .2             b     o      .3             b     o      .4                 b   1.0           b   1.1           b   1.2           b   1.3           b   1.4               b   2.0           b   2.1           b   2.2           b   2.3           b   2.4               b   3.0           b   3.1           b   3.2           b   3.3           b   3.4           ]          [           a   x               a   y               a   z               a   o             255         ]                       
     where r i  is the result pixel and a i  is the A operand pixel. Matrix must be 5 columns by 4 rows. 
     FIG. 140 illustrates a block diagram of the multiplier-adders that perform the matrix multiplication in the main data path unit  242 . It includes multipliers to multiply the matrix coefficients with the pixel channels, adders to add the products together, and logic to clamp and find the absolute value of the output if required. 
     The complete matrix multiplication takes 2 clock cycles to complete. At each cycle the multiplexers are configured differently to select the right data for the multipliers and adders. 
     At cycle  0 , the least significant 2 bytes of the pixel are selected by the multiplexers  1851 ,  1852 . They then multiply the coefficients on the left 2 columns of the matrix, i.e. the matrix coefficients on line  0  in the cache. The results of the multiplication, and the constant term in the matrix, are then added together and stored. 
     At cycle  1 , the more significant 2 bytes of the pixel are selected by the top multiplexers. They then multiply the coefficients on the right 2 columns of the matrix. 
     The result of the multiplication is then added  1854  to the result of the last cycle. The sum of the adder is then rounded  1855  to 8 bits. 
     The ‘operand logic’  1856  rearranges the outputs of the multipliers to form four of the inputs of the adder  1854 . It rearranges the outputs of the multipliers so that they can be added together to form the true product of the 24-bit coefficient and 8-bit pixel component. 
     The ‘AC (Absolute value-clamp/wrap) logic’  1855  firstly rounds off the bottom 12 bits of the adder output. It then finds the absolute value of the rounded result if it is set to do so. After that it clamps or wraps the result according to how it is set up. If the ‘AC logic’ is set to clamp, it forces all values less than 0 to 0 and all values more than 255 to 255. If the ‘AC logic’ is set to wrap, the lower 8 bits of the integer part is passed to the output. 
     Apart from the image processing operations above, the main data path unit  242  can be configured to perform other operations. 
     The foregoing description provides a computer architecture that is capable of performing various image processing operations at high speed, while the cost is reduced by design reuse. The computer architecture described is also highly flexible, allowing any external programming agent with intimate knowledge of the architecture to configure it to perform image processing operations that were not initially expected. Also, as the core of the design mainly comprizes a number of those multifunction blocks, the design effort is reduced significantly. 
     3.18.6 Data Cache Controller and Cache 
     The data cache controller  240  maintains a four-kilobyte read data cache  230  within the coprocessor  224 . The data cache  230  is arranged as a direct mapped RAM cache, where any one of a group of lines of the same length in external memory can be mapped directly to the same line of the same length in cache memory  230  (FIG.  2 ). This line in cache memory is commonly referred to as a cache-line. The cache memory comprizes a multiple number of such cache-lines. 
     The data cache controller  240  services data requests from the two operand organizers  247 ,  248 . It first checks to see if the data is resident in cache  230 . If not, data will be fetched from external memory. The data cache controller  240  has a programmable address generator, which enables the data cache controller  240  to operate in a number of different addressing modes. There are also special addressing modes where the address of the data requested is generated by the data cache controller  240 . The modes can also involve supplying up to eight words (256 bits) of data to the operand organizers  247 ,  248  simultaneously. 
     The cache RAM is organized as 8 separately addressable memory banks. This is needed for some of the special addressing modes where data from each bank (which is addressed by a different line address) is retrieved and packed into 256 bits. This arrangement also allows up to eight 32-bits requests to be serviced simultaneously if they come from different banks. 
     The cache operates in the following modes, which will be discussed in more detail later. Preferably, it is possible to automatically fill the entire cache if this is desired. 
     1. Normal Mode 
     2. Single Output General Color Space Conversion Mode 
     3. Multiple Output General Color Space Conversion Mode 
     4. JPEG Encoding Mode 
     5. Slow JPEG Decoding Mode 
     6. Matrix Multiplication Mode 
     7. Disabled Mode 
     8. Invalidate Mode 
     FIG. 141 shows the address, data and control flow of the data cache controller  240  and data cache  230  shown in FIG.  2 . 
     The data cache  230 , consists of a direct mapped cache of the type previously discussed. The data cache controller  240 , consists of a tag memory  1872  having a tag entry for each cache-line, which tag entry comprizes the most significant part of the external memory address that the cache-line is currently mapped to. There is also a line valid status memory  1873  to indicate whether the current cache-line is valid. All cache-lines are initially invalid. 
     The data cache controller  240  can service data requests from operand organizer B  247  (FIG. 2) and operand organizer C  248  (FIG. 2) simultaneously via the operand bus interface  1875 . In operation, one or both of the operand organizers  247  or  248  (FIG.  2 ), supplies an index  1874  and asserts a data request signal  1876 . The address generator  1881  generates one or more complete external addresses  1877  in response to the index  1874 . A cache controller  1878  determines if the requested data is present in cache  230  by checking the tag memory  1872  entries for the tag addresses of the generated addresses  1877  and checking the line valid status memory  1873  for the validity of the relevant cache-line(s). If the requested data is present in cache memory  230 , an acknowledgment signal  1879  is supplied to the relevant operand organizer  247  or  248  together with the requested data  1880 . If the requested data is not present in the cache  230 , the requested data  1870  is fetched from external memory, via an input bus interface  1871  and the input interface switch  252  (FIG.  2 ). The data  1870  is fetched by asserting a request signal  1882  and supplying the generated address(es)  1877  of the requested data  1870 . An acknowledgement signal  1883  and the requested data  1870  are then sent to the cache controller  1878  and the cache memory  230  respectively. The relevant cache-line(s) of the cache memory  230  are then updated with the new data  1870 . The tag addresses of the new cache-line(s) are also written into tag memory  1872 , and the line valid status  1873  for the new cache-line(s) are asserted. An acknowledgment signal  1879  is then sent to the relevant operand organizer  247  or  248  (FIG. 2) together with the data  1870 . 
     Turning now to FIG. 142, which shows the memory organization of the data cache  230 . The data cache  230  is arranged as a direct mapped cache with 128 cache-lines C 0 , . . . ,C 127  and a cache-line length of 32 bytes. The cache RAM consists of 8 separately addressable memory banks B 0 , . . . ,B 7 , each having 128 bank-lines of 32 bits, with each cache-line Ci consisting of the corresponding 8 bank-lines B 0 i, . . . ,B 7 i of the 8 memory banks B 0 , . . . B 7 . 
     The composition of the generated complete external memory address is shown in FIG.  143 . The generated address is a 32-bit word having a 20-bit tag address, a 7-bit line address, a 3-bit bank address and a 2-bit byte address. The 20-bit tag address is used for comparing the tag address with the tag stored in the tag memory  1872 . The 7-bit line address is used for addressing the relevant cache-line in the cache memory  1870 . The 3-bit bank address is used for addressing the relevant bank of the memory banks of the cache memory  1870 . The 2-bit byte address is used for addressing the relevant byte in the 32-bit bank line. 
     Turning now to FIG. 144, which shows a block diagram of the data cache controller  240  and data cache  230  arrangement. In this arrangement, a 128 by 256 bit RAM makes up the cache memory  230 , and as noted previously is organized as 8 separately addressable memory banks of 128 by 32 bits. This RAM has a common write enable port (write), a common write address port (write_addr) and a common write data port (write_data). The RAM also has a read enable port (read), eight read address ports (read_addr) and eight read data output ports (read_data). A write enable signal is generated by the cache controller block  1878  for supply to the common write enable port (write) for simultaneously enabling writing to all of the memory banks of the cache memory  230 . When required, the data cache  230  is updated by one or more lines of data from external memory via the common write data port (write_data). A line of data is written utilizing the 8:1 multiplexer MUX supplying the line address to the write address port (write_addr). The 8:1 multiplexer MUX selects the line address from the generated external addresses under the control of the data cache controller (addr_select). A read enable signal is generated by the cache controller block  1878  for supply to the common read port (read) for simultaneously enabling reading of all the memory banks of cache memory  230 . In this way, eight different bank-lines of data can be simultaneously read from eight read data ports (read_data) in response to respective line addresses supplied on the eight read address ports (read_addr) of the memory banks of the cache memory  230 . 
     Each bank of the cache memory  230  has its own programmable address generator  1881 . This allows eight different locations to be simultaneously accessed from the respective eight banks of memory. Each address generator  1881  has a dcc-mode input for setting the mode of operation of the address generator  1881 , an index-packet input, a base-address input and an address output. The modes of operation of the programmable address generator  1881  include 
     (a) Random access mode where a signal on the dcc-mode input sets each address generator  1881  to the random access mode and complete external memory address(es) are supplied on the index-packet input(s) and outputted on the address output of one or more of the address generators  1881 ; and 
     (b) JPEG encoding and decoding, color space conversion, and matrix multiplication modes, where a signal on the dcc-mode input sets each address generator  1881  to the appropriate mode. In these modes, each address generator  1881  receives an index on the index-packet input and generates an index address. The index addresses are then added to a fixed base address supplied on the base-address input resulting in a complete external memory address which is then outputted on the address output. Depending upon the mode of operation, the address generators are able to generate up to eight different complete external memory addresses. 
     The eight address generators  1881  consist of eight different combinational logic circuits each having as their inputs; a base-address, a dcc-mode and an index and each having a complete external memory address as an output. 
     A base-address register  1885  stores the current base address that is combined with the index packet and a dcc-mode register  1888  stores the current operational mode (dcc-mode) of the data cache controller  240 . 
     The tag memory  1872  comprizes one block of 128 by 20 bit, multi-port RAM. This RAM has one write port (update-line-addr), one write enable port (write), eight read ports (read 0 line-addr, . . . ,read 7 line-addr) and eight read output ports (tag 0 _data, . . . ,tag 7 _data). This enables eight simultaneous lookups on the ports (read 0 line-addr, . . . ,read 7 line-addr) by the eight address generators  1881  to determine, for each line address of the one or more generated memory addresses, the tag addresses currently stored for those lines. The current tag addresses for those lines are outputted on the ports (tag 0 -data, . . . tag 7 -data) to the tag comparator  1886 . When required, a tag write signal is generated by the cache controller block  1878  for supply to the write port (write) of the tag memory  1872  to enable writing to the tag memory  1872  on the port (update-line-addr). 
     A 128-bit line valid memory  1873  contains the line valid status for each cache-line of the cache memory  230 . This is 128 by 1 bit memory with one write port (update-line-addr), one write enable port (update), eight read ports (read 0 line-addr, . . . ,read 7 line-addr) and eight read output ports (linevalid 0 , . . . , linevalid 7 ). In a similar manner to the tag memory, this allows eight simultaneous lookups on the ports (read 0 line-addr, . . . ,read 7 line-addr) by the eight address generators  1881  to determine. for each line address of the one or more generated memory addresses, the line valid status bits currently stored for those lines. The current line valid bits for those lines are outputted on the ports (linevalid 0 , . . . ,linevalid 7 ) to the tag comparator  1886 . When required, a write signal is generated by the cache controller block  1878  for supply to the write port (update) of the line valid status memory  1873  to enable writing to the line valid status memory  1873  on the port (update-line-addr). 
     The tag comparator block  1886  consists of eight identical tag comparators having; tag_data inputs for respectively receiving the tag addresses currently stored in tag memory  1872  at those lines accessed by the line addresses of the currently generated complete external addresses, tag_addr inputs for respectively receiving the tag addresses of the currently generated complete external memory addresses, a dcc_input for receiving the current operational mode signal (dcc_mode) for setting the parts of the tag addresses to be compared, and a line_valid input for receiving the line valid status bits currently stored in the line valid status memory  1873  at those lines accessed by the line addresses of the currently generated complete external memory addresses. The comparator block  1886  has eight hit outputs for each of the eight address generators  1881 . A hit signal is asserted when the tag address of the generated complete external memory address matches the contents of the tag memory  1872  at the location accessed by the line address of the generated complete external memory address, and the line valid status bit  1873  for that line is asserted. In this particular embodiment, the data structures stored in external memory are small, and hence the most significant bits of the tag addresses are the same. Thus it is preferable to compare only those least significant bits of the tag addresses which may vary. This is achieved by the current operational mode signal (dcc_mode) setting the tag comparator  1886  for comparing those least significant bits of the tag addresses which may vary. 
     The cache controller  1878  accepts a request (proc_req)  1876  from the operand B  247  or operand C  248  and acknowledges (pro_cack)  1879  this request if the data is available in cache memory  230 . Depending on the mode of operation, up to eight differently addressed data items may be requested, one from each of the eight banks of cache memory  230 . The requested data is available in cache memory  230  when the tag comparator  1886  asserts a hit for that line of memory. The cache controller  1878  in response to the asserted hit signal (hit 0 , . . . ,hit 7 ) generates a read enable signal on the port (cache_read) for enabling reading of those cache-lines for which the hit signal has been asserted. When a request (proc_req)  1876  is asserted, but not the hit signal (hit 0 , . . . ,hit 7 ), a generated request (ext_req)  1890  is sent to the external memory together with the complete external memory address for that cache-line of data. This cache-line is written into the eight banks of cache memory  230  via the input (ext_data) when it is available from the external memory. When this happens, the tag information is also written into the tag memory  1886  at that line address, and the line status bit  1873  for that line asserted. 
     Data from the eight banks of cache memory  230  is then outputted through a series of multiplexers in a data organizer  1892 , so that data is positioned in a predetermined manner in an output data packet  1894 . In one operational mode, the data organizer IS  1892  is able to select and output eight 8-bit words from the respective eight 32-bit words outputted from the eight memory banks by utilising the current operational mode signal (dcc_mode) and the byte addresses (byte_addr) of the current generated complete external memory addresses. In another operational mode, the data organizer  1892  directly outputs the eight 32-bit words outputted from the eight memory banks. As noted previously, the data organizer arranges this data in a predetermined manner for output. 
     A request would comprize the following steps: 
     1) The processing unit requests a packet of data by supplying an address to the processing unit interface of the cache controller  1878 : 
     2) Each of the eight address generator units  1881  then generate a separate address for each block of cache memory depending on the mode of operation; 
     3) The Tag portion of each of the generated addresses is then compared to the Tag address stored in the four blocks of triple-port Tag memory  1886  and addressed by each of the corresponding line part of the eight generated addresses; 
     4) If they match, and the line valid status  1873  for that line is also asserted, the data requested for that block of memory is deemed to be resident in the said cache memory  230 : 
     5) Data that is not resident is fetched via the external bus  1890  and all eight blocks of the cache memory  230  are updated with that line of data from external memory. The Tag address of the new data is then written to the Tag memory  1886  at the said line address, and the line valid status  1873  for that line asserted; 
     6) When all requested data items are resident in cache memory  230 , it is presented to the processing unit in a predetermined packet format. 
     As previously noted, all the modules (FIG. 2) of the coproccessor  224  include a standard cBus interface  303  (FIG.  20 ). For more details on the standard cBus interface registers for the data cache controller  240  and cache  230 , reference is made to pages B 42  to B 46  of Appendix B. The settings in these registers control the operation of the data controller  240 . For the sake of simplicity only two of these registers are shown in FIG. 153, i.e. baseaddress and dcc_mode. 
     Once the data cache controller  240  and data cache  230  are enabled, the data cache controller intially operates in the normal mode with all cache lines invalid. At the end of an instruction, the data cache controller  240  and cache  230  always reverts to the normal mode of operation. In all of the following modes except the “Invalidate” mode, there is an “Auto-fill and validate” option. By setting a bit in the dcc_cfg 2  register, it is possible to fill the entire cache starting at the address stored in the base_address register. During this operation, the data requests from the operand organizers B and C  247 , 248  are locked out until the operation is complete. The cache is validated at the end of this operation. 
     a. Normal Cache Mode 
     In this mode, the two operand organizers supply the complete external memory addresses of the data requested. The address generator  1881  outputs the complete external memory addresses which are then checked independently using the internal tag memory  1872  to see that if the data requested is resident in the memory cache  230 . If both requested data items are not in cache  230 , data will be requested from the input interface switch  252 . Round Robin scheduling will be implemented to service persistent simultaneous requests. 
     For simultaneous requests, if one of the data items is resident in cache, it will be placed on the least significant 32 bits of each requestor&#39;s data bus. The other data will be requested externally via the input interface switch. 
     b. The Single Output General Color Space Conversion Mode 
     In this mode, the request comes from operand organizer B in the form of a 12-bit byte address. The requested data items are 8-bit color output values as previously discussed with reference to FIG.  60 . The 12-bit address is fed to the index_packet inputs of the address generators  1881  and the eight address generators  1881  generate eight different 32-bit complete external memory addresses of the format shown in FIG.  96 . The bank, line and byte addresses of the generated complete addresses are determined in accordance with Table 12 and FIG.  61 . The external memory address is interpreted as eight 9-bit line and byte addresses, which are used to address a byte from each of the eight banks of RAM. The cache is accessed to obtain the eight byte values from each bank which are returned to the operand organizers for subsequent interpolation by the main data path  242  in accordance with the principles previously discussed with reference to FIG.  60 . As the single output color value table is able to fit entirely within the cache memory  230 , it is preferable to load the entire single output color value table within the cache memory  230  prior to enabling the single color conversion mode. 
     c. Multiple Output General Color Space Conversion Mode 
     In this mode, a 12-bit word address is received from operand organizer B  247 . The requested data items are 32-bit color output values as previously discussed with reference to FIG.  62 . The 12-bit address is fed to the index packet inputs of the address generators  1881  and the eight address generators  1881  generate eight different 32-bit complete external memory addresses of the format shown in FIG.  96 . The line and tag addresses of the complete external memory addresses are determined in accordance with table 12 and FIG.  63 . The completed external memory address is interpreted as eight 9-bit addresses with the 9-bit address being decomposed into a 7-bit line address and a 2-bit tag address as discussed previously with reference to FIG.  63 . Upon the tag address not being found, the cache stalls while the appropriate data is loaded from the input interface switch  252  (FIG.  2 ). Upon the data being available, the output data is returned to the operand organizers. 
     d. JPEG Encoding Mode 
     In this mode, the necessary tables for JPEG encoding and other operational sub-sets are stored in each bank of cache RAM. The storage of tables being previously described in the previous discussion of the JPEG encoding mode (Tables 14 and 16). 
     e. Slow JPEG Decoding Mode 
     In this mode, the data is organized in accordance with Table 17. 
     f. Matrix Multiplication Mode 
     In this mode, the cache is utilized to access 256 byte lines of data. 
     g. Disabled Mode 
     In this mode, all requests are passed through to the input interface switch  252 . 
     h. Invalidate Mode 
     In this mode, the contents of the entire cache are invalidated by clearing all the line valid status bits. 
     3.18.1 Input Interface Switch 
     Returning again to FIG. 2, the input interface switch  252  performs the function of arbitrating data requests from the pixel organizer  246 , the data cache controller  240  and the instruction controller  235 . Further, the input interface switch  252  transmits addresses and data as required to the external interface controller  238  and local memory controller  236 . 
     The input interface switch  252  stores in one of its configuration register the base address or the memory object in the host memory map. This is a virtual address that must be aligned on a page boundary, hence 20 address bits are required. For each request made by the pixel organizer data cache controller, instruction controller the input interface switch  252  first subtracts the co-processor&#39;s base address bits from the most significant 6 bits of the start address of the data. If the result is negative, or the most significant 6 bits of the result are non-zero, this indicates that the desired destination is the PCI bus. 
     If the most significant 6 bits of the result are zero, this indicates that the data maps to a co-processor&#39;s memory location. The input interface switch  252  then needs to check the next 3 bits to determine if the co-processor&#39;s location is legal or not. 
     The legal co-processor&#39;s locations that may act as a source of data are: 
     1) 16 Mbytes occupied by the Generic interface, beginning at an offset of 0×01000000 from the co-processor&#39;s base address. 
     2) 32 Mbytes occupied by the local memory controller (LMC), starting at an offset of 0×02000000 from the base address of the co-processor&#39;s memory object. 
     Requests that map to an illegal co-processor&#39;s location are flagged as errors by the Input Interface Switch. 
     The PCI bus is the source of data corresponding to any addresses that map outside of the range occupied by the co-processor&#39;s memory object. An i-source signal is used by the input interface switch to indicate to the EIC whether requested data is to originate from the PCI bus or the Generic interface. 
     After the address decoding process, legal requests are routed to the appropriate IBus interface when the bus is free. The EIC or LMC is busy with a data transaction to the input interface switch when they have their i-ack signal asserted. However, the input interface switch does not keep a count for the number of incoming words, and so must monitor the i-oe signal, controlled by the pixel organizer, instruction controller or data cache controller, in order to determine when the current data transaction has completed. 
     The input interface switch  252  must arbitrate between three modules: the pixel organizer, data cache controller and instruction controller. All of these modules are able to request data simultaneously, but not all requests can be instantly met since there are only two physical resources. The arbitration scheme used by the input interface switch is priority-based and progranmnable. Control bits within a configuration register of the input interface switch specify the relative priorities of the instruction controller. data cache controller and pixel organizer. A request from the module with the lower priority is granted when neither of the other two modules are requesting access to the same resource as it is. Assigning the same priority to at least two of the requesters results in the use of a round robin scheme to deduce the new winners. 
     As immediate access to a resource may not be possible, the input interface switch needs to store the address, burst length and whether to prefetch data provided by each requester. For any given resource, the arbitration process only needs to determine a new winner when there is not an IBus transaction in progress. 
     Turning to FIG. 145, there is illustrated the instruction interface switch  252  in more detail. The switch  252  includes the standard CBus interface and register file  860  in addition to two IBus transceivers  861  and  862  between an address decoder  863  and arbiter  864 . 
     The address decoder  863  performs address decoding operations for requests received from the pixel organizer, data cache controller and instruction controller. The address decoder  863  checks the address is a legal one and performs any address re-mapping required. The arbiter  864  decides which request to pass from one IBus transceiver  661  to a second IBus transceiver  862 . Preferrably, the priority system is programmable. 
     The IBus transceivers  861 ,  862  contain all the necessary multiplexing/demultiplexing and tristate buffering to enable communication over the various interfaces to the input interface switch. 
     3.18.8 Local Memory Controller 
     Returning again to FIG. 2, the local memory controller  236  is responsible for all aspects of controlling the local memory and handling access requests between the local memory and modules within the co-processor. The local memory controller  236  responds to write requests from the result organizer  249  and read requests from the input interface switch  252 . Additionally, it also responds to both read and write requests from the peripheral interface controller  237  and the usual global CBus input. The local memory controller utilizes a programmable priority system and further utilizes FIFO buffers to maximize throughput. 
     In the present invention, a multi-port burst dynamic memory controller is utilized in addition to using First-In-First-Out (FIFO) buffers to de-couple the ports from a memory array. 
     FIG. 146 depicts a block diagram of a four-port burst dynamic memory controller according to a first embodiment of the present invention. The circuit includes two write ports (A  1944  and B  1946  ) and two read ports (C  1948  and D  1950  ) that require access to a memory array  1910 . The data paths from the two write ports pass through separate FIFOs  1920 ,  1922  and to the memory array  1910  via a multiplexer  1912 , while the data paths of the read ports  1948 ,  1950  pass from the memory array  1910  via separate FIFOs  1936 ,  1938 . A central controller  1932  coordinates all port accesses as well as driving all the control signals necessary to interface to the dynamic memory  1910 . A refresh counter  1934  determines when dynamic memory refresh cycles for the memory array  1910  are required and coordinates these with the controller  1932 . 
     Preferably, the data is read from and written to the memory array  1910  at twice the rate that data is transferred from the write ports  1944 ,  1946  to the FIFOs  1920 ,  1922  or from the FIFOs  1936 ,  1938  to the read ports  1948 ,  1950 . This results in as little time as possible being taken up doing transfers to or from the memory array  1910  (which is the bottleneck of any memory system) relative to the time taken to transfer data through the write and read ports  1944 ,  1946 ,  1948 ,  1950 . 
     Data is written into the memory array  1910  via either one of the write ports  1944 ,  1946 . The circuits connected to the write ports  1944 ,  1946  see only a FIFO  1920 ,  1922  which are initially empty. Data transfers through the write ports  1944 ,  1946  proceed unimpeded until the FIFO  1920 ,  1922  is filled, or the burst is ended. When data is first written into the FIFO  1920 ,  1922 , the controller  1932  arbitrates with the other ports for the DRAM access. When access is granted, data is read out of the FIFO  1920 ,  1922  at the higher rate and written into the memory array  1910 . A burst write cycle to DRAM  1910  is only initiated when a preset number of data words have been stored in the FIFO  1920 ,  1922 , or when the burst from the write port ends. In either case, the burst to DRAM  1910  proceeds when granted and continue until the FIFO  1920 ,  1922  is emptied, or there is a cycle request from a higher priority port. In either event, data continues to be written into the FIFO  1920 ,  1922  from the write port without hindrance, until the FIFO is filled, or until the burst ends and a new burst is started. In the latter case, the new burst cannot proceed until the previous burst has been emptied from the FIFO  1920 ,  1922  and written to the DRAM  1910 . In the former case, data transfers recommences as soon as the first word is read out of the FIFO  1920 ,  1922  and written to DRAM  1910 . Due to the higher rate of data transfers out of the FIFO  1920 ,  1922 , it is only possible for the write port  1944 ,  1946  to stall if the controller  1832  is interrupted with cycle requests from the other ports. Any interruption to the data transfers from the write ports  1944 ,  1946  to the FIFOs  1920 .  1922  is preferably kept to a minimum. 
     The read ports  1948 ,  1950  operate in a converse fashion. When a read port  1948 ,  1950  initiates a read request, a DRAM cycle is immediately requested. When granted, the memory array  1910  is read and data is written into the corresponding FIFO  1936 ,  1938 . As soon as the first data word is written into the FIFO  1936 ,  1938 , it is available for read-out by the read port  1948 ,  1950 . Thus there is an initial delay in obtaining the first datum word but after that there is a high likelihood that there are no further delays in retrieving the successive data words. DRAM reads will be terminated when a higher priority DRAM request is received, or if the read FIFO  1936 ,  1938  becomes full, or when the read port  1948 ,  1950  requires no more data. Once the read has been terminated in this way, it is not restarted until there is room in the FIFO  1936 ,  1938  for a preset number of data words. Once the read port terminates the cycle, any data remaining in the FIFO  1936 ,  1938  is discarded. 
     In order to keep DRAM control overheads to a minimum, rearbitration for the DRAM access is restricted so that bursts cannot be interrupted until a preset number of data words have been transferred (or until the corresponding write FIFO  1920 ,  1922  is emptied, or read FIFO  1936 ,  1938  is filled). 
     Each of the access ports  1944 ,  1946 ,  1948 .  1950  has an associated burst start address which is latched in a counter  1942  at the start of the burst. This counter holds the current address for transactions on that port so that, should the transfer be interrupted, it can be resumed at any time at the correct memory address. Only the address for the currently active DRAM cycle is selected by multiplexer  1940  and passed on to the row address counter  1916  and column address counter  1918 . The low order N bits of address are inputted to the column counter  1918  while the higher order address bits are inputted to the row counter  1916 . Multiplexer  1914  outputs row addresses from the row counter  1916  to the memory array  1910  during the row address time of the DRAM and passes column addresses from the column counter  1918  during column address time of the DRAM. The row address counter  1916  and the column address counter  1918  are loaded at the start of any burst to the memory array DRAM  1910 . This is true both at the start of a port cycle and at the continuation of an interrupted burst. The column address counter  1918  is incremented after each transfer to memory has taken place while the row address counter  1916  is incremented when the column address counter  1918  rolls over to a count of zero. When the latter happens, the burst must be terminated and restarted at the new row address. 
     In the preferred embodiment it is assumed that memory array  1910  comprizes 4×8 bit byte lines making up a 32 bits per word. Further there is associated with each write port  1944 ,  1946  a set of four byte write enable signals  1950 ,  1952  which individually allow data to be written to each 8-bit portion of each 32-bit data word in the memory array  1910 . Since it is possible to arbitrarily mask the writing of data to any byte within each word that is written to the memory array  1910 , it is necessary to store the write enable information along with each data word in corresponding FIFOs  1926 ,  1928 . These FIFOs  1926 ,  1928  are controlled by the same signals that control the write FIFOs  1920 ,  1922  but are only 4 bits wide instead of the 32 bits required for the write data in FIFOs  1920 ,  1922 . In like fashion, multiplexer  1930  is controlled in the same manner as the multiplexer  1912 . The selected byte write enables are inputted to the controller  1932  which uses the information to selectively enable or disable writing to the addressed word in the memory array  1910  in synchronization with the write data being inputted to the memory array  1910  by way of multiplexer  1912 . 
     The arrangement of FIG. 146 operates under the control of the controller  1932 . FIG. 147 is a state machine diagram depicting the detail of operation of the controller  1932  of FIG.  146 . After power up and at the completion of reset the state machine is forced into state IDLE  100  in which all DRAM control signals are driven inactive (high) and multiplexer  1914  drives row addresses to the DRAM array  1910 . When a refresh or cycle request is detected, the transition is made to state RASDELL  1962 . On the next clock edge the transition to state RASDEL 2   1964  is made. On the next clock edge, if the cycle request and refresh have gone away, the state machine returns to state IDLE  1900 , otherwize, when the DRAM tRP (RAS precharge timing constraint) period has been satisfied, the transition to state RASON  1966  is made at which time the row address strobe signal, RAS, is asserted low. After tRCD (RAS to CAS delay timing constraint) has been satisfied, the transition to state COL  1968  is made, in which the multiplexer  1914  is switched over to select column addresses for inputting to the DRAM array  1910 . On the next clock edge the transition to state CASON  1970  is made and the DRAM column address strobe (CAS) signal is driven active low. Once the tCAS (CAS active timing constraint) has been satisfied, the transition to state CASOFF  1972  is made in which the DRAM column address strobe (CAS) is driven inactive high once again. At this point, if further data words are to be transferred and a higher priority cycle request or refresh is not pending or if it is too soon to rearbitrate anyway, and once the tCP (CAS precharge timing constraint) has been satisfied, the transition back to state CASON  1970  will be made in which the DRAM column address strobe (CAS) is driven active low again. If no further data words are to be transferred, or if rearbitrating is taking place and a higher priority cycle request or refresh is pending, then the transition is made to state RASOFF  1974  instead, providing tRAS (RAS active timing constraint) and tCP (CAS precharge timing constraint) are both satisfied. In this state the DRAM row address strobe (RAS) signal is driven inactive high. On the next clock edge the state machine returns to state IDLE  1860  ready to start the next cycle. 
     When in state RASDEL 2   1964  and a refresh request is detected, the transition will be made to state RCASON  1980  once tRP (RAS precharge timing constraint) has been satisfied. In this state DRAM column address strobe is driven active low to start a DRAM CAS before RAS refresh cycle. On the next clock edge the transition to state RRASON  1978  is made in which DRAM row address strobe (RAS) is driven active low. When tCAS (CAS active timing constraint) has been met, the transition to state RCASOFF  1976  will be made in which DRAM column address strobe (CAS) is driven inactive high. Once tRAS (RAS active timing constraint) has been met, the transition to state RASOFF  1974  is made in which DRAM row address strobe (RAS) is driven inactive high effectively ending the refresh cycle. The state machine then continues as above for a normal DRAM cycle, making the transition back to state IDLE  1960 . 
     The refresh counter  1934  of FIG. 146 is simply a counter that produces refresh request signals at a fixed rate of once per 15 microseconds, or other rate as determined by the particular DRAM manufacturer&#39;s requirements. When a refresh request is asserted, it remains asserted until acknowledged by the state machine of FIG.  147 . This acknowledgement is made when the state machine enters state RCASON  1980  and remains asserted until the state machine detects the refresh request has been de-asserted. 
     In FIG. 148, there is set out in pseudo code form, the operation of the arbitrator  1924  of FIG.  146 . It illustrates the method of determining which of four cycle requesters is granted access to the memory array  1910 , and also a mechanism for modifying the cycle requester priorities in order to maintain a fair access regime. The symbols used in this code are explained in FIG.  149 . 
     Each requester has 4 bits associated with it that represent that requester&#39;s priority. The two high order bits are preset to an overall priority by way of configuration values set in a general configuration register. The two low order bits of priority are held in a 2-bit counter that is updated by the arbitrator  24 . When determining the victor in an arbitration, the arbitrator  1924  simply compares the 4-bit values of each of the requesters and grants access to the requester with the highest value. When a requester is granted a cycle its low order 2-bit priority count value is cleared to zero, while all other requesters with identical high order 2-bit priority values and whose low order 2-bit priority is less than the victor&#39;s low order 2-bit priority have their low order 2-bit priority counts incremented by one. This has the effect of making a requester that has just been granted access to the memory array  1910  the lowest priority among requesters with the same priority high order 2-bit value. The priority low order 2-bit value of other requesters with priority high order 2-bit value different to that of the winning requester are not affected. The high order two bits of priority determine the overall priority of a requester while the low order two bits instil a fair arbitration scheme among requesters with identical high order priority. This scheme allows a number of arbitration schemes to be implemented ranging from hard-wired fixed priority (high order two bits of each requester unique) through part rotating and part hard-wired (some high order 2-bit priorities different to others, but not all) to strictly fair and rotating (all priority high order 2-bit fields the same). 
     FIG. 149 depicts the structure of the priority bits associated with each requester and how the bits are utilized. It also defines the symbols used in FIG.  148 . 
     In the preferred embodiment, the various FIFOs  1920 ,  1922 ,  1938  and  1936  are 32 bits wide and 32 words deep. This particular depth provides a good compromise between efficiency and circuit area consumed. However, the depth may be altered, with a corresponding change in performance, to suit the needs of any particular application. 
     Also, the four port arrangement shown is merely a preferred embodiment. Even the provision of a single FIFO buffer between the memory array and either a read or write port will provide some benefits. However, the use of multiple read and write ports provides the greatest potential speed increase. 
     3.18.9 Miscellaneous Module 
     The miscellaneous module  239  provides clock generation and selection for the operation of the co-processor  224 , reset synchronization, multiplexing of error and interrupt signals by routing of internal diagnostic signals to external pins as required, interfacing between the internal and external form of the CBus and multiplexing of internal and generic Bus signals onto a generic/external CBus output pins. Of course, the operation of the miscellaneous module  239  varies in accordance with clocking requirements and implementation details depending on the ASIC technology utilized. 
     3.18.10 External Interface Controller 
     The following described apsects of the invention relate to a method and an apparatus for providing virtual memory in a host computer system having a co-processor that shares the virtual memory. The embodiments of the invention seek to provide a co-processor able to operate in a virtual memory mode in conjunction with the host processor. 
     In particular, the co-processor is able to operate in a virtual memory mode of the host processor. The co-processor includes a virtual-memory-to-physical-memory mapping device that is able to interrogate the host processor&#39;s virtual memory tables, so as to map instruction addresses produced by the co-processor into corresponding physical addresses in the host processor&#39;s memory. Preferably, the virtual-memory-to-physical-memory mapping device forms part of a computer graphics co-processor for the production of graphical images. The co-processor may include a large number of modules able to form various complex operations on images. The mapping device is responsible for the interaction between the co-processor and the host processor. 
     The external interface controller (EIC)  238  provides the co-processors interface to the PCI Bus and to a generic Bus. It also provides memory management to translate between the co-processor&#39;s internal virtual address space and the host system physical address space. The external interface controller  238  acts as a master on the PCI Bus when reading the data from the host memory in response to a request from the input interface switch  252  and when writing data to host memory in response to a request from the result organizer  249 . The PCI Bus access is implemented in accordance the well known standard with “PCI Local Bus specification, draft 2.1”. PCI special interest group, 1994. 
     The external interface controller  238  arbitrates between simultaneous requests for PCI transactions from the input interface switch  252  and the result organizer  249 . The arbitration is preferably configurable. The types of requests received include transactions for reading less than one cache line of the host co-processor at a time, reading between one and two cache lines of the host and reading two or more cache lines of the host. Unlimited length write transactions are also implemented by the external interface controller  238 . Further, the external interface controller  238  optionally also performs prefetching of data. 
     The construction of the external interface controller  238  includes a memory management unit which provides virtual to physical address mapping of host memory accesses for all of the co-processor&#39;s internal modules. This mapping is completely transparent to the module requesting the access. When the external interface controller  238  receives a request for host memory access, it initiates a memory management unit operation to translate the requested address. Where the memory management unit is unable to translate the address, in some cases this results in one or more PCI Bus transaction to complete the address translation. This means that the memory management unit itself can be another source of transaction requests on the PCI Bus. If a requested burst from the input interface switch  252  or results organizer  249  crosses the boundary of a virtual page, the external interface controller  238  automatically generates a memory management unit operation to correctly map all virtual addresses. 
     The memory management unit (MMU) ( 915  of FIG. 150) is based around a 16 entry translation look aside buffer (TLB). The TLB acts as a cache of virtual to physical address mappings. The following operations are possible on the TLB: 
     1) Compare: A virtual address is presented, and the TLB returns either the corresponding physical address, or a TLB miss signal (if no valid entry matches the address). 
     2) Replace: A new virtual-to-physical mapping is written into the TLB. replacing an existing entry or an invalid entry. 
     3) Invalidate: A virtual address is presented: if it matches a TLB entry, that entry is marked invalid. 
     4) Invalidate All. All TLB entries are marked invalid. 
     5) Read: A TLB entry&#39;s virtual or physical address is read, based on a four bit address. Used for testing only. 
     6) Write: A TLB entry&#39;s virtual and physical address is written, based on a four bit address. 
     Entries within the TLB have the format shown in FIG.  151 . Each valid entry consists of a 20-bit virtual address  670 , a 20-bit physical address  671 , and a flag which indicates whether the corresponding physical page is writable. The entries allow for page sizes as small as 4 kB. A register in the MMU can be used to mask off up to 10 bits of the addresses used in the comparison. This allows the TLB to support pages up to 4 MB. As there is only one mask register, all TLB entries refer to pages of the same size. 
     The TLB uses a “least-recently-used” (LRU) replacement algorithm. A new entry is written over the entry which has the longest elapsed time since it was last written or matched in a comparison operation. This applies only if there are no invalid entries; if these exist, they are written to before any valid entries are overwritten. 
     FIG. 152 shows the flow of a successful TLB compare operation. The incoming virtual address  880  is divided into 3 parts  881 - 883 . The lower 12 bits  881  are always part of the offset inside a page and so are passed directly on to the corresponding physical address bits  885 . The next 10 bits  882  are either part of the offset, or part of the page number, depending on the page size, as set by the mask bits. A zero in the mask register  887  indicates that the bit is part of the page offset, and should not be used for TLB comparisons. The 10 address bits are logically “ANDED” with the 10 mask bits to give the lower 10 bits of the virtual page number  889  for TLB lookups. The upper 10 bits  883  of the virtual address are used directly as the upper 10 bits of the virtual page number  889 . 
     The 20-bit virtual page number thus generated is driven into the TLB. If it matches one of the entries, the TLB returns the corresponding physical page number  872 , and the number of the matched location. The physical address  873  is generated from the physical page number using the mask register  887  again. The top 10 bits of physical page number  872  are used directly as the top 10 bits of the physical address  873 . The next 10 bits of physical address  872  are chosen  875  from either the physical page number (if the corresponding mask bit is 1), or the virtual address (if the mask bit is 0). The lower 12 bits  885  of physical address come directly from the virtual address. 
     Finally, following a match, the LRU buffer  876  is updated to reflect the use of the matched address. 
     A TLB miss occurs when the input interface switch  252  or the results organizer  249  requests an access to a virtual address which is not in the TLB  872 . In this case, the MMU must fetch the required virtual-to-physical translation from the page table in host memory  203  and write it into the TLB before proceeding with the requested access. 
     The page table is a hash table in the hosts main memory. Each page table entry consists of two 32-bit words, with the format shown in FIG.  153 . The second word comprizes the upper 20 bits for the physical address and the lower 12 bits are reserved. The upper 20 bits of the corresponding virtual address are provided in the first word. The lower 12 bits include a valid (V) bit and writable (W) or a “read-only” bit, with the remaining 10 bits being reserved. 
     The page table entry contains essentially the same information as the TLB entry. Further flags in the page table are reserved. The page table itself may be, and typically is, distributed over multiple pages in main memory  203 , which in general are contiguous in virtual space but not physical space. 
     The MMU contains a set of 16 page table pointers, setup by software, each of which is a 20-bit pointer to a 4 kB memory region containing part of the page table. This means the co-processor  224  supports a page table 64 kB in size, which holds 8 k page mappings. For systems with a 4 kB page size, this means a maximum of 32 MB of mapped virtual address space. Preferably, the page table pointers always reference a 4 kB memory region, regardless of the page size used in the TLB. 
     The operation of the MMU following a TLB miss is shown  690  in FIG. 154, as follows: 
     1. Execute the hash function  892  on the virtual page number  891  that missed in the TLB, to produce a 13-bit index into the page table. 
     2. Use the top 4 bits  894  of the page table index  894 ,  896  to select a page table pointer  895 . 
     3. Generate the physical address  890  of the required page table entry, by concatenating the 20-bit page table pointer  895  with the lower 9 bits of the page table index  896 , setting the bottom 3 bits to 000 (since page table entries occupy 8 bytes in host memory). 
     4. Read 8 bytes from host memory, starting at the page table entnr physical address  898 . 
     5. When the 8-byte page table entry  900  is returned over the PCI bus. the virtual page number is compared to the original virtual page number that caused the TLB miss, provided that the VALID bit is set to 1. If it does not match, the next page table entry is fetched (incrementing the physical address by 8 bytes) using the process described above. This continues until a page table entry with a matching virtual page number is found, or an invalid page table entry is found. If an invalid page table entry is found, a page fault error is signalled and processing stops. 
     6. When a page table entry with a matching virtual page number is found, the complete entry is written into the TLB using the replace operation. The new entry is placed in the TLB location pointed to by the LRU buffer  876 . 
     The TLB compare operation is then retried, and will succeed, and the originally requested host memory access can proceed. The LRU buffer  876  is updated when the new entry is written into the TLB. 
     The hash function  892  implemented in the EIC  238  uses the following equation on the 20 bits of virtual page number (vpn): 
     
       
         index=(( vpn&gt;&gt;S   1 ) XOR ( vpn&gt;&gt;S   2 ) XOR ( vpn&gt;&gt;S   3 )) &amp;  Ox 1 fff;   
       
     
     where s 1 , S 2  and S 3  are independently programmable shift amounts (positive or negative), each of which can take on four values. 
     If the linear search through the page table crosses a 4 kB boundary, the MMU automatically selects the next page table pointer to continue the search at the correct physical memory location. This includes wrapping around from the end of the page table to the start. The page table always contains at least one invalid (null) entry, so that the search always terminates. 
     Whenever the software replaces a page in host memory, it must add a page table entry for the new virtual page, and remove the entry corresponding to the page that has been replaced. It must also make sure that the old page table entry is not cached in the TLB on the co-processor  224 . This is achieved by performing a TLB invalidation cycle in the MMU. 
     An invalidation cycle is performed via a register write to the MMU, specifying the virtual page number to be invalidated, along with a bit that causes the invalidation operation to be done. This register write may be performed directly by the software, or via an instruction interpreted by the Instruction Decoder. An invalidation operation is performed on the TLB for the supplied virtual page number. If it matches a TLB entry, that entry is marked invalid, and the LRU table updated so that the invalidated location is used for the next replace operation. 
     A pending invalidate operation has priority over any pending TLB compares. When the invalidate operation has completed, the MMU clears the invalidate bit, to signal that it can process another invalidation. 
     If the MMU fails to find a valid page table entry for a requested virtual address, this is termed a page fault. The MMU signals an error, and stores the virtual address that caused the fault in a software accessible register. The MMU goes to an idle state and waits until this error is cleared. When the interrupt is cleared, the MMU resumes from the next requested transaction. 
     A page fault is also signalled if a write operation is attempted to a page that is (not marked writable) marked read only. 
     The external interface controller (EIC)  238  can service transaction requests from the input interface switch  252  and the result organizer  249  that are addressed to the Generic bus. Each of the requesting modules indicates whether the current request is for the Generic Bus or the PCI bus. Apart from using common buses to communicate with the input interface switch  252  and the results organizer  249 , the EIC&#39;s operation for Generic bus requests is entirely separate from its operation for PCI requests. The EIC  238  can also service CBus transaction types that address the Generic bus space directly. 
     FIG. 150 shows the structure of the external interface controller  238 . The IBus requests pass through a multiplexer  910 , which directs the requests to the appropriate internal module, based on the destination of the request (PCI or Generic Bus). Requests to the Generic bus pass on to the generic bus controller  911 , which also has RBus and CBus interfaces. Generic bus and PCI bus requests on the RBus use different control signals, so no multiplexer is required on this bus. 
     IBus requests directed to the PCI bus are handled by an IBus Driver (IBD)  912 . Similarly, an RBus Receiver (RBR)  914  handles the RBus requests to PCI. Each of the IBD  912  and RBR  914  drive virtual addresses to the memory management unit (MMU)  915 , which provides physical addresses in return. The IBD, RBR and MMU can each request PCI transactions, which are generated and controlled by the PCI master mode controller (PMC)  917 . The IBD and the MMU request only PCI read transactions, while the RBR requests only PCI write transactions. 
     A separate PCI Target Mode Controller (PTC)  918  handles all PCI transactions addressed to the co-processor as a target. This drives CBus master mode signals to the instruction controller, allowing it to access all other modules. The PTC passes returned CBus data to be driven to the PCI bus via the PMC, so that control of the PCI data bus pins comes from a single source. 
     CBus transactions addressed to EIC registers and module memory are dealt with by a standard CBus interface  7 . All submodules receive some bits from control registers, and return some bits to status registers, which are located inside the standard CBus interface. 
     Parity generation and checking for PCI bus transactions is handled by the parity generate and check (PGC) module  921 , which operates under the control of the PMC and PTC. Generated parity is driven onto the PCI bus, as are parity error signals. The results of parity checking are also sent to the configuration registers section of the PTC for error reporting. 
     FIG. 155 illustrates the structure of the IBus driver  912  of FIG.  150 . Incoming IBus address and control signals are latched  930  at the start of a cycle. An or-gate  931  detects the start of the cycle and generates a start signal to control logic  932 . The top address bits of the latch  930 , which form the virtual page number, are loaded into a counter  935 . The virtual page number is passed to the MMU  915  (FIG. 150) which returns a physical page number which is latched  936 . 
     The physical page number and the lower virtual address bits are recombined according to the mask  937  and form the address  938  for PCI requests to the PMC  717  (FIG.  102 ). The burst count for the cycle is also loaded into a counter  939 . Prefetch operations use another counter  941  and an address latch and compare circuit  943 . 
     Data returned from the PMC is loaded into a FIFO  944 , along with a marker which indicates whether the data is part of a prefetch. As data becomes available at the front of the FIFO  944 , it is clocked out by the read logic via synchronization latches  945 , 946 . The read logic  946  also generates the IBus acknowledge signal. 
     A central control block  932 , including state machines, controls the sequencing of all of the address and data elements, and the interface to the PMC. 
     The virtual page number counter  935  is loaded at the start of an IBus transaction with the page number bits from the IBus address. The top 10 bit of this 20-bit counter always come from the incoming address. For the lower 10 bits, each bit is loaded from the incoming address if the corresponding mask bit  937  is set to 1: otherwize, the counter bit is set to 1. The 20-bit value is forwarded to the MMU interface. 
     In normal operation the virtual page number is not used after the initial address translation. However, if the IBD detects that the burst has crossed a page boundary, the virtual page counter is incremented, and another translation is performed. Since the low order bits that are not part of the virtual page number are set to 1 when the counter is loaded, a simple increment on the entire 20-bit value always causes the actual page number field to increment. The mask bits  937  are used again after an increment to set up the counter for any subsequent increments. 
     The physical address is latched  936  whenever the MMU returns a valid physical page number after translation. The mask bits are used to correctly combine the returned physical page number with the original virtual address bits. 
     The physical address counter  938  is loaded from the physical address latch  936 . It is incremented each time a word is returned from the PMC. The count is monitored as it increments, to determine whether the transaction is about to cross a page boundary. The mask bits are used to determine which bits of the counter should be used for the comparison. When the counter detects that there are two or less words remaining in the page, it signals the control logic  932 , which the terminates the current PCI request after two more data transfers, and requests a new address translation if required. The counter is reloaded after the new address translation, and PCI requests resumed. 
     The burst counter  939  is a 6-bit down counter which is loaded with the IBus burst value at the beginning of a transaction. It is decremented every time a word is returned from the PMC. When the counter value is two or less, it signals to the control logic  932 , which can then terminate the PCI transaction correctly with two more data transfers (unless prefetching is enabled). 
     The prefetch address resister  943  is loaded with the physical address of the first word of any prefetch. When the subsequent IBus transaction starts, and the prefetch counter indicates that at least one word was successfully prefetched, the first physical address of the transaction is compared to the value in the prefetch address latch. If it matched, the prefetch data is used to satisfy the IBus transaction, and any PCI transaction requests start at the address after the last prefetched word. 
     The prefetch counter  941  is a four bit counter which is incremented whenever a word is returned by the PMC during a prefetch operation, up to a maximum count equal to the depth of the input FIFO. When the subsequent IBus transaction matches the prefetch address, the prefetch count is added to the address counter, and subtracted from the burst counter, so that PCI requests can start at the required location. Alternatively, if the IBus transaction only requires some of the prefetched data, the requested burst length is subtracted from the prefetch count, and added to the latched prefetch address, and the remaining prefetch data is retained to satisfy further requests. 
     The Data FIFO  944  is a 8 word by 33 bit asynchronous fall through FIFO. Data from the PMC is written into the FIFO, along with a bit indicating whether the data is part of a prefetch. Data from the front of the FIFO is read out and driven onto the IBus as soon as it becomes available. The logic that generates the data read signals operates synchronously to clk, and generates the IBus acknowledge output. If the transaction is to be satisfied using prefetched data, signals from the control logic tell the read logic how many words of prefetched data should be read out of the FIFO. 
     FIG. 156 illustrates the structure of the RBus Receiver  914  of FIG.  150 . Control is split between two state machines  950 ,  951 . The Write state machine  951  controls the interface to the RBus. The input address  752  is latched at the start of an RBus burst. Each data word of the burst is written in a FIFO  754 , along with its byte enables. If the FIFO  954  become full r-ready is deasserted by the write logic  951  to prevent the results organiser from attempting to write any more words. 
     The write logic  951  notifies the main state machine  950  of the start of an RBus burst via a resynchronized start signal to prevent the results organizer from trying to write any more words. The top address bits, which form the virtual page number, are loaded into a counter  957 . The virtual page number is passed to the MMU, which returns a physical page number  958 . The physical page number and the lower bits of the virtual address are recombined according to the mask, and loaded into a counter  960 , to provide the address for PCI requests to the PMC. Data and byte enables for each word of the PCI request are clocked out of the FIFO  954  by the main control logic  950 , which also handles all PMCM interface control signals. The main state machine indicates that it is active via a busy signal, which is resynchronized and returned to the write state machine. 
     The write state machine  951  detects the end of an RBus burst using r-final. It stops loading data into the FIFO  954 , and signals the main state machine that the RBus burst has finished. The main state machine continues the PCI requests until the Data FIFO has been emptied. It then deasserts busy, allowing the write state machine to start the next RBus burst. 
     Returning to FIG. 150, the memory management unit  915  is responsible for translating virtual page numbers into physical page numbers for the IBus driver (IBD)  912  and the RBus receiver (IBR)  914 . Turning to FIG. 157, there is illustrated the memory management unit in further detail. A 16 entry translation lookaside buffer (TLB)  970  takes its inputs from, and drives its outputs to, the TLB address logic  971 . The TLB control logic  972 , which contains a state machine, receives a request, buffered in the TLB address logic, from the RBR or IBD. It selects the source of the inputs, and selects the operation to be performed by the TLB. Valid TLB operations are compare, invalidate, invalidate all, write and read. Sources of TLB input addresses are the IBD and RBR interfaces (for compare operations), the page table entry buffer  974  (for TLB miss services) or registers within the TLB address logic. The TLB returns the status of each operation to the TLB control logic. Physical page numbers from successful compare operations are driven back to the IBD and RBR. The TLB maintains a record of its least recently used (LRU) location, which is available to the TLB address logic for use as a location for write operations. 
     When a compare operations fails, the TLB control logic  972  signals the page table access control logic  976  to start a PCI request. The page table address generator  977  generates the PCI address based on the virtual page number, using its internal page table pointer registers. Data returned from the PCI request is latched in the page table entry buffer  974 . When a page table entry that matches the required virtual address is found, the physical page number is driven to the TLB address logic  977  and the page table access control logic  976  signals that the page table access is complete. The TLB control logic  972  then writes the new entry into the TLB, and retries the compare operation. 
     Register signals to and from the SCI are resynchronized  980  in both directions. The signals go to and from all other submodules. A module memory interface  981  decodes access from the Standard CBus Interface to the TLB and page table pointer memory elements. TLB access are read only, and use the TLB control logic to obtain the data. The page table pointers are read/write, and are accessed directly by the module memory interface. These paths also contain synchronization circuits. 
     3.18.11 Peripheral Interface Controller 
     Turning now to FIG. 158, there is illustrated one form of peripheral interface controller (PIC)  237  of FIG. 2 in more detail. The PIC  237  works in one of a number of modes to transfer data to or from an external peripheral device. The basic modes are: 
     1) Video output mode. In this mode, data is transferred to a peripheral under the control of an external video clock and clock/data enables. The PIC  237  drives output clock and clock enable signs with the required timing with respect to the output data. 
     2) Video input mode. In this mode, data is transferred from a peripheral under the control of an external video clock and data enable. 
     3) Centronics mode. This mode transfers data to and from the peripheral according to the standard protocol defined in IEEE  1284  standard. 
     The PIC  237  decouples the protocol of the external interface from the internal data sources or destination in accordance with requirements. Internal data sources write data into a single stream of output data, which is then transferred to the external peripheral according to the selected mode. Similarly, all data from an external peripheral is written into a single input data stream, which is available to satisfy a requested transaction to either of the possible internal data destinations. 
     There are three possible sources of output data: the LMC  236  (which uses the ABus), the RO  249  (which uses the RBus), and the global CBus. The PIC  237  responds to transactions from these data sources one at a time—a complete transaction is completed from one source before another source is considered. In general, only one source of data should be active at any time. If more than one source is active, they are served with the following priority—CBus, then ABus, then RBus. 
     As usual, the module operates under the control of the standard CBus interface  990  which includes the PIC&#39;s internal registers. 
     Further, a CBus data interface  992  is provided for accessing and controlling peripheral devices via the co-processor  224 . An ABus interface  991  is also provided for handling memory interactions with the local memory controller. Both the ABus interface  991  and CBus data interface  992  in addition to the result organizer  249  send data to an output data path  993  which includes a byte—wide FIFO. Access to the output data path is controlled by an arbiter which keeps track of which source has priority or ownership of the output stream. The output data path in turn interfaces with a video output controller  994  and centronics control  997  depending on which of these is enabled. Each of the modules  994 ,  997  reads one byte at a time from the output data path&#39;s internal FIFO. The centronics controller  997  implements the centronics data interfacing standard for controlling peripheral devices. The video output controller includes logic to control output pads according to the desired video output protocols. Similarly, a video input controller  998  includes logic to control any implemented video input standard. The video input controller  998  outputs to an input data path unit  999  which again comprizes a byte wide input FIFO with data being written into the FIFO asynchronously, one byte at a time, by either the video input controller  998  or centronics controller  997 . 
     A data timer  996  contains various counters utilized to monitor the current state of FIFO&#39;s within output data paths  993  and input data path  999 . 
     It can be seen from the foregoing that the co-processor can be utilized to execute dual streams of instructions for the creation of multiple images or multiple portions of a single image simultaneously. Hence, a primary instruction stream can be utilized to derive an output image for a current page while a secondary instruction stream can be utilized, during those times when the primary instruction stream is idle, to begin the rendering of a subsequent page. Hence, in a standard mode of operation, the image for a current page is rendered and then compressed utilising the JPEG coder  241 . When it is required to print out the image, the co-processor  241  decompresses the JPEG encoded image, again utilising the JPEG coder  241 . During those idle times when no further portions of the JPEG decoded image are required by an output device, instructions can be carried out for the compositing of a subsequent page or band. This process generally accelerates the rate at which images are produced due to the overlap operating of the co-processor. In particular, the co-processor  224  can be utilized to substantial benefit in the speeding up of image processing operations for printing out by a printer attached to the co-processor such that rendering speeds will be substantially increased. 
     It will be evident from the foregoing that discussion of the preferred embodiment refers to only one form of implementation of the invention and modifications, obvious to those skilled in the art, can be made thereto without departing from the scope of the invention.