Abstract:
A controller operable to control an array of processing elements comprises a retrieval unit operable to retrieve instruction items for each of a plurality of instructions streams, each instruction stream having a plurality of instructions items, a combining unit operable to combine the plurality of instruction streams into a serial instruction stream, and a distribution unit operable to distribute the serial instruction stream to an array of processing elements.

Description:
[0001]     The present invention relates to parallel data processing apparatus, and in particular to SIMD (single instruction multiple data) processing apparatus.  
       BACKGROUND OF THE INVENTION  
       [0002]     Increasingly, data processing systems are required to process large amounts of data. In addition, users of such systems are demanding that the speed of data processing is increased. One particular example of the need for high speed processing of massive amounts of data is in the computer graphics field. In computer graphics, large amounts of data are produced that relate to, for example, geometry, texture, and colour of objects and shapes to be displayed on a screen. Users of computer graphics are increasingly demanding more lifelike and faster graphical displays which increases the amount of data to be processed and increases the speed at which the data must be processed.  
         [0003]     A previously proposed processing architecture for processing large amounts of data in a computer system uses a Single Instruction Multiple Data (SIMD) array of processing elements. In such an array all of the processing elements receive the same instruction stream, but operate on different respective data items. Such an architecture can thereby process data in parallel, but without the need to produce parallel instruction streams. This can be an efficient and relatively simple way of obtaining good performance from a parallel processing machine.  
         [0004]     However, the SIMD architecture can be inefficient when a system has to process a large number of relatively small data item groups. For example, for a SIMD array processing data relating to a graphical display screen, for a small graphical primitive such as a triangle, only relatively few processing elements of the array will be enabled to process data relating to the primitive. In that case, a large proportion of the processing elements may remain unused while data is being processed for a particular group.  
         [0005]     It is therefore desirable to produce a system which can overcome or alleviate this problem.  
       SUMMARY OF THE INVENTION  
       [0006]     Various aspects of the present invention are exemplified by the attached claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is a block diagram illustrating a graphics data processing system;  
         [0008]      FIG. 2  is a more detailed block diagram illustrating the graphics data processing system of  FIG. 1 ;  
         [0009]      FIG. 3  is a block diagram of a processing core of the system of  FIG. 2 ;  
         [0010]      FIG. 4  is a block diagram of a thread manager of the system of  FIG. 3 ;  
         [0011]      FIG. 5  is a block diagram of a array controller of the system of  FIG. 3 ;  
         [0012]      FIG. 6  is a block diagram of an instruction issue state machine of the channel controller of  FIG. 3 ;  
         [0013]      FIG. 7  is a block diagram of a binning unit of the system of  FIG. 3 ;  
         [0014]      FIG. 8  is a block diagram of a processing block of the system of  FIG. 3 ;  
         [0015]      FIG. 9  is a flowchart illustrating data processing using the system of FIGS.  1  to  8 ;  
         [0016]      FIG. 10  is a more detailed block diagram of a thread processor of the thread manager of  FIG. 4 ;  
         [0017]      FIG. 11  is a block diagram of a processor unit of the processing block of  FIG. 8 ;  
         [0018]      FIG. 12  is a block diagram illustrating a processing element interface;  
         [0019]      FIG. 13  is a block diagram illustrating a block I/O interface;  
         [0020]      FIG. 14  is a block diagram of part of the processor unit of  FIG. 11 ; and  
         [0021]      FIG. 15  is a block diagram of another part of the processor unit of  FIG. 11 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0022]     The data processing system described below is a graphics data processing system for producing graphics images for display on a screen. However, this embodiment is purely exemplary, and it will be readily apparent that the techniques and architecture described here for processing graphical data are equally applicable to other data types, such as video data. The system is of course applicable to other signal and/or data processing techniques and systems.  
         [0023]     An overview of the system will be given, followed by brief descriptions of the various functional units of the system. A graphics processing method will then be described by way of example, followed by detailed description of the functional units.  
         [0024]     Overview  
         [0025]      FIG. 1  is a system level block diagram illustrating a graphics data processing system  3 . The system  3  interfaces with a host system (not shown), such as a personal computer or workstation, via an interface  2 . Such a system can be provided with an embedded processor unit (EPU) for control purposes. For example, the specific graphics system  3  includes an embedded processing unit (EPU)  8  for controlling the overall function of the graphics processor and for interfacing with the host system. The system includes a processing core  10  which processes the graphical data for output to the display screen via a video output interface  14 . Local memory  12  is provided for the graphics system  3 .  
         [0026]     Such a data processing can be connected for operation to a host system or could provide a stand alone processing system, without the need for a specific host system. Examples of such application include a “set top box” for receiving and decoding digital television and internet signals.  
         [0027]      FIG. 2  illustrates the graphics processing system in more detail. In one particular example, the graphics system connects to the host system via an advanced graphics port (AGP) or PCI interface  2 . The PCI interface and AGP  2  are well known.  
         [0028]     The host system can be any type of computer system, for example, a PC  99  specification personal computer or a workstation.  
         [0029]     The AGP  2  provides a high bandwidth path from the graphics system to host system memory. This allows large texture databases to be held in the host system memory, which is generally larger than local memory associated with the graphics system. The AGP also provides a mechanism for mapping memory between a linear address space on the graphics system and a number of potentially scattered memory blocks in the host system memory. This mechanism is performed by a graphics address re-mapping table (GART) as is well known.  
         [0030]     The graphics system described below is preferably implemented as a single integrated circuit which provides all of the functions shown in  FIG. 1 . However, it will be readily apparent that the system may be provided as separate circuit card carrying several different components, or as a separate chipset provided on the motherboard of the host, or integrated with the host central processing unit (CPU), or in any suitable combination of these and other implementations.  
         [0031]     The graphics system includes several functional units which are connected to one another for the transfer of data by way of a dedicated bus system. The bus system preferably includes a primary bus  4  and a secondary bus  6 . The primary bus is used for connection of latency intolerant devices, and the secondary bus is used for connection of latency tolerant devices. The bus architecture is preferably as described in detail in the Applicant=s co-pending UK patent applications, particularly GB 9820430.8. It will be readily appreciated that any number of primary and secondary buses can be provided in the bus architecture in the system. The specific system shown in  FIG. 2  includes two secondary buses.  
         [0032]     Referring mainly to  FIG. 2 , access to the primary bus  4  is controlled by a primary arbiter  41 , and access to the secondary buses  6  by a pair of secondary arbiters  61 . Preferably, all data transfers are in packets of 32 bytes each. The secondary buses  6  are connected with the primary bus  4  by way of respective interface units (SIP)  62 .  
         [0033]     An auxiliary control bus  7  is provided in order to enable control signals to be communicated to the various units in the system.  
         [0034]     The AGP/PCI interface is connected to the graphics system by way of the secondary buses  6 . This interface can be connected to any selection of the secondary buses, in the example shown, to both secondary buses  6 . The graphics systems also includes an embedded processing unit (EPU)  8  which is used to control operation of the graphics system and to communicate with the host system. The host system has direct access to the EPU  8  by way of a direct host access interface  9  in the AGP/PCI  2 . The EPU is connected to the primary bus  4  by way of a bus interface unit (EPU FBI)  90 .  
         [0035]     Also connected to the primary bus is a local memory system  12 . The local memory system  12  includes a number, in this example four, of memory interface units  121  which are used to communicate with the local memory itself. The local memory is used to store various information for use by the graphics system.  
         [0036]     The system also includes a video interface unit  14  which comprises the hardware needed to interface the graphics system to the display screen (not shown), and other devices for exchange of data which may include video data. The video interface unit is connected to the secondary buses  6 , via bus interface units (FBI).  
         [0037]     The graphics processing capability of the system is provided by a processing core  10 . The core  10  is connected to the secondary buses  6  for the transfer of data, and to the primary bus  4  for the transfer of instructions. As will be explained in more detail below, the secondary bus connections are made by a core bus interface (Core FBI)  107 , and a binner bus interface (Binner FBI)  111 , and the primary bus connection is made by a thread manager bus interface (Thread Manager FBI)  103 .  
         [0038]     As will be explained in greater detail below, the processing core  10  includes a number of control units: thread manager  102 , array controller  104 , channel controller  108 , a binning unit  1069  per block and a microcode store  105 . These control units control the operation of a number of processing blocks  106  which perform the graphics processing itself.  
         [0039]     In the example shown in  FIG. 2 , the processing core  10  is provided with eight processing blocks  106 . It will be readily appreciated that any number of processing blocks can be provided in a graphics system using this architecture.  
         [0040]     Processing Core  
         [0041]      FIG. 3  shows the processing core in more detail. The thread manager  102  is connected to receive control signals from the EPU  8 . The control signals inform the thread manager as to when instructions are to be fetched and where the instructions are to be found. The thread manager  102  is connected to provide these instructions to the array controller  104  and to the channel controller  108 . The array and channel controllers  104  and  108  are connected to transfer control signals to the processing blocks  106  dependent upon the received instructions.  
         [0042]     Each processing block  106  comprises an array  1061  of processor elements (PEs) and a mathematical expression evaluator (MEE)  1062 . As will be described in more detail below, a path  1064  for MEE coefficient feedback is provided from the PE memory, as is an input/output channel  1067 . Each processing block includes a binning unit  1069  unit  1068  and a transfer engine  1069  for controlling data transfers to and from the input/output channel under instruction from the channel controller  108 .  
         [0043]     The array  1061  of processor elements provides a single instruction multiple data (SIMD) processing structure.  
         [0044]     Each PE in the array  1061  is supplied with the same instruction, which is used to process data specific to the PE concerned.  
         [0045]     Each processing element (PE)  1061  includes a processor unit  1061  a for carrying out the instructions received from the array controller, a PE memory unit  1061   c  for storing data for use by the processor unit  1061   a,  and a PE register file  1061   b  through which data is transferred between the processor unit  1061   a  and the PE memory unit  1061   c.  The PE register file  1061   b  is also used by the processor unit  1061   a  for temporarily storing data that is being processed by the processor unit  1061   a.    
         [0046]     The provision of a large number of processor elements can result in a large die size for the manufacture of the device in a silicon device. Accordingly, it is desirable to reduce the effect of a defective area on the device. Therefore, the system is preferably provided with redundant PEs, so that if one die area is faulty, another can be used in its place.  
         [0047]     In particular, for a group of processing elements used for processing data, additional redundant processing elements can be manufactured. In one particular example, the processing elements are provided in Apanels@ of 32 PEs. For each panel a redundant PE is provided, so that a defect in one of the PEs of the panel can be overcome by using the redundant PE for processing of data. This will be described in more detail below.  
         [0048]     Thread Manager  
         [0049]     The array of processing elements is controlled to carry out a series of instructions in an instruction stream. Such instruction streams for the processing blocks  106  are known as Athreads@. Each thread works co-operatively with other threads to perform a task or tasks. The term “multithreading” refers to the use of several threads to perform a single task, whereas the term “multitasking” refers to the use of several threads to perform multiple tasks simultaneously. It is the thread manager  102  which manages these instruction streams or threads.  
         [0050]     There are several reasons for providing multiple threads in such a data processing architecture. The processing element array can be kept active, by processing another thread when the current active thread is halted. The threads can be assigned to any task as required. For example, by assigning a plurality of threads for handling data I/O operations for transferring data to and from memory, these operations can be performed more efficiently, by overlapping I/O operations with processing operations. The latency of the memory I/O operations can effectively be masked from the system by the use of different threads.  
         [0051]     In addition, the system can have a faster response time to external events. Assigning particular threads to wait on different external events, so that when an event happens, it can be handled immediately.  
         [0052]     The thread manager  102  is shown in more detail in  FIG. 4 , and comprises a cache memory unit  1024  for storing instructions fetched for each thread. The cache unit  1024  could be replaced by a series of first-in-first-out (FIFO) buffers, one per thread. The thread manager also includes an instruction fetch unit  1023 , a thread scheduler  1025 , thread processors  1026 , a semaphore controller  1028  and a status block  1030 .  
         [0053]     Instructions for a thread are fetched from local memory or the EPU  8  by the fetch unit  1023 , and supplied to the cache memory  1024  via connecting logic.  
         [0054]     The threads are assigned priorities relative to one another. Of course, although the example described here has eight threads, any number of threads can be controlled in this manner. At any particular moment in time, each thread may be assigned to any one of a number of tasks. For example, thread zero may be assigned for general system control, thread  1  assigned to execute 2D (two dimensional) activities, and threads  2  to  7  assigned to executing 3D activities (such as calculating vertices, primitives or rastering).  
         [0055]     In the example shown in  FIG. 4 , the thread manager includes one thread processor  1026  for each thread. The thread processors  1026  control the issuance of core instructions from the thread manager so as to maintain processing of simultaneously active program threads, so that each the processing blocks  106  can be active for as much time as possible. In this particular example the same instruction stream is supplied to all of the processing blocks in the system.  
         [0056]     It will be appreciated that the number of threads could exceed the number of thread processors, so that each thread processor handles control of more than one thread. However, providing a thread processor for each thread reduces the need for context switching when changing the active thread, thereby reducing memory accesses and hence increasing the speed of operation.  
         [0057]     The semaphore controller  1028  operates to synchronise the threads with one other.  
         [0058]     Within the thread manager  102 , the status block  1030  receives status information  1036  from each of the threads. The status information is transferred to the thread scheduler  1025  by the status block  1030 . The status information is used by the thread scheduler  1025  to determine which thread should be active at any one time.  
         [0059]     Core instructions  1032  issued by the thread manager  102  are sent to the array controller  104  and the channel controller  108  ( FIG. 3 ).  
         [0060]     Array Controller  
         [0061]     The array controller  104  directs the operation of the processing block  106 , and is shown in greater detail in  FIG. 5 .  
         [0062]     The array controller  104  comprises an instruction launcher  1041 , connected to receive instructions from the thread manager. The instruction launcher  1041  indexes an instruction table  1042 , which provides further specific instruction information to the instruction launcher.  
         [0063]     On the basis of the further instruction information, the instruction launcher directs instruction information to either a PE instruction sequencer  1044  or a load/store controller  1045 . The PE instruction sequencer receives instruction information relating to data processing, and the load/store controller receives information relating to data transfer operations.  
         [0064]     The PE instruction sequencer  1044  uses received instruction information to index a PE microcode store  105 , for transferring PE microcode instructions to the PEs in the processing array.  
         [0065]     The array controller also includes a scoreboard unit  1046  which is used to store information regarding the use of PE registers by particular active instructions. The score board unit  1046  is functionally divided so as to provide information regarding the use of registers by instructions transmitted by the PE instruction sequencer  1044  and the load/store controller  1045  respectively.  
         [0066]     In general terms, the PE instruction sequencer  1044  handles instructions that involve data processing in the processor unit  1061   a.  The load/store controller  1045 , on the other hand, handles instructions that involve data transfer between the registers of the processor unit  1061   a  and the PE memory unit  1061   c.  The load/store controller  1045  will be described in greater detail later.  
         [0067]     The instruction launcher  1041  and the score board unit  1046  maintain the appearance of serial instruction execution whilst achieving parallel operation between the PE instruction sequencer  1044  and the load/store controller  1045 .  
         [0068]     The remaining core instructions  1032  issued from the thread manager  102  are fed to the channel controller  108 . This controls transfer of data between the PE memory units and external memory (either local memory or system memory in AGP or PCI space).  
         [0069]     Channel Controller  
         [0070]     The channel controller  108  operates asynchronously with respect to the execution of instructions by the array controller  104 . This allows computation and external I/O to be performed simultaneously and overlapped as much as possible. Computation (PE) operations are synchronised with I/O operations by means of semaphores in the thread manager, as will be explained in more detail below.  
         [0071]     The channel controller  108  also controls the binning units  1068  which are associated with respective processing blocks  106 . This is accomplished by way of channel controller instructions.  
         [0072]      FIG. 6  shows the channel controller=s instruction issue state machine, which lies at the heart of the channel controller=s operation, and which will be described in greater detail later.  
         [0073]     Each binning unit  1069  ( FIG. 3 ) is connected to the I/O channels of its associated processing block  106 . The purpose of the binning unit  1069  is to sort primitive data by region, since the data is generally not provided by the host system in the correct order for region based processing.  
         [0074]     The binning units  1068  provide a hardware implemented region sorting system, (shown in  FIG. 7 ), which removes the sorting process from the processing elements, thereby releasing the PEs for data processing.  
         [0075]     Memory Access Consolidation  
         [0076]     In a computer system having a large number of elements which require access to a single memory, or other addressed device, there can be a significant reduction in processing speed if accesses to the storage device are performed serially for each element.  
         [0077]     The graphics system described above is one example of such a system. There are a large number of processor elements, each of which requires access to data in the local memory of the system. Since the number of elements requiring memory access exceeds the number of memory accesses that can be made at any one time, accesses to the local and system memory involves serial operation. Thus, performing memory access for each element individually would cause a degradation in the speed of operation of the processing block.  
         [0078]     In order to reduce the effect of this problem on the speed of processing of the system, the system of  FIGS. 1 and 2  includes a memory access consolidating function.  
         [0079]     The memory access consolidation is also described below with reference to  FIGS. 12 and 13 . In general, however, the processing elements that require access to memory indicate that this is the case by setting an indication flag or mark bit. The first such marked PE is then selected, and the memory address to which it requires access is transmitted to all of the processing elements of the processing block. The address is transmitted with a corresponding transaction ID. Those processing elements which require access (ie. have the indication flag set) compare the transmitted address with the address to which they require access, and if the comparison indicates that the same address is to be accessed, those processing elements register the transaction ID for that memory access and clear the indication flag.  
         [0080]     When the transaction ID is returned to the processing block, the processing elements compare the stored transaction ID with the incoming transaction ID, in order to recover the data.  
         [0081]     Using transaction IDs in place of simply storing the accessed address information enables multiple memory accesses to be carried, and then returned in any order. Such a Afire and forget@ method of recovering data can free up processor time, since the processors do not have to await return of data before continuing processing steps. In addition, the use of transaction ID reduces the amount of information that must be stored by the processing elements to identify the data recovery transaction. Address information is generally of larger size than transaction ID information.  
         [0082]     Preferably, each memory address can store more data than the PEs require access to. Thus, a plurality of PEs can require access to the same memory address, even though they do not require access to the same data. This arrangement can further reduce the number of memory accesses required by the system, by providing a hierarchical consolidation technique. For example, each memory address may store four quad bytes of data, with each PE requiring one quad byte at any one access.  
         [0083]     This technique can also allow memory write access consolidation for those PEs that require write access to different portions of the same memory address.  
         [0084]     In this way the system can reduce the number of memory accesses required for a processing block, and hence increase the speed of operation of the processing block.  
         [0085]     The indication flag can also be used in another technique for writing data to memory. In such a technique, the PEs having data to be written to memory signal this fact by setting the indication flag. Data is written to memory addresses for each of those PEs in order, starting at a base address, and stepped at a predetermined spacing in memory. For example, if the step size is set to one, then consecutive addresses are written with data from the flagged PEs.  
         [0086]     Processing Blocks  
         [0087]     One of the processing blocks  106  is shown in more detail in  FIG. 8 . The processing block  106  includes an array of processor elements  1061  which are arranged to operate in parallel on respective data, items but carrying out the same instruction (SIMD). Each processor element  1061  includes a processor unit  1061   a,  a PE register file  1061   b  and a PE memory unit  1061   c.  The PE memory unit  1063   c  is used to store data items for processing by the processor unit  1061   a.  Each processor unit  1061   a  can transfer data to and from its PE memory unit  1061   c  via the PE register file  1061   b.  The processor unit  1061   a  also uses the PE register file  1061   b  to store data which is being processed. Transfer of data items between the processor unit  1061   a  and the memory unit  1061   c  is controlled by the array controller  104 .  
         [0088]     Each of the processing elements is provided with a data input from the mathematical expression evaluator (MEE)  1062 . The MEE operates to evaluate a mathematical expression for each of the PEs. The mathematical expression can be a linear, bi-linear, cubic, quadratic or more complex expression depending upon the particular data processing application concerned.  
         [0089]     One particular example of a mathematical expression evaluator is the linear expression evaluator (LEE). The LEE is a known device for evaluating the bi-linear expression:  
         [0090]     for a range of values of x i  and y j .  
         [0091]     The LEE is described in detail in U.S. Pat. No. 4,590,465. The LEE is supplied with the coefficient values a, b and c for evaluating the bi-linear expression, and produces a range of outputs corresponding to different values of x i  and y j . Each processing element  1061  represents a particular (x i , y j ) pair and the LEE produces a specific value of the bi-linear expression for each processor element.  
         [0092]     The bi-linear expression could, for example, define a line bounding one side of a triangle that is to be displayed. The linear expression evaluator then produces a value to indicate to the processor element whether the pixel for which the processor element is processing data lies on the line, to one side or the other of the line concerned. Further processing of the graphical data can then be pursued.  
         [0093]     The mathematical expression evaluator  1062  is provided with coefficients from a feedback buffer (FBB)  1068  or from a source external to the processing block (known as immediates). The feedback buffer  1068  can be supplied with coefficients from a PE register file  1061   b,  or from a PE memory unit  1061   c.    
         [0094]     The bus structure  1064  is used to transfer data from the processor elements (register file or memory unit) to the FBB  1068 . Each PE is controlled in order to determine if it should supply coefficient data to the MEE.  
         [0095]     In one example, only one PE (at a time is enabled) to transfer data to the feedback buffer FBB  1068 . The FBB queues the data to be fed to the MEE  1062 . In another example, multiple PEs can transfer data to the FBB at the same time, and so the handling of the transfer of data would then depend upon the nature of the MEE feedback bus structure  1064 . For example, the bus could be a wired-OR so that if multiple data is written, the logical OR of the data is supplied to the MEE  1062 .  
         [0096]     The MEE operand feedback path can also effectively be used to communicate data from one processor element to all the others in the block concerned, by setting the a and b coefficients to zero, and supplying the data to be communicated as the c coefficient. All of the MEE results would then be equal to the coefficient c, thus transferring the data to the other processor elements.  
         [0097]     In the present system the processing blocks  106  are provided with opcodes (instructions) and operands (data items) for the expression evaluator separately from one another. Previously, instructions and data are provided in a single instruction stream. This stream must be produced during processing which can result in a slowing of processing speed, particularly when the operands are produced in the array itself.  
         [0098]     In the present system, however, since the opcode is separated from the operand, opcodes and operands can be produced by different sources and are only combined when an operation is to be performed by the MEE  1062 .  
         [0099]     Graphics Data Processing  
         [0100]      FIG. 9  illustrates simplified steps in a graphics data processing method using the system of FIGS.  1  to  8 . The host system prepares data concerning the vertices of the primitive graphical images to be processed and displayed by the graphics system. The data is then transferred, either as a block of vertex data, or vertex by vertex as it is prepared by the host system to the graphics system.  
         [0101]     The data is loaded into the PEs of the graphics system so that each PE contains data for one vertex. Each PE then represents a vertex of a primitive that can be at an end of a line or part of a two dimensional shape such as a triangle.  
         [0102]     The received data is then processed to transform it from the host system reference space to the required screen space. For example, three dimensional geometry, view, lighting and shading etc. is performed to produce data depending upon the chosen viewpoint.  
         [0103]     Each PE then copies its vertex data to its neighbouring PEs so that each PE then has at least one set of vertex data that corresponds to a graphical primitive, be that a line, a triangle or a more complex polygon. The data is then organised on a primitive per PE basis.  
         [0104]     The primitive data is then output from the PEs to the local memory in order that it can be sorted by region. This is performed by the binning unit  1069  of  FIG. 3 , as will be described in more detail below. The binning unit  1069  sorts primitive data by region, since the data is generally not provided by the host system in the correct order for region based processing.  
         [0105]     The binning units  1068  provide a hardware implemented region sorting system which removes the sorting process from the processing elements, thereby releasing the PEs for data processing.  
         [0106]     All of the primitive data is written into local memory, each primitive having one entry. When data for a particular primitive is written, its extent is compared with the region definitions. Information regarding the primitives that occur in each region is stored in local memory. For each region in which at least part of a primitive occurs, a reference is stored to the part of local memory in which the primitive data is stored. In this way, each set of primitive data need only be stored once.  
         [0107]     Once the primitive information has been stored in local memory, it is read back into the individual PEs. However, at this stage, all of the PEs in one processing block contain data concerning respective primitives occurring in a single region. From this point, a given processing block operates on data associated with a single region of the display.  
         [0108]     Each PE then transfers, in turn, its data concerning its primitive to the MEE for processing into pixel data. For example, a PE will supply coefficient data to the MEE which define a line that makes up one side of a triangular primitive. The MEE will then evaluate all of the pixel values on the basis of the coefficients, and produce results for each pixel which indicate whether a pixel appears above, below or on the line. For a triangle, this is carried out three times, so that it can be determined whether or not a pixel occurs within the triangle, or outside of it. Each PE then also includes data about a respective pixel (i.e., data is stored on a pixel per PE basis).  
         [0109]     Once each pixel is determined to be outside or inside the triangle (primitive) concerned, the processing for the primitive can be carried out only on those pixels occurring inside the primitive. The remainder of the PEs in the processing block do not take any further part in the processing until that primitive is processed.  
         [0110]     Detailed Description of the Functional Units Described Above Thread Manager  
         [0111]     A detailed description will now be given of the thread manager  102 , which as mentioned above with reference to  FIG. 4 , comprises a cache memory unit  1024  for storing instructions fetched for each thread. The cache unit  1024  could be replaced by a series of first-in-first-out (FIFO) buffers, one per thread. The thread manager also includes an instruction fetch unit  1023 , a thread scheduler  1025 , thread processors  1026 , a semaphore controller  1028  and a status block  1030 .  
         [0112]     Instructions for a thread are fetched from local external memory  103  or from the EPU  8  by the fetch unit  1023 , and supplied to the cache memory  1024  via connecting logic.  
         [0113]     At a given time, only one thread is executing, and the scheduling of the time multiplexing between threads is determined by the dynamic conditions of the program execution. This scheduling is performed by a thread scheduler in the thread manager  102 , which ensures that each processor block  106  is kept busy as much as possible. The switching from one thread to another involves a state saving and restoring overhead. Therefore, the priority of threads is used to reduce the number of thread switches, thereby reducing the associated overheads.  
         [0114]     Core instructions issued by the thread manager  102  are sent to one of two controller units, the array controller  104  or channel controller  108 .  
         [0115]     Determining Which Thread Should be Active  
         [0116]     The thread scheduler, when running, recalculates which thread should be active whenever one of the following scheduling triggers occur:  
         [0117]     A thread with higher priority than the current active thread is READY, or The thread is (not Ready) and YIELDING.  
         [0118]     The thread scheduler is able to determine this because each thread reports the status of whether it is READY or YIELDING back to the thread scheduler, and are examined in a register known as the Scheduler-Status register.  
         [0119]     In determining the above, a thread is always deemed to be READY, unless it is:  
         [0120]     waiting on an instruction cache miss,  
         [0121]     waiting on a zero semaphore;  
         [0122]     waiting on a busy execution unit, or  
         [0123]     waiting on a HALT instruction.  
         [0124]     When a thread stops operation, for example because it requires memory access, it can be Ayielding@ or Anot yielding@. If the thread is yielding, then if another thread is ready, then that other thread can become active. If the thread is not yielding, then other threads are prevented from becoming active, even though ready. A thread may not yield, for example, if that thread merely requires a short pause in operation. This technique avoids the need to swap between active threads unnecessarily, particularly when a high priority thread simply pauses momentarily.  
         [0125]     In the event that a scheduling trigger occurs as described above, the scheduler comes into effect, and carries out the following. First, it stops the active thread from running, and waits a cycle for any semaphore decrements to propagate.  
         [0126]     If the previously active thread is yielding, the scheduler activates the highest priority READY thread, or the lowest priority thread if no thread is ready (since this will cause another immediate scheduling trigger).  
         [0127]     If the previously active thread is not yielding, the scheduler activates the highest priority thread which is READY which has higher priority than the previously active thread. If there is no such thread, the scheduler reactivates the previously active thread (which will cause another scheduling trigger if that thread has not become READY).  
         [0128]     The thread scheduler can be disabled through the EPU interface. When the scheduler is disabled the EPU is able to control activation of the threads. For example, the EPU could start and stop the active thread, set the active thread pointer to a particular thread, and single step through the active thread.  
         [0129]     The thread manager  102  only decodes thread manager instructions or semaphore instructions. In addition, each thread has its own thread processor  1026 , as shown in  FIG. 10 . The thread processor  1026  can be divided into several parts in order to aid understanding of its operation.  
         [0130]     Each thread processor comprises a byte alu  540 , a predicate alu  550 , a branch unit  520 , an instruction cache  530 , an instruction assembler  510  and an enable unit  500 .  
         [0131]     The purpose of the thread processor  1026  is to allow high level flow control to be performed for a thread, (such as looping and conditional branches), and to assemble instructions to be issued to the array controller  104  and channel controller  108 .  
         [0132]     An enable unit  500  is used to determine whether a thread is READY, as outlined in the text above.  
         [0133]     The instruction cache  530  receives addresses for instructions from the branch unit  520  and fetches them from the cache  5301 . During start up, the EPU can program the program counters in the branch unit. If the cache  5301  does not contain the instruction, a cache miss is signalled, and an instruction fetch from local memory is initiated. If there is no miss, the instruction is latched into the instruction register  5302 .  
         [0134]     The branch adder  520  controls the address of the next instruction. In the normal course of events, it simply increments the last address, thus stepping sequentially through the instructions in memory. However, if a branch is requested, it calculates the new address by adding an offset (positive or negative) to the current address, or by replacing the current address with an absolute address in memory. If the thread processor is halted, a PC 0  register  5201  provides the last address requested, as a PC 1  register  5202  will already have been changed.  
         [0135]     The byte alu section  540  provides a mechanism for performing mathematical operations on the  16 -bit registers contained in the thread processor  102 . The programmer can use thread manager instructions to add, subtract and perform logical operations on the thread processor general registers  5402 , thereby enabling loops to be written. Information can also be passed to the array controller  104  from the general registers by using the byte alu  540  and the instruction assembler  510 .  
         [0136]     The predicate alu  550  contains sixteen 1 bit predicate registers  5501 . These represent true or false conditions. Some of these predicates indicate carry, overflow, negative, most significant bit status for the last byte alu operation. The remaining predicates can be used by the programmer to contain conditions. These are used to condition branches (for loop termination), and can receive status information from the array controller  104  indicating Aall enable registers off@ (AEO) in the array.  
         [0137]     The instruction assembler  510  assembles instructions for the various controllers such as channel controller  108  and array controller  104 . Most instructions are not modified and are simply passed on to the respective controllers. However, sometimes fields in the various instructions can be replaced with the contents of the general registers. The instruction assembler  510  does this before passing the instruction to the relevant controller. The instruction assembler  510  also calculates the yield status, the wait status and the controller signal status sent to the enable unit  500  and the scheduler in the thread manager  102 .  
         [0138]     Semaphore Controller  
         [0139]     Synchronisation of threads and control of access to other resources is provided by the semaphore controller  1028 .  
         [0140]     Semaphores are used to achieve synchronisation between threads, by controlling access to common resources. If a resource is in use by a thread, then the corresponding semaphore indicates this to the other threads, so that the resource is unavailable to the other threads. The semaphore can be used for queueing access to the resource concerned.  
         [0141]     In a particular example, the semaphore controller  1028  uses a total of eighty semaphores, split into four groups in dependence upon which resources the semaphores relate to.  
         [0142]     Semaphore Count and Overflow  
         [0143]     The semaphores have an eight bit unsigned count. However, the msb (bit 7 ) is used as an overflow bit, and thus should never be set. Whenever any semaphore&#39;s bit  7  is set, the semaphore overflow flag in the thread manager status register is set. If the corresponding interrupt enable is set the EPU is interrupted. The semaphore overflow flag remains set until cleared by the EPU.  
         [0144]     Semaphore Operations  
         [0145]     The following operations are provided for each semaphore:  
         [0146]     Preset: A thread can preset the semaphore value. The thread should issue a preset instruction only when it is known that there are no pending signals for the semaphore.  
         [0147]     Wait: A thread can perform a wait operation on the semaphore by issuing a wait instruction. If the semaphore is nonzero the semaphore is decremented. If it is zero the thread is paused waiting to issue the wait instruction.  
         [0148]     Signal: The semaphore is incremented. This operation can be performed by the threads, the PE Sequencer, the Load/Store Unit, or the Channel Controller. But in general a semaphore can only be signalled by one of these, as discussed below.  
         [0149]     The EPU  8  can read and write the thread semaphore counts anytime. In general, the core should not be executing instructions when the EPU accesses the other semaphore values.  
         [0150]     Semaphore Groups  
         [0151]     The semaphores are broken into four groups according to which execution units they can be signalled by.  
                                               number of                   sems in   semaphore   semaphores in group can       group id   group   group name   be signalled by                   0   32   Thread   threads and EPU       1   16   Channel   channel controller       2   16   Load/Store   load/store unit       3   16   PE   PE sequencer                  
 
         [0152]     The EPU can read and write all semaphore values when the core is frozen. In addition, the EPU can preset, increment, and decrement a thread semaphore at any time as follows:  
         [0153]     Increment: the EPU can atomically increment the semaphore by writing its increment register (an atomic operation is an operation that cannot be interrupted by other operations, as is well known).  
         [0154]     Decrement: the EPU can atomically decrement the semaphore by reading its decrement register. If the semaphore is nonzero before decrementing the read returns TRUE.  
         [0155]     Otherwise the read returns FALSE and the semaphore is left at zero.  
         [0156]     Each thread semaphore has a separately enabled nonzero interrupt. When this interrupt is enabled the semaphore interrupts the EPU when nonzero. The EPU would typically enable this interrupt after receiving a FALSE from a semaphore decrement. Upon receiving the interrupt, it is preferable to attempt the decrement again.  
         [0157]     Array Controller  
         [0158]     A detailed description will now be given of the array controller  104 , as shown in  FIG. 5 . The array controller  104  directs the operation of the processing block  106 . The array controller  104  comprises an instruction launcher  1041 , connected to receive instructions from the thread manager. The instruction launcher  1041  indexes an instruction table  1042 , which provides further specific instruction information to the instruction launcher.  
         [0159]     On the basis of the further instruction information, the instruction launcher directs instruction information to either a PE instruction sequencer  1044  or a load/store controller  1045 . The PE instruction sequencer receives instruction information relating to data processing, and the load/store controller receives information relating to data transfer operations.  
         [0160]     The PE instruction sequencer  1044  uses received instruction information to index a PE microcode store  105 , for transferring PE microcode instructions to the PEs in the processing array.  
         [0161]     The array controller also includes a scoreboard unit  1046  which is used to store information regarding the use of PE registers by particular active instructions. The scoreboard unit  1046  is functionally divided so as to provide information regarding the use of registers by instructions transmitted by the PE instruction sequencer  1044  and the load/store controller  1045  respectively.  
         [0162]     The instruction launcher  1041  and the scoreboard unit  1046  maintain the appearance of serial instruction execution whilst achieving parallel operation between the PE instruction sequencer  1044  and the load/store controller  1045 .  
         [0163]     The remaining core instructions  1032  issued from the thread manager  102  are fed to the channel controller  108 . This controls transfer of data between the PE memory units and external memory (either local memory or system memory in AGP or PCI space).  
         [0164]     In order to maintain the appearance of serial instruction execution, the PE instruction sequencer or Load/store controller stalls the execution of an instruction when that instruction accesses a PE register which is locked by a previously launched, still executing instruction from the load/store controller and PE instruction sequencer resepctively. This mechanism does not delay the launching of instructions. Instruction execution is stalled only when a lock is encountered in the instruction execution.  
         [0165]     The PE register accesses which cause a stall are:  
         [0166]     Any access to a locked register  
         [0167]     Write to the enable stack (used as enable for load/store)  
         [0168]     Write to a P register ( FIG. 4 ) (used as indexed address for load/store)  
         [0169]     Write to a V register ( FIG. 4 ) (used as enable for MEE feedback)  
         [0170]     The Instruction Launcher  1041  determines which registers an instruction accesses and locks these registers as the instruction is launched. The registers are unlocked when the instruction completes. For load/store instructions, determining the accessed registers is straight forward. This is because the accessed registers are encoded directly in the instruction. For PE instructions the task is more complex because the set of accessed registers depends on the microcode. This problem is solved by using nine bits of the PE instruction to address the instruction table  1042  (which is preferably a small memory), which gives the byte lengths of the four operands accessed by the instruction. The instruction table  1042  also determines whether the instruction modifies the enable stack, P register, or V register. Furthermore, it also contains the microcode start address for the instruction.  
         [0171]     When a PE instruction is launched, the instruction table  1042  is accessed to determine the set of registers accessed. These registers are marked in the scoreboard  1046  as locked by that instruction. The registers are unlocked when the instruction completes. Load/Store instructions are stalled when they access or use a register locked by the PE instruction sequencer  1044 .  
         [0172]     When a load/store instruction is launched, all register file registers (R 31 -R 0 ) which are loaded or stored by that instruction are locked. The registers are unlocked when the instruction completes. PE instructions are stalled when they access a register locked by the load/store controller.  
         [0173]     Writes to the P registers stall execution of the Load/Store unit as follows (V register and enable stack are similar). When a PE instruction is launched, it locks the P register if the instruction table lookup indicates that the instruction modifies the P register. The P register remains locked until the instruction completes. A load/store instruction stalls while the P register is locked if the load/store instruction&#39;s Indirect bit is set. A load/store instruction stalls while the V register is locked if the load/store instruction writes the feedback buffer. A load/store instruction stalls while the enable stack is locked if the load/store instruction=s Condition bit is set.  
         [0174]     As mentioned earlier, the instruction table  1042  may be a small memory (RAM),  512  words deep by 64 bits wide. The table is addressed by the instruction index field of PE instructions to determine the instruction start address and type. The table is written with the Load Address and Load Data housekeeping instructions and is read via I address and I data registers on the EPU bus.  
         [0175]     Load/Store Controller  
         [0176]     A detailed description will now be given of the load/store controller  1045 .  
         [0177]     In a particular example, PE memory cycles are nominally at one quarter of the PE clock rate, but can be geared to any desired rate, such as one sixth of the PE clock rate. The memory is 128 bits wide (a page), and has a quadbyte (32-bit) wide interface to the PE register file. This register file interface runs at four times the memory cycle rate, so the register file interface runs at full memory speed.  
         [0178]     Load/store controller instructions execute in one memory cycle (nominally four PE cycles) unless they are stalled by the instruction launcher  1041  or by cycles stolen for refresh or I/O.  
         [0179]     Each load/store instruction transfers part or all of a single memory page. No single load/store instruction accesses more than one page.  
         [0180]     Memory Operations Performed by the Load/Store Controller  
         [0181]     The load/store controller  1045  performs the following operations on PE memory  1063 :  
         [0182]     loads and stores from PE memory  1063  to PE register files  
         [0183]     reads from PE memory  1063  to the MEE feedback buffers  
         [0184]     copies from PE memory to PE memory  
         [0185]     PE memory refresh  
         [0186]     I/O channel transfers  
         [0187]     Loading and Storing from PE Memory to PE Register Files  
         [0188]     The Load and Store instructions transfer the number of bytes indicated between a single memory page and four quadbytes of the register file as follows:  
         [0189]     The memory access begins at the indicated memory byte address (after applying address manipulations, see below) and proceeds for the indicated number of bytes, wrapping from the end of the page (byte  15 ) to the start of the page (byte  0 ).  
         [0190]     The register file access is constrained to four quadbytes of the register file. The access begins at the indicated register and proceeds through four quadbytes, then wraps to byte  0  of the first quadbyte accessed.  
         [0191]     Once the transfer is initiated it executes in one memory cycle.  
         [0192]     Reading from PE Memory to the LEE Feedback Buffers  
         [0193]     All or part of a memory page may be copied to the MEE feedback buffer. The page address can be modified with the Memory Base Register mechanism (see below). Each quadbyte of the page can be copied into any subset of the A, B, or C parts of the MEE feedback buffer, with a feedback buffer push available after each quadbyte.  
         [0194]     Cycle Priorities  
         [0195]     Memory refresh has priority over all other memory operations. The Load/Store versus I/O Channels priority is selected by a status register bit.  
         [0196]     Refresh  
         [0197]     The PE Memory is dynamic and must be refreshed. This may be achieved in software by ensuring all pages are read every refresh period. However, the preferred method is to include a hardware refresh in the architecture.  
         [0198]     Address Manipulations  
         [0199]     The memory addresses used by the load/store controller  1045  can be manipulated with either or both of the following two mechanisms:  
         [0200]     Memory Base Register (MBR)  
         [0201]     The Memory Base Register is optionally added to the page address specified by appropriate instructions, conditioned by a bit in the instruction.  
         [0202]     Each thread has its own MBR in the array controller. Threads load their MBR with a housekeeping instruction. The MBR can be read over the EPU bus.  
         [0203]     Address Indexing  
         [0204]     When an instruction=s Index bit is set, the low five bits of the instruction=s memory quadbyte address are ORed per PE with the low five bits of the PE=s P register.  
         [0205]     Channel Controller  
         [0206]     A detailed description now follows of the channel controller  108 . As mentioned above, the channel controller controls the transfer of data between external memory and PE memory. At each processing block  106 , a transfer engine carries out Direct Memory Access DMA transfers between the block I/O registers and the bus architecture. Depending upon the channel instruction, the data transfers go through a binning unit  1069 , or directly to/from external memory.  
         [0207]     The channel controller  108  operates on an instruction set which is spilt into three fundamental parts:  
         [0208]     Read instructions which transfer data from external memory to PE memory,  
         [0209]     Write instructions which transfer data from PE memory to external memory,  
         [0210]     Housekeeping instructions which manipulate register values within the channels and binning units.  
         [0211]     Instructions from the thread manager  102  are pushed into three separate instruction FIFOs for low priority, high priority, and binner instructions. Each FIFO has its own Afull@ indication which is sent to the thread manager  102 , so that a thread blocked on a full instruction FIFO will not prevent another thread from pushing an instruction into a non-full instruction FIFO.  
         [0212]      FIG. 6  shows an instruction state machine which controls the operation of the channel controller  108 .  
         [0213]     All instructions are launched from the idle state  1081 . The highest priority ready instruction is launched, where the instruction readiness is determined according to preset rules.  
         [0214]     There are three priorities for channel instructions: Addressed and Strided instructions can be specified as low or high priority. Binning instructions are always treated as very high priority. Lower priority instructions may be interrupted or pre-empted by higher priority ones. When a transfer instruction is pre-empted, the contents of the PE page registers are returned to the PE memory pages from which they came. They can then be restarted at a later time when the higher priority instruction has completed.  
         [0215]     Addressed instruction are data transfers between PE memory and external memory where every PE specifies the external memory address of the data it wishes to read or write.  
         [0216]     The data transfer is subject to the consolidation process, so that, for example, four PEs that each write to different bytes of a 32 byte packet address result in a single memory access of 32 bytes, any subset of which may contain valid data to be written to external memory. Also, any number of PEs which wish to read data from the same packet address have their accesses consolidated into a single access to external memory.  
         [0217]     In a Write Addressed instruction, each PE supplies 8 bytes of data together with the external memory address it is to be written to, and 8 bits which serve as byte enables. Any number of PEs which wish to write data to the same packet address have their accesses consolidated into a single access to external memory.  
         [0218]     In a Read Addressed instruction, each PE supplies an address for the data it wishes to read, and sixteen bytes of data (one half of a memory packet) are delivered back to the PE.  
         [0219]     AStrided@ memory accesses are data transfers between PE memory and external memory where the external memory address of each PEs data is generated by the transfer engine. Addresses are stepped from a base register by a predetermined step size, such that the selected PEs send to or receive from spaced external memory addresses. For example, if the step size is set to one, then the selected PEs access consecutive memory addresses. This has the advantage over AAddressed@ transfers in that PEs can use all their I/O page register data, instead of using some of it for address information. The base address for the transfer can be specified with a channel controller instruction or written by the EPU.  
         [0220]     For a Write Strided instruction, each PE outputs 16 bytes of data. Data from two PEs is combined into a 32 byte data packet and written to an external memory address generated by the transfer engine. Consequently packets are written to incrementing addresses. Optionally in the instruction, the external address that each PE=s data was written to can be returned to the PE I/O page registers.  
         [0221]     For potential Read Strided instructions, each PE in turn receives 16 bytes of data from stepped addresses under control of the transfer engine.  
         [0222]     Binning instructions relate to data transfers between PE memory and external memory where the data flows through the binning unit of each core block between the block I/O bus and a system bus to external memory. The binning unit contains a number of control registers that are set with special instructions. It generates external memory addresses for all the data being written to or read from external memory. It contains logic for the support of binning primitives into the regions that they fall in, and for merging multiple bin lists that are held in external memory. It also performs management of bin lists in external memory. Data flow between PEs and the binning unit are buffered in a FIFO.  
         [0223]     Binning Function  
         [0224]     As mentioned above, each processing block  106  has an associated binning unit  1069 , which is attached between the block I/O bus and the system bus  6 . The binning unit provides specific support for the writing and reading of primitive pointers in bin lists in external memory.  
         [0225]     The binning process must maintain primitive order between the geometry and rasterisation phases due to requirements of most host systems. Since both phases are block parallel, there needs to be a mechanism for transferring data between any block to any of the bins and between any bin and any block. This is implemented by creating multiple bin lists per region, one for every processing block  106  that is processing geometry data. This allows the geometry output phase to proceed in block parallel mode. Then, during the rastering phase, each region is processed by just one processing block  106 , and a merge sort of the multiple bin lists in memory for that region is performed.  
         [0226]     The binning unit  1069  only handles pointers. Primitive data itself can be written to memory using normal channel write operations. It can also be read using normal channel read operations once the binner hardware has provided each PE with a primitive pointer.  
         [0227]     A record is kept of how many primitives are written to each bin, so that regions can be sorted into similar size groups for block parallel rasterisation. In addition, primitive Aattribute@ flags are recorded per region. This allows optimisation of rasterisation and shade code per region by examining the bitwise AOR@ of a number of defined flags of every primitive in a region. In this way regions requiring similar processing can be grouped for parallel processing, which results in reduced processing time.  
         [0228]     After the PE array  1061  has computed bounding boxes for primitives, the binner hardware offloads the binitization process from the PE array  1061 , and turns it into a pure I/O operation. This enables it to be overlapped with some further data processing, for example the next batch of processing geometry data.  
         [0229]     Writing—On writing the primitive pointers at the end of a geometry pass, the PEs output the pointers, flags and bounding box information for primitives on the channel. The binning unit  1069  appends the pointer to the bin list of every region included in the bounding box for that primitive. It also updates the primitive count and attribute flags for that region. The binner is responsible for maintaining the bin lists only for its processing block  106 , and the bin list state is preserved across multiple geometry passes.  
         [0230]     Reading—The binning unit  1069  supplies ordered primitive pointers to the processing block  106 , one per PE that requests, for a specific region. It traverses the multiple bin lists for that region, with a merge sort to restore original primitive order. Bin list state is preserved across multiple rasterisation passes.  
         [0231]     Binning Memory Organisation  
         [0232]     The bin lists are created in external memory, by outputting list data to memory. The bin lists indicate the locations of the contents of the bin within memory. Maintenance of such linked list structures requires additional storage in the form of pointer arrays. The binner hardware accesses these structures in memory directly.  
         [0233]     Binning Hardware  
         [0234]     The binning hardware is shown in detail in  FIG. 7 , and is responsible for handling the computation involved in the binnitization process needed to enable the PE array  1061  to read and write primitive pointers to external memory.  
         [0235]     Instruction decoder  1101  receives instructions from the channel controller  108 , and triggers the state machine  1102  into operation. The state machine  1102  is the logic that sequences the other parts of the binning unit to perform a particular function such as reading or writing primitive pointers to or from external memory. The state machine  1102  may be implemented as several communicating state machines. Control signals to all other parts of the binning unit are not shown.  
         [0236]     The binnitization function is executed by the binning unit according to a set of internal registers  1103  that define the current binning context, that is the location of bin lists in external memory, the region to be rasterised next, the operation mode and so on. This set of Astate@ registers  1103  is multiple ported to the channel controller  108 , the block I/O bus and the EPU  8  (ie. the registers have a number of ports that can be used simultaneously).  
         [0237]     Between the block I/O bus and the binning unit  1069  itself there is a data buffer FIFO  1104 , which is regarded as being part of the binning unit  1069 . The purpose of the data buffer  1104  is to buffer data flowing between the PE I/O page registers and the binning unit  1069 , to smooth out the indeterminate timing of the binning unit  1069 . Data is transferred to/from the binning unit  1069  in bursts of size that depends on the buffer depth. The binning unit  1069  presents the status of this buffer to the rest of the block control logic, and by looking at the status of all the binning unit buffers  1104 , the channel controller  108  can schedule data transfer bursts to the binning units  1068  in an efficient way.  
         [0238]     The binning unit  1069  of each block has its own register set interface  1105  to the EPU  8 . The EPU  8  performs the following set of binning unit  1069  tasks via the interface  1105 :  
         [0239]     Initialisation  
         [0240]     Allocation of bin list memory  
         [0241]     Save and restore of binning state on context switch  
         [0242]     When the binning unit  1069  is executing a Write Binner instruction, it needs an unknown amount of memory to be allocated for the creation of bin lists. It requests this memory a portion at a time from the EPU  8 , and assigns it to whichever bin lists require it. The binner unit  1068  assigns small chunks (portions) of 32 bytes to bin lists, but this would load the EPU intolerably if it were to be allocated at this level. Instead, the EPU provides large portions of data of whatever size it decides is appropriate (for example, 64 kBytes, but any convenient multiple of 32 bytes) and the binner unit  1068  divides this up into individual chunks, using the chunk generator  1106 . The transfer of large amounts of data from the EPU is more efficient for the EPU, and the processing of small amounts of data for the binning unit  1069  is more efficient for the binning unit  1069 .  
         [0243]     During pointer writing, primitive data from PEs is lodged in a register set  1107 , and passed to the data logic  1112  as required.  
         [0244]     A Y stepper  1108  is used to step the y axis region co-ordinate across the primitive bounding box during pointer writing as part of the binitization process. It comprises a counter and register pair with an equality comparator.  
         [0245]     A X stepper  1109  is used to step the X axis region coordinate across the primitive bounding box during pointer writing as part of the binitization process. It also comprises a counter and register pair with an equality comparator. However, since the X stepper must also run the same sequence of values for every value of the Y stepper  1108 , the counter is loaded and reloaded from an extra register that contains the initial value.  
         [0246]     To merge block bin lists for a region during the pointer read process, there is provided a dedicated hardware section  1110 . So that primitives can be ordered through the binning process, a batch id code is added to the bin lists. The batch id code relates to the geometry ordering, since host requires geometry to be returned in the correct order. Under control of the state machine  1102 , and aided by a block counter  1117 , the binning unit  1069  evaluates which bin list has the lowest batch ID and directs pointer reading from that list.  
         [0247]     When a further batch ID is encountered in that list, or a NULL terminator encountered, the block selection is re-evaluated. The block counter  1117  provides a loop counter for the state machine  1102  when it is evaluating the next bin list to process (in conjunction with the bin list selection unit  1110 ).  
         [0248]     The Data logic unit  1112  is the data processing block of the binning unit  1069 . It is able to increment pointers, merge attribute flags and format different data types for writing to external memory via the data cache  1115 .  
         [0249]     A region number unit  1116  computes a linear region number from the X and Y region coordinates outputted from the X/Y steppers  1108 / 1109 . This number, together with the output of the data logic unit  1112  and state registers  1103 , are used by an address compute unit  1113 , to compute a memory address for bin list array entries.  
         [0250]     The data cache  1115  is provided for decoupling all memory references from the external memory bus. It exploits the address coherence of the binning unit memory accesses to reduce the external memory bandwidth, and to reduce the stall time that would be cased by waiting for data to arrive.  
         [0251]     The data cache  1115  has an address tag section  1114 . This indicates to the binning unit  1069  whether any particular external memory access is a hit or a miss in the data cache. On miss, the binning unit  1069  is stalled until the required data packet is fetched from memory.  
         [0252]     Processing Elements  
         [0253]      FIG. 11  shows a processor unit  1061   a  and PE register file  1061   b  which form part of the processing element shown in  FIGS. 3 and 8 . The PE  1061  includes an arithmetic logic unit (alu)  214  which is connected to receive data values from a block of 8 bit registers  202 ,  204 ,  206 ,  208  (designated R, S, V and P) via multiplexers  210  and  212  (A and B).  
         [0254]     The PE register file  1061   b  which operates to buffer data between the PE and its associated PE memory, and to store temporarily data on which the processor unit  1061   a  is processing.  
         [0255]     The RSVP registers  202 ,  204 ,  206 ,  208  operate to supply operands to the alu  214 . The A multiplexer  210  receives data values from the R and S registers and so controls which of those register values is supplied to the alu  214 . The B multiplexer  212  is connected to receive data values from the V and P registers and also from the MEE  1062 , and so controls which of those values is to be supplied to the alu.  
         [0256]     The processor unit  1061   a  further includes a shifter  200  which can perform a left or right shift on the data output from the S, V and P registers.  
         [0257]     The R register can hold its previous value, or can be loaded with a byte from the register file, or the result from the alu. The alu result is 10 bits wide, and so the R register can receive the first 8 bits (bits  7  to  0 ) or bits  9  to  2 , for a Booth multiply step. Booth multiplication is a well known way of providing multiplication results in one clock cycle.  
         [0258]     The S register can hold its previous value, or can be loaded with a shifted version of its previous value. The S register can also be loaded with the alu result, a bit from the register file or the low 2 bits from the alu concatenated with the high 6 bits of the S registers previous value (for the Booth multiply step).  
         [0259]     The V and P registers can both be loaded with the alu result, or a byte from the register file. The Isb of the V register is used to determine the set of processor elements which are participating in MEE feedback transfer. The five low bits of the P register are used to modify the memory address in memory accesses.  
         [0260]     Using four registers R, S, V and P provides the system with improved performance over previously known systems because any of the registers are able to provide data to the alu  214 . In addition, any of the registers can be loaded with data from the PE register file  1061  b, which improves the generality of the system, and provides better support for floating point operations. Since the R register input is never shifted, the R register can be used to store and modify the exponent of floating point numbers.  
         [0261]     The alu  214  receives instructions from the array controller (not shown) and supplies its output to the PE register file  1061   b.  The PE register file  1061   b  is used to store data for immediate use by the PE, for example, the register file  1061   b  can store 16 words of 16 bits in length.  
         [0262]     Data to be written to the register file is transferred via a write port, and data to be read from the register file is transferred via a read port. Data is transferred to and from the register file from the PE memory via a load/store port under the control of the load/store controller.  
         [0263]     The PE register file  1061   b  can receive data to be stored through its write port in a number of ways: a 16 bit value can be received from the processor element which form the element=s left or right neighbour, a 16 bit value can be received from a status/enable register, or an 8 bit value can be received from the alu result. In the case that the alu result is supplied to the register file, the 8 bit value is copied into both the high and low bytes of the register file entry concerned.  
         [0264]     The write port is controlled on the basis of the source of data, and is usually controlled by way of the contents of the enable stack. It is possible to force a register file write regardless of the enable stack contents.  
         [0265]     The processor unit  1061   a  also includes an enable stack which is used to determine when the alu  214  can process data. The enable stack provides 8 enable bits which indicate if the alu can operate on the data supplied to it. In a preferred example, the alu  214  will only operate if all 8 bits are set to logical 1. A stack of enable bits is particularly useful when the alu is to perform nested conditional instructions. Such nested instructions tend to occur most often in IF, ELSE, ENDIF instruction sequences.  
         [0266]     By providing an enable stack of multiple bits in hardware, it is possible to remove the need for software to save and load the contents of a single enable bit when the alu is processing a nested instruction sequences.  
         [0267]     The read and write ports of the PE register file  1061   b  enable a 16 bit data word to be copied to the PE register file of at least one of the neighbouring PEs.  
         [0268]     The load and store operations can be issued in parallel with microcoded alu instructions from the array controller. The PE register file  1061   b  provides several performance advantages over previous systems in which the alu has directly accessed a memory device. The PE register file  1061   b  provides faster access to frequently used data values than a processor element to memory or memory to memory architecture can provide. In addition, there are no restrictions on the order in which data values are ordered in the register file, which further aids speed of processing and programming flexibility.  
         [0269]      FIG. 12  is a block diagram illustrating a processing element, and data input and output lines to that element. As previously described, the processing element includes a processor unit  1061   a,  a PE register file  1061   b,  and a PE memory unit  1061   c.  The memory unit  1061   c  is preferably DRAM which is able to store 128 pages of 16 bytes. Alternatively, other memory configurations could be used for the PE memory unit. Data items can be transferred between the PE register file  1061   b  and the PE memory unit  1061   c  by way of memory read data and memory write data lines  1078  and  1079 .  
         [0270]     In addition, data can be transferred out of the processor element, and indeed out of the processor block in which the element is situated, by way of a block I/O data out bus  1067   d,  and can be transferred into the processor block by way of a block I/O data in bus  1067   c.  Address transaction ID and data transaction ID information can be transferred to the processor block by way of busses  1067   a  and  1067   b.  The MEE feedback data is transferred from the PE memory unit  1061   c  or the PE register file  1061   b  to the MEE feedback buffer (not shown) by way of a MEE feedback data out bus  1064 .  
         [0271]      FIG. 13  shows the block I/O interface in more detail. PE memory read and write data buses  1078  and  1079  interface with a block I/O register file  1071  for transferring data between the register and the processing unit and the memory unit. Data to be read out from the processing element is output from the block I/O register file  1071  onto the block I/O data out bus  1067   c,  and data to be read into the processing element concerned is input to the block I/O register file  1071  from the block I/O in bus  1067   d.    
         [0272]     The processing elements that require access to memory indicate that this is the case by setting an indication flag or mark bit. The first such marked PE is then selected, and the memory address to which it requires access is transmitted to all of the processing elements of the processing block. The address is transmitted with a corresponding transaction ID. Those processing elements which require access (ie. have the indication flag set) compare the transmitted address with the address to which they require access, and if the comparison indicates that the same address is to be accessed, those processing elements register the transaction ID for that memory access and clear the indication flag.  
         [0273]     All those PEs requiring access to memory (including the selected PE) then compare the required address with the address transmitted on the block I/O inbus  1067   d,  by way of an address compare unit  1073 . If the result of the address compare demonstrates that the selected address is required for use, then the byte mask is unset and the transaction ID for the memory access concerned is stored in a transaction ID register  1075 . The address transaction ID is supplied on the address transaction ID bus  1067   a.  Later, the required data carrying the same transaction ID returned along the block I/O data inbus  1067   d.  Simultaneously, or just before the data is returned, the transaction ID is returned along the data transaction ID bus  1067   b  all of the processor elements compare the returned data transaction ID with transaction ID stored in the transaction ID register  1075  by means of comparator  1076 . If the comparison indicates that the returned transaction ID is equivalent to the stored transaction ID, the data arriving on the block I/O data inbus  1067   d  is input into the PE register file  1061   b.  When the transaction ID is returned to the processing block, the processing elements compare the stored transaction ID with the incoming transaction ID, in order to recover the data.  
         [0274]     Using transaction IDs in place of simply storing the accessed address information enables multiple memory accesses to be carried, and then returned in any order.  
         [0275]     Booth multiplication is achieved using the B multiplexer  212 , which is shown in more detail in  FIG. 14 . The B multiplexer  212  receives inputs  230  from the V and P registers and from the MEE  1602 . The B multiplexer  212  includes a Booth recode table  218  and a shift and complement unit  220 . The Booth recode table  218  receives inputs  224 ,  226  from the two least significant bits of the S register and from a Booth register (S reg and Boothreg). Booth recoding is based on these inputs and the Booth recode table transforms these bits into shift, transport and invert control bits which are fed to the shift and complement unit  220 . The shift and complement unit  220  applies shift, transport and invert operations to the contents of the V register. The shift operation shifts the V register one bit to the left, shifting in a 0, and the transport and invert bits cause the possibly shifted result to be transported, inverted or zeroed or a combination of those.  
         [0276]      FIG. 15  shows a block diagram of the alu  214  of the processor element shown in  FIG. 13 . The alu  214  receives 10 bit inputs  234  from the A and B multiplexers  210  and  212 , and also receives inputs  244  and  246  from the BoothCarryin and CarryReg registers. The alu  214  also receives instructions from the controller. The alu  214  includes a carry propagate unit  236 , a carry generate unit  238  and a carry select unit  242 . The alu also includes an exclusive OR (XOR) gate  250  for determining the alu result output. A CarryChain unit  240  receives inputs from Carry propagate unit  236  and the carry generate unit  238 , and outputs a result to the XOR gate  250 .  
         [0277]     The various units in the alu  214  operate to carry out instructions issued by the controller.