Abstract:
Systems and methods are disclosed to transfer data between a first bus internal to a system-on-chip (SOC) device and a second bus external to the SOC device, each bus having a plurality of bus segments shared among a plurality of peripheral devices communicating over one or more bus segments. When reading data from a peripheral device, the system packs data by enabling each effected first bus data segment in sequence until requested data is packed; and when writing data to a peripheral device, the system unpacks data by enabling each effected second bus data segment in sequence until requested data is unpacked.

Description:
COPYRIGHT NOTICE 
     A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever. 
     BACKGROUND 
     The present invention relates to data transfer. 
     Wireless data services now enable a new generation of high-performance, low-power-consumption mobile devices to access network-centric applications and content anywhere, anytime. Handheld devices include personal digital assistants (PDAs), email companions, and other data-centric mobile products such as Palm OS, Symbian, and Pocket PC products. The main functionality of such devices has been for personal information manager (PIM) applications. But as more of these devices get network connectivity options, applications such as voice and email are becoming important. Additionally, next-generation mobile phones are hybrid devices that extend the voice-centric nature of current generation (2 G) handsets. These devices are connected to packet-based networks, which deliver data-services in addition to voice-services. Handsets connected to 2.5 G networks such as GPRS and PHS allow always-on data network connection. This enables further proliferation of multimedia- and graphics-based applications in the consumer segment of this market. 3 G Handsets have been designed from the ground up to interface to high-speed, packet-based networks that deliver speeds from 20 Kbps to 2 Mbps. These handsets, in addition to the features of 2.5 G phones, have the capability to support 2-way video, share pictures and video clips, use location-based information, provide a rich web experience and support next-generation server-based applications for business like always-on email. 
     As mobile applications become richer and more complex, the ability to optimally process multimedia becomes a necessity on mobile devices such as PDAs and smart-phones. Applications such as video mail, mapping services, reading PDF files, and graphics-rich games all require high performance graphics and multimedia capabilities. These capabilities enable new applications that benefit from rich images and system performance in ways that were previously unavailable to most handheld users. These mobile devices face the challenge of providing a compelling user experience while reducing overall system energy consumption and cost. 
     To reduce cost, system-on-chip (SOC) solutions have appeared. The SOC solutions integrate various circuits such as a memory controller, a hard disk controller, a graphics/video controller, a communications controller, and other peripheral controllers such as serial and USB onto a single device. A clock signal is used to synchronize data transfers between circuits. The circuits also communicate over a central bus. Processing performance is influenced in part by the width of a data bus that transfers data between components within the SOC device and external devices such as memory. A data width is typified by, for example, 8 bits, 16 bits, 32 bits, 64-bits and 128 bits, which are a power of 2. If a large data bus width is adopted, data transfer capacity is increased for memory intensive applications. However, a large data bus width increases the number of wiring conductors for physically connecting the data bus among the SOC devices and the overall size of the system is inevitably increased. Additionally, many peripherals such as serial ports and USB ports do not need high data transfer rate and typically communicate over 8-bit or 16-bit buses. 
     Since each SOC device has a number of components that must communicate with each other, a system for packing and unpacking data from components or peripherals with varying bus widths is needed. Alignment is important for functional reasons because an unaligned data access may cause a bus error resulting in a system crash. Alignment is also important for performance reasons because unaligned data access, which can be handled with hardware or software alignment correction tools, will likely become more expensive as processor speeds continue to increase. 
     Data stored in memory or disk is typically heterogeneous, in the sense that it consists of elements with varying alignment requirements. The storage space allocated for the data, in the absence of alignment requirements, can be optimized by packing the elements one after another. Data packing and unpacking are frequently used procedures when there are transfers between devices with different data bus widths. However, imposing alignment requirements on the data elements may force the introduction of padding to fill holes in storage caused by the alignment requirements. This padding may increase the amount of storage required to store the data elements. The amount of storage required to store the data elements may depend on the order in which the data elements are arranged in storage. This is because the padding necessary to accommodate the data alignment requirements may be different depending on the order that the data elements are stored. 
     SUMMARY 
     Systems and methods are disclosed to perform data packing and unpacking. A power efficient approach minimizing overall cost is used for data packing and unpacking in the read and write path of the external bus interface module of a system-on chip solution. The external bus is shared among different type of memories/devices with different data bus width. The system utilizes gated clocks for the packing and unpacking of data. 
     The system conserves power by driving only the effected data bus segment of the external shared data bus in case of write accesses. It conserves the power by enabling the relative segment of the data bus, and holding it to pack the captured data up-to 32 bits. The write out data bus is divided into four-data segments; each is 8 bit. The external memories are sharing the same bus (sharing all or segments of it). The supported external bus width is 32 bit. The memories less than 32 bit data bus width can be driving upper or lower bits of the shared bus (specified by register programming or boot mode option). The supported bus sizes for the memories 8, 16, or 32 bits. The unpacking logic controls the generation of the gated clock enables by taking the width, location, and the least significant bits of the address for the targeted external memory, also by taking into account the size of the requested signal. Only the desired data segments are driven by enabling the related clock (each clock controls flip-flops, for example synchronous D flip-flops). Most of the internal requests are 32 bit requests. When 32 bit read accesses are requested from the memories with 8/16 bit, the data packing logic enables the effected internal data segment; this is done in a sequence until the requested data is packed and ready to be latched by the originator. 
     The advantages of the approach can be summarized as following. The system eliminates the data hold multiplexing (hereinafter muxing) logic for data packing. When data is captured 8 bit or 16 bit at a time in a sequence to form 32 bit internal data, some muxing logic is needed to hold the previously captured data segment(s). Instead of using muxes and flip-flops, flip-flops triggered by gated clocks are used. The clocks are enabled in sequence according to the least significant bits of the start address and the data bus size of the target memory. 
     Other advantages include performing packing and unpacking operations in real time while requiring minimal hardware resources. The system provides compatibility and wiring space minimization and allows the re-use of peripheral cores in their original bus widths. The system also supports efficient data transfer among diverse peripherals with different bus widths. When data having different bit widths (for example, 8/16/32 bits) is transferred over a wide bus (such as a 32-bit width, for example), wasteful power consumption is minimized for circuits supporting the unused portion of the wide data bus. Other advantages include a compact implementation and the sharing of many operations using the same circuitry to allow space reduction while maintaining a highly efficient algorithm. A power efficient implementation is achieved. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       In order that the manner in which the above-recited and other advantages and features of the invention are obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof, which are illustrated, in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
         FIG. 1  shows one implementation of a data packing and unpacking device. 
         FIG. 2  shows an exemplary read packing unit. 
         FIG. 3  shows an exemplary write unpacking unit. 
         FIG. 4  shows a computer system used with the data packing and unpacking device. 
     
    
    
     DESCRIPTION 
     In the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be obvious to one skilled in the art that the present invention may be practiced without these specific details. In other instances well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the present invention. While the following detailed description of the present invention describes its application in the area involving a graphics/display controller, it is to be appreciated that the present invention is also applicable to any application involving multiple data paths such as communications, core logic, central processing units (CPU), and others. 
     Referring now to the drawings in greater detail, there is illustrated therein structure diagrams for processes that a system will utilize to pack and unpack data accesses, as will be more readily understood from a study of the diagrams. 
     Referring now made to  FIG. 1 , a block diagram illustrating a 32-bit data packing/unpacking engine is shown. In the embodiment of  FIG. 1 , a four byte-wide packing unit  12  communicates data from a bus A to a bus B. Correspondingly, a four byte-wide unpacking unit  14  handles data transfers from the bus B to the bus A. The packing unit  12  receives data generated by any byte-wide peripheral connected to any arbitrary byte  0 - 3  on the bus A and transfers the byte to any arbitrary byte on the bus B. The packing unit  12  also receives data generated by any short word peripheral connected to any contiguous two bytes on the bus A and transfers the short-word to any arbitrary contiguous two bytes on the bus B. 
     Correspondingly, the unpacking unit  14  transfers any arbitrary byte  0 - 3  from the bus B to any arbitrary position on the bus A. For short-word transfers, the unpacking unit  14  transfers any arbitrary contiguous pairs of bytes from the bus B to any arbitrarily selected pair of bytes on the bus A. 
     Referring now to  FIG. 2 , an exemplary read packing unit  12  is shown. Four byte-wide flip-flops have their outputs connected to bytes  0 -byte  3  of the internal bus B, respectively. Each flip-flop  24 ,  28 ,  32  or  36  is individually clocked. A plurality of multiplexers  22 ,  26 ,  30  and  34  are connected to bytes  0 - 3  of the bus A and adapted to move data from any byte of bus A to any arbitrary byte on the bus B. The multiplexer  22  drives the input of a byte-wide flip-flop  24 . The output of the flip-flop  24  drives byte  0  of the internal bus B. Correspondingly, the multiplexer  26  drives the input of a byte-wide flip-flop  28 . The output of the flip-flop  28  drives byte  1  of the internal bus B. The multiplexer  30  drives the input of a byte-wide flip-flop  32 . The output of the flip-flop  32  drives byte  2  of the internal bus B. Additionally, the multiplexer  34  drives the input of a byte-wide flip-flop  36 , whose output drives byte  3  of the internal bus B. Each of the flip-flops  24 ,  28 ,  32  and  36  is clocked by a clock signal BYTE 0 CLK, BYTE 1 CLK, BYTE 2 CLK, and BYTE 3 CLK, respectively. 
     In  FIG. 2 , an 8-bit device on the external bus A can communicate over any byte of bus A. Thus, it can communicate data over byte  0  (bits  7 : 0 ), byte  1  (bits  15 : 8 ), byte  2  (bits  23 : 16 ) or byte  3  (bits  31 : 24 ). Correspondingly, a 16-bit device connected to the external bus A can communicate over any 16-bit word of bus A (bits  15 : 0 ,  23 : 7  or  31 : 16 ). 
     The read packing unit  12  is highly flexible in that it can place the output of any byte-wide external device to any byte on the internal bus B. Also, the read packing unit  12  can place the output of any short word-wide external device onto any two consecutive bytes on the internal bus B. 
     Next, exemplary operations of the circuit of  FIG. 2  are discussed: 
     8-Bit Peripheral Connected to the First Byte (Bits  7 : 0 ) of the External Bus A. 
     In case 1, an internal requester such as a processor  101  ( FIG. 4 ) or a DMA controller  108  ( FIG. 3 ) ( FIG. 4 ) connected to the internal bus B can read 32-bits of data. In this case, the packing unit  12  selects the first byte (bits  7 : 0 ) of the external bus A and enables the byte clocks BYTE 0 CLK, BYTE 1 CLK, BYTE 2 CLK, and BYTE 3 CLK, one at a time, until four bytes of data are packed into a single 32-bit word for reading by the internal requester. 
     In case 2, the internal requester needs 16-bits of data. The packing unit  12  selects the first byte (bits  7 : 0 ) of the external bus A and enables the byte clocks BYTE 0 CLK, BYTE 1 CLK sequentially until two bytes of data are packed into a 16-bit word for reading by the internal requester. 
     In case 3, the internal requester needs 8-bits of data. The packing unit  12  selects the first byte (bits  7 : 0 ) of the external bus A and enables the byte clock BYTE 0 CLK whenever the data from the external peripheral connected on the external bus A is ready to transfer data. 
     In case 4, the internal requester needs 16-bits of data. The packing unit  12  selects the first byte (bits  7 : 0 ) of the external bus A and enables the byte clocks BYTE 2 CLK, BYTE 3 CLK sequentially until two bytes of data are packed into a 16-bit word for reading by the internal requester. 
     In case 5, the internal requester needs 8-bits of data. The packing unit  12  selects the first byte (bits  7 : 0 ) of the external bus A and enables the byte clock BYTE 1 CLK whenever the data from the external peripheral connected on the external bus A is ready to transfer data. 
     In case 6, the internal requester needs 8-bits of data. The packing unit  12  selects the first byte (bits  7 : 0 ) of the external bus A and enables the byte clock BYTE 2 CLK whenever the data from the external peripheral connected on the external bus A is ready to transfer data. 
     In case 6, the internal requester needs 8-bits of data. The packing unit  12  selects the first byte (bits  7 : 0 ) of the external bus A and enables the byte clock BYTE 3 CLK whenever the data from the external peripheral connected on the external bus A is ready to transfer data. 
     8-Bit Peripheral Connected to the Second Byte (Bits  15 : 8 ) of the External Bus A. 
     In this case, the internal requester on the internal bus B can read 32-bits of data. In this case, the packing unit  12  selects the second byte (bits  15 : 8 ) of the external bus A and enables the byte clocks BYTE 0 CLK, BYTE 1 CLK, BYTE 2 CLK, and BYTE 3 CLK, one at a time, until four bytes of data are packed into a single 32-bit word for reading by the internal requester. 
     In the case where the internal requester can handle 16-bits of data, the packing unit  12  selects the second byte (bits  15 : 8 ) of the external bus A and enables the byte clocks BYTE 1 CLK, BYTE 2 CLK sequentially until two bytes of data are packed into a 16-bit word for reading by the internal requester. 
     In the case where the internal requester can handle 16-bits of data, the packing unit  12  selects the second byte (bits  15 : 8 ) of the external bus A and enables the byte clocks BYTE 2 CLK, BYTE 3 CLK sequentially until two bytes of data are packed into a 16-bit word for reading by the internal requester. 
     In the next case where the internal requester needs 8-bits of data, the packing unit  12  selects the second byte (bits  15 : 8 ) of the external bus A and enables the byte clock BYTE 1 CLK whenever the data from the external peripheral connected on the external bus A is ready to transfer data. 
     In the next case where the internal requester needs 8-bits of data, the packing unit  12  selects the second byte (bits  15 : 8 ) of the external bus A and enables the byte clock BYTE 2 CLK whenever the data from the external peripheral connected on the external bus A is ready to transfer data. 
     In the next case where the internal requester needs 8-bits of data, the packing unit  12  selects the second byte (bits  15 : 8 ) of the external bus A and enables the byte clock BYTE 3 CLK whenever the data from the external peripheral connected on the external bus A is ready to transfer data. 
     Turning now to  FIG. 3 , an exemplary unpacking unit  14  is shown. Four byte-wide multiplexers  42 ,  46 ,  50  and  54  have their outputs connected to bytes  0 -byte  3  of the internal bus B, respectively. The multiplexers  42 ,  46 ,  50  and  54  are adapted to move data from any byte of bus B to the input of byte-wide flip-flops  44 ,  48 ,  52  and  56 , respectively. Each flip-flop  44 ,  48 ,  52  or  56  is individually clocked. The multiplexer  42  drives the input of a byte-wide flip-flop  44 . The output of the flip-flop  44  drives byte  0  of the external bus A. Correspondingly, the multiplexer  46  drives the input of a byte-wide flip-flop  48 . The output of the flip-flop  48  drives byte  1  of the external bus A. The multiplexer  50  drives the input of a byte-wide flip-flop  52 . The output of the flip-flop  52  drives byte  2  of the external bus A. Additionally, the multiplexer  54  drives the input of a byte-wide flip-flop  56 , whose output drives byte  3  of the external bus A. Each of the flip-flops  44 ,  48 ,  52  and  56  is clocked by a clock signal BYTE 0 WRCLK, BYTE 1 WRCLK, BYTE 2 WRCLK, and BYTE 3 WRCLK, respectively. 
     In the embodiment of  FIG. 3 , an internal data generator such as the processor  101  or the DMA engine  108  can write to an 8-bit device that is arbitrarily connected to any byte  0  . . .  3  of the external bus A. Alternatively, the internal data generator can write to any 16-bit device that is connected to two consecutive bytes of the external bus A. 
     An exemplary process for correspondingly unpacking data from bus B to bus A is discussed next. In this process, a 32-bit word entry is read from external memory. When the output from bus B is valid, the processor  101  ( FIG. 4 ) enables one or more of the multiplexers  42 ,  46 ,  50  and  54  to appropriately route the particular byte from bus B onto the appropriate position on bus A. Next, exemplary operations of the circuit of  FIG. 3  are discussed: 
     8-Bit Peripheral Connected to the First Byte (Bits  7 : 0 ) of the External Bus A. 
     In one case, an internal data generator such as the processor  101  or the DMA controller  108  connected to the internal bus B can write 32-bits of data. In this case, the unpacking unit  14  selects the first byte (bits  7 : 0 ) of the external bus A and enables the write clock BYTE 0 WRCLK, selects the second byte (bits  15 : 8 ) of the external bus A and enables the write clock BYTE 0 WRCLK, selects the third byte (bits  23 : 16 ) of the external bus A and enables the write clock BYTE 0 WRCLK, and selects the fourth byte (bits  31 : 24 ) of the external bus A and enables the write clock BYTE 0 WRCLK, one at a time, until four bytes of data are unpacked into a single byte for transmission to the external 8-bit external device by the internal data generator. 
     In another case, the internal data generator writes 16-bits of data. The unpacking unit  14  selects the first byte (bits  7 : 0 ) of the external bus A and enables the byte clocks BYTE 0 WRCLK. Next, the unpacking unit  14  selects the second byte (bits  15 : 8 ) of the external bus A and enables BYTE 0 WRCLK such that two bytes of data are unpacked into a byte for transmission to the 8-bit external device by the internal data generator. 
     8-Bit Peripheral Connected to the Third Byte (Bits  23 : 16 ) of the External Bus A. 
     In this case, an internal data generator such as the processor  101  or the DMA controller  108  connected to the internal bus B can write 32-bits of data. In this case, the unpacking unit  14  selects the first byte (bits  7 : 0 ) of the external bus A and enables the write clock BYTE 2 WRCLK, selects the second byte (bits  15 : 8 ) of the external bus A and enables the write clock BYTE 2 WRCLK, selects the third byte (bits  23 : 16 ) of the external bus A and enables the write clock BYTE 2 WRCLK, and selects the fourth byte (bits  31 : 24 ) of the external bus A and enables the write clock BYTE 2 WRCLK, one at a time, until four bytes of data are unpacked into a single byte for transmission to the 8-bit external device by the internal data generator. 
     16-Bit External Device Connected to the Bits  31 : 16  of the External Bus A. 
     The unpacking unit  14  selects the first byte (bits  7 : 0 ) of the external bus A and enables the byte clock BYTE 2 WRCLK. At the same time, the unpacking unit  14  also selects the second byte (bits  15 : 8 ) of the external bus A and enables BYTE 3 WRCLK such that four bytes of data are unpacked into a short word for transmission to the external 16-bit peripheral device. To complete the rest of the 32-bit unpack, the unpacking unit  14  selects the third byte (bits  23 : 16 ) and enables BYTE 2 WRITECLK. Additionally (or at the same time), the unpacking unit  14  also selects the fourth byte (bits  31 : 24 ) and enables BYTE 3 WRITECLK. 
     Reference is now made to  FIG. 4  which illustrates, for example, a high-level diagram of computer system  100  upon which the present invention may be implemented or practiced. More particularly, computer system  100  may be a laptop or hand-held computer system. It is to be appreciated that computer system  100  is exemplary only and that the present invention can operate within a number of different computer systems including desk-top computer systems, general purpose computer systems, embedded computer systems, and others. 
     In  FIG. 4 , computer system  100  is a highly integrated system which includes of integrated processor circuit  101 , peripheral controller  102 , read-only-memory (ROM)  103 , and random access memory (RAM)  104 . The highly integrated architecture allows power to be conserved. Computer system architecture  100  may also include a peripheral controller if there is a need to interface with complex and/or high pin-count peripherals that are not provided in integrated processor circuit  101 . 
     While peripheral controller  102  is connected to integrated processor circuit  101  on one end, ROM  103  and RAM  104  are connected to integrated processor circuit  101  on the other end. Integrated processor circuit  101  comprises a processing unit  105 , memory interface  106 , graphics/display controller  107 , direct memory access (DMA) controller  108 , and core logic functions including encoder/decoder (CODEC) interface  109 , parallel interface  110 , serial interface  1131 , and input device interface  112 . Processing unit  105  integrates a central processing unit (CPU), a memory management unit (MMU), together with instruction/data caches. 
     CODEC interface  109  provides the interface for an audio source and/or modem to connect to integrated processor circuit  101 . Parallel interface  110  allows parallel input/output (I/O) devices such as hard disks, printers, etc. to connect to integrated processor circuit  101 . Serial interface  111  provides the interface for serial I/O devices such as universal asynchronous receiver transmitter (UART) to connect to integrated processor circuit  101 . Input device interface  112  provides the interface for input devices such as keyboard, mouse, and touch pad to connect to integrated processor circuit  101 . 
     DMA controller  108  accesses data stored in RAM  104  via memory interface  106  and provides the data to peripheral devices connected to CODEC interface  109 , parallel interface  110 , serial interface  111 , or input device interface  112 . The memory interface  106  provides the unpacking/packing functions of  FIG. 1 . 
     Graphics/display controller  107  requests and accesses the video/graphics data from RAM  104  via memory interface  106 . Graphics/display controller  107  then processes the data, formats the processed data, and sends the formatted data to a display device such as a liquid crystal display (LCD), a cathode ray tube (CRT), or a television (TV) monitor. In computer system  100 , a single memory bus is used to connect integrated processor circuit  101  to ROM  103  and RAM  104 . 
     In one embodiment, digital system  100  includes an ECC processor (EP)  124  that communicates with DMA controller  108  and memory interface  106 . In another embodiment, DMA controller  108  is part of a NAND-flash controller that further includes the ECC processor. DMA controller  108  moves data from a peripheral device, such as a flash memory card, directly to system memory without requiring the involvement of CPU  105 . The DMA controller  108  allows the system to continue processing other tasks while new data is being retrieved. ECC processor  124  performs ECC related operations to compensate for errors caused by defects and to maintain data integrity. ECC processor  124  also provides status information to Error Correction software which may be stored in ROM  103  and executed by CPU  105  to facilitate error correction and also provides an appropriate indication of the existence of errors. 
     Memory interface  106  is fed by and electrically connected to DMA controller  108  and ECC processor  124 . Memory interface  106  drives a communications bus that feeds RAM  104  that can include DRAM  132 . A NAND-Flash Controller (NFC)  123  drives a NAND-Flash (NF) memory  130 . Memory interface  106  performs the standard interface functions, such as code conversion, protocol conversion, and buffering, required for communications to and from a peripheral. Memory interface  106  allows a number of independent devices with varying protocols to communicate with each other. NF  130  is representative of any well-known NAND-flash memory, which is an electrically erasable, non-volatile memory device that retains its data even after the power is removed. NAND-flash memory devices are well-suited for cellular phones, digital music players, hand-held computers, digital cameras, camcorders, and digital voice recorders, where performance is critical. 
     The above system can perform the real-time image capture/compression/display process within a hand-held device such as a PDA or a cellular phone that takes advantage of the data packing and unpacking operations. In this case, a liquid crystal display (LCD) can have a 16-bit interface, the processor can have a 32-bit that interfaces with application-specific integrated circuit (ASIC), and a video camera that also interfaces with the ASIC over an 8-bit bus, for example. 
     The video camera can be a charge coupled device (CCD) which captures images associated with the pictures. The analog information can be encoded by the transmitter in analog form and transmitted. Alternatively, the transmission can be digital where a suitable analog to digital converter (ADC) receives and digitally converts the analog video information from the CCD. Suitable actuators can be provided to physically control camera settings. For example, a lens opening control unit can be provided to adjust light levels to be received by the CCD. Further, a lens focusing unit can be used to automatically focus the images, based on information provided by one of the sensors. Further, the lens may be automatically switched with additional lens to provide different views. Additionally, the lens have one or optional filters to filter lights coming to the lens. 
     The above operations are controlled by a processor or an application specific integrated circuit (ASIC). In one embodiment, a processor is embedded and the processor can be a reduced instruction set computer (RISC) processor or a complex instruction set computer (CISC) processor. In one embodiment, the processor is a low power CPU such as the MC68328V DragonBall device available from Motorola Inc. The processor is connected to a read-only-memory (ROM) for receiving executable instructions as well as certain predefined data and variables. The processor is also connected to a random access memory (RAM) for storing various run-time variables and data arrays, among others. The RAM size is sufficient to store user application programs and data. In this instance, the RAM can be provided with a back-up battery to prevent the loss of data even when the computer system is turned off. However, it is generally desirable to have some type of long term storage such as a commercially available miniature hard disk drive, or non-volatile memory such as a programmable ROM such as an electrically erasable programmable ROM, a flash ROM memory in addition to the ROM for data back-up purposes. 
     It is to be understood that various terms employed in the description herein are interchangeable. Accordingly, the above description of the invention is illustrative and not limiting. Further modifications will be apparent to one of ordinary skill in the art in light of this disclosure. Thus, although primarily intended to be used in audio-visual environment such as camera-enabled cellular telephones or portable computers and PDAs, this invention is also applicable in any multimedia environment. Examples of such environment include but are not limited to software and games delivery systems, digital books and collaborative creation of documents. Moreover, although the invention has been discussed with reference to JPEG, a variety of different video coding standards, including MPEG-1, MPEG-2, MPEG-4, MPEG-7, H.261, and H.263, can be used as well. 
     The invention has been described in terms of specific examples which are illustrative only and are not to be construed as limiting. The invention may be implemented in digital electronic circuitry or in computer hardware, firmware, software, or in combinations of them. Apparatus of the invention may be implemented in a computer program product tangibly embodied in a machine-readable storage device for execution by a computer processor; and method steps of the invention may be performed by a computer processor executing a program to perform functions of the invention by operating on input data and generating output. Suitable processors include, by way of example, both general and special purpose microprocessors. Storage devices suitable for tangibly embodying computer program instructions include all forms of non-volatile memory including, but not limited to: semiconductor memory devices such as EPROM, EEPROM, and flash devices; magnetic disks (fixed, floppy, and removable); other magnetic media such as tape; optical media such as CD-ROM disks; and magneto-optic devices. Any of the foregoing may be supplemented by, or incorporated in, specially-designed application-specific integrated circuits (ASICs) or suitably programmed field programmable gate arrays (FPGAs). 
     While the preferred forms of the invention have been shown in the drawings and described herein, the invention should not be construed as limited to the specific forms shown and described since variations of the preferred forms will be apparent to those skilled in the art. Thus the scope of the invention is defined by the following claims and their equivalents.