Patent Publication Number: US-6708254-B2

Title: Parallel access virtual channel memory system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 09/438,001, entitled “Parallel Access Virtual Channel Memory System”, filed Nov. 10, 1999, now U.S. Pat. No. 6,477,621. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a memory system for a semiconductor memory. More particularly, the present invention relates to a memory system which provides a plurality of virtual access channels to facilitate access by a plurality of memory masters. 
     2. Description of the Prior Art 
     Conventional data processing systems typically include multiple processors/processes which share a system memory. The multiple processors/processes (i.e., memory masters) access the system memory (e.g., general system memory or graphic frame buffer memory) in a multi-tasking manner. The memory masters can include central processing units (CPUs), graphics processors, PCI bus masters and EISA/ISA bus masters. Each memory master accesses portions of the system memory which exhibit an address locality, a time locality and/or a particular block size. It would therefore be desirable to have a memory system which allows multiple memory masters to access a system memory in an efficient manner. It would further be desirable if such a memory system could be dynamically modified to accommodate different types of memory masters. 
     FIG. 1 is a block diagram of a multi-processing system  100  which employs a shared memory architecture. System  100  includes processors  101   a - 101   c , dedicated cache memories  102   a - 102   c , dedicated cache controllers  103   a - 103   c , system bus  104 , global main memory  105  and memory controller  106 . Processors  101   a - 101   c  share main memory  105  through common parallel system bus  104 . Cache memories  102   a - 102   c  are typically constructed using relatively high speed SRAM arrays. Main memory  105  is typically constructed using relatively low speed and low cost DRAM arrays. Systems such as system  100  are described in the following references: (1) “Protocols Keep Data Consistent”, John Gallant, EDN Mar. 14, 1991, pp.41-50 and (2) “High-Speed Memory Systems”, A. V. Pohm and O. P. Agrawal, Reston Publishing, 1983, pp.79-83. 
     Dedicated cache memories  102   a - 102   c  reduce the frequency with which each of processors  101   a - 101   c  access main memory  105 . This reduces the amount of traffic on system bus  104 . However, cache memories  102   a - 102   c  are relatively expensive. In system  100 , an expensive cache memory must be added for each added processor. In addition, system  100  requires control logic to maintain the consistency of data in cache memories  102   a - 102   c  and main memory  105  (i.e., cache coherence). The problem of cache coherence is described in more detail in “Scalable Shared Memory Multiprocessors”, M. Dubois and S. S. Thakkar, Kluwer Academic Publishers, 1992, pp.153-166. The control logic required to provide cache coherence increases the cost and decreases the performance of system  100 . In addition, the efficiency of main memory  105  and system bus  104  suffers if the data values fetched into cache memories  102   a - 102   c  are not used. 
     FIG. 2 is a block diagram of another conventional multi-processor system  200  which includes a global main memory  204  which is divided into modules  206   a - 206   c . Each of main memory modules  206   a - 206   c  is attached to a single corresponding cache memory module  205   a - 205   c , respectively. Each of cache memory modules  205   a - 205   c  is attached to a main memory bus  202 . Processors  201   a - 201   c  are also attached to main bus  202 . Processors  201   a - 201   c  share cache memory modules  205   a - 205   c  and main memory modules  206   a - 206   c . System  200  is described in, “High-Speed Memory Systems”, Pohm et al., pp.75-79. When the number of processors is approximately equal to the number of memory modules (i.e., cache memory modules), cache thrashing can occur. Cache thrashing refers to the constant replacement of cache lines. Cache thrashing substantially degrades system performance. 
     To minimize the cost of SRAM cache memories, some prior art systems use additional prefetch buffers for instructions and data. These prefetch buffers increase the cache-hit rate without requiring large cache memories. Such prefetch buffers are described in PCT Patent Application PCT/US93/01814 (WO 93/18459), entitled “Prefetching Into a Cache to Minimize Main Memory Access Time and Cache Size in a Computer System” by Karnamadakala Krishnamohan et al. The prefetch buffers are used in a traditional separate cache memory configuration, and memory bandwidth is consumed by both the prefetch operations and the caching operations. A robust prefetch algorithm (with a consistently high probability of prefetching the correct information) and an adequate cache size and organization (to provide a high cache hit rate) is required to deliver any performance improvement over traditional caching schemes. 
     Other conventional systems use the sense-amplifiers of a DRAM array as a cache memory. (See, e.g., PCT Patent Publication PCT/US91/02590, by M. Farmwald et al.) Using the sense-amplifiers of a DRAM array as cache memory provides low cost, high transfer bandwidth between the main memory and the cache memory. The cache hit access time, equal to the time required to perform a CAS (column access) operation, is relatively short. However, the cache miss access time of such a system is substantially longer than the normal memory access time of the DRAM array (without using the sense amplifiers as a cache memory). This is because when the sense amplifiers are used as cache memory, the DRAM array is kept in the page mode (or activated mode) even when the DRAM array is not being accessed. A cache miss therefore requires that the DRAM array perform a precharge operation followed by RAS (row access) and CAS (column access) operations. The time required to perform the precharge operation (i.e., the precharge time) is approximately twice as long as the time required to perform the RAS operation. The total memory access time is therefore equal to the sum of the precharge time, the RAS access time and the CAS access time of the DRAM array. In contrast, during normal operation of the DRAM array, the DRAM array is in precharged mode when it is not being accessed, and the memory access time is equal to the RAS access time plus the CAS access time of the DRAM array. 
     Another prior art cache memory system includes an SRAM cache memory which is integrated into a DRAM array. The DRAM array includes four banks which collectively serve as the main system memory. The SRAM cache memory includes a cache row register which has the capacity to store a complete row of data from one of the banks of the DRAM array. A last row read (LRR) address latch stores the address of the last row read from the DRAM array. When the row address of a current read access is equal to the row address stored in the LRR address latch, the requested data values are read from the row register, rather than the DRAM array. Thus, there is one cache entry in the cache row register which is shared by each of the four banks in the DRAM array. This prior art memory system is described in more detail in  DM  2202  EDRAM  1  MB× 4  Enhanced Dynamic RAM , Preliminary Datasheet, Ramtron International Corp., pp. 1-18. 
     It is therefore desirable to have a memory system which overcomes the previously described shortcomings of the prior art memory systems. 
     SUMMARY 
     In accordance with the present invention, a memory system includes a main memory and a plurality of virtual access channels connected in parallel to the main memory. The main memory typically includes a plurality of memory banks. Each of the virtual access channels includes a set of memory access resources for accessing the main memory. These memory access resources can include, for example, cache resources, burst access control resources, and memory precharge resources. Each of the virtual access channels is independently addressable by an external memory master. 
     By enabling the virtual access channels to be addressed by external memory masters, the virtual access channels can be flexibly assigned to serve different memory masters as required by the data processing system to which the memory system is connected. For example, one memory master can be assigned to access two virtual access channels, while several other memory masters can be assigned to share the access of a single virtual access channel. These assignments can be static or can be changed dynamically during normal operation of the memory system. These assignments can also be modified for connection to different data processing systems. 
     In one embodiment, the virtual access channels include a plurality of cacheable virtual access channels which perform caching operations. In such an embodiment, each cacheable virtual access channel includes a cache data memory for storing one or more cache data entries, and a corresponding cache address memory for storing one or more corresponding cache address entries. By assigning a cacheable virtual access channel to service each of the memory masters, each of the memory masters is advantageously provided with a dedicated cache memory resource. The virtual access channels can also include a non-cacheable virtual access channel which enables the cacheable virtual access channels to be bypassed when a cache miss occurs. 
     The present invention also includes a method of accessing a memory array which includes the steps of: (1) coupling a virtual access system to the memory array, wherein the virtual access system has a plurality of virtual access channels connected in parallel to the memory array, each virtual access channel providing a set of memory access resources for accessing the memory array, (2) assigning each of the memory masters to access one or more of the virtual access channels, (3) providing an access address from the memory masters to the virtual access system, and (4) accessing a selected one of the virtual access channels in response to the access address. 
     This method can also include the steps of (5) storing a cache entry and a corresponding cache address entry in the selected virtual access channel, (6) comparing the access address with the cache address entry, and (7) accessing the cache entry if the access address matches the cache address entry. If the access address does not match the cache address entry, then the memory array can be accessed through a bus bypass circuit. In this case, the cache entry of the selected virtual access channel is updated to reflect the data value accessed through the bus bypass circuit, and the cache address entry is updated to reflect the address accessed. 
     In a variation of this method, two of the virtual access channels can be activated at the same time, with one of the virtual access channels performing operations at the interface between the virtual access channels and the memory masters, while another one of the virtual access channels is performing operations at the interface between the virtual access channels and the memory array. This advantageously provides for improved concurrency of operations within the memory system. 
     In another variation of this method, the operating modes of each of the virtual access channels are independently programmed. For example, each virtual access channel can be individually programmed to have specific cache chaining modes, burst lengths and precharge modes. This enables the virtual access channels to be individually tailored to best serve the operating needs of the corresponding memory master. 
     The present invention will be more fully understood in light of the following detailed description taken together with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a conventional multi-processor system which employs a shared memory architecture; 
     FIG. 2 is a block diagram of another conventional multi-processor system which includes a global main memory divided into modules; 
     FIGS. 3A and 3B are block diagrams of a memory system in accordance with one embodiment of the present invention; 
     FIG. 4 is a schematic diagram of a channel row cache memory and cache row address register in accordance with one embodiment of the invention; 
     FIG. 5 is a block diagram of an accelerated graphic processor system which implements the memory system of FIG. 3 in accordance with one embodiment of the invention; and 
     FIG. 6 is a block diagram of a pipelined graphic system which implements the memory system of FIG. 3 in accordance with another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 3A and 3B are block diagrams of a memory system  300  in accordance with one embodiment of the present invention. Memory system  300  includes memory banks  301 - 304 , bank row interface circuits  401 - 404 , memory bank interface bus  310 , virtual channel system  320 , memory system interface bus  312 , memory system interface  314  and system bus  315 . Memory system interface  314  is coupled to one or more external memory masters (not shown) through the system bus  315 . 
     The virtual channel system  320  includes memory bank interface bus  310 , channel row cache interface circuits  501 - 507 , channel row cache memories  601 - 607 , cache row address registers  701 - 707 , channel row cache interface circuits  801 - 807 , bus bypass circuit  311 , memory system interface bus  312 , and virtual channel control circuit  325 . 
     In general, virtual channel system  320  provides a plurality of independent memory access resources, which are hereinafter referred to as “virtual access channels”. Each virtual access channel is located between memory banks  301 - 304  and memory system interface  314 . Virtual channel system  320  includes eight virtual access channels 1-8. Seven cacheable virtual access channels 1-7 are defined by the seven channel row cache circuits  601 - 607 , their seven corresponding cache row address registers  701 - 707 , and their corresponding channel row cache interface circuits  501 - 507  and  801 - 807 . One non-cacheable virtual access channel 8 is defined by the bus bypass circuit  311 . As described in more detail below, each cacheable virtual access channel is capable of acting as an independent cache memory. The non-cacheable virtual channel 8 provides for direct accessing of the memory banks  301 - 304 . 
     Each of the virtual access channels 1-8 is individually selectable externally via the pins of the chip on which memory system  300  is located. The virtual access channels 1-8 are individually addressable through virtual channel control circuit  325 . Thus, a memory master coupled to system bus  315  can provide a 3-bit address signal to virtual channel control circuit  325  (through memory system interface  314 ). In response to this 3-bit address signal, the virtual channel control circuit  325  enables or selects one of the eight virtual access channels 1-8. In this manner, a different memory master (e.g., CPU, memory controller or graphic controller) can be mapped to each virtual access channel. Table 1 below lists a virtual access channel addressing scheme in accordance with one embodiment of the invention. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 3-bit Virtual 
                 Addressed Virtual Channel 
               
               
                 Channel Address 
                 (Defined Element Reference Nos.) 
               
               
                   
               
             
            
               
                 000 
                 1 (inc. 501, 601, 701, 801) 
               
               
                 001 
                 2 (inc. 502, 602, 702, 802) 
               
               
                 010 
                 3 (inc. 503, 603, 703, 803) 
               
               
                 011 
                 4 (inc. 504, 604, 704, 804) 
               
               
                 100 
                 5 (inc. 505, 605, 705, 805) 
               
               
                 101 
                 6 (inc. 506, 606, 706, 806) 
               
               
                 110 
                 7 (inc. 507, 607, 707, 807) 
               
               
                 111 
                 8 (inc. 311) 
               
               
                   
               
            
           
         
       
     
     The virtual access channels provide each memory master accessing the memory system  300  with its own channel for high speed unimpeded access to the memory banks  301 - 304 . As a result, multiple memory masters can operate concurrently, each with its own virtual access channel. Each memory master operates at its own speed, according to its own processing needs, to access memory banks  301 - 304  as needed. The single bus  310  between memory banks  301 - 304  and virtual access channels 1-8 indicates that there can only be one operation (read/write) between the memory banks  301 - 304  and the virtual access channels 1-8 at any given time. The separate bus  312  between virtual access channels 1-8 and memory system interface  314  indicates that there can only be one operation between the virtual access channels 1-8 and the memory system interface  314  at any given time. However, operations between the memory banks  301 - 304  and virtual access channels 1-8 and operations between the virtual access channels 1-8 and memory interface system  314  can take place concurrently. 
     Memory system  300  provides very fast read and write access to the external memory masters once the virtual access channels 1-7 have been loaded. The virtual access channels are loaded using the same operating and timing sequence as a standard synchronous DRAM (SDRAM) operation. In the described embodiment, a two phase command/data format is implemented in which commands are decoded in a first phase of a clock signal and executed in a second phase of the clock signal. The cacheable virtual access channels 1-7 operate at a peak hit rate, with little or no cache thrashing, because all of the cacheable virtual access channels 1-7 are directly managed from an external host memory controller which is able to map different channels to different memory masters to avoid mutual interference. By allowing the host memory controller to directly manage the channels, the cacheable virtual access channels 1-7 are effective in automatically tracking various dynamically changing spatial and temporal localities, and provide fast cache accesses for all processes in a multi-tasking system environment. 
     Memory system  300  exhibits the access speed of an SDRAM when the cacheable virtual access channels 1-7 are not performing a caching function, and exhibits the access speed of a PC second level (L-2) cache memory when the cacheable virtual access channels 1-7 are performing a caching function. The speed of memory system  300  is thereby improved to the full access speed of the cacheable virtual access channels 1-7. This performance is, of course, limited by the performance bandwidth of the memory system interface  314  and system bus  315 . In particular embodiments, a single memory master can access more than one virtual access channel, thereby minimizing the probability of cache thrashing. 
     The present invention will now be described in detail. In the described embodiment, memory banks  301 - 304  are conventional dynamic random access memory (DRAM) arrays. DRAM arrays are used because of the relatively small layout area required for each memory cell. However, the present invention is not limited by the type of memory used to implement memory banks  301 - 304 . In other embodiments, memory banks  301 - 304  can be implemented using static random access memory (SRAM) arrays. 
     In the described embodiment, each of memory banks  301 - 304  includes 512 rows and 8192 columns of memory cells. The 8192 columns define 1024 8-bit bytes. Thus, the capacity of each of memory banks  301 - 304  is 512 kBytes, and the total capacity of memory banks  301 - 304  is 2 MBytes. The invention is not limited to the memory capacity or row/column configuration described in the present embodiment. The organization of the memory banks in terms of numbers of rows and columns is determined by the desirability of parallelisms within the memory banks and the limits of die size efficiency. 
     Moreover, although four memory banks  301 - 304  are illustrated in FIGS. 3A and 3B, any number of memory banks can be used in other embodiments of the invention. For example, in different embodiments, memory banks  301 - 304  can be replaced by a single memory bank, two memory banks, eight memory banks or sixteen memory banks. The number of banks in memory system  300  is determined by the need to provide concurrence of operations. As a predefined total memory capacity is divided into a greater number of memory banks, the granularity of memory system  300  increases. In addition, as more memory banks are used, a greater number of memory banks can be activated (i.e., connected to their corresponding sense amplifier circuits in preparation for the arrival of a column address to perform read or write operations) to perform various operations while other banks are occupied with un-interruptable operations such as activation, precharge, read, write or refresh operations. At the same time, the number of memory banks used to provide a given memory capacity is constrained by the practicality of die size and the cost of the memory chip which contains memory system  300 . 
     Each of memory banks  301 - 304  includes a sense amplifier circuit. Each sense amplifier circuit is coupled to the bit (column) lines within the corresponding memory bank, such that each sense amplifier circuit stores an entire row of data (1024 bytes) during a read or write operation. Such sense amplifier circuits are well known in the art. The sense amplifier circuits of memory banks  301 - 304  are coupled to bank row interface circuits  401 - 404 , respectively. Bank row interface circuits  401 - 404 , which are controlled by address signals received from virtual channel control circuit  325 , provide data pathways between the sense amplifier circuits of memory arrays  301 - 304  and the memory bank interface bus  310 . Bank row interface circuits  401 - 404  are controlled to ensure that only one of the memory banks  301 - 304  drives the memory bank interface bus  310  at any given time. Bank row interface circuits  401 - 404  are also controlled such that one or more of the memory banks  301 - 304  can simultaneously receive data from the memory bank interface bus  310 . 
     In different embodiments, the memory bank interface bus  310  can be wider or narrower than the sense amplifier circuits of memory banks  301 - 304 . In the described embodiment, the width of each sense amplifier circuit is an integer multiple of the width of memory bank interface bus  310 . More specifically, each sense amplifier circuit is 1024 bytes wide (i.e., the physical row width of memory banks  301 - 304 ), and the memory bank interface bus  310  is only 64 bytes (512 bits) wide In this case, during a read operation, the selected memory bank provides a 1024 byte row of data to its associated sense amplifier circuit. The bank row interface circuit corresponding to the selected memory bank then passes one of the sixteen 64-byte words present in the 1024 byte row to the memory bank interface bus  310 . In this manner, the selected bank row interface circuit performs a multiplexing function. 
     Conversely, during a write operation, a 64-byte word to be written to a selected memory bank is provided to memory bank interface bus  310 . The appropriate bank row interface circuit performs a de-multiplexing function to pass this 64-byte word to a selected one of the sixteen 64-byte word locations within the corresponding sense amplifier circuit. In this manner, the bank row interface circuits  401 - 404  perform bi-directional multiplexing/de-multiplexing functions. 
     Alternatively, the memory bank interface bus  310  can have a width which is an integer multiple of the width of the sense amplifier circuits. In this case, each of bank row interface circuits  401 - 404  performs a de-multiplexing function to route data read from the selected memory banks (i.e., multiple banks can be read at the same time) to a selected subset of the bus lines of the memory bank interface bus  310 . Similarly, each of bank row interface circuits  401 - 404  performs a multiplexing function to route data to be written to memory banks  301 - 304  from a subset of the bus lines of the memory bank interface bus  310  to the selected sense amplifier circuit. 
     Memory bank interface bus  310  extends in parallel from bank row interface circuits  401 - 404  to channel row cache interface circuits  501 - 507  and bus bypass circuit  311 . Channel row cache interface circuits  501 - 507  provide an interface between the memory bank interface bus  310  and the corresponding channel row cache circuits  601 - 607 . The width of the channel row cache circuits  601 - 607  may be wider or narrower than the width of the memory bank interface bus  310 . As such, channel row cache interface circuits  501 - 507  may perform multiplexing and de-multiplexing functions between the memory bank interface bus  310  and the channel row cache circuits  601 - 607  as necessary. In the described embodiment, the channel row cache circuits  601 - 607  and the memory bank interface bus  310  each have a width of 64 bytes. Thus, no multiplexing function is necessary in this embodiment. As described in more detail below, the operations of channel row cache interface circuits  501 - 507  are controlled by virtual channel control circuit  325 . 
     Implicit in the design of the memory bank interface bus  310  is a timing control that gates the signals from memory banks  301 - 304  or channel row cache circuits  601 - 607  to the memory bank interface bus  310 , depending upon whether a read or a write operation is being performed by the memory banks  301 - 304 . More specifically, implicit timing controls ensure that the channel row cache interface circuits  501 - 507  only drive the memory bank interface bus  310  during write operations to the memory banks  301 - 304 . Similarly, the timing controls ensure that bank row interface circuits  401 - 404  only drive the memory bank interface bus  310  during read operations from memory banks  301 - 304 . 
     As previously described, each channel row cache interface circuit  501 - 507  is coupled to a corresponding one of channel row cache memories  601 - 607 . The number of channel row cache memories  601 - 607  defines the number of cacheable virtual access channels in memory system  300 . Although seven cacheable virtual access channels 1-7 are present in the described embodiment, any integer number of cacheable virtual access channels are possible. For example, in another embodiment, 15 cacheable virtual access channels are used. The number of cacheable virtual access channels may be greater than, equal to, or less than the number of memory banks. Each of channel row cache memories  601 - 607  has an integer number, N, of independently loadable and addressable cache entries, where N is an integer equal to or greater than one. That is, each of channel row cache memories  601 - 607  has at least one cache entry. Each cache entry has the capacity (width) to store a number of data bytes corresponding to a full or partial row of one of the memory banks  301 - 304 . Each of channel row cache memories  601 - 607  can be parallel loaded from any one of the memory banks  301 - 304 . 
     Each of channel row cache memories  601 - 607  is associated with a corresponding one of cache row address registers  701 - 707 . Each of cache row address registers  701 - 707  stores N cache addresses. That is, each cache entry in channel row cache memories  601 - 607  has a corresponding cache address stored in a corresponding one of cache row address registers  701 - 707 . 
     FIG. 4 is a schematic diagram of channel row cache memory  601  and cache row address register  701  in accordance with one embodiment of the invention. In this embodiment, N is equal to two, such that there are two cache entries  601   a ,  601   b  in channel row cache memory  601  and two corresponding cache addresses  701   a ,  701   b  in cache row address register  701 . The first cache entry 601 a  in channel row cache memory  601  is a 64-byte word B 0 -B 63 . The first cache address  701   a  in channel row address register  701  stores an address which identifies: (1) the memory bank, (2) the row address, and (3) the column address corresponding to the 64-byte word B 0 -B 63 . For example, the first cache address  701   a  in cache row address register  701  could indicate that the first cache entry  601   a  in the channel row cache memory  601  was received from memory bank  302 , row number 256, and column position  64 . Similarly, the second cache entry  601   b  is a 64-byte word B /   0 -B /   63  which is identified by the second cache address  701   b.    
     Although cache row address registers  701 - 707  are shown as being part of memory system  300 , in other embodiments, these cache row address registers can be implemented in the controllers of the memory masters which access memory system  300 . 
     Channel row cache interface circuits  801 - 807  couple the channel row cache memories  601 - 607  and the cache row address registers  701 - 707  to the memory system interface bus  312 . Note that FIGS. 3A and 3B explicitly illustrate the data paths, but not the address paths, within memory system  300 . Because the channel row cache memories  601 - 607  have a width of 64 bytes, and the memory system interface bus  312  has a width of 1 byte, the channel row cache interface circuits  801 - 807  perform multiplexing and de-multiplexing functions to enable data transfer between these elements. Channel row cache interface circuits  801 - 807  are controlled by virtual channel control circuit  325  as described in more detail below. 
     Bus bypass circuit  311  provides a non-cacheable virtual access channel between memory bank interface bus  310  and memory system interface bus  312 . Because the memory bank interface bus  310  has a width of 64 bytes, and the memory system interface bus  312  has a width of 1 byte, the bus bypass circuit  311  performs multiplexing and de-multiplexing functions to enable data transfer between these buses. Bus bypass circuit  311  is controlled by virtual channel control circuit  325  as described in more detail below. 
     The memory system interface bus  312  is a parallel bus which couples the channel row cache interface circuits  801 - 807 , the bus bypass circuit  311  and the virtual channel control circuit  325  to the memory system interface  314 . The memory system interface bus  312  can be a multiplexed address/data bus. Alternatively, dedicated data and address paths can be provided by memory system interface bus  312 . The memory system interface  314  includes the input/output pin drivers associated with memory system  300 . If the memory system interface  314  is coupled to an address/data multiplexed bus, then the appropriate de-multiplexers are also provided in the memory system interface  314 . Memory system interface  314  also provides circuitry required to perform any logic voltage conversions. 
     Virtual channel control circuit  325  includes the control logic for controlling bank row interface circuits  401 - 404 , channel row interface circuits  501 - 507 , cache row address registers  701 - 707 , channel row interface circuits  801 - 807 , and bus bypass circuit  311 . 
     Operation of memory system  300  proceeds as follows. Initially, the external processing system which is coupled to system bus  315  determines which memory masters will access which virtual access channels. That is, the external processing system assigns each memory master to one or more of the virtual access channels. Each virtual access channel is accessed by one or more external memory masters. These assignments are determined by the memory controller logic, the system BIOS, or the memory management manager part of the operating system. The assignments may be static (i.e., never change once assigned), or dynamic (i.e., the memory manager juggles the virtual channel assignments as the system resources change). The virtual channels can be assigned as needed by the system processors. 
     To initiate a memory access, an external memory master provides the following information to memory system interface  314 : (1) a current access address, which includes the bank, row and column addresses of the current access (2) a read/write signal which identifies the access as a read or a write operation, (3) a write data byte, if the current access is a write operation, and (4) a plurality of virtual channel select signals which identify the virtual access channel being accessed by the memory master. 
     The memory system interface  314  provides the virtual channel select signals to virtual channel control circuit  325 , thereby informing virtual channel control circuit  325  of the virtual access channel being accessed by the memory master. The memory system interface  314  also provides the current access address to the virtual channel control circuit  325 . In response, the virtual channel control circuit  325  compares the current access address with the cache addresses stored in the cache row address register of the selected virtual access channel. For example, if the virtual channel select signals identify the virtual access channel which includes channel cache row memory  601  and cache row address register  701 , then virtual channel control circuit  325  compares the current access address with the cache addresses  701   a ,  701   b  in cache row address register  701  to determine whether a match (cache hit) exists. 
     Read Operations 
     If a match does not exist (cache miss), and the read/write signal indicates that the memory access is a read operation, then the virtual channel control circuit  325  causes a row of 1024 data bytes to be read from one of memory banks  301 - 304 . The particular row and memory bank are selected in response to the current access address. The selected memory bank is accessed using a conventional SDRAM RAS-CAS command sequence. The addressed row of data bytes is provided to the bank row interface circuit corresponding to the addressed memory bank. The virtual channel control circuit  325  further causes this corresponding bank row interface circuit to selectively pass a 64-byte word of from the row of 1024 data bytes to memory bank interface bus  310 . This 64-byte word is selected in response to the column address of the current access address. The 64-byte word is selected to include the data byte corresponding to the current access address and 63 other consecutive data bytes. The addressed data byte is routed through the bus bypass circuit  311  to the memory system interface  314  and provided to the memory master. Note that bus bypass circuit  311  only passes one selected byte of the received 64-byte word. 
     In addition, the 64-byte word is routed from memory bank interface bus  310  to channel row cache interface circuits 501-507. The virtual channel control circuit  325  enables only the channel row interface circuit which corresponds to the selected virtual access channel, thereby causing the 64-byte word to be written as a cache entry to the channel row cache memory of the selected virtual access channel. For example, the virtual channel control circuit  325  could enable cache row interface circuit  501  to cause the 64-byte word to be written to cache entry  601   a  of channel row cache memory  601 . Virtual channel control circuit  325  further causes the bank address, the row address and a portion of the column address (i.e., that portion defining the column location of the 64-byte word) of the current access address to be written to the cache row address register of the selected virtual access channel. For example, the virtual channel control circuit  325  could cause the appropriate portion of the current access address to be written as cache address  701   a  in cache row address register  701 . 
     If there is only one cache entry per virtual access channel, a cache miss results in the overwriting of the previous cache entry and the previous corresponding cached address in the selected virtual access channel. If the virtual access channel depth is greater than one, a cache miss results in the updating of one of the cache entries and its corresponding cache address within the selected virtual access channel. The methods used to select the cache entry to be updated can include, but are not limited to, one or more of the following methods: (1) overwriting the least recently used cache entry, (2) overwriting the entry on a round-robin basis, (3) randomly overwriting the cache entry, and (4) overwriting the cache entry with the row address closest to the new row address. 
     The virtual channel control circuit  325  detects if the current access address matches a cache address stored in the cache row address register of the virtual access channel identified by the virtual channel select signals (cache hit). Upon detecting a cache hit, the virtual channel control circuit  325  enables the cache row interface circuit ( 801 - 807 ) of the selected virtual access channel to pass the addressed data byte from the channel row cache memory of the selected virtual access channel. For example, the virtual channel control circuit  325  can cause cache row interface circuit  801  to pass data byte B 3  from cache entry  601   a  of channel row cache memory  601  when the current access address matches the cache address  701   a  stored in cache row address register  701 . Accessing channel row cache memories  601 - 607  when cache hits occur eliminates the need to access the slower memory banks  301 - 304 . 
     Virtual channel control circuit  325  can also facilitate burst read accesses. In such an embodiment, a memory master requesting a burst read access asserts a burst enable control signal to inform the virtual channel control circuit  325  that the read operation is to be a burst read access. In this embodiment, the virtual channel control circuit  325  includes a burst access control register (not shown). The burst access control register can be, for example, a 40-bit register which is partitioned into eight 5-bit entries. Each 5-bit entry identifies the burst access length for a corresponding one of the eight virtual access channels. For example, the virtual access channels can have burst access lengths of 2, 4, 8, 16, 32, or 64 bytes. Different virtual access channels can have different burst access lengths, depending upon the characteristics of the particular memory masters accessing the virtual access channels. The burst access control register is programmed by the external data processing system in view of the characteristics of the memory masters of the data processing system. The virtual channel control circuit  325  controls the burst access operations. 
     In a preferred embodiment, channel row cache aliasing is implemented. In channel row cache aliasing, the virtual channel control circuit  325  determines whether the current access address matches a cache address stored in any of the cache row address registers  701 - 707 . That is, the current access address is compared with all of the cache row addresses, instead of only the cache row addresses of the addressed virtual access channel. If such a match exists, then the virtual channel control circuit  325  causes the desired data byte to be retrieved from the virtual access channel which contains the desired data byte, even if this virtual access channel is not the virtual access channel identified by the virtual channel select signals. 
     Write Operations 
     If the current access address does not match a cache address of any of the virtual access channels (cache miss), and the read/write signal indicates that the memory access is a write operation, then the virtual channel control circuit  325  causes the write data byte provided by the memory master to be written to the channel row cache memory  601 - 607  which is identified by the virtual channel select signals. The virtual channel control circuit  325  additionally updates the corresponding cache row address register  701 - 707  to reflect the current access address. Because only one data byte is written to the channel row cache memory (which stores 64 bytes), the other 63 bytes are provided to the channel row cache memory from the appropriate memory bank. 
     The following example will clarify the write operation for a cache miss. First, the current access address, the virtual channel select signals, the write control signal and the data byte are routed from the memory master to the virtual channel control circuit  325 . In the present example, it is assumed that the virtual channel select signals identify the cacheable virtual access channel 1 (as identified by elements  501 ,  601 ,  701  and  801 ), and that the data byte, B 3 , is the fourth data byte of a 64-byte word within one of memory banks  301 - 304 . 
     The virtual channel control circuit  325  compares the current access address with the contents of the cache row address registers  701 - 707  and determines that no match exists. In response, the virtual channel control circuit  325  writes the current access address to cache address  701   a  in cache row address register  701 . The virtual channel control circuit  325  also writes the data byte B 3  to the fourth byte position in cache entry  601   a  of channel row cache memory  601 . This write operation is performed through cache row interface circuit  801 . Substantially in parallel with this write operation, virtual channel control circuit  325  accesses the 64-byte data word at the current access address in memory banks  301 - 304 . This 64-byte word is routed to cache row interface circuit  501 . Virtual channel control circuit  325  controls cache row interface circuit  501  to perform a byte write masking function. That is, cache row interface circuit  501  only passes Bytes  0 - 2  and  4 - 63  of the 64-byte word to cache entry  601   a  of channel row cache memory  601 . In this manner, a current 64-byte cache entry is provided to channel row cache memory  601 . 
     The data byte B 3  is not immediately written through to memory banks  301 - 304  (i.e., write through is eliminated). Instead, data byte B 3  remains in channel row cache memory  601  until cache entry  601   a  is restored. Note that any subsequent read accesses to the current access address will cause the data byte B 3  to be properly read from channel row cache memory  601  (due to cache row aliasing). Consequently, channel row aliasing eliminates the need for cache snooping. That is, there is no need to continuously monitor new write addresses (snoop) to determine whether a new write operation overwrites memory banks  301 - 304  without updating a corresponding cache entry. 
     If the current access address matches a cache address of any of the virtual access channels (cache hit), and the read/write signal indicates that the memory access is a write operation, then the virtual channel control circuit  325  causes the write data byte provided by the memory master to be written to the cache entry which resulted in the cache hit. 
     For example, if a cache hit exists because the current access address corresponds to the cache address  701   a  stored in channel row address register  701 , then virtual channel control circuit  325  causes the new data byte to be routed through cache row interface circuit  801  to overwrite one of the bytes in the 64-byte cache entry  601   a . The byte to be overwritten is determined by the current access address. This new data byte is routed to the appropriate byte location within the 64-byte cache entry  601   a  by cache row interface circuit  801 . Cache row interface circuit  801  performs this de-multiplexing function in response to control signals received from virtual channel control circuit  325 . In this manner, data coherence within memory system  300  is maintained. 
     The cache entry  601   a  is not immediately restored to the memory banks  301 - 304 . Instead, the restore operation is delayed until some later time. This enables a plurality of consecutive data bytes to be written to cache entry  601   a  without requiring an access to memory banks  301 - 304  after each data byte. A memory master can issue a restore command which causes the contents of a cache entry to be written to the appropriate row of the appropriate memory bank whenever the memory master anticipates a new cache entry will be required. A restore command can be implemented concurrently with another operation (read or write) between a virtual access channel and memory system interface  314 . In the described embodiment, multiple restore operations can be concurrently implemented in the multiple memory banks  301 - 304 . The concurrent restore operations among the banks are staggered, by virtue of the fact that the restore commands are initiated at different times by the memory masters. This staggering ensures that the memory bank interface bus  310  is used at different times to complete the data transfer operations from the virtual access channels 1-8 to the memory banks  301 - 304 . To enable the host memory controller to easily track the status of bus  310 , the restore operation is defined to have a duration of a fixed number of clock cycles. 
     In accordance with another aspect of the present invention, other memory access functionalities can be implemented by the virtual access channels. For example, the virtual access channels can also implement prefetch operations within memory system  300 . Prefetch refers to the retrieving of data bytes from memory arrays  301 - 304  during the time that other data bytes are being read from the virtual access channels. The host memory controller is controlled to ensure that any necessary restore operations are performed prior to performing a prefetch operation. 
     A prefetch command can be issued by a memory master. The prefetch command specifies that a particular row is to be read from a particular one of memory banks  301 - 304  and loaded into a particular entry of a specified one of the virtual access channels 1-8, via the memory bank interface bus  310 . The prefetch command can be executed concurrently with an operation that involves the reading or writing of data between the virtual access channels 1-8 and the memory system interface  314  on the memory system interface bus  312 . Furthermore, each of the memory banks  301 - 304  can be performing separate prefetch operations with respect to different cache entries in different virtual access channels. Of course, these concurrent prefetch operations among the memory banks  301 - 304  are pipelined (staggered at different phase of operations) by virtue of the fact that the prefetch commands are issued/initiated at different times. Furthermore, the staggered stages of execution also ensure that there is no conflict in the use of the memory bank interface bus  310  in transferring the data from the memory banks  301 - 304  to the virtual access channels 1-8. 
     In accordance with another aspect of the invention, the virtual channel control circuit  325  can include an 8-bit precharge control register which includes 1 precharge control bit for each of the eight virtual access channels. If a precharge control bit has a first logic value, then auto-precharge is implemented in the corresponding virtual access channel. In auto-precharge mode, after each write operation to a channel row cache memory, a corresponding write operation is performed to memory banks  301 - 304 . If a precharge control bit has a second logic value, then a non-automatic precharge is implemented in the corresponding virtual access channel. 
     As previously discussed, only one operation can be performed on memory bank interface bus  310  at any given time. Similarly, only one operation can be performed on memory system interface bus  312  at any given time. However, concurrent operations can be performed on memory bank interface bus  310  and memory system interface bus  312 . For example, data can be prefetched from a memory bank to a channel row cache memory at the same time that data is being read from a channel row cache memory to memory system interface  314 . Similarly, a restore operation can be performed from a channel row cache memory to a memory bank at the same time that data is being read from a channel row cache memory to memory system interface  314 . 
     In an alternative embodiment, multiple memory systems (chips) can be connected in parallel to create a larger channel row cache memory and a wider output bus. For example if eight memory systems identical to memory system  300  were connected in parallel, the result would be a 64-bit output bus. Eight virtual access channels would exist, with each virtual access channel consisting of a channel row cache memory within each memory system. Thus, each of the eight virtual access channels would have a width of 512 bytes (8×64 bytes). 
     In another embodiment, the effective length of the channel row cache memory can be increased by chaining two of the channel row cache memories. For example, channel row cache memories  601  and  602  can be chained as follows. First, a 64-byte word of a row within memory bank  301  is written to a cache entry within channel row cache memory  601 . This first 64 byte word can be, for example, the first 64 bytes of the 1024 byte row of data read from the memory bank. The first 64 byte word can then be read out from channel row cache memory  601 , one byte at a time. As the individual bytes of the first 64 byte word are sequentially read out of channel row cache memory  601 , a second 64 byte word is read from memory bank  301  and stored in channel row cache memory  602 . This second 64 byte word can be, for example, the second 64 bytes of the memory row. The second 64 byte word is stored in channel row cache memory  602  before the last byte of the first 64 byte word is read out of channel row cache memory  601 . The second 64 byte word can therefore be read out from channel row cache memory  602  in a seamless manner after the first 64 byte word. 
     In an alternative embodiment, a third 64 byte word can then be read out of memory bank  301  and stored in channel row cache memory  601  while the bytes of the second 64 byte word are read out of channel row cache memory  602 . In this manner, any number of 64 byte words can be chained together, thereby creating a channel row cache memory having an infinite length. Such a recirculating chaining scheme is particularly useful for memory masters which tend to “walk and play hopscotch” through a large block of memory. Such memory masters include those which engage in screen refresh operations, reading a texture map in reverse mapping operation 3D graphics, and video stream processing. 
     Virtual channel control circuit  325  can include a chain control register which controls the chaining mode within the various virtual access channels. In one embodiment, the chain control register would be a 16-bit register, with a 2-bit entry corresponding to each of the eight virtual access channels. Each of these two bit entries would identify one of the following chaining options: (1) no chaining, (2) chaining of two cache entries, and (3) infinite chaining. The chain control register is programmed by the external data processing system in accordance with the operating characteristics of the memory masters. 
     In the foregoing manner, each virtual access channel can be provided with its own independent operating characteristics, thereby enabling the virtual access channels to be modified to match the operating characteristics of the individual memory masters. 
     The previously described memory systems can be used in the data processing systems illustrated in FIGS. 5 and 6. FIG. 5 is a block diagram of a personal computer (PC)  1000  which is equipped with an Accelerated Graphic Processor (AGP). PC  1000  implements a multi-tasking system memory  1009  in accordance with the previously described embodiments. System memory  1009  includes a plurality of parallel connected memory systems  330   a - 300   n , each of which is substantially identical to previously described memory system  300  (FIGS.  3 A and  3 B). The system memory  1009  is independently accessed (through the north bridge circuit  1008 ) by the processor  1001 , the AGP 3-D graphic processor  1002 , the south bridge circuit  1003  (which is the master for the EISA/ISA bus  1004 ) and the PCI bus master  1005  (which interfaces with the PCI devices  1006 ). The AGP circuit  1007  provides an interface between the AGP processor  1002  and the north bridge circuit  1008 . In a particular implementation, each of these bus masters (i.e., processor  1001 , AGP processor  1002 , south bridge circuit  1003  and PCI bus master  1005 ) assumes control of one or more of the virtual channels present in the system memory  1009 . The assignment of the specific virtual channels can be hardwired, static (as determined by the BIOS during power on), or dynamically managed by memory management codes below the operating system kernel. Four cached virtual channels are assigned to (1) processor  1001  instruction access, (2) processor  1001  data access, (3) AGP processor  1002  texture mapping access, and (4) PCI bus master  1005  access. The non-cached virtual channel is assigned to (1) memory refresh and (2) ISA bus master  1003 . 
     FIG. 6 is a block diagram of a pipelined graphic system  1010  implementing multi-tasking memory system  300 . The Bit Blit In engine  1011 , the Bit Blit Out engine  1012 , the screen refresh engine  1013  and the polygon draw engine  1014  (each being conventional processing engines) can each independently act as a memory master to access the low latency memory  1016  through the memory request interleaving logic  1015 . Again, memory  1016  includes a plurality of memory systems  300   a - 300   n  which are substantially identical to previously described memory system  300 . Again, each of the memory masters  1011 - 1014  is assigned to access one or more of the virtual channels within the system memory  1016 . 
     In another example, the memory system of the present invention is used as a 2D/3D graphic frame buffer. In this example, four cached virtual channels are respectively mapped to a screen refresh process, a 3D rendering process, a 2D drawing and polygon fill process, and a texture mapped write process. The non-cached virtual channel is mapped to a memory refresh process. 
     In yet another embodiment, the present invention is implemented in connection with an SDRAM array. In such an embodiment, three additional pins are added to a conventional SDRAM memory chip to enable the selection of four cached virtual channels and a non-cached virtual channel. A performance gain of 2.0 to 2.5 times can be realized on small and large block accesses, respectively. 
     In yet another embodiment, the present invention is implemented in connection with an RDRAM (RAMBUS DRAM) array. RDRAM typically has two banks, each with 2 KBytes of write through cache. This is modified to include four virtual channels of 1 KByte row cache. Bits in the RDRAM request packets are used to select the virtual access channels. Multiple memory masters reside in a single pipelined multi-tasking controller. A performance gain of 80% can be realized in an RDRAM memory chip. 
     In general, each system processor may include a plurality of memory masters, each of which performs a particular type of processing task. These memory masters can access the memory at different frequencies, as determined by the tasks performed. Each memory master will also exhibit a different behavior in terms of numbers of consecutive bytes read/written for every address locality. Thus, even if there is only one physical device accessing memory system  300 , a multiplicity of memory masters may still be present. In most cases, there are multiple physical devices attached to the memory system  300 , and each of these physical devices may include one or more memory masters. 
     The foregoing has described the principles and preferred embodiments of the present invention. However, the invention should not be construed as being limited to the particular embodiments described. The above-described embodiments should be regarded as illustrative rather than restrictive. Variations can be made to those embodiments by workers skilled in the art without departing from the scope of the present invention as defined by the following claims.