Abstract:
A method and apparatus that provides for the atomic transfer of data associated with a command to be transferred to a device consisting of a cache memory which supports a plurality of virtual devices. The atomic transfer enables the device to initiate and complete execution of the command immediately with respect to a particular virtual device without having to wait for data to come during subsequent bus transfers. This insures that the state of the device will be consistent during execution of the command.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of cache memory. More particularly, the present invention relates to the field of high speed caches and command processing. 
     2. Art Background 
     Computer components and systems are constantly improved upon to process data faster. One technique to increase processing speed is to increase the speed of the clock which drives the component or system. Another technique is to minimize the number of clock cycles needed to perform an operation. 
     In a system environment, the central processing unit (CPU) is typically connected to a plurality of components, such as memory, storage devices and other peripheral devices through a system bus. The CPU communicates commands and associated data over the bus lines interconnecting the CPU and peripheral device. 
     Certain devices coupled to the CPU may require processing at speeds faster than the CPU. In addition, these devices are bidirectional and may process data received externally as well as internally from the CPU. One example of a device is an input/output (I/O) device such as a network interface which communicates data bidirectionally over a high speed network. It is desirable that the network interface be able to operate at high speeds comparable to the network transmission speeds. Such communication can require multiple clock cycles to transfer all the data. If the state of the device operating a high speeds is affected by not only the CPU, problems can arise when the CPU attempts to transfer data or issue a command if the data or command issued requires more than one bus cycle to transfer. 
     A particular example is a cache memory located on the high speed network interface. When data is received or transferred over channels through the network, the state of the cache may change to reflect channel activity. If the state of the cache changes prior to the completion of transmission of a command, errors may occur. More particularly, in the prior art, when the command and associated data is transmitted from the CPU to the device over multiple clock cycles, the device typically begins process of the command upon receipt of the partial information received during the first clock cycle. However, activity on the network may cause the channel states, stored in the cache, to change at a rate faster than the CPU can operate. 
     When the host computer attempts to issue a command to a particular channel, such as disabling a particular channel, and data regarding identification of the channel is transferred during a first bus transfer, the device responds by performing a form of lookup to determine the physical location of the entry in the cache corresponding to the channel upon which the command is to be performed. The data regarding the command to be executed, however, is transferred during a subsequent cycle. Therefore, it is possible that the cache has been updated in the interim and the corresponding physical location is no longer correct. It follows that if a write operation to is to be performed, the cache will write the data received to the wrong cache line because the line of memory located at the cache location changed. If a read operation is to be performed, data from a different memory address will be read. 
     It is therefore advantageous to provide a mechanism that enables transfers of commands and data atomically to assure that the device is in a determined state when the command is executed. 
     SUMMARY 
     The method and apparatus of the present invention enables the atomic transfer of data associated with a command to be transferred to a device consisting of a cache memory which supports a plurality of virtual devices. Therefore, the device can initiate and complete execution of the command immediately with respect to a particular virtual device without having to wait for data to come during subsequent bus transfers. This insures that the state of the device will be consistent during execution of the command. 
     In the present embodiment, the device includes a single command register to which the host transfers commands directed to any one of the plurality of virtual devices supported. Thus the register is accessed through a number of different addresses, one address for each supported virtual device. 
     To provide sufficient bandwidth to transfer the data, the data is transferred over the data bus and over the address bus; the address bus is configured to transfer address and command data associated with a particular virtual device. Therefore, in the present embodiment, the processor transfers the command, address and data information over the address and data busses in one bus cycle. The receiving device, for example the cache, receives the information into the single command register. The address stored in the register identifies the virtual device the command and data apply to. The information is parsed to extract the command and data for a particular entry in the cache reflective of the state of a particular virtual device. The operation identified by the command received is performed on the virtual device identified by the address using the data received. 
     In one embodiment the receiving device is a network interface device which comprises a cache memory which stores entries indicative of active channels used for data transfer between the host computer system and a network. Up to 1024 virtual channels are provided; entries of those active or enabled channels are stored in the cache which provides up to 128 entries. When the processor wishes to perform an operation with respect to a particular channel, such as disabling the channel, a write operation is performed to the virtual address of the channel. The data transferred with the command includes an identification that a virtual channel command is to be performed, the type of command, the virtual channel it applies to (VCI ID) and the actual command to be performed by the device. To accommodate the all the data in a single bus transfer, the identification that a VCI command is to be performed, the command type and the VCI ID are transferred over predetermined bits of the address bus. The remaining bits available on the address bus are used to transfer the address of the slot on the computer system bus where the network interface device is located. 
     Logic is provided on the device to process the address and data transferred across the address bus and data bus. The slot number notifies the device to latch the information transferred across the address and data busses. The logic parses the data transferred. If the data includes an identification that a VCI command is to be performed, the logic in the card causes the remaining data to be transferred to a relevant subcircuit in accordance with the VCI command type, for example the data transfer subcircuit or data receive subcircuit. Logic within the subcircuit further parses the pertinent data from the information latched and stores the data needed to perform the operation in registers. In the present embodiment, only one register is needed; the register stores the VCI ID (received over the address bus) and the command (which was originally transferred over the data bus). Using the VCI ID, the circuitry performs a lookup to the cache which stores the corresponding physical location of the VCI and certain status bits such as the state of the channel, and executes the command stored in the register. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The objects, features and advantages of the present invention will be apparent from the following detailed description in which: 
     FIG. 1 is an exemplary computer network which implements an asynchronous transfer mode protocol of the transfer of information. 
     FIG. 2 is a simplified block diagram of a network interface device connected to a host computer system. 
     FIG. 3 is a simplified block diagram of a system core illustrating the receive blocks which operates in accordance with the teachings of the present invention. 
     FIG. 4a rates information transferred over the data bus; FIG. 4b illustrates information transferred over the address bus; and FIG. 4c is an illustration of the registers used to store the data utilized to perform a command in accordance with the teachings of the present invention. 
     FIG. 5 is a flow chart generally illustrating the steps performed which enable the transmission of command and data to the device during one bus transmission. 
     FIG. 6 is a simplified block diagram of the receive cache. 
     FIG. 7a and 7b is a flow chart illustrating the process for updating the cache. 
     FIG. 8a, 8b, 8c, 8d and 8e illustrate the process for updating the receive cache. 
     FIG. 9a, 9b, 9c, 9d and 9e illustrate the process for updating the receive cache when the entry is the least recently used entry. 
    
    
     DETAILED DESCRIPTION 
     The system and method of the present invention enables the atomic transmission of commands and associated data such that errors due to a change in state of a high speed cache in the receiving device during a multiple bus cycle transmission is minimized. The present invention will be described in the context of a asynchronous transfer mode (ATM) interface card and, in particular, the cache sub-system which maintains state information regarding the network communication channels established over the network. However, the present invention is not limited as such, and can be utilized in similar environments. 
     FIG. 1 illustrates an exemplary computer system network incorporating the ATM network interface circuit which utilizes the method and apparatus of data transfer coordination of the present invention. The computer system network 10 includes host computer systems (not shown) which incorporate one or more of the ATM network interface circuits (NIC) 12. The NICs 12 are coupled through a local ATM switch 14 to a public ATM switch 16 to enable asynchronous transfer of data between host computer systems coupled to the network 10. Alternately, the NICs 12 can be coupled directly to the public ATM switch 16. As shown in FIG. 1, the computer system network 10 may also include computer systems which incorporate the use of a Local Area Network (&#34;LAN&#34;) emulation 15 which serves as a gateway for connecting other networks such as Ethernet or token ring networks 17 which utilize the ATM network as a supporting framework. 
     FIG. 2 is a simplified system diagram illustrating the architecture of the ATM NIC 12 which utilizes the method and apparatus of data transfer coordination in accordance with a one embodiment of the present invention. The ATM NIC 12 interfaces a host computer system 48 coupled through system bus 38 to the network ATM Cell Interface 40 operating in accordance with the ATM protocol. 
     The ATM NIC 12 shown includes a System Bus interface 20, a Generic Input/Output (&#34;GIO&#34;) interface 24, an ATM Layer Core 22, a Local Slave interface 26, a transmit (TX) FIFO 28, a receive (RX) FIFO 30, a Cell Interface block 32, an External Buffer Memory Interface 34 and a clock synthesis circuit 36. 
     Together, the elements 20-36 of NIC 12 cooperate to transfer data between the host computer 48 and the other computers in the network through multiple, dynamically allocated channels in multiple bandwidth groups. Collectively, the elements of the network interface circuit 12 function as a multi-channel intelligent direct memory access (DMA) controller coupled to the System Bus 38 of the host computer system 48. In one embodiment, multiple transmit and receive channels are serviced as virtual connections utilizing a full duplex 155/622 Mbps (Mega bits per second) physical link. Multiple packets of data, subscribed to different channels over the System Bus 38 to the external buffer memory 42, via the External Buffer Memory Interface 34, are segmented by the System and ATM Layer Core 22 into transmit cells for transmission to the ATM Cell Interface 40 through Cell Interface block 32. The Core 22 also comprises reassembly logic to facilitate reassembly of the received cells to packets. 
     Three memory sub-systems are associated with the operation of the NIC 12. These include the host memory 49 located in the host computer system 48, the external buffer memory 42 external to the NIC 12 and storage block 44 located in the Core 22. The NIC 12 manages two memory areas: the external buffer memory 42 and the storage block 44. The external buffer memory 42 contains packet data for all transmit and receive channels supported by the NIC 12. The storage block 44 contains DMA state information for transmit and receive channels and pointers to data structures in host memory 49 for which DMA transfers are performed. The storage block 44 also contains the data structure specifics to manage multiple transmit and receive buffers for packets in transition between the host 48 and the ATM Cell Interface 40. 
     The host computer system 48 includes host memory 49 which contains data packets and pointers to the packets being transmitted and received. As noted previously, the NIC 12 also shields the cell delineation details of asynchronous transfer from the applications running on the host computer system. For present purposes, it is assumed that software running on the host computer system 48 manage transmit and receive data using wrap around transmit and receive rings with packet interfaces as is well known in the art. 
     The TX and RX buffers, for example, TX and RX FIFOS 28 and 30, coupled between the Core 22 and the Cell Interface block 32, are used to stage the transmit and receive cell payloads of the transmit and receive packets respectively. The Cell Interface block 32 transmits and receives cells to the ATM Cell Interface 40 of the network, driven by clock signals provided by Clock Synthesis Circuit 36. Preferably, the ATM Cell Interface 40, and therefore the Cell Interface block 32, conforms to the Universal Test and Operations Physical Interface for ATM (&#34;UTOPIA&#34;) standard, as described by the ATM Forum specification. To conform to the UTOPIA specification, the clock synthesis circuit 36 provides either a clock signal of 20-25 MHz or 40-50 MHz to enable the Cell Interface block 32 to support an 8-bit stream at 20-25 MHz for 155 Mbps or a 16-bit stream at 40-50 MHz for a 622 Mbps data stream. 
     52-byte data cells each having a 4-byte cell header and a 48-byte payload are transferred from the TX FIFO 28 from TX Buffer Memory 26 via the external buffer memory interface 34 under the control of the Core 22, to the Cell Interface block 32 in groups of 4 bytes. When the Cell Interface block 32 receives the data cells through the TX FIFO 28, it inserts into each cell a header checksum as a fifth byte to the cell header prior to providing the 53-byte data cell to the ATM Cell Interface 40 at either 155 or 622 Mbps. Conversely, when the Cell Interface block 32 receives cells from the ATM Cell Interface 40, it examines the header checksum in the fifth byte of each cell to determine if the checksum is correct. If so, the byte representing the checksum is stripped from the cell and the 52-byte data cell is forwarded to the RX FIFO 30 4 bytes at a time at either 155 or 622 Mbps, otherwise the entire cell is dropped. Transferred bytes are stored in the RX Buffer Memory 45 via the external Buffer Memory Interface 34 under the control of Core 22. 
     In one embodiment, the TX and RX FIFOS 28 and 30 are 33 bits wide, of which 32 bits are used for transmitting data and 1 bit is used as a tag. The tag bit is used to differentiate the 4-byte cell header from the 8-byte cell payload. The tag bit is generated by a TX circuit 50 located within the Core 22. In one embodiment, the tag bit is set to 1 to indicate the start of a cell header and the tag bit is reset to 0 to indicate a cell payload. Thus, the tag bit is 1 for the first 4 bytes of the cell (header) and then the tag bit is 0 for the remaining 48 bytes of the cell (cell payload). 
     Upon receiving the data cells from the TX FIFO 28, a TX circuit 54 located within the Cell Interface block 32 examines the tag bit. If the tag bit is a 1, the TX circuit 54 decodes the corresponding 32 bits as the header of the cell. If the tag bit is 0, the TX circuit 54 decodes the corresponding 32 bits as data. Conversely, when the Cell Interface block 32 receives data cells from the ATM Cell Interface 40, a RX circuit 56 in the Cell Interface block 32 generates a tag bit to differentiate the 4-byte cell header from the 48-byte cell payload. The Cell Interface block 32 then dispatches the data cells in groups of 4 bytes to RX FIFO 30. Upon receipt of the cell data from the RX FIFO 30, an RX circuit 52 in the Core 22 decodes the cell data in accordance with the value of the tag bit as discussed above. 
     Two synchronous clocks signals, a 20 MHz signal and a 40 MHz signal, are provided to the Cell Interface block 32 from the ATM Cell Interface Clock via the Clock Synthesis circuit 36. A 40 MHz clock is supplied to provide a 16-bit data stream at 40 MHz for 622 Mbps in accordance with the specifications of UTOPIA. A divide by 2 of the 40 MHz clock signal is performed in the Clock Synthesis circuit 36 to provide an 8-bit data stream at 20 MHz for 155 Mbps in accordance with the specifications of UTOPIA. The 40 MHz clock signal is also provided to the external buffer memory interface 34 for providing a 1.2 Gbps transmission rate. In addition, the GIO 24 uses the 40 MHz clock signal for transmitting and receiving data. 
     The TX Buffer Memory 46 provides 32 bits of data to the TX FIFO 28 and the RX Buffer Memory 45 reads 32 bits of data from the RX FIFO 30 at every cycle of the 40 MHz clock signal. However, the ATM Cell Interface 40 reads 4 bytes of data from TX FIFO 28 every two clock cycles when operating at 622 Mbps, and reads 4 bytes of data from TX FIFO 28 every 8 clock cycles when operating at 155 Mbps. In the same manner, the Cell Interface block provides 4 bytes of data to TX FIFO 28 every two clock cycles when operating at 622 Mbps, and provides 4 bytes of data to TX FIFO 28 every 8 clock cycles when operating at 155 Mbps. Although the cell burst rate of the Core 22 is different from the cell burst rate of the Cell Interface block 32, the data rate between TX FIFO 28 and the Cell Interface block 32 is, on average, the same as the data rate between the between TX FIFO 28 and the Core 22. Similarly, the data rate between RX FIFO 30 and the Cell Interface block 32 is on average, the same as the date rate between the RX FIFO 28 and the Core 22. This is because the data rate between TX and RX FIFOS 28 and 30 and the Core 22 is dependent the rate that data is read or written by the Cell Interface block 32 respectively. In one embodiment, the depth of the TX FIFO 28 is 18 words or 11/2 cells long and the depth of the RX FIFO 30 is 70 words long. 
     The System Bus Interface 20 and GIO interface 24 insulate the host computer system 48 from the specifics of the transfer to the ATM Cell Interface 40. Furthermore, the Core 22 is insulated from the specifics of the system bus 38 and host specifics. In the present embodiment, the System Bus is an S-Bus, as specified in the Institute of Electronics and Electrical Engineers (&#34;IEEE&#34;) standard 1496 specification. The System Bus Interface 20 is configured to communicate in accordance with the specifications of the system bus, in the present illustration, the S-Bus. It is contemplated that the System Bus Interface 20 can be configured to conform to different host computer system busses. The System Bus Interface 20 is also configured to transfer and receive data in accordance with the protocols specified by the GIO interface 24. The GIO interface 24 provides a singular interface through which the Core 22 communicates with the host computer. Thus, the Core 22 does not change for different embodiments of the NIC 12 which interface to different host computer systems and busses. 
     FIG. 3 is a simplified block diagram of a portion of System and ATM Layer Core 22 showing the transmit (TX) and receive (RX) blocks 310, 315 which respectively function to transmit and receive data from the network. The present invention will be described in the context of the receive block 315; however, it is contemplated that the present invention is also applicable to the transmit block 310. 
     The receive block 315 includes an RX unload 320 and RX load 325 blocks. Cells containing received data from the network are stripped of the cell header, loaded into the RX load block 325, transferred to the RX unload block 320 and subsequently transferred to the host processor. The receive block 315 can support up to 1024 virtual channels, a channel being an identified connection between the host processor and a second host system coupled to the ATM network. To minimize the amount of hardware needed to support 1024 channels, virtual channel support is provided. This is transparent to the host processor. The DMA hardware used to transfer received data to the host memory functions in accordance with state information stored in a memory. The state information enables the receive block to perform transfers with respect to a particular channel. The state information can include status bits indicating physical location/codes of channels, data rates, memory locations to transfer received data to, error information and other information needed to perform transfers in accordance with system specifications and network specifications. 
     To the host processor, the device contains 1024 channels that are individually addressable by the host. As noted above, the RX load hardware a single set of hardware is utilized which operates in one of 1024 states corresponding to the 1024 available channels. Preferably, the states are stored in an external memory. A cache is utilized to store a subset of the different channel states, for example, the cache is large enough to store 128 states. The cache operates at sufficiently high speed to timely process data received over the high speed ATM network. 
     To minimize the risk of error, commands originating from the slower host processor are transferred atomically by utilizing both the address bus and data bus to transfer the information necessary to perform and operation relative to a particular virtual channel. In the present embodiment, the host processor knows the set of addresses that correspond to a channel. The address specifies the command to be performed and the channel the command is to be performed on. Thus the virtual channel command and the address of the channel is transmitted over the address bus and command data, which consists of the data/information needed to perform the command, is transferred over the data bus. Exemplary data transferred over the data bus is illustrated in FIG. 4a and data transferred over the address bus is illustrated in FIG. 4b. 
     Referring to FIG. 4b, the channel address utilized by the host consists of the slot bus address for card on which the device is located, an address indicating that the information is directed to the VCI register (typically a device can contain many other registers that are accessible by the host processor), the type of VCI command and the identification of the channel (VCI ID). This information is transparent to the host. In the preferred embodiment, the host stores an array of addresses for each type of VCI command; therefore the host processor simply selects a particular channel addresses from the array of addresses available for a particular command to be performed. 
     Referring back to the embodiment represented by FIG. 3, the information transferred over the data bus 340 and address bus 345 is received by receive circuit 355 of the card. As is described below, logic at various stages decodes predetermined bits to route the data such that the type of VCI command, VCI ID and command data are stored in the VCI register for subsequent execution. Preferably, logic in the receive circuit 355 parses the slot ID from the bits received across the address bus. Logic on the card (not shown) then determines the chip on the card the information is to be forwarded to from bits 27-20 of the data received over the address bus. The remaining bits and the 32 bits received over the data bus are forwarded to the chip identified in the present example, chip 305. 
     Logic on the component 305 preferably parses bits off bits received from the address bus 345 to determine whether to transfer data to the receive block 315 or to transmit block 310. Receive block 315 includes logic to determine that the data and remaining address information is forwarded to the RX load block 325. If logic in the receive circuit determines from the VCI register address that the information is to be forwarded to the VCI register, the remaining address bits and data bits are forwarded to the VCI command logic 335 which includes VCI register 360. The VCI ID, command type and command data are stored in the VCI register 360. This is illustrated in FIG. 4c. Although the VCI register 360 illustrated in FIG. 4c is represented as three registers, it is readily apparent that the register can be implemented as one or more registers. VCI command logic processes the type of VCI command to determine that the operation is to be performed with respect to one of the virtual channels, and the RX block subsequently executes the command with respect to the identified virtual connection by first retrieving the corresponding state information from the cache 330 (FIG. 3). 
     The VCI ID is used to index into a table to determine whether the information for a particular channel is currently located in the cache 330 (FIG. 3). Referring to FIG. 6, the VCI map 606 identifies whether the VCI is currently cached, item 615, and if it is cached, the location in the cache, item 610. 
     A most recently used (MRU), doubly linked, list 630 is preferably used to maintain pointers into the cache and determine the cache entries to remove first (i.e., the least recently accessed channel). Each entry in the MRU list preferably contains a used field 640, a VCI ID field 645, a previous pointer 650 and next pointer 655. The previous pointer 650 points to the entry next more recently used and the next pointer 655 points to the next entry lesser recently used. The cache 670 caches the information needed by the receive block to process data for the corresponding channel. When the channel information is removed from the cache 670 it is stored in slower memory 680, which provides storage of channel information for all 1024 channels. 
     The MRU list also contains two pointers referred to herein as MRU --  first and MRU --  last. The MRU --  first pointer points to the most recently used entry and the MRU --  last pointer points to the least recently used entry. When a new channel state needs to be brought into the cache it replaces the current least recently used entry, identified by MRU --  last. Preferably MRU --  last is updated during all cache operations. 
     The cache memory is extremely fast to accommodate the high speeds specified by the ATM protocol. Therefore it is not uncommon for the states of the entries in the cache 670 to change quickly. If the system requires multiple bus cycles to transfer the data, and the VCI command block initiates execution upon receipt of a first portion of the information during a first bus cycle and performs a lookup in the VCI Map 605 and MRU list to determine whether the channel information is cached and if cached, the location of the information in the cache 670, and by the time the remaining information has transferred, the channel information is no longer in the cache, the execution of the command will cause an error. By receiving all necessary information atomically such problems are avoided. 
     The process followed to transfer the channel information to cause a command to be executed with respect to a particular virtual channel is illustrated by the flow chart of FIG. 5. At step 505, the host processor prepares the address and data to be transferred to the network interface card. Although only 128 devices are supported at any one time, the complete set of 1024 devices appear available to the host processor. As noted above, the slot address of the network interface card is first removed once the data is received in the core, step 510. The information which identifies that a VCI command and the type of command are then processed and removed from the data stream as this information is used by the logic to direct the data stream to the receive block&#39;s VCI circuit (335 FIG. 3). At step 515, the VCI ID and the command to be performed are placed in registers in the VCI circuit, step 520. The VCI circuit is then able to process the command with respect to a particular channel, step 525. 
     In order to increase efficiency over a random replacement algorithm, the updating of the cache is controlled by a MRU list which provides the information as to which cache entries to remove when another entry needs to be placed in the cache. The MRU list is updated in an innovative manner in order to reduce the number of clock cycles needed to update the list when an entry is accessed or removed. As shown in FIG. 7a, the MRU list is doubly linked, through a series of previous pointers 705 and next pointers 710. Updating of the cache and the supporting structure is preferably controlled by a state machine. 
     The speed enhancements achieved by this structure are evident for the common occurrence of a cached VCI which has just been accessed. This will be explained using the diagrams of FIGS. 8a, 8b, 8c, 8d, 8e and the flow chart of FIGS. 7a and 7b. The cache mechanism will be explained in terms of the cell receiving process and structure; however, it is readily apparent it is equally applicable to the transmit process and structure. 
     Referring to FIGS. 7a and 7b, at step 705, if a cell is to be processed, the cell header is decoded and the channel is identified by reference to the VCI Map, step 710. The DMA state identification and whether it is cached is determined from the VCI map. If the channel information is not cached, step 712, the channel information is read from memory and placed in the cache, step 715, and the operation is performed on the cell, step 740. If the channel information is already cached and is not already located at the top of the list, at step 740, the operation is first performed on the cell. 
     Once the operation is performed on the cell, the MRU list is updated rapidly to move the accessed entry to the top of the list in a minimum number of clock cycles. At step 745, the entry in the MRU list corresponding to the accessed channel is identified through the DMA state. Preferably, at step 747, a test is performed to determine if the entry is the first entry in the MRU list as identified by MRU --  first. If it is the first entry, there is no need to update MRU list and the update process is complete. If the entry is not the first entry, the update process continues at step 750. At step 750, the previous pointer of the current top of the list is updated to point to the accessed entry. 
     At step 753, a test is performed to determine if the entry is pointed by the MRU --  last pointer indicating that it is the least recently used entry. If the entry is not the least recently used entry, at step 755, the previous pointer of the entry pointed to by the next pointer of the accessed entry is updated to point to the entry pointed to by the previous pointer of the accessed entry. 
     If the entry is the least recently used entry, the process continues at step 760 in which the next pointer of the entry pointed to by the previous pointer of the accessed entry is updated to point to the entry pointed to by the next pointer of the accessed entry. The MRU pointer (MRU --  first) which points to the entry at the top of the MRU list is now updated to point to the accessed entry. The next pointer of the accessed entry is also updated to point to the current top of the list and the previous pointer of the accessed entry is updated not point to anything, step 765. After these steps have been performed, the accessed entry functions as the top of the MRU list (770). 
     Thus significant time savings are achieved. Prior techniques could require a significant number of clock cycles just to determine the where the accessed, next and previous entries were located. Using the process and structure described herein, the MRU list is updated after 6 clock cycles. This is illustrated with respect to FIGS. 8a-8e. Referring to FIGS. 8a-8e, the first 3 clock cycles are used to perform a lookup to the VCI Map, identification of the accessed entry in MRU list and update of the previous pointer of the top entry to point to the accessed entry (FIGS. 8a, 8b). During the third clock cycle, the next pointer of the entry prior to the accessed entry is updated (FIG. 8c) and during the fourth clock cycle, the previous pointer of the entry subsequent to the accessed entry is updated (FIG. 8d). Once the above noted pointers have been updated, the last three pointers can be updated during the same clock cycle. In particular, as shown in FIG. 8e, the MRU --  first pointer is updated to point to the accessed entry, and the previous and next entries of the accessed entry are updated. 
     For the instance when the DMA state corresponds to the least recently used entry in the list, the MRU list is updated in accordance with the steps set forth in FIGS. 9a-9e. Referring to FIGS. 9a-9e, the steps performed are cycles quite similar to those described in FIGS. 8a-8e except the MRU --  last pointer is updated during the fourth clock cycle as there is no entry subsequent to the accessed entry to update. 
     The invention has been described in conjunction with the preferred embodiment. It is evident that numerous alternatives, modifications, variations and uses will be apparent to those skilled in the art in light of the foregoing description.