Network interface for changing byte alignment transferring on a host bus according to master and slave mode memory and I/O mapping requests

[0002] 1. Technical Field

[0003] The present invention relates to network interfacing and more particularly to a network interface for efficiently supplying data from a buffer memory to a host bus.

[0004] 2. Background Art

[0005] Network interface devices are commonly used to transmit data between a host computer and network communication systems, such as a local area network. Typical network interface devices include Ethernet-type and IEEE 802.3. One of the primary functions of a network interface is to control the transfer of data between a buffer and the host bus. The data stored in the buffer is retrieved as a result of one of two types of requests, namely master and slave.

[0006] In master mode, a transfer is initiated by a master device which must arbitrate for use of the host bus with a host CPU prior to retrieving the data. In slave mode, the host CPU provides a target device with sufficient information to access the buffer and retrieve the data. A slave access can be performed using two different types of mapping, namely memory mapping and Input/Output (I/O) mapping.

[0007] Transmission of data from the buffer memory to the host bus has traditionally been accomplished by providing specific logic for each type of request. FIG. 9 is a block diagram illustrating a typical buffer architecture 200 for accessing data from buffer memory. An interface unit 202 receives master and slave requests to access the buffer memory 204. The request is directed to a transfer logic 206 that transfers data to or from the buffer memory 204. The transfer logic 206 must be capable of handling each type of request individually. Thus, the interface unit 202 transfers the request to a specific logic portion in the transfer logic 206 based on the nature of the request. For example, the transfer logic 206 includes a first logic circuit 208 that services a master request, a second logic circuit 210 that services an I/O mapped slave request, and a third logic circuit 212 that services memory mapped slave requests.

[0008] Hence, a primary disadvantage associated with current methods of transferring data is the excessive amount of logic necessary to service the different types of requests. Another disadvantage is the increased latency encountered during the data transfer process. This latency can be defined as the delay between the time when data is retrieved from the buffer to the time it is delivered to the host bus. As previously mentioned, additional delays are encountered during transfers initiated by a master device, because the master device must arbitrate for access to the bus with other master devices that also require use of the bus.

[0009] There is a need for an arrangement for supplying data frames to a host bus that minimizes the logic necessary to access a buffer memory using different types of requests.

[0010] There is also a need for an arrangement for supplying data frames to a host bus that minimizes the latency encountered during the transfer of data from a buffer memory.

[0011] There is also a need for an arrangement for supplying data frames to a host bus that is capable of maintaining appropriate byte alignment regardless of the type of transfer.

[0012] These and other needs are attained by the present invention, where master and slave accesses to the buffer are serviced by a single logic circuit that eliminates the need for arbitration, reduces the amount of latency encountered during the data transfer process, and maintains appropriate byte alignment.

[0013] In accordance with one aspect of the present invention, a network interface for supplying data frames to a host bus comprises a buffer memory for storing a data frame received from a network according to a first byte alignment. A memory management unit is configured to transfer the stored data frame from the buffer memory in response to the one transfer request. The memory management unit includes request logic for generating a generic request to access the buffer memory in response to the one transfer request. A bus interface unit is configured for outputting the data frame onto a host bus according to a second byte alignment based on one of a master transfer request and a slave transfer request. The bus interface unit also includes a byte packing circuit configured for changing the first byte alignment of a portion of the stored data frame to the second byte alignment, prior to transfer to the host bus, based on the generic request. The generic request is serviced by a single logic circuit, eliminating the need for specialized logic to individually service master and slave requests. Hence, the network interface can perform master transfers or slave transfers with minimal logic. In addition, the byte packing circuit maintains the appropriate byte alignment regardless of the type of transfer.

[0014] Another aspect of the invention provides a method for accessing a buffer memory in a network interface. The method comprises generating a generic request in response to the detection of one of a master transfer request and a slave transfer request, reading a data frame from the buffer memory based on the generic request, determining a byte alignment for transmission of the data frame onto a host bus based on the generic request, and shifting bytes of the data frame for transfer onto the host bus based on the determined byte alignment. Generating a generic request enables a single logic circuit to service master and slave requests (both I/O and memory mapped) to access the buffer memory. Hence, a data frame from a buffer memory can be transferred to a host bus in accordance with a prescribed byte alignment for different transfer requests using minimal logic.

[0015] Additional objects, advantages, and novel features of the present invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumalities and combinations particularly pointed out in the appended claims.

[0016] Reference is made to the attached drawings, wherein elements having the same reference numeral designations represent like elements throughout and wherein:

[0017] FIG. 1 is a block diagram illustrating a network interface for supplying data frames from a buffer memory to a host bus according to an embodiment of the present invention.

[0018] FIG. 2 is a block diagram illustrating the buffer architecture of the network interface of FIG. 1 according to an embodiment of the present invention.

[0019] FIGS. 3A and 3B are diagrams illustrating the structure of the buffer memory of FIG. 2.

[0020] FIG. 4 is block diagram illustrating a request logic for generating generic requests according to an embodiment of the present invention.

[0021] FIG. 5 is a block diagram illustrating circuitry configured for data transfer from the buffer memory of FIG. 2 according to an embodiment of the present invention.

[0022] FIG. 6 is a timing diagram illustrating a master data transfer according to an embodiment of the present invention.

[0023] FIG. 7 is a diagram illustrating alignment of data from the buffer memory of FIG. 2 to a system memory according to an embodiment of the present invention.

[0024] FIG. 8 is a timing diagram illustrating a slave transfer according to an embodiment of the present invention.

[0025] FIG. 9 is a block diagram illustrating a prior art buffer architecture for servicing requests to access a buffer memory.

[0026] The present invention will be described with the example of a network interface in a packet switched network, such as an Ethernet (IEEE 802.3) network. A description will first be given of a network interface architecture, followed by the arrangement for transferring data from a buffer memory to a system memory using a single logic circuit configured for responding to master and slave requests. It will become apparent, however, that the present invention is also applicable to other network interface systems.

[0027] FIG. 1 is a block diagram of an exemplary network interface 10 that accesses the media of an Ethernet (ANSI/IEEE 802.3) network according to an embodiment of the present invention.

[0028] The network interface 10, preferably a single-chip, 32-bit Ethernet controller, provides an interface between a local bus 12 of a computer, for example a peripheral component interconnect (PCI) local bus, and an Ethernet-based media 50.

[0029] The interface 10 includes a PCI bus interface unit 16, a buffer memory portion 18, and a network interface portion 20. The PCI bus interface unit 16 includes a PCI slave interface 16aand a direct memory access (DMA) interface 16b. The slave interface 16amanages PCI control and status information including reading and programming of the PCI status registers, but may also be configured for managing slave transfers via the PCI bus with a host CPU. The DMA interface 16bmanages DMA transfers by the network interface 10 to and from system memory. Hence, the PCI bus interface unit 16 can be selectively configured for PCI transfers in slave and/or master (e.g., DMA) mode.

[0030] The memory portion 18 includes a 32 bit static random access memory (SRAM) implemented directly on the network interface chip 10. According to the disclosed embodiment, the SRAM 18 may be accessed in a random access manner under the control of a memory management unit 22, or may be segmented into a receive portion 18aand a transmit portion 18bfor receive and transmit paths, respectively.

[0031] The network interface 10 also includes a buffer management unit 24 configured for managing DMA transfers via the DMA interface 16b. The buffer management unit 24 manages DMA transfers based on DMA descriptors in host memory that specify start address, length, etc. The buffer management unit 24 initiates a DMA read from system memory into the transmit buffer 18bby issuing an instruction to the DMA interface 16b, which translates the instructions into PCI bus cycles. Hence, the buffer management unit 24 contains descriptor management for DMA transfers, as well as pointers associated with storing and reading data from the memory portion 18. Although the buffer management unit 24 and the memory management unit 22 are shown as discrete components, the two units may be integrated to form a memory management unit (MMU) 52 controlling all transfers of data to and from the memory unit 18, as seen with additional reference to FIG. 2.

[0032] The network interface 20 includes a media access control (MAC) core 26, a general purpose serial interface (GPSI) 28, a media independent interface (MII) 30 for connecting to external 10 Mb/s or 100 Mb/s physical (PHY) transceivers, an external address detection interface (EADI) 32, an attachment unit interface (AUI) 34 having a Manchester encoder and decoder, and a 10/100 Mb/s twisted pair transceiver media attachment unit (MAU) 36.

[0033] The network interface 10 also includes a network port manager 38 configured for performing MII handshaking between two devices on an MII bus via the MI port 30. Such MII handshaking may include link information, programming information at the MII layer using a management data clock (MDC), and management data input/output (MDIO) paths.

[0034] The auto-negotiation portion 40 performs IEEE-compliant negotiation with a link partner on the PHY layer to exchange data indicating whether the link partner is capable of operating at 10 Mb/s, 100 Mb/s, and whether the link should be half-duplex or full-duplex.

[0035] The LED controller 44 selectively controls the generation of LED output signals based upon the internal decoding logic and network interface status registers (not shown). The network interface 10 also includes an IEEE 1149.1-compliant JTAG boundary scan test access port interface 36.

[0036] The EEPROM interface 42 connects to an EEPROM on either a network interface adapter card or the motherboard of the host computer via a serial interface link. The EEPROM (not shown in FIG. 1) will be programmed with configuration information related to the network interface 10, enabling the network interface 10 to be configured during initialization via the EEPROM interface 42. Once initialized, the network interface 10 stores the configuration information in internal registers (not shown), enabling the network interface 10 to operate independently of the host computer in the event the host computer is powered down. Hence, the network interface 10 can be configured to operate while the host computer is in a stand-by mode, enabling the network interface 10 to output power up information to logic within the host computer to enable the host computer to automatically turn on in response to data packets received from the network and having a specific protocol, described below.

[0037] FIG. 2 is a block diagram illustrating the buffer architecture of the network interface 10 according to an embodiment of the present invention. As shown in FIG. 2, transfer of data frames between the PCI bus interface unit 16, also referred to as the bus interface unit (BIU), and the MAC 20 is controlled by a memory management unit (MMU) 52 including the buffer management unit 24 and the SRAM MMU 22 of FIG. 1. The memory management unit 52 controls the reading and writing of data to the SRAM 18, illustrated in FIG. 2 as a receive SRAM portion 18aand a transmit SRAM portion 18bfor convenience. It will be recognized in the art that the receive SRAM (RX_SRAM) 18aand the transmit SRAM (TX_SRAM) 18bmay be implemented as a single memory device, or alternatively as two separate SRAM devices.

[0038] As shown in FIG. 2, the memory management unit includes the buffer management unit 24, also referred to as the descriptor management unit, the SRAM MMU 22, and an arbitration unit 54. The arbitration unit 54 arbitrates DMA requests for data transmission, data reception, descriptor lists from the descriptor management block 24, and status.

[0039] The SRAM MMU 22 includes separate controllers for each SRAM 18aand 18b, for both read and write operations. According to the disclosed embodiment, the network interface 10 operates in two generic clock domains, namely a host computer bus clock domain 56a, and a network clock domain 56b. Since the network interface 10 needs to send and receive data across two independent clock domains 56, divided by the dotted line 58, the SRAM MMU 22 must be capable of reading and writing data to each SRAM 18aand 18bin a manner that tracks memory status independent of the PCI clock in the host computer domain and the MAC clock generated from network activity in the network domain 56b.

[0040] According to the disclosed embodiment, the SRAM MMU includes a transmit-data bus-side memory management unit (XB_MMU) 22a, a transmit-data MAC-side memory management unit (XM_MMU) 22b, a receive-data bus-side memory management unit (RM_MMU) 22c, a receive-data MAC-side memory management unit (RM_MMU) 22d, and a synchronization circuit 60. The XB_MMU 22aand the RM_MMU 22doperate as write controllers configured for writing frame data into the SRAMs 18band 18a, respectively. The XB_MMU 22aand the RB_MMU 22coperate according to the PCI bus clock (CLK). The RM_MMU 22doperates according to the receive MAC clock (RX_CLK) received by the MAC 20, and the XM_MMU 22boperates under the control of the MAC transmit clock (TX_CLK) received by the MAC 20. The XM_MMU 22band the RB_MMU 22coperate as read controllers configured for reading frame data from the SRAMs 18band 18a, respectively. Hence, receive data from the MAC 20 is written into the RX_SRAM 18aunder the control of the write controller 22dsynchronous to the receive clock (RX_CLK) in the network clock domain 56b. Frame data stored in the RX_SRAM 18ais read and output to the BIU 16 via data path 62aunder the control of the receive-data read controller 22c, which reads the frame synchronous to the PCI bus clock signal.

[0041] Similarly, transmit data to be output onto the network by the MAC 20 is written into the TX_SRAM 18bvia data path 62bunder the control of the transmit-data write controller 22a, configured for writing the frame data synchronized to the PCI bus clock (CLK). The stored transmit data is read and output from the TX_SRAM 18bto the MAC 20 under the control of the transmit-data read controller 22baccording to the MAC transmit clock (TX_CLK) within the network clock domain 56b.

[0042] The presence of two separate clock domains 56aand 56bin writing and reading to a random access memory 18 requires that the write controller and read controller devices be coordinated and synchronized to ensure that no contention issues arise due to the relative independence of the two clock domains 56aand 56b. The SRAM MMU 22 includes a synchronization circuit 60 that asynchronously monitors the status of the RX_SRAM 18aand 18b, enabling the memory controllers to read and write to the memory 18 between the two clock domains 56aand 56b. Thus, problems that would ordinarily arise between the two clock domains in the individual memory management units 22a, 22b, 22cand 22dare avoided by use of the synchronization circuit 60 according to a prescribed arbitration logic.

[0043] FIGS. 3A and 3B illustrate the structure of data frames 70, 70′ stored in the SRAM 18. Each data frame 70 is 33 bits wide, 1 bit corresponding to an ENF parameter 74 and a 32 bit double word (DWORD) corresponding to data or control information. The various types of DWORDs in the data frame 70, include a frame track 72, frame data 86, an upper status 88, and a lower status 90. The frame track 72 functions as a header for the frame 70 and includes a FRM parameter 76, a LONG parameter 78, a CNT parameter 80, an ENF_ADDR parameter 82, and a BE_L parameter 84. The FRM parameter 76 is a 1 bit parameter that, when set to 1, indicates the entire received frame 70 stored in the SRAM 18 is valid. The LONG parameter 78 is a 1 bit parameter that, when set to 1, indicates the expected receive frame length is greater than a preset length. The CNT 80 parameter is a 14 bit byte count that indicates the total number of bytes in the receive frame 70. The ENF_ADDR. parameter 82 is a 12 bit parameter that indicates the end address of the frame 70. The CNT parameter 80 and the ENF_ADDR parameter 82 are only valid when the FRM bit 76 is set to 1. The BE_L parameter 84 is a 4 bit active low parameter used to indicate which bytes are valid in the last DWORD (D LAST) of the frame 70. The ENF bit 74 is set to 1 to identify the final data segment 86 in the frame 70, and set to 0 for all other DWORDs. The upper status and lower status DWORDs 88, 90 provide control information, such as the total number of bytes of receive data in the frame, necessary to perform slave transactions.

[0044] FIG. 4 is a block diagram illustrating a request logic 96 for generating generic transfer requests according to an embodiment of the present invention. The bus interface unit 16 is configured for outputting different types of requests to access the buffer memory 18athrough a request logic 96 in order to generate a generic request. The requests can be in the form of master or slave. In addition, the slave requests can be either I/O mapped or memory mapped. As shown in FIG. 4, the request logic 96 is implemented as an OR gate 98. The OR gate 98 generates the generic request which is output to the read controller 22c(RB_MMU) in order to access the buffer memory 18a.

[0045] FIG. 5 is a diagram illustrating a logic circuit 100 for transferring data from the SRAM 18ato the BIU 16 according to an embodiment of the present invention. The logic circuit 100 includes a bus side control block 102 for reading stored frame data from the SRAM 18a, and a byte packing circuit 104 for aligning portions of the frame data prior to transfer to the host bus (e.g., the PCI bus 12). The bus side control block 102 includes an address register 106, a read line control register 108, an increment circuit 110, and an advance signal generator 112. The address register 106 stores an address pointer value (RB_ADDR) that identifies a memory location in the SRAM 18afor reading a corresponding 32-bit double-word (DWORD) of frame data. The address register 106 includes a multiplexer 106aand a delay flip flop 106b. The increment circuit 110 supplies an incremented address pointer value (RB_ADDR) which is stored in the address register 106. The address register 106 stores the address pointer value in response to a bus-side advance signal (RB_ADV) supplied to the selection input of multiplexer 106a. The advance signal generator 112 generates the bus-side advance signal (RB_ADV) in response to an advance signal (AD_ADV) generated by the BIU 16. A read control line register 108, which includes a multiplexer 108aand a delay flip-flop 108b, outputs an active-low read signal (RB_RD_L) to the RX_SRAM 18ain response to the bus-side advance signal (RB_ADV), causing the RX_SRAM 18ato output the stored DWORD from the location addressed by the address pointer value (RB_ADDR).

[0046] The byte packing circuit 104 includes a first holding register 114, a second holding register 116, and a third holding register 118. As shown in FIG. 5, the first holding register 114 includes a multiplexer 114athat selectively outputs the output frame data received from the RB_SRAM_DATA. signal path based on the bus-side advance signal (RB_ADV), and a delay flip flop 114bthat latches the frame data at the rising edge of a clock signal (BCLK). The second holding register 116 also includes a multiplexer 116awhich selectively outputs frame data received from the RB_DATA signal path based on the bus-side advance signal (RB_ADV), and a delay flip flop 116b. Similarly, the third holding register 118 includes a multiplexer 118aand a delay flip flop 118b.

[0047] SRAM 18aoutputs the 32-bit DWORD of frame data to the first holding register 114. A 32-bit multiplexer 120 selectively supplies (on a bit-by-bit basis) the outputs of the first holding register 114 (via RB_DATA) or the second holding register 116 (via DMA_DFF), in response to a 32-bit selection signal from a BIU state machine 122, to the third holding register 118 where the data is held until it is ready for transfer onto the host bus 12 along the AD signal path. The advance signal (AD_ADV) generated by the BIU 16 indicates that frame data has been successfully transferred to the host bus 12 and that the BIU 16 is ready to accept new frame data. The advance signal (AD_ADV) generated by the BIU 16 drives an advance signal generator 112 to produce the bus-side advance signal (RB_ADV). The BIU state machine 122 receives various signals from the DMA descriptor management block 24, the RB_MMU 22c, and the BIU 16 which are used to generate the control signals output to multiplexer 120 and the multiplexer 118a. The signals include active low byte enable values for master transfers (DMA_BE_L), active low byte enable values for slave transfers (SLV_BE_L), and the advance signal (AD_ADV).

[0048] FIG. 6 is a timing diagram illustrating a master (DMA) transfer according to an embodiment of the present invention. The bus interface unit (BIU) 16 and the read controller 22c(RB_MMU) are both inactive at the start of clock cycle 1. At the start of clock cycle 2, the DMA descriptor management block 24 asserts DMA_REQ to request transfer of data to or from a host memory, and outputs a DMA address value (ADDR1) along signal path DMA_ADDR. The BIU 16 deasserts a bus request signal (REQ#) on the host bus 12 and outputs an internal grant signal (DMA_GNT) to the descriptor management block 24 during clock cycle 2 and in response to the DMA request signal (DMA_REQ). The DMA descriptor management block 24 also outputs a DMA count value (DMA_XFR_CNT) to the BIU 16 and the read controller 22c(RB_MMU) indicating the number of PCI data transfers to be completed during the transaction. As shown in FIG. 6, the DMA address (ADDR1) and the number of data transfers (DMA_XFR_CNT) are valid as long as the DMA_REQ remains asserted. The DMA descriptor management block 24 also outputs active low byte enable signals (DMA_BE_L), representing a value (BEL1) of valid bytes for the first data block, to the BIU 16 and read controller 22c(RB_MMU) during clock cycle 1. The SRAM 18aoutputs the first block (D1) of data as a 32-bit double word (DWORD) to the first holding register 114 via the signal path RB_SRAM_DATA during clock cycle 1 in response to the corresponding address (RB_ADDR) output by the address register 106 and the read signal (RB_RD_L) output by the read control line register 108.

[0049] Assertion of DMA_REQ and DMA_GNT cause the BIU 16 to assert an advance signal (AD_ADV) during clock cycle 3, indicating that the BIU 16 is ready to transfer data. The advance signal (AD_ADV) drives the advance signal generator 112 used to produce the bus-side signal RB_ADV. As shown in FIG. 5, the bus-side advance signal (RB_ADV) controls the address register 106, the read control line register 108, the first holding register 114, and the second holding register 116. During clock cycle 3, the PCI arbiter (e.g. the host CPU) deasserts GNT# in response to REQ#, thus granting ownership of the host bus 12 to the network interface 10 at the next cycle.

[0050] At the start of clock cycle 4, the network interface 10 asserts FRAME# on the host bus 12. The first data block, D1, is also output to the first holding register 114 from the SRAM along the RB_SRAM_DATA path at the start of clock cycle 4. Deassertion of REQ# and GNT# causes the BIU 16 to keep AD_ADV asserted during clock cycle 4. Assertion of AD_ADV causes the count (ADV_CNT) to be decremented each clock cycle. In response to AD_ADV at clock cycle 4, a byte enable value (BEL2) for the second data block D2 is output to DMA_BE_L, and the value of read line pointer (RB_ADDR) in the address register 106 is incremented to the value A2. The first data block D1 is simultaneously transferred from the first holding register 114 to the second holding register 116 and to multiplexer 120 along the signal path RB_DATA, while the second data block, D2, is loaded into the first holding register 114 along the signal path RB_SRAM_DATA during clock cycle 4.

[0051] The BIU 16 asserts GNT# during clock cycle 5, and the byte enable signal (BEL3) for the third data block D3 is output to DMA_BE_L. The third data block D3 is also output from the SRAM 18avia the RB_SRAM_DATA path to the first holding register 114, and the second data block D2 is output from the first holding register 114 to the second holding register 116 via signal path RB_DATA. The first data block, D1, is transferred to the third holding register 118 as a host bus (e.g., PCI) data block D′1 where it will be output to host bus 12 along the AD signal path.

[0052] The data blocks output by the SRAM 18aon the RB_SRAM_DATA path are aligned according to a first byte alignment. The PCI data blocks output to the host bus 12 by the third holding register 118 are aligned according to a second byte alignment. Consequently, the PCI data block D′1 may not be identical to the first data block D1 because the first data block, D1, must be properly aligned for output to the host bus 12 before it can be transferred to the system memory. FIG. 7 illustrates alignment of data from the SRAM for a variety of data transfer methods. A sample data frame contains six data blocks that must be transferred to the system memory. Each data block in the SRAM 18acontains 4 bytes of information, with the exception of the last data block, which may contain 1 to 4 bytes of information. The data is shifted and moved to the system memory according to the second byte alignment, based on the byte enable values. For example, the SRAM receive frame shown in FIG. 7 contains a DWORD having a first byte alignment “03 02 01 00”. The data in the SRAM 18ais contiguous except for the last DWORD, which contains only two bytes. However, the frame data in the SRAM 18amust be properly shifted, according to the second byte alignment, (e.g., “02 01 00 XX”) before it can be moved to the system memory.

[0053] In order to properly align the data retrieved from the SRAM on the host bus, a byte enable value (BEL) is output by the descriptor management block 24 to the bus interface unit 16 for each data block. The BIU state machine 122 outputs byte selection signals to multiplexer 120 in response to byte enable values so that selected bytes (or bits) may be transferred to the third holding register 118. The first byte enable value, BEL1, specifies the number bytes from the first data block, D1, which may be transferred to the third holding register 118 for transfer to the host bus 12 along signal path AD. The second holding register 116 retains the value of the first data block, D1 so that the BIU state machine 122 can control the multiplexer 120 may output selected bytes of the first data block, D1, or the second data block, D2. Hence, the output of the second holding register 116 is shifted by multiplexer 120, so that the PCI data block D′1 output to the third holding register 118 will be properly aligned on the host bus. Accordingly, it should be understood that the BIU state machine 122 can control the multiplexer 120 for bit-wise or byte-wise manipulation of the data blocks. Alternatively, a plurality of 1 bit multiplexers, corresponding to the length of the data block in bits, may be substituted.

[0054] The network interface 10 and the target device respectively deassert IRDY# and TRDY#, al the start of clock cycle 6, to indicate readiness to complete transfer of the first PCI data block, D′1, to the host bus 12. Once transfer of the first PCI data block is complete, transfer of a second PCI data block D′2 may be initiated. During clock cycle 6, the DMA descriptor management block 24 outputs a byte enable value (BEL4) for the fourth data block, D4, along the signal path DMA_BE_L to the BIU state machine 122. The value for the read pointer (RB_ADDR) stored in the address register 106 also is incremented, and the fourth data block, D4, is output to the first holding register 114 via the RB_SRAM_DATA signal path. The third data block, D3, is output from the second holding register 116. The second byte enable value (BEL2) is used by the BIU state machine 122 to control multiplexer 120 for aligning data from the second data block, D2, with data from the first data block, D1, which was retained in the second holding register 116, and output the second PCI data block, D′2, to the host bus 12. The byte alignment process is repeated until all the data transfers are complete.

[0055] During clock cycle 6, the BIU 16 asserts an acknowledgement signal (BUS_XFR) which is output to the DMA descriptor management block 24 to indicate successful transmission of a PCI data block. The acknowledgement signal remains asserted throughout the entire data transfer process, clock cycle 7 to clock cycle 14. At the start of clock cycle 13, the network interface 10 asserts FRAME# to indicate that it is ready to complete the data transfer phase and relinquish control of the host bus 12. At the start of clock cycle 14, the BIU 16 outputs a termination signal (DMA_DONE) to the RB_MMU indicating that the data transfer sequence is successfully completed. Similarly, the network interface 10 and the target device assert IRDY# and TRDY#, respectfully, to indicate the last transaction has occurred.

[0056] FIG. 8 is a timing diagram illustrating a slave transfer according to an embodiment of the present invention. The host CPU deasserts FRAME# at clock cycle 1 to indicate the start of a transaction, and supplies the starting address (ADDR1) and the transaction type (RD) on the AD and CBE# paths of the host bus 12, respectively. According to the disclosed embodiment, the two least significant bits of the starting address (ADDR1) are used to encode byte enable values for the transaction. Since each data block is in the form of DWORD, the two least significant bits do not effect the data block specified by a particular address location. For example, a starting address whose last two hexadecimal digits are 10, converts to a binary value of 10000. The two least significant bits. 00, represent the encoded byte enable values. Similarly hexadecimal addresses whose last two digits are 11, 12, and 13 generate encoded byte enable values of 01, 10, and 11, respectively. The least significant bits are converted to 4-bit byte enable values (SLV_BE_L) as shown in Table 1:

[0057] Hence, the same logic may be used to interpret byte enables for master or slave requests.

[0058] In response to the signals asserted by the host CPU, the read controller 22c(RB_MMU) outputs a value (A1) for the read pointer (RB_ADDR) to the address register 106. The SRAM 18aoutputs a first data block, D1, to the first holding register 114 via the signal path RB_SRAM_DATA in response to the corresponding address (RB_ADDR) output by the address register 106 and the read signal (RB_RD_L) output by the read control line register 108. As shown in FIG. 8, the value of the read pointer (RB_ADDR) in the address register 106 and the data in the first holding register 114 are valid for three clock cycles.

[0059] During clock cycle 2, the host CPU enters a wait state on the address signal path (AD), and outputs a byte enable signal along the command signal path (C/BE#) having a byte enable value, for example, of zero. The host CPU deasserts IRDY# to indicate its readiness to commence the next data transfer. The slave device (e.g., the target network interface addressed by the host CPU) deasserts DEVSEL# during clock cycle 3 in response to the starting address (ADDR1). The BIU 16 asserts an internal chip select CS_RB on the read controller 22c(RB_MMU) to indicate a slave read access from the SRAM 18a. The BIU 16 decodes the starting address (ADDR1) and outputs a byte enable signal (SLV_BE_L) to BIU state machine 122. The BIU 16 also asserts AD_ADV to indicate that it is ready to accept data.

[0060] During clock cycle 4, the second data block, D2, is output by the SRAM 18ato the first holding register 114 along the signal path RB_SRAM_DATA, and the first data block, D1, is concurrently transferred to the second holding register 116 and multiplexer 120 along the signal path RB_DATA in response to the advance signal (AD_ADV).

[0061] During clock cycle 5, the slave device deasserts TRDY# on the host bus 12 to indicate that it is ready to complete the current data phase. The third data block, D3, is loaded into the first holding register 114, and the second data block, D2, is transferred to the second holding register 116. The first data block, D1, is aligned for transfer onto the host bus 12 according to the byte aligning process previously described with reference to the BIU state machine 122.

[0062] During clock cycle 6, the BIU 16 asserts BUS_XFR to indicate successful transfer of the first PCI data block D′1, and the SRAM 18aoutputs the fourth data block, D4, to the first holding register 114. The third data block, D3, is transferred from the first holding register 114 to the second holding register 116, and a second PCI data block, D′2, is output to the host bus 12. The data transfer process continues until clock cycle 9, at which point the fourth PCI data block D′4 has been output on the host bus 12.

[0063] During clock cycle 9, the host CPU asserts IRDY# to halt the current data transfer. During clock cycle 10, the host CPU deasserts FRAME# and outputs a new starting address (ADDR5) to the address path on the host bus 12 to continue the data transfer. The host CPU also outputs a read command (RD) along the command signal path (CBE#) of the host bus 12 to indicate a read transaction from the SRAM 18a. The BIU deasserts CS_RB and the advance signal (AD_ADV). During clock cycle 10, the slave device (e.g., the network interface) also continues reading data from the SRAM 18a. During clock cycle 11, the host CPU inserts a wait state into the address path of the host bus 12, and begins to load a byte enable value on the command signal path (CBE#). The host CPU deasserts IRDY# to indicate that it is ready to initiate data transfer.

[0064] The BIU 16 deasserts DEVSEL# and CS_RB during clock cycle 12 in response to deassertion of IRDY# by the host CPU. The BIU 16 also asserts AD_ADV. During clock cycle 13, the sixth address for the read pointer is loaded into the address register 106, D5, the sixth data block, D6, is read into the first holding register 114, and the fifth data block, D5, is transferred to the second holding register 116. During the fourteenth clock cycle the slave device deasserts TRDY#, indicating that it is ready to receive data. The address for the read pointer (RB_ADDR) in the address register 106 is incremented, and the seventh data block D7 is loaded into the first holding register 114. The sixth data block, D6, is transferred from the first holding register 114 to the second holding register 116. The fifth data block, D5, is manipulated and transferred to the host bus 12 as a fifth PCI data block, D′5, as previously described until the data transfer is completed.

[0065] As shown in FIGS. 5 and 8, slave accesses of the SRAM 18aare performed by retrieving the address from the AD signal path during the PCI address phase, followed by incrementing the read pointer value in the address register 106 based on the bus side advance signal (RB_ADV). Hence, the disclosed embodiment uses a generic logic configuration that can perform slave transfers independent of whether I/O mapped transfers or memory mapped transfers are used by the host CPU.

[0066] According to the disclosed embodiment, a generic transfer logic is configured for performing data transfers from a buffer memory to a host data bus, independent of whether the data transfer is a master access or a slave access. In addition, a byte packing circuit ensures that appropriate byte alignment is maintained between data output from the receive buffer 18aand the host bus 12, regardless of whether a master transfer or slave transfer is performed.

[0067] While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.