Apparatus and method for transferring data between buses of different widths

Abstract: A byte tracking system (60) has a 32-bit wide system data bus (34). System bus interface unit (48) connects the system bus (34) to the transmit FIFO buffer memory (42) to supply 4-byte data words (62) to the FIFO buffer memory (42). The FIFO buffer memory (42) has word write and read accessibility. A 4:1 multiplexer (64) is connected at the output side of the FIFO buffer memory (42) by a 32-bit wide bus (66). The multiplexer (64) is used to multiplex correct bytes (63) from the data words (62) to an 8-bit output bus (68). A byte tracker circuit (70) controls the multiplexer (64) and determines which byte (63) is to be sent to the output bus (68). The bytes (63) supplied to output bus (68) are converted by a parallel to serial converter (69) to a serial bit stream, which is supplied to Manchester encoder/decoder (36).

Description: The present invention relates generally to an apparatus and method for transferring data between two data busses having different widths. More particularly, it relates to such apparatus and method including a digital storage buffer designed to interface the two data busses, where information is passed to the buffer sequentially from one bus and retrieved from the buffer sequentially to the other bus. Most especially, the invention relates to such apparatus and method including a logic circuit to interface a wider data bus to a second bus having a size equal to the largest common denominator of the two busses through a First-In-First-Out (FIFO) buffer memory. In such systems, data presented from the wider bus may contain a full word, which represents the full width of the bus, or a partial word, which means that only part of the bus has valid data associated with it. The interface must recognize the invalid data in the partial word and must not pass the invalid data to the second bus. It is known to use a perfect data aligner to recognize and eliminate the invalid data before passing the aligned data to the FIFO memory buffer. The data aligner typically consists of a multiplexer which packs data to eliminate the invalid data fields. A temporary holding register is used to store data during the packing process. Any part of a word not written to the FIFO buffer memory immediately has to be stored in the temporary holding register, and any subsequent word has to be aligned with these stored data. The resulting logic requires a substantial amount of control logic, which is a speed limitation for a high speed bus interface. We will describe an arrangement to eliminate the need for a perfect aligner in a system and method for transferring data between two data busses having different bus widths. We will describe a system and method in which speed limitations introduced by control logic for a perfect aligner are avoided. The present arrangement circumvents the limitations of perfect data aligners and their control logic by eliminating the need for a perfect aligner. The data word, full or partial, is written in its entirety to the FIFO buffer memory, with invalid data marked as holes in an associated hole register. A byte tracker is installed at the output end of the FIFO buffer memory, where the invalid data is skipped while the data is being read out of the buffer. The attainment of these and related objects is addressed through use of the novel byte tracking system and method as claimed. A byte tracking system in accordance with this invention transfers data in multiple byte words between a first bus and a second bus having different widths. A system interface is connected to the first bus. A first-in-first-out (FIFO) buffer memory has an input connected to the system interface by a system interface bus with a width corresponding to the first bus. The first-in-first-out buffer memory has an output connected by a buffer memory output bus with a width corresponding to the first bus. The buffer memory output bus is connected to a multiplexer. The multiplexer is connected to the second bus. The apparatus is characterised by a first-in-first-out buffer memory configured to store each multiple byte word in a multiple byte data field with an associated multiple bit byte validity field, a system interface generating byte validity data to the multiple bit byte validity field in the FIFO buffer memory, and a byte tracker connected to the byte validity fields of the first-in-first-out buffer memory and to the multiplexer to control operation of the multiplexer to select and provide valid bytes from the first-in-first-out buffer memory to the second bus, in response to data in the byte validity field. A byte tracking method in accordance with this invention transfers data in multiple byte words between a first bus and a second bus having different widths. The method comprises the steps of receiving the multiple byte words by means of system interface, storing the words in a first-in-first-out (FIFO) buffer memory, supplying the multiple byte words from the FIFO to a multiplexer and providing bytes from the multiplexer to the second bus. The method is characterised by generating corresponding byte validity data for each multiple byte word, storing the corresponding byte validity data in the FIFO buffer memory in association with the multiple byte word, providing the byte validity data from the FIFO buffer memory to a byte tracker (70), and, controlling the multiplexer (64) using the byte tracker (70) to select valid bytes from the FIFO buffer memory. The attainment of the foregoing and related objects, advantages and features of the invention should be more readily apparent to those skilled in the art, after review of the following more detailed description of the invention, taken together with the drawings, in which: BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram of a system in accordance with the invention. Figure 2 is a functional block diagram of a portion of the system of Figure 1. Figure 3 is a representative data format used in the system of Figure 1. Figure 4 is a more detailed block diagram of part of the system portion shown in Figure 3. Figure 5 is a circuit diagram corresponding to the part of the system portion shown in Figure 4. DETAILED DESCRIPTION OF THE INVENTION Turning now to the drawings, more particularly to Figure 1, there is shown a media access controller (MAC) 30 used as an interface in an Ethernet data processing network. The MAC 30 controls the passage of information between a shared Ethernet serial data path 32 and a system bus 34. The MAC 30 includes a Manchester encoder/decoder 36 and a MAC core 38, which in the preferred embodiment, is implemented in accordance with the IEEE 802.3 standard. The MAC 30 also includes a receive FIFO 40, a transmit FIFO 42 and FIFO control logic 44 as well as command and control registers 46. A system bus interface unit 48 is logically disposed between the receive and transmit FIFOs 40 and 42 and the system bus 34. The MAC core 38 includes a station address detect (SAD) unit 50 which determines whether packets received by the MAC 30 in fact are addressed to it and should be captured in their entirety, or are addressed to a different MAC station (not shown) and should be rejected. The byte tracking system of this invention is incorporated in the system bus interface unit 48, transmit FIFO 42, MAC core 38 and Manchester encoder/decoder 36 of the MAC 30. Further background information on the Ethernet data processing network and the MAC 30 are provided in our USDPatent application No 07/841113, corresponding to US Patent 5305321, entitled, "Ethernet Media Access Controller with External Address Detection Interface and Associated Method,". Details of byte tracking system 60 are shown in Figure 2. System bus 34 is a 32-bit wide data bus. System bus interface unit 48 connects the system bus 34 to the transmit FIFO buffer memory 42 to supply 4-byte data words 62 to the FIFO buffer memory 42. The FIFO buffer memory 42 has word write and read accessibility. A 4:1 multiplexer 64 is connected at the output side of the FIFO buffer memory 42 by a 32-bit wide bus 66. The multiplexer 64 is used to multiplex correct bytes 63 from the data words 62 to an 8-bit output bus 68. A byte tracker circuit 70 controls the multiplexer 64 and determines which byte 63 is to be sent to the output bus 68. The bytes 63 supplied to output bus 68 are converted by a parallel to serial converter 69 to a serial bit stream, which is supplied to Manchester encoder/decoder 36. The FIFO buffer memory 42 has control logic (not shown) to implement the First-In-First-Out function. The system and method is independent of the depth of the FIFO buffer memory. Each line of the FIFO buffer memory 42 consists of a 4 byte data field 72 and a 4 bit hole field 74. Figure 3 shows a generalized format for the data as stored in the FIFO buffer memory 42. As shown, each multiple byte word consists of n data bytes n-1:0 (where n is 4 in the case of the system of Figures 1-2) and n bits of status tag called HOLE n-1:0. Each word is written entirely into the FIFO memory 42 during a write access. For each byte in the word, the write access logic generates the corresponding HOLE bit to indicate the validity of the byte. At the output side of the FIFO buffer memory 42, the entire word is read out and passed over to the multiplexer 64. The byte tracker 70 determines which byte is to be passed to the data output bus 68 of the byte tracking system 60. For each word that is read from the FIFO buffer memory 42, the byte tracker scans the HOLE bits starting from the lowest order bit and searches for the first bit that is not set. The bit that is not set corresponds to the next valid byte that would be sent to the output of the system 60 when a byte is requested. After this byte is read out, the byte tracker 70 proceeds to search for the next valid byte by looking at the holes again, skipping the bits that are set or have already been scanned. Figures 4 and 5 show details of the byte tracker 70. The byte tracker 70 scanning is implemented with three elements: (1) Logic 80 determines the current valid bytes 63. It does so using the relationship, VALID(i) == not (HOLE(i)) and not (MASK(i)). VALID(n-1:0) indicates which of the n bytes of the word remain valid. The HOLE information provided in the status tag is filtered by the MASK, which is updated after every access. Any byte that has already been accessed would be masked to zero in the corresponding VALID field. When HOLE is presented for each fresh word, none of the bytes 63 have been accessed, and the MARK is zero. VALID corresponds to all the bytes 63 not marked as holes. After each VALID is accessed, MASK is updated to exclude this current byte 63, and VALID would then indicate the location of the remaining valid bytes 63. (2) A priority encoder 82 with VALID(n-1:0) as its output. The priority encoder 82 determines which of the valid bytes 63 is to be sent out to the output of the multiplexer 64. VALID indicates the location of all the remaining valid bytes in the word. COL is a column signal that is sent out to the multiplexer 64, and the corresponding byte 63 will be multiplexed to the output. Only one of the n COL lines will be asserted any time. The priority encoder 82 will assert the COL line corresponding to the least significant bit of VALID(n-1:0). (3) A mask state machine 84. The input to the mask state machine 84 includes: a) a RESET signal, which resets the MASK(n-1:0) to all zero, implying none of the hole bits would be masked in the beginning. b) a READBYTE request signal, which tells the byte tracker 70 to output a byte 63; c) VALID(n-1:0) signals, which are the primary inputs to the state machine 84, and will determine the next MASK value; d) a CLOCK signal. The output of the state machine 84 includes: a) a MASK(n-1:0) signal, which is used to mask out the invalid bytes 63 and the bytes 63 that have already been sent out in prior accesses; b) a READWORD signal, which signals the FIFO buffer memory 42 to access the next word 62. The mask state machine 84 accepts the VALID(n-1:0) signals as primary inputs. The next state of the MASK signal is determined by flipping the least significant 0-bit of VALID(n-1:0) to a one, unless LASTBYTE is indicated. LASTBYTE detection logic 86 determines if there is only one valid byte 63 left, which is indicated by a single zero in VALID(n-1:0), and the next state of the MASK signal is all zero. READWORD is also asserted to signal the FIFO buffer memory 42 to access the next word. Figure 6 is a truth table for the mask state machine 84. The truth table incorporates the above rules for the state machine 84. The operation of the system 60 can be further understood by examining the five lines 62 of data in the FIFO buffer memory 42. In the first line 62, all four bytes V0-V3 in the data field 72 are valid, as shown by the four zeros in the hole field 74. In the second line 62, the first byte is invalid data, as indicated by the one in the first bit of the hole field 74, and the remaining bytes V4-V6 are valid as indicated by the zeros in the second through fourth bits of the hole field. Similarly, the first two bytes of the third line 62 contain invalid data, the first three bytes of the fourth line 62 contain invalid data, and the first and third bytes of the fifth line contain invalid data. As a result of operation of the multiplexer 64 and the byte tracker 70, the valid bytes are provided as a sequential output V0...VN with no interruption by the invalid bytes. It should further be apparent to those skilled in the art that various changes in form and details of the invention as shown and described may be made. It is intended that such changes be included within the scope of the claims appended hereto.

Claims: Apparatus for transferring data in multiple byte words between a first bus (34) and a second bus (32) having different widths, which comprises a system interface (48) connected to said first bus, a first-in-first-out (FIFO) buffer memory (42) having an input connected to said system interface by a system interface bus having a width corresponding to said first bus, said first-in-first-out buffer memory having an output connected by a buffer memory output bus (66) having a width corresponding to said first bus, said buffer memory output bus being connected to a multiplexer (64), said multiplexer being connected to said second bus, characterised by said first-in-first-out buffer memory being configured to store each multiple byte word in a multiple byte data field with an associated multiple bit byte validity field, said system interface (48) generating byte validity data to said multiple bit byte validity field in said first-in-first-out buffer memory, and a byte tracker (70) connected to a byte validity field of said FIFO buffer memory and to said multiplexer to control said multiplexer to select and provide valid bytes from said FIFO buffer memory to said second bus, in response to the data in the byte validity field. The apparatus of claim 1 additionally comprising a parallel to serial converter (69) connected to said second bus to provide the valid bytes as a serial bit stream. The apparatus of claim 1 in which said byte tracker comprises logic for determining valid bytes, a mask state machine (84) connected to said logic for determining valid bytes, and a priority encoder (82) connected to said logic for determining valid bytes. A method for transferring data in multiple byte words between a first bus (34) and a second bus (68) having different widths, which comprises receiving the multiple byte words from said first bus (34) by means of a system interface (48), storing said words in a first-in-first-out (FIFO) buffer memory (42), supplying the multiple byte words from the FIFO to a multiplexer (64) the connections between the system interface, the FIFO and the multiplexer having a width corresponding to said first bus and providing bytes from said multiplexer to said second bus (68), characterised by the system interface (48) generating corresponding byte validity data for each multiple byte word, storing said corresponding byte validity data in said FIFO buffer memory in association with said multiple byte word, providing said byte validity data from said FIFO buffer memory to a byte tracker (70), and, controlling said multiplexer (64) using the byte tracker (70) to select and provide valid bytes from said FIFO buffer memory to said second bus in response to the byte validity data. The method of claim 4 additionally comprising converting the valid bytes supplied to the second bus from a parallel data stream to a serial bit stream.

Inventors: CHOY HENRY SIU-FAI, CHOY, HENRY SIU-FAI
Applicants: ADVANCED MICRO DEVICES INC, ADVANCED MICRO DEVICES, INC.
Application year: 1993
Main classification: G06F13
IPC codes: G06F13