Memory system

Abstract: A memory system comprises a memory (12) for transferring m data bytes (e.g. 16 bytes) at a time, first and second buses (A,B) each having widths (e.g. 4 bytes) of less than m data bytes, first parallel m-byte wide read and write registers (16,18) connected between the first bus (A) and the memory (12) and second parallel m-byte wide read and write registers (20,22) connected between the second bus (B) and the memory (12). With this arrangement data can be transferred m bytes at a time to and from the memory (12), with transfer between the read and write registers and the associated buses (A,B) taking place independently of memory transfer, and with a number of bytes corresponding to the associated bus width.

Description: The present invention relates to a memory system of the kind used in data processing systems. Two important elements in a data processing system are the processor and the memory. The processor operates on data stored in the memory and generally controls the transfer of data into and out of the memory. Other elements of the system may also require access to the memory. In order to avoid conflicts such as overlapping requests for memory access, the processor may control access to the memory by arbitrating between the various requests. As the data processing system becomes more complex, an increasing number of elements require access to the memory. For example, multiple processors and control units require individual access to the memory. Although multiple processors and control units can provide increased efficiency, speed and functionality, they also create a memory access bottleneck. This can result in more wait states for each element which can deteriorate system performance. Another factor contributing to memory access delays is related to the use of the data bus interconnecting the various elements and the memory. The data bus is used for data transfers between the various elements as well as for memory access. Since simultaneous use of the bus is not permitted, an element may have to wait for access to the bus as well as having to wait for access to the memory. In other words, if all the elements of a system are connected to the same bus there is competition not only to access the memory but also to have access to the bus. A dual port memory system allows access to a single memory from two busses. A known dual port memory system is disclosed in U.S. Patent No. 4,796,232. The known system provides access to a multiple bank, DRAM memory through two ports. A logic circuit arbitrates between read/write requests from the ports and DRAM refresh requests. The logic circuit allows one memory bank to be refreshed while another bank is accessed by a read or write to a port. The known system also uses a data register between each bus and the memory banks. A data register will accept, for example, a data element written from a bus thereby freeing that bus for other activity. The known system has the disadvantage of a limited operating speed. EP-A-0290172, Advanced Micro Devices Inc, discloses a bi-directional FIFO device for changing the data path width when transmitting data between a wide and a narrow data bus, but the problem of dual access to a single bus is not considered. It is an object of the present invention to provide a memory system permitting a high speed of memory access. Therefore, according to the present invention, there is provided a memory system, including a memory adapted to store m data bytes at each addressable location therein, and first and second data buses arranged to transfer data to and from said memory, characterized in that said first and second data buses have respective widths of n bytes and p bytes, where n and p are both less than m, in that m byte-wide first write registers are connected in parallel between said first bus and said memory, in that m byte-wide first read registers are connected in parallel between said first bus and said memory, in that m byte-wide second write registers are connected in parallel between said second bus and said memory, in that m byte-wide second read registers are connected in parallel between said second bus and said memory, the connections between the registers and the memory being made by an m-byte wide bus adapted to transfer up to m data bytes to or from a register in a single memory access and in that data bytes from consecutive transfers between one of said write or read registers and the associated first and second data bus are transferrable in a single memory access to or from said one register. In a memory system according to the invention, data can be transferred m bytes at a time to and from the memory, with transfer between the read and write registers and the associated buses taking place independently of memory transfer, and with a number of bytes corresponding to the associated bus width, thereby achieving high speed memory access. One embodiment of the present invention will now be described by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a data path block diagram of a dual port memory system according to one form of the present invention; Figure 2 is a signal path diagram for the dual port memory system of Figure 1; Figures 3A-3U show a diagram of the data registers of Figure 1; and Figure 4 is a circuit for generating WRITE ENABLE signals. Figure 1 shows a schematic data path diagram of a dual port memory system 10 according to the present invention, the structure and operation of which will first be briefly described. System 10 includes a single ported memory 12 which will typically consist of a number of DRAM memory chips. Data bytes are transferred into and out of memory 12 over data bus 14. Bus 14 has a multiple byte width, meaning that a plurality of data bytes may be simultaneously transferred into and out of memory 12. In the embodiment described in Figure 3, bus 14 is sixteen bytes wide. Thus, for eight bit data bytes, 128 bits of data may be written into or read from memory 12 in a single memory access. Memory 12 is accessible from two independent busses A and B. Each bus A and B has a width less than that of bus 14. In the Figure 3 embodiment, the width of each bus A and B is four bytes. For best performance, the width of each bus A and B is an integer diversor of the width of bus 14. However, there is no requirement that the width of bus A and B be the same. Memory 12 is connected to bus A through write registers 16 and read registers 18, and is connected to bus B through write registers 20 and read registers 22. Write registers 16 and read registers 18 are connected in parallel and are used respectively for writes to and reads from memory 12. Similarly, write registers 20 and read registers 22 are connected in parallel and are used respectively for writes to and reads from memory 12. Each of the registers 16, 18, 20 and 22 have the same number of byte wide data registers as the byte width of bus 14. For example, if bus 14 has a width of sixteen bytes, write register 16 will have sixteen parallel registers connected to respective byte lines in bus 14. In operation, data transfers between bus A and registers 16 and 18 may occur simultaneously with data transfers between bus B and registers 20 and 22. For example, assume bus 14 has a width of 16 bytes and busses A and B each have a width of four bytes. Further assume consecutive data bytes are to be written into memory 12 from bus A, at the same time a memory read request from bus B is received. Four consecutive writes of four bytes each from bus A to write registers 16 may immediately occur before access to memory 12 is required. At the same time, read registers 22 may be filled with sixteen bytes of data by a single read access to memory 12. If all sixteen data bytes in read registers 22 are requested, four consecutive reads of four bytes each from registers 22 can now occur before access to memory 12 is again required by bus B. During the reading of registers 22, the data in registers 16 may be written into memory 12, and, in addition, other accesses to memory 12 such as a refresh operation may take place. In this manner, the transfer of data into and out of memory 12 is highly efficient with minimum delays for reads or writes from either bus. The above described scenario assumes that the four consecutive writes to registers 16 and the four consecutive reads from registers 22 each contain adjacent data bytes for a common sixteen byte address in memory 12. However, even if the bytes are not all to the same address, improved efficiency is achieved. Referring now to Figure 2, a signal flow path diagram is shown. Data bus A is connected through port A to the A registers block 24 which contains write registers 16 and read registers 18 shown in Figure 1. Data bus B is connected through port B to the B registers block 26 which contains write registers 20 and read registers 22 shown in Figure 1. A signal generator 28 receives address signals A_ADD associated with bus A and address signals B_ADD associated with bus B. In addition, signal generator 28 receives a read/write signal R/W associated with each of busses A and B, and provides acknowledge and ready signals over each bus 30 and 32. Signal generator 28 provides MEMORY ADDRESS and RAS and CAS signals to memory 12, A_OUTPUT_EN, A_WRITE_LAT, WORD_ADD_A and A_BYTE_EN signals to A registers 24, B_OUTPUT_EN, B_WRITE_LAT, WORD_ADD_B and B_BYTE_EN signals to B registers 26, and DATA_LAT, A/B and R/W signals to both A registers 24 and B registers 26. The generation of MEMORY ADDRESS and RAS and CAS signals is well known and may be done in a conventional manner. Generator 28 arbitrates between memory access requests from bus A and bus B and may also arbitrate memory refresh operations. The various signals provided to A registers 24 and B registers 26 may also be generated in a conventional manner. Their routing and operation will be described with respect to Figures 3A-3U. Reference is now made to Figures 3A-3U which are assembled according to the map on Figure 3A. Figure 3N shows memory 12 with a single port for bus 14 for the transfer of data into and out of memory 12. Each of the lines 34a, 34b, 34c, ..., 34p, comprising bus 14 carries a byte (eight bits) of data. The embodiment discloses 16 of such byte wide data lines. However, the invention is not limited to 16 byte lines, nor is it limited to eight bit data bytes. Each addressable location in memory 12 stores the same number of data bytes as the number of lines in bus 14. Memory 12 also receives MEMORY ADDRESS, WRITE ENABLE, RAS and CAS signals for the transfer of data into and out of memory 12. Bus A is four bytes wide and is connected to memory 12 by write registers 16a, 16b, ..., 16p, and by read registers 18a, 18b, ..., 18p. Write registers 16a, 16b, ..., 16p, are connected in parallel with read registers 18a, 18b, ..., 18p. Bus B is also four bytes wide and is connected to memory 12 by write registers 20a, 20b, ..., 20p, and by read registers 22a, 22b, ..., 22p. Write registers 20a, 20b, ..., 20p, are connected in parallel with read registers 22a, 22b, ..., 22p. It should be noted that bus A and bus B each has a width of less than bus 14, but the two buses do not necessarily have the same bus width. Each of the various write and read registers are a single byte wide. Each write register 16a, 16b, ..., 16p, and each write register 20a, 20b, ..., 20p, is connected to memory 12 through a multiplexer 36a, 36b, ..., 36p, respectively. Referring to Figures 3A-3C, each write register 16a, 16b, ..., 16p, is connected to an address line for receiving a word address WORD_ADD_A signal, to a line for receiving a timing A_WRITE_LAT signal, and to one of the control lines for receiving enable A_BYTE_EN signals. Every fourth write register, e.g. 16a, 16e, 16i, and 16m, is connected to one of the A_BYTE_EN signal lines, with exactly four of the write registers connected to each line. Referring to Figures 3O-3Q, each write register 20a, 20b, ..., 20p, is connected to an address line for receiving a word address WORD_ADD_B signal, to a line for receiving a timing B_WRITE_LAT signal, and to one of the control lines for receiving enable B_BYTE_EN signals. Every fourth write register, e.g. 20a, 20e, 20i, and 20m, is connected to one of the B_BYTE_EN signal lines, with exactly four of the write registers connected to each line. Each write register 16a, 16b, ..., 16p, may include a respective flag register (not shown) to generate a WRITE HISTORY A signal each time a data byte is written into the respective write register. Similarly, each write register 20a, 20b, ..., 20p, may include a respective flag register (not shown) to generate a WRITE HISTORY B signal each time a data byte is written into the respective write register. Each multiplexer 36a, 36b, ..., 36p, is also connected to a control line for receiving a control signal A/B for enabling the transfer of data bytes from registers 16a, 16b, ..., 16p, or registers 20a, 20b, ..., 20p, to memory 12. Each multiplexer 36a, 36b, ..., 36p, is connected to memory 12 through a tristate device 38a, 38b, ..., 38p, respectively. Each tristate device receives a WRITE ENABLE signal which deactivates the devices for a read operation and selectively enables the devices for a write operation. The generation of the WRITE ENABLE signals is shown in Figure 4. A multiplexer 48 has inputs for receiving WRITE HISTORY A signals and WRITE HISTORY B signals and a control input A/B for selecting between the signals. The output of multiplexer 48 is connected to one input of an AND gate 50. The other input of AND gate 50 receives the R/W signal, and the output of AND gate 50 are the WRITE ENABLE signals provided to tristate devices 38a, 38b, ..., 38p. In this manner, only those write registers having a data byte will be connected to memory 12. Read registers 18a, 18b, ..., 18p, (Figures 3D-3G) are connected to a control line for receiving an enable A/B signal and a clock DATA_LAT signal. Read registers 22a, 22b, ..., 22p, (Figures 3R-3U) are connected to a control line for receiving an enable A/B signal and a clock DATA_LAT signal. Read registers 18a, 18b, ..., 18p, respond to one level of the A/B signal and registers 22a, 22b, ..., 22p, respond to its complement so that only registers 18a, 18b, ..., 18p, or 22a, 22b, ..., 22p, are enabled at a time. Referring to Figures 3D-3E, multiplexers 40a, 40b, 40c and 40d are connected between the output of selected read registers 18a, 18b, ..., 18p, and bus A. Each multiplexer 40a, 40b, 40c and 40d is connected to four read registers. For example, multiplexer 40a is connected to the output of read registers 18a, 18e, 18i and 18m. Each multiplexer 40a, 40b, 40c and 40d is connected to an address line for receiving the word address WORD_ADD_A signal. The output of each multiplexer 40a, 40b, 40c and 40d is connected to bus A through tristate device 42a, 42b, 42c and 42d, respectively. Each tristate device is connected to a control line for receiving an enable A_OUTPUT_EN signal for transferring data bytes from predetermined ones of the read registers 18a, 18b, ..., 18p. Similarly, referring to Figures 3R-3S, multiplexers 44a, 44b, 44c and 44d are connected between the output of selected read registers 22a, 22b, ..., 22p, and bus B. Each multiplexer 44a, 44b, 44c and 44d is connected to four read registers. For example, multiplexer 44a is connected to the output of read registers 22a, 22e, 22i and 22m. Each multiplexer 44a, 44b, 44c and 44d is connected to an address line for receiving the word address WORD_ADD_B signal. The output of each multiplexer 44a, 44b, 44c and 44d is connected to bus B through tristate device 46a, 46b, 46c and 46d, respectively. Each tristate device is connected to a control line for receiving an enable B_OUTPUT_EN signal for transferring data bytes from predetermined ones of the read registers 22a, 22b, ..., 22p. In operation, both data busses A and B may be active at the same time. Writes to memory 12 from bus A are started by filling write registers 16a, 16b, ..., 16p. Four data bytes may be simultaneously transferred from bus A. If bus A is transferring sixteen consecutive data bytes to an addressable location in memory 12, all four A_BYTE_EN signals will be activated. In addition, the address bits of the first four-byte data word will enable write registers 16a, 16b, 16c and 16d. The timing signal A_WRITE_LAT will then clock the first four data bytes from bus A into write registers 16a, 16b, 16c and 16d, respectively. The next four data bytes will be transferred into registers 16e, 16f, 16g and 16h, respectively, and so forth until all write registers 16a, 16b, ..., 16p, are filled. The sixteen data bytes in registers 16a, 16b, ..., 16p, may be transferred to memory 12 by selecting the A registers with the A/B signal to multiplexers 36a, 36b, ..., 36p, and enabling tristate devices 38a, 38b, ..., 38p, with the WRITE ENABLE signals. If less than a full sixteen data bytes are to be transferred to memory 12 from bus A, tristate devices 38a, 38b, ..., 38p, will be selectively enabled. At the same time only those locations in memory 12 to receive a data byte will receive a WRITE ENABLE signal. The sixteen bytes are stored in memory 12 by receipt of MEMORY ADDRESS, RAS, CAS and WRITE ENABLE signals. Similarly, writes to memory 12 from bus B are started by filling write registers 20a, 20b, ..., 20p. Four data bytes may be simultaneously transferred from bus B. If bus B is transferring sixteen consecutive data bytes to an addressable location in memory 12, all four B_BYTE_EN signals will be activated. In addition, the address bits of the first four-byte data word will enable write registers 20a, 20b, 20c and 20d. The timing signal B_WRITE_LAT will then clock the first four data bytes from bus B into write registers 20a, 20b, 20c and 20d, respectively. The next four data bytes will be transferred into registers 20e, 20f, 20g and 20h, respectively, and so forth until all write registers 20a, 20b, ..., 20p, are filled. The sixteen data bytes in registers 20a, 20b, ..., 20p, may be transferred to memory 12 by selecting the B registers with the A/B signal to multiplexers 36a, 36b, ..., 36p, and enabling tristate devices 38a, 38b, ..., 38p, with the WRITE ENABLE signals. If less than a full sixteen data bytes are to be transferred to memory 12 from bus B, tristate devices 38a, 38b, ..., 38p, will be selectively enabled. At the same time only those locations in memory 12 to receive a data byte will receive a WRITE ENABLE signal. It should be clear that data bytes may be transferred from busses A and B to write registers 16 and 20 simultaneously. Alternatively, bus A can read registers 18a, 18b, ..., 18p, and/or bus B can read registers 22a, 22b, ..., 22p. For example, if an addressable location in memory 12 is to be read by bus A, tristate devices 38a, 38b, ..., 38p, are placed in a tristate mode by the WRITE ENABLE signal, read registers 18a, 18b, ..., 18p, are selected with the A/B signal and then filled upon receipt of the DATA_LAT clock signal. Four consecutive data bytes for transfer from registers 18 to bus A can be selected by the WORD_ADD_A signal provided to multiplexers 40a, 40b, 40c and 40d. The WORD_ADD_A signal selects a predetermined four byte word within the sixteen bytes held in read registers 18a, 18b, ..., 18p. The four selected data bytes are transferred to bus A by enabling tristate devices 42a, 42b, 42c and 42d with the A_OUTPUT_EN signal. Similarly, if an addressable location in memory 12 is to be read by bus B, tristate devices 38a, 38b, ..., 38p, are placed in a tristate mode by the WRITE ENABLE signal, read registers 22a, 22b, ..., 22p, are selected with the A/B signal and then filled upon receipt of the DATA_LAT clock signal. Four consecutive data bytes for transfer from registers 18 to bus B can be selected by the WORD_ADD_B signal provided to multiplexers 44a, 44b, 44c and 44d. The WORD_ADD_B signal selects a predetermined four byte word within the sixteen bytes held in read registers 22a, 22b, ..., 22p. The four selected data bytes are transferred to bus B by enabling tristate devices 46a, 46b, 46c and 46d with the B_OUTPUT_EN signal. Data registers 18a, 18b, ..., 18p, are always completely filled for a read from bus A. Similarly, data registers 22a, 22b, ..., 22p, are always filled for a read from bus B. In this manner, if the next read from either bus is for a sequential data byte, the respective read registers 18a, 18b, ..., 18p, or 22a, 22b, ..., 22p, can be read without another access to memory 12 and without interfering with a memory access from the other bus.

Claims: A memory system, including a memory (12) adapted to store m data bytes at each addressable location therein, and first (A) and second (B) data buses arranged to transfer data to and from said memory (12), characterized in that said first and second data buses (A,B) have respective widths of n bytes and p bytes, where n and p are both less than m; m byte-wide first write registers (16a-16p) are connected in parallel between said first bus (A) and said memory (12); m byte-wide first read registers (18a-18p) are connected in parallel between said first bus (A) and said memory (12); m byte-wide second write registers (20a-20p) are connected in parallel between said second bus (B) and said memory (12); and m byte-wide second read registers (22a-22p) are connected in parallel between said second bus (B) and said memory (12), the connections between the registers (16,18,20,22) and the memory (12) being made by an m-byte wide bus (14) adapted to transfer up to m data bytes to or from a register (16, 18, 20, 22) in a single memory access and data bytes from consecutive transfers between one of said write or read registers(16,18,20,22) and the associated first and second data bus (A, B) are transferrable in a single memory access to or from said one register (16,18,20,22). A memory system according to claim 1, characterized by a first address line (WORD ADD A) connected to each of said first write registers (16a-16p), n first control lines (A BYTE EN), each connected to selected ones of said first write registers (16a-16p), a second address line (WORD ADD B) connected to each of said second write registers (20a-20p), and p second control lines (B BYTE EN) each connected to selected ones of said second write register (20a-20p), whereby said first and second address line (WORD ADD A, WORD ADD B) provide signals which selectively enable said first and second write registers (16a-16p, 20a-20p) to receive one or more data bytes from the associated data bus (A,B). A memory system according to claim 2, characterized by multiplexing means (36a-36p, 38a-38p), connected between said first and second write registers (16a-16p, 20a-20p) and said memory (12), and responsive to control signals (A/B, WRITE ENABLE) to selectively enable the transfer of up to m data bytes from said first or second write registers (16a-16p, 20a-20p) into said memory (12). A memory system according to claim 2 or claim 3, characterized by third and fourth control lines (A/B, DATA LAT), connected respectively to each said first read register (18a-18p) and each said second read register (22a-22p) whereby said first or second read register (18a-18p, 22a-22p) are enabled to receive and latch m data bytes from said memory (12). A memory system according to claim 4, characterized by n first multiplexers (40a-40d) and p second multiplexers (44a-44d) connected between said first and second read registers (18a-18p, 22a-22p) and said first and second buses (A,B) respectively, whereby a selected group of n of said first read registers (18a-18p) and/or a selected group of p of said second read registers (22a-22p) provides data signals to said first bus (A) and/or said second bus (B), respectively. A memory system according to claim 5, characterized in that said first address line (WORD ADD A) is connected to said first multiplexers (40a-40d) and said second address line (WORD ADD B) is connected to said second multiplexers (44a-44d). A memory system according to any one of the preceding claims, characterized in that n and p are integer divisors of m. A memory system according to any one of the preceding claims, characterized in that n = p. A memory system according to claim 1, characterized in that the bytes transferred to or from a register (16, 18, 20, 22) have different addressable locations in said memory (12).

Inventors: HILLEY MICHAEL R, KASS WILLIAM J, HILLEY, MICHAEL R., KASS, WILLIAM J.
Applicants: NCR INT INC, NCR INTERNATIONAL, INC.
Application year: 1991
Main classification: G06F13
IPC codes: G06F12, G06F13, G11C11