MULTIPROCESSOR SHARED PIPELINE CACHE MEMORY

Abstract: A cache memory unit is constructed to have a two-stage pipeline shareable by a plurality of sources which include two independently operated central processing units (CPUs). Apparatus included within the cache memory unit operates to allocate alternate time slots to the two CPUs which offset their operations by a pipeline stage. This permits one pipeline stage of the cache memory unit to perform a directory search for one CPU while the other pipeline stage performs a data buffer read for the other CPU. Each CPU is programmed to use less than all of the time slots allocated to it. Thus, the processing units operate conflict-free while pipeline stages are freed up for processing requests from other sources, such as replacement data from main memory or cache updates.

Description: BACKGROUND OF THE INVENTION The present invention relates to computer systems and multiprocessor computer systems. More particularly, the present invention relates to multiprocessor systems which include a cache memory system. It is well known that cache memories have been highly effective in increasing the throughput of small and large uniprocessor and multiprocessor systems. In multiprocessor systems, cache memories are normally configured in either of two ways. The first is a shared cache configuration in which a cache memory is utilized for one or several main memory modules. It is accessible by all of the processors within the system. The second configuration is a private cache arrangement in which the cache memory is dedicated to a single processor. These configurations are described in an article titled "Effects of Cache Coherency in Multiprocessors" by Michael Dubois and Fay A. Briggs, IEEE Transactions on Computers, Volume C-31, No. 11, November, 1982. Additionally, multiprocessor systems have been configured to share a common control unit which includes a cache memory. U.S. Patent Nos. 4,378,591 and 4,392,200 are examples of these types of systems. In such systems, the processing units connect to a common bus and include arbitration circuits for allocating available bus cycles for accessing the cache memory. It has been found that considerable time is expended in resolving access conflicts among processors. This in turn reduces system performance, in addition to adding to the complexity of the system. Additionally, in the system disclosed in U.S. Patent No. 4,378,591 other sources of requests for bus cycles, such as a first in first out (FIFO) memory are included within the cache subsystem. This resource must be granted access to the cache memory via the local bus on a priority basis. The FIFO is granted a higher priority than the processing units, so that information transfers which are commonly main memory write operations would take precedence. That is, the cache update operations are assigned a higher priority than the processor requests which further slow down the performance of the system. One prior art uniprocessor system utilizes a memory system which contains a cache and main memory implemented by fully segmented pipelines. The system is a single personal computer and as such can only accommodate a single user system. For a discussion of the system, reference may be made to the article entitled "The Memory System of a High-Performance Personal Computer" by Douglas W. Clark, Butler W. Lampson and Kenneth A. Pier, IEEE Transactions on Computers, Volume C-30, No. 10, October, 1981. From US patent 4,349,871 a duplicate tag store for a cached multiprocessor system is known which is concerned with "pipelining" the resources of the system i.e., the bus and the common control elements. The processors are interfaced to a common control (CC) and to each other by a high-speed, synchronous, non-interlocked backplane bus ("P-bus"). Memory management circuitry, address translation circuitry, the cache, the cache hit check circuitry, and the P-bus are allocated to a requesting processor in the sequence named when a memory read request is made. A succeeding requesting processor may be allocated these same resources in the same sequence, but no earlier than one resource after the earlier processor. There is no disclosure of a feature which allows two processors to access the cache at the same time. Accordingly, it is the main object of this invention to provide a cache system with improved performance which is able to handle requests from a plurality of sources thus permitting independently operating sources to share a cache unit on a conflict-free basis. This object is achieved in the cache system according to the characterizing features of claim 1. Further advantageous embodiments may be taken from the subclaims. The inventive cache system is concerned with a dual-cycle cache, one which allows two processors access thereto at the same time. Thus, this dual-cycle cache allows one processor to search the directory at the same time that an earlier searching processor that caused a directory hit is accessing the cache store to retrieve the "found" data. This form of operation enables a two-fold increase in the throughput of the cache by avoiding idle elements thereof. The novel features of the invention together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying drawings. DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram of a data processing system which includes the cache memory of the present invention. Figure 2 is a block diagram of one of the central subsystems of Figure 1 constructed according to the present invention. Figures 3a-3c show in greater detail, the timing and control circuits of Figure 2. Figure 4 is a timing diagram used to explain the operation of the cache memory of the present invention. DESCRIPTION OF A PREFERRED EMBODIMENT In the preferred embodiment which is described in the following, a cache memory subsystem is constructed to have two pipeline stages which are shareable by a plurality of sources including a number of independently operated central processing units. The first pipeline stage of the cache memory provides for a directory search and compare operation while the second pipeline stage performs the operations of fetching the requested data from the cache buffer memory and its transfer to the source. The cache memory further includes timing and control apparatus which couples to the sources and allocates each processing unit's time slots to offset their operations by a pipeline stage. By having the cache memory control apparatus synchronize the operation of the sources, the opportunity for conflict between sources is effectively eliminated thereby increasing system performance. Thus, the cache memory is cycled continuously permitting conflict-free operation of the pipeline stages by all sources. Additionally, in the preferred embodiment, each central processing unit source is programmed to request use of less than all of the time slots allocated to it by the cache unit. That is, the preferred embodiment, each central processor source requests every other time slot allocated to it. The remaining time slots are then made available to other sources. In the preferred embodiment, one such other source is a first in first out (FIFO) buffer which handles update and replacement requests. The arrangement of the present invention allows these requests to be handled during the free time slots at a rate which prevents any overflow of the buffer. It is convenient to have each central processing unit microprogram controlled such that alternate microinstructions are coded for generating requests every other microinstruction cycle of operation. Synchronization circuits included within the cache memory generate signals for synchronizing the clock circuits of each central processing unit at different time intervals. These intervals are offset by a sufficient number of time slots so as to provide for a conflict-free operation of such subsystem. DESCRIPTION OF THE SYSTEM OF FIGURE 1 Figure 1 shows a multiprocessor data processing system 10 which includes a plurality of subsystems 14 through 30 which couple in common to a system bus 12. The illustrative subsystems include a plurality of central subsystems 14 through 16, a plurality of memory subsystems 20 through 28 and a peripheral subsystem 30. Each subsystem includes an interface area which enables the unit or units associated therewith to transmit or receive requests in the form of commands, interrupts, data or responses/status to another unit on system bus 12 in an asynchronous manner. That is, each interface area can be assumed to include bus interface logic circuits such as those disclosed in U.S. Patent No. 3,995,258, entitled "Data Processing System Having a Data Integrity Technique", invented by George J. Barlow. The organization of each of the central subsystems 14 through 16 is the same. Figure 2 shows in block diagram form central subsystem 14 organized according to the present invention. Subsystem 14 includes a pair of central processing unit (CPU) subsystems 14-2 and 14-4 coupled to share a cache subsystem 14-6. The cache subsystem 14-6 couples to system bus 12 through a first in first out (FIFO) subsystem 14-10 which can be considered as being included within interface area 14-1. As seen from Figure 2, both CPU subsystems 14-2 and 14-4 are identical in construction. That is, each CPU subsystem includes a 32-bit central processing unit (CPU) (i.e., CPU's 14-20 and 14-40), and a virtual memory management unit (VMMU) (i.e., VMMU 14-26 and 14-46) for translating CPU virtual addresses into physical addresses for presentation to cache subsystem 14-6 as part of the memory requests. Also, each CPU subsystem includes a read only store (ROS) and a 16-bit ROS data output register (RDR) (i.e., ROS 14-24, 14-44 and RDR 14-25, 14-45). At the beginning of each cycle, each ROS is conditioned to read out a 16-bit microinstruction word into its data output (RDR) register which defines the type of operation to be performed during the cycle (firmware step/box). The clock circuits within each CPU subsystem (i.e., circuits 14-22 and 14-42) establish the basic timing for its subsystem under the control of cache subsystem 14-6 as explained herein. The elements of each CPU subsystem can be constructed from standard integrated circuit chips. As seen from Figure 2, cache subsystem 14-6 is organized into a source address generation section and two separate pipeline stages, each with its own decode and control circuits. The source address generation section includes blocks 14-62 and 14-64 which perform the functions of source address selecting and incrementing. The first pipeline stage is an address stage and includes the circuits of blocks 14-66 through 14-76, arranged as shown. This stage performs the functions of latching the generated source address and directory searching and hit comparing. The first pipeline stage provides as an output information in the form of a level number and a column address. The operations of the first pipeline stage are clocked by timing signals generated by the timing and control circuits of block 14-60. The information from the first stage is immediately passed onto the second pipeline stage leaving the first stage available for the next source request. The second pipeline stage is a data stage and includes the circuits of blocks 14-80 through 14-96, arranged as shown. This stage performs the functions of accessing the requested data from the buffer memories 14-88 and 14-90, or replacing/storing data with data received from FIFO subsystem 14-10. Thus, the second pipeline stage provides a 36-bit data word for transfer to one of the CPU subsystems. Again, the operations of the second pipeline stage are clocked by timing signals generated by the timing and control circuits of block 14-60. The different blocks of the first and second pipeline stages are constructed from standard integrated circuits, such as those described in "The TTL Data Book, Volumn 3", Copyrighted 1984, by Texas Instruments Inc. and in the "Advanced Micro Devices Programmable Array Logic Handbook", Copyright 1983, by Advanced Micro Devices, Inc. For example, the address selector circuit of block 14-62 is constructed from two sets of six 74AS857 multiplexer chips cascaded to select one of four addresses. The swap multiplexer of block 14-92 is constructed from the same type chips. The latches of blocks 14-68 and 14-72 are constructed from 74AS843 D-type latch chips. The swap multiplexer and data register circuits of block 14-70 are constructed from a single clocked programmable array logic element, such as part number AMPA16R6A, manufactured by Advanced Micro Devices, Inc. The directory memories 14-74 and 14-76 are constructed from 8-bit slice cache address comparator circuits having part number TMS2150JL, manufactured by Texas Instruments Incorporated. The address and data registers 14-80 through 14-84 and 14-94 and 14-96 are constructed from 9-bit interface flip-flops having part number SN74AS823, manufactured by Texas Instruments, Inc. The buffer memories are constructed from 4K x 4-bit memory chips having part number IMS1420, manufactured by INMOS Corporation. The address increment circuits of block 14-64 are constructed from standard ALU chips designated by part number 74AS181A and a programmable array logic element having part number AmPAL16L8A, manufactured by Advanced Micro Devices, Inc. The first and second levels of command register and decode circuits of block 14-66 and 14-86, respectively, utilize clocked programmable array logic elements having part numbers AmPAL16R4A and AmPAL16R6A, manufactured by Advanced Micro Devices, Inc. These circuits generate the required selection, read and write control signals as indicated in Figure 2 (i.e., signals SWAPLT+00, SWAPRT+00, P0LDDT-OL, P1LDDT-OL, P0LDDFOR, P1LDDT-OR). For further details, reference may be made to the equations of the Appendix. As seen from Figure 2, cache subsystem 14-6 is organized into even and odd sections which permit two data words to be accessed simultaneously in response to either an odd or even memory address. For further information about this type of cache addressing arrangement, reference may be made to U.S. Patent No. 4,378,591 which is assigned to the same assignee as named herein. Figure 2 also shows in block form, FIFO subsystem 14-10 which includes the FIFO control and clocking circuits of block 14-11 which couples to a replacement address register 14-12 and to system bus 12. FIFO subsystem 14-10 receives all of the information transferred between any two subsystems on system bus 12. When the information is for updating data in main memory, the information is coded to indicate such updating or replacement operation. FIFO subsystem 14-10 also receives any new data resulting from a memory request being forwarded to system bus 12 by cache subsystem 14-6. Both update and new data are stored as requests within a buffer memory included within subsystem 14-10. FIFO control circuits decode each request and initiate the appropriate cycles of operation which result in address, data and commands being applied to different parts of cache subsystem 14-6 as seen from Figure 2. For the purpose of the present invention, FIFO subsystem can be considered conventional in design and take the form of the FIFO circuits disclosed in U.S. Patent No. 4,195,340 which is assigned to the same assignee as named herein. The basic timing for each of the subsystems of Figure 2 is established by the timing and control circuits of block 14-60. In accordance with the present invention, such control permits the conflict-free sharing of cache subsystem 14-6 by CPU subsystems 14-2 and 14-4 and FIFO subsystem 14-10. The circuits of block 14-60 are shown in greater detail in Figures 3a through 3c. Figure 3a shows address select logic circuits of block 14-600 which generate control signals PT0SEL+00, PT1SEL+00, SLFIAD+00 and SLRPAD+00. These signals are generated in response to CPU cache request signals P0RD02+10 and P1RD02+10 from CPU subsystems 14-2 and 14-4, CPU port busy signals PT0BSY-00 AND PT1BSY-00 from interface area 14-1, FIFO signals FIUPDT+00 and FISHBA+00 from FIFO subsystem 14-10 and address timing signals TM1AD2+00 and TM3AD4+00 from the timing circuits of block 14-640. These signals condition address selector 14-62 to select one of the subsystems 14-2, 14-4 and 14-10 as a request address source. As seen from Figure 3a, the circuits include AND gates 14-602 through 14-608 and OR gate 14-610. Also, the Figure shows bit position 2 of RDR registers 14-25 and 14-45 and part of FIFO block 14-11 including an AND gate 14-112 which generates signals FIUPDT+00 and FISHBA+00. Figure 3b shows the pipeline clock circuits of block 14-620. These circuits define the different types of cache memory cycles which can initiate the start of the pipeline which results in the generation of signal STPIPE+00. Signal STPIPE+00 conditions a clocked register 14-632 which generates a predetermined sequence of signals in response to each request. In greater detail, each of the NAND gates 14-622 through 14-628 provides a signal which indicates a type of activity required by one of the subsystems of Figure 2. That is, signals P0CARD-00 and P1CARD-00 respectively indicate a cache request for service by CPU0 subsystem 14-2 and CPU1 subsystem 14-4 while signals FISHBA-10 and FIUPDT-10 indicate cache requests for service by FIFO subsystem 14-10. These requests can be summarized as follows: 1. CPU0 READ CYCLE A CPU0 read occurs in response to a cache request initiated by ROS 14-24 (i.e., signal P0RD02+10=1) during a first time slot/interval (i.e., signal TIME01+00=1) when CPU port 0 within interface 14-1 is not busy (i.e., signal PT0BSY-10=1). The address supplied by CPU0 subsystem 14-2 is furnished to the first pipeline stage and the directory is read. When a hit is detected, indicating that the requested data is stored in the data buffer, the buffer is read and the data is clocked into the CPU0 data register. When a miss is detected, the CPU0 port is made busy, the request is forwarded to memory to fetch the requested data. 2. CPU1 READ CYCLE A CPU1 read occurs in response to a cache request initiated by ROS 14-44 (i.e., signal P1RD02+10=1) during a third time slot/interval (i.e., signal TIME03+00+1) when CPU port 1 within interface 14-1 is not busy (i.e., signal PT1BSY-10=1). 3. SECOND HALF BUS CYCLE A second half bus cycle occurs in response to a first type of cache request initiated by FIFO subsystem 14-10 for data requested from either main memory or an I/O device being returned on system bus 12 (i.e., signal FISHBA=1) during a first or third time slot/interval (i.e., signal TM1OR3+00=1) when FIFO subsystem 14-10 has a request stored (i.e., signal FIHAVE+10=1). When FIFO subsystem 14-10 furnishes data from an I/O device to the first pipeline stage, it passes therethrough without changing the states of any memories and is clocked into the appropriate CPU data register. Data from main memory is written into the cache data buffers and is clocked into the appropriate CPU data registers. 4. MEMORY WRITE UPDATE CYCLE A memory write update cycle occurs in response to a second type of cache request initiated by FIFO subsystem 14-10 for replacement or update data received from system bus 12 (i.e., signal FIUPDT+00=1) upon acknowledgement of such data (i.e., signal F1ACKR+00=1) during a first or third time slot/interval (i.e., signal TM1OR3+00=1) when FIFO subsystem 14-10 has a request stored (i.e., signal FIHAVE+10=1) FIFO subsystem 14-10 furnishes data to the first pipeline stage resulting in the reading of the directory memory. When a hit is detected, the replacement data is written into the buffer memory. When any one of signals P0CARD-00 through F1UPDT-10 is forced low to a binary ZERO, a NOT OR gate 14-630 (equivalent to a NAND gate) forces start pipe signal STPIPE+00 high to a binary ONE. This conditions register 14-632 to start counting in response to each clock signal MCLOCK+00. Register 14-632 is connected as a ring counter such that the register outputs are fed back to successive register input stages directly or after being inverted by an inverter circuit 14-634 as shown. The register 14-632 continues to count by advancing the pulse loaded into the first stage in response to each occurrence of clock signal MCLOCK+00. This produces a sequence of pulses which corresponds to signals PIPE0A+0A through PIPE0B-0B. Clock signals MCLOCK+00 are generated by crystal oscillator timing circuits, not shown, included within block 14-60. The clock signals MCLOCK+00 are also applied as inputs to CPU clock circuits of blocks 14-22 and 14-42. Figure 3c shows the master counter and sync control circuits of block 14-640 and the FIFO cycle select logic circuits of block 14-660. The circuits of block 14-640, in response to clock signal MCLOCK+00, generate the required sequence of timing signals TIME01+00 through TM1OR3+00 which define the various time slots/intervals during which cache subsystem 14-6 carries out the required operations for processing subsystem cache requests. The master counter includes a register 14-650 and associated NOT AND and OR gates 14-642 and 14-644 through 14-648 respectively connected as shown. The arrangement operates as a ring counter which is continously incremented or advanced by each clock signal MCLOCK+00. The master counter applies timing signals TIME01+00 and TIME03+00 to the sync control circuits of block 14-652. These circuits generate signal P0MSYN+00 and signal P1MSYN+00 which are applied to the CPU0 and 1 clock circuits of blocks 14-22 and 14-24, respectively. Each such signal synchronizes the operation of the CPU clock circuits with the cache subsystem timing and control circuits. More specifically, sync signal P0MSYN+00 is applied to an input of the time 02 flip-flop stage of a ring counter register included within block 14-22 while sync signal P1MSYN+00 is applied to an input of the time 02 flip-flop stage of a ring counter register included within block 14-44. Each such signal when present enables the advancement of the CPU clocking circuits at the appropriate time intervals. Signals P0MSYNC+00 and P1MSYNC+00 are generated so that they remain offset from one another by two time slots/intervals (see Appendix). The FIFO cycle select logic circuits of block 14-660 include a pair of NOT OR gates 14-662 and 14-664, a pair of AND gates 14-666 and 14-668, a NOR gate 14-670, a NOT AND gate (NAND) 14-672 and a clocked D-type register flip-flop stage 14-674 which connect in series as shown. These circuits generate FIFO cycle signal CYFIFO+00 and FIFO signal DOAFIF+00 during time slots/intervals TIME02 and TIME04 which synchronize the operation of FIFO subsystem 14-10 with that of cache subsystem 14-6 as explained herein. DESCRIPTION OF OPERATION With reference to Figures 1 through 3c, the operation of the preferred embodiment of the present invention will now be described with reference to the timing diagram of Figure 4. It is assumed that the ring counter circuits of blocks 14-620 and 14-640 of Figures 3b and 3c are cycling or operating (i.e., counting in response to clock signals MCLOCK+00) and that both ROS stores 14-24 and 14-44 have been initialized to predetermined states. As mentioned previously, the ROS words of each ROS store are appropriately coded so as to provide patterns of alternate ONE's and ZERO's defining the cycles allocated to CPU subsystems 14-2 and 14-4. This coding is illustrated in Figure 2 wherein bit position 2 of each ROS word is appropriately coded. At the leading edge of signal TIME01+00, the ROS word is clocked into CPU0 ROS data register 14-25. The command field of the ROS word is examined (i.e., bits 0 and 1). When bits 0 and 1 are "10", this specifies that cache subsystem 14-6 is to execute a system bus request wherein data is written or read from a device connected to system bus 12. Assuming that the cache request specifies a main memory read operation, cache subsystem 14-6 reads the cache memory. At this time, ROS data bit 2 is examined to establish whether or not CPU0 is going to utilize the next upcoming cache pipeline cycle which corresponds to time intervals T2 and T3. When bit 2 is a ZERO, this indicates that CPU0 subsystem 14-2 is not going to use the next cache cycle. However, when bit 2 is a ONE, this indicates that CPO subsystem 14-2 is going to use the next cache cycle. It is assumed that the port circuits for CPU0 subsystem 14-2 are not busy processing another system bus request (i.e., signal PT0BSY-10=1). As seen from Figure 3b, NAND gate 14-622 forces signal P0CARD-00 to a ZERO which causes NOT OR gate 14-630 to force start pipe signal STPIPE+00 to a binary ONE. As seen from Figure 4, this signal is valid by the trailing edge of signal TIME01+00. That is, the start pipe signal STPIPE+00 as indicated by the hash marked area is valid until the leading edge of the signal designated as CPU0 cycle in Figure 4. It will also be noted that during the time signal STPIPE+00 is being generated, signal P0RD02+10, corresponding to ROS data word bit 2, together with timing signal TM1AD2+00 and port busy signal PT0BSY-00, cause AND gate 14-602 of Figure 3a to force address select signal PT0SEL+00 to a ONE. This conditions or configures address selector 14-62 to select as the address to be applied to odd and even latches 14-68 and 14-72, the 33-bit address from CPU0 VMMU 19-26. The start pipe signal STPIPE+00 is applied to ring counter 14-623 of Figure 3b and clocked into the first bit position on the leading edge of the second clock signal MCLOCK+00. As seen from Figure 4, this results in the generation of signal PIPE0A+0A. The first occurrence of start pipe signal STPIPE+00 defines a CPU0 cycle. Signal PIPE0A+0A is applied as a clock input to the address odd and even latches 14-68 and 14-72. The same signal is applied to the first level register and decode circuits of block 14-66 and swap multplexer data register 14-70. At the leading edge of signal PIPE0A+0A, the latches 14-68 and 14-72 are enabled and at the trailing edge of the signal, the latches are conditioned to store the cache request address generated by CPU0 VMMU 14-26. That is, even address latches 14-72 store an even address value previously incremented by one by increment circuit 14-64 if the original address was odd. The unincremented odd address value is stored in odd address latches 14-68. The odd and even address contents of latches 14-68 and 14-72 are applied to odd and even directory memories 14-74 and 14-76. Assuming that the requested data resides in cache, the directory memories 14-74 and 14-76 read out the level and column information designating the locations in the respective buffer memories 14-88 and 14-90 where the requested data resides. The decode circuits of block 14-66 generate the appropriate directory read signals which are clocked into the programmable array logic output register in response to signal P1PE0A+0A. This completes the operations performed by the first pipeline stage. As seen from Figure 4, the directory level and column information is loaded into the odd and even address register 14-80 and 14-84 of the second pipeline stage at the leading edge of signal PIPE0B+0A. This signal is generated by ring counter 14-632 in response to third clock signal MCLOCK+00. Signal PIPE0A+0B, which is generated in response to the second clock signal MCLOCK+00 is not used. At the same time, the even and odd address registers 14-80 and 14-84 are clocked by signal PIPE0B+0A, the second level command register and decode circuits of block 14-86 decode the command word resulting in the generation of left and right swapping signals SWAPLT+00 and SWAPRT+00 which are applied to swap multiplexer 14-92. The swapping signals, as well as the other signals produced by the circuits of block 14-86, are clocked into the programmable array logic output register, in response to signal P1PE0B+0A. The odd and even data words read out from odd and even buffer memories 14-88 and 14-90 are transferred through swap multiplexer 14-92 as specified by signals SWAPLT+00 and SWAPRT+00. Also, the circuits of block 14-86 generate left half word and right half word signals P0LDDT-0L and P0LDDT-0R which enable a single or double word to be clocked into CPU0 data register 14-94, in response to clock signal PIPE0B-0B. The data words are thereafter transferred under microprogram control to CPU subsystem 14-2. As seen from Figure 3b, the signal PIPE0B-0B is the complement of signal PIPE0B+0B of Figure 4 which is generated by ring counter 14-632, in response to the fourth clock signal MCLOCK+00. This completes the operations of the second pipeline stage. As seen from Figure 4, a similar sequence of operations is performed by the first and second pipeline stages in processing a cache main memory request for CPU1 subsystem 14-4. That is, a second start pipe signal STPIPE+00 is generated during time T3 by NAND gate 14-624 forcing signal P1CARD-00 to a binary ZERO. During the timing interval T4, when the buffer memory addresses for the CPU0 cache request are being loaded into the registers 14-80 and 14-84, the cache request odd and even addresses from CPU1 subsystem 14-4 are being latched into the odd and even address latches 14-68 and 14-72. Next, in the case of another cache hit, the directory memories 14-74 and 14-76 read out the level and column information designating the locations in odd and even cache buffer memories 14-88 and 14-90 where the requested data resides. In response to signal PIPE0B+0A, this information is loaded into odd and even address registers 14-80 and 14-84. At the same time, the circuits of block 14-86 by command decoding generate left and right swapping signals SWAPLT+00 and SWAPRT+00, as well as signals P1LDDT-0L and P1LDDT-0R. The result is that the data words read out from odd and even buffer memories 14-80 and 14-84 and transferred via swap multiplexer 14-92 are clocked into CPU1 data register 14-96. This completes the processing of the second cache request. It is now assumed that FIFO subsystem 14-10 requires servicing which results in the generation of a third start pipe signal STPIPE+00. This cycle can result from either one of the two types of requests as discussed above. According to the present invention, FIFO subsystem 14-10 is serviced whenever there is a free pipeline stage. A free pipeline stage occurs during time T1 when CPU0 ROS data bit 2 is a ZERO or during time T3 when a CPU1 ROS data bit 2 is a ZERO. This enables FIFO subsystem 14-10 to be serviced during these times. Hence, FIFO cycles occur at time 2 and time 4 when the corresponding CPU subsystems do not request cache cycles. In Figure 4, it is assumed that CPU0 subsystem 14-2 and CPU1 subsystem 14-4 do not use the following T1 and T3 time slots/cycles. When FIFO subsystem 14-10 generates a cache write update request, the source of the cache request address is from the FIFO address portion of the command furnished by the circuits 14-11. That is, address selector 14-62 is conditioned or configured to transfer this address into odd and even latches 14-68 and 14-72. Signal PIPE0A+0A clocks the information into the latches while at the same time, data from FIFO subsystem circuits 14-11 is clocked into the swap register of block 14-70. The programming array logic circuits of block 14-70 are conditioned by signals from the FIFO subsystem 14-10 (i.e., address bit 22) to properly align the left and right data word halves which are then clocked into the swap register on the leading edge of signal PIPE0A+0A. As seen from Figure 4, in the case of a hit, signal PIPE0B+0A loads the level and column information into odd and even address registers 14-80 and 14-84 designating locations in odd and even cache buffer memories 14-88 and 14-90 where the data to be updated resides. At the same time, the update data is clocked into data register 14-82. Thereafter, the update data is written into odd and even buffer memories 14-88 and 14-90 under the control of write signals generated by the decode circuits of block 14-86. As seen from Figure 3c, the circuits of block 14-660 force signal DOAFIF100 to a binary ONE when signal P0RD02+10 is a ZERO during time T1 (i.e., signal TIME01+00=1). As seen from Figure 4, signal DOAFIF+00 forces signal CYFIFO+00 to a binary ONE in response to clock signal MCLOCK+00. Signal CYFIFO+00 is applied to the FIFO circuits of block 14-11 and results in incrementing internal FIFO counter circuits which complete the processing of the request stored within FIFO subsystem 14-10. This also completes the processing of the FIFO request by cache subsystem 14-6. It is assumed that during the next FIFO cycle, the FIFO subsystem 14-10 furnishes a cache replacement request which causes NAND gate 14-626 of Figure 3b to force signal FIUPDT-10 to a ZERO. This results in the generation of the fourth start pipe signal STPIPE+00. In this instance, the RAR register 14-12 will have been loaded when the request was initially forwarded to system bus 12 in response to a CPU cache request. Accordingly, a similar sequence of operations is carried out by cache subsystem 14-6. It will be noted that FIFO cycles are initiated by signal DOAF1F+00 at time T1 and time T3, respectively, when signal P0RD02+00 or PT0BSY-10 is a ZERO and signal P1RD02+10 or PT1BSY-10 is a ZERO. This causes FIFO cycles to occur at times T2 and T4. In each case, when both signals (i.e., P0RD02+10, PT0BSY-10 AND P1RD02+10, PT1BSY-10) are ONEs, this precludes the generation of either signal SLFIF0+0A or signal SLFIF0+0B which precludes the occurrence of a FIFO cycle. Thus, the FIFO cycles are overridden when the CPU subsystem allocated the time slot/cycle is utilizing the cycle. Under worst case conditions, there can be at most two requests stacked up in FIFO subsystem 14-10. Thus, there can be up to two FIFO cycles occurring back to back when the FIFO subsystem 14-10 is full as illustrated in Figure 4 as discussed above. The arrangement of the present invention ensures that there are always enough free pipeline stages to service FIFO subsystem 14-6 at the maximum rate at which it receives requests from system bus 12. This is achieved by allowing each CPU subsystem to utilize a cache cycle every other microinstruction word/firmware box. For CPU subsystem 14-2, every other microinstruction word read out each time T1 can specify a cache request. The same is true for CPU subsystem 14-4, for the microinstruction word read out every time T3. In this manner, cache subsystem 14-6 can process cache requests from a number of sources without conflict or contention. It will be appreciated that when cache subsystem 14-6 detects a miss condition in response to a CPU cache request, this will result in interface area circuits 14-1 forcing the port circuits to a busy state (i.e., signal PT0BSY-10 or PT1BSY-10 is forced to a ZERO). This, in turn, is forwarded back to the CPU subsystem causing it to stall its operation. Since this operation is not pertinent to the understanding of the present invention, it will not be further discussed. From the above, it is seen how the present invention permits expeditious processing of cache requests from a plurality of sources. It permits the time sharing of a cache subsystem among the different subsystems of a multiprocessor system on a conflict-free basis. This allows the continued processing of cache requests from other sources when the data requested by one source does not reside in cache. It will be appreciated by those skilled in the art that many changes may be made to the preferred embodiment of the present invention. For example, the system timing may be changed as, for example, different time slots may be allocated to the subsystems, as well as the coding of ROS memories 14-24 and 14-44 may be altered to utilize different cycles (e.g. every third, fourth, etc.). Also, the number of bits and cache width may be altered (i.e., process single, double or quad words). Other changes will be as apparent to those skilled in the art.

Claims: A cache system for providing concurrent access to a dual-cycle cache unit (14-74, 76, 88, 90) by a plurality of accessing units (14-2, 4), wherein each of said accessing units, upon requiring access to said cache unit, supplies an address representation of a main memory location, said cache unit being characterized by: a timer-controller (14-60) coupled to each of said accessing units for synchronizing their operations with said cache unit, said timer-controller generating sequences of signals, each sequence defining a number of time slot intervals, at least a different one of said time slot intervals being allocated to each of a number of said accessing units, so that only one of said accessing units at a time is permitted initial access to said cache unit; a first stage (14-68, 70, 72) coupled to said timer-controller, said first stage being conditioned by said timer-controller during a predetermined one of said time slot intervals for receiving said address representation and for searching in the directory (14-74, 76) of said cache unit for a corresponding address representation and, if the search is successful, delivering output signals representing the location of corresponding data held in the cache store (14-88, 90); and a second stage (14-80, 82, 84, 92) coupled to said timer-controller and to said first stage, said second stage being conditioned by said timer-controller during a time interval succeeding said predetermined one of said time slot interval to receive said output signals and to access the designated data at the location in the cache store (14-88, 90) represented by said output signals for transfer to the requesting accessing unit; wherein said timer-controller concurrently allows a first one of said accessing units access to said first stage and a second one of said accessing units access to said second stage immediately following a successful search in said directory, thereby enabling conflict-free processing of requests from said plurality of accessing units. The cache system of claim 1, characterized in that an address selector (14-62) is coupled to each of said accessing units for receiving said address representations, and said timer-controller (14-60) is coupled to said accessing units and said address selector, and is provided to cause said address selector to transfer to said first stage the address representation supplied by said first one of said accessing units whose access is allowed to said first stage by said timer-controller. The cache system of claim 1, characterized in that said timer-controller (14-60) synchronizes said accesses of said first and second accessing units by synchronizing their respective clock circuits (14-22, 14-42) to enable said first and second accessing units alternate initial access intervals to said cache unit. The cache system of claim 3, characterized by an additional accessing unit (14-10), whereby at least one of said first and second synchronized accessing units being programmed to selectively waive one of its access intervals to allow initial access to said cache unit by said additional accessing unit. The cache system of claim 4, characterized in that said synchronized first and second accessing units are CPU subsystems, and said additional accessing unit (14-10) is a FIFO subsystem for presenting additional requests for access to said cache unit.

Inventors: JOYCE, THOMAS F., KEELEY, JAMES W.
Applicants: BULL HN INFORMATION SYSTEMS INC.
Application year: 1985
Main classification: G06F12
IPC codes: G06F12