PRIVATE CACHE MISS AND ACCESS MANAGEMENT IN A MULTIPROCESSOR COMPUTER SYSTEM EMPLOYING PRIVATE CACHES FOR INDIVIDUAL CENTRAL PROCESSOR UNITS AND A SHARED CACHE

Abstract: A computer system including a group of CPUs, each having a private cache which communicates with its CPU to receive requests for information blocks and for servicing such requests includes a CPU bus coupled to all the private caches and to a shared cache. Each private cache includes a cache memory and a cache controller having: a processor directory for identifying information blocks resident in the cache memory, logic for identifying cache misses on requests from the CPU, a cache miss output buffer for storing the identifications of a missed block and a block to be moved out of cache memory to make room for the requested block and for selectively sending the identifications onto the CPU bus, a cache miss input buffer stack for storing the identifications of all recently missed blocks and blocks to be swapped from all the CPUs in the group, a comparator for comparing the identifications in the cache miss output buffer stack with the identifications in the cache miss input buffer stack and control logic, responsive to the first comparator sensing a compare (indicating a request by another CPU for the block being swapped), for inhibiting the broadcast of the swap requirement onto the CPU bus and converting the swap operation to a "siphon" operation to service the request of the other CPU.

Description: PRIVATE CACHE MISS AND ACCESS MANAGEMENT IN A MULTIPROCESSOR COMPUTER SYSTEM EMPLOYING PRIVATE CACHES FOR INDIVIDUAL CENTRAL PROCESSOR UNITS AND A SHARED CACHE Cross Reference to Related Provisional Application This application claims the benefit of the filing date of U.S. Provisional Patent Application Serial No. 60/ , filed December 16, 1996, entitled MULTIPROCESSOR COMPUTER SYSTEM EMPLOYING PRIVATE CACHES FOR INDIVIDUAL CENTRAL PROCESSOR UNITS AND A SHARED CACHE by William A. Shelly et al. Field of the Invention This invention relates to the art of computer hardware and, more particularly, to a multiprocessor computer system employing private cache memories for individual central processor units and at least one shared cache memory bridging diverse central processor and system busses. Background of the Invention In the generation-to-generation development of maύifrarne computer systems, increased speed of operation is sought for a give class of system, and it is therefor useful to minimize the time required to carry out operations which are performed often. This is particularly the case in multiprocessor systems because frequent operations which cause interaction between the processors, or in which one processor briefly takes over a common facility, affects the performance of the entire system. Some modern computer systems employ multiple processors, each of which has its own private cache memory. As is well known in the art, if a given processor asks its cache for a block of information which is not currently resident, the cache requests the block from one or more additional units in the system, at least one of which should be able to fill the request The other units may include the private caches of the other processors and a main memory. Other systems, such as that contemplated, interpose a secondary, shared cache between the several private caches and a main memory (and possibly other information sources). In this configuration, a private cache miss in a given processor results in an request to at least die shared cache and, as in the present system, to all the private caches in a group of processors. If none of these units have a valid copy of the requested block, the shared cache seeks it in main memory or elsewhere. When a processor requests a block which is not in the private cache, it is known that the missed block will be promptly brought into the private cache such that room must be made for the incoming block. This requirement causes a decision algorithm(usually , least recently used) to select the block to be moved out of the private cache to a larger memory such as main memory or, as in the present system, to the shared cache (which is typically much larger than the private caches). This is called a "swap" operation. It often happens, in a multiprocessor system employing private caches for each processor and a shared cache, that a request, resulting from a cache miss, originating in a given processor has been recently preceded by a request from another processor of the block which the given processor has decided to swap out to the shared cache. The block being swapped is therefore temporarily unavailable to the other processor which has a detrimental effect on system performance. In another aspect of multiprocessor systems employing private caches for each processor, a private cache must know what blocks are currently resident, a task performed by a cache directory which usually includes an appropriate associative memory. As blocks are siphoned from private cache as a result of serviced misses from other processors, the cache directory must be accordingly updated, but the updating process may interfere with its ongoing task of checking processor requests against the current contents of the private cache. This is another load on system performance. Thus, it will be clear to those skilled in the art of multiprocessing, once these problems are understood, that if cache swap operations could be converted to siphon operations on the fly when another processor has requested a block being swapped and processor directory updates can be performed without interrupting ongoing processor/directory operation, a significant improvement in system speed could be achieved, and it to these ends that the present invention is directed. Objects of the Invention It is therefore a broad object of this invention to provide efficient private cache miss and access management in a multiprocessor computer system employing private caches for individual central processor units and a shared cache. It is a more specific object of this invention to provide logic to achieve such efficient management which includes, in each private cache for each processor in the system, first and second cache directories and associated control logic by the use of which swap operations are converted to siphon operations on the fly when a second processor is requesting a block being swapped out and directory updating is performed seamlessly with respect to processor/directory operation. Summary of the Invention Briefly, these and other objects of the invention are achieved with a computer system including one or more groups of central processor units, each having a private cache module which communicates bilaterally with its CPU to receive requests for instruction and operand information blocks and for servicing such requests. The computer system further includes, for each group of CPUs, a CPU bus coupled to all the private cache modules in the group for bilateral communication of address and control signals and information blocks. Within each group of CPUs, a shared cache unit is coupled to the CPU bus for bilateral communication with the several CPUs of address and control signals and information blocks. Each private cache memory module includes a cache memory and a cache controller having: a processor directory for identifying information blocks resident in the cache memory, cache miss detection logic for detecting that a block of information requested by the private cache memory module's CPU is not resident in the cache memory, a command/address output buffer stack for temporarily storing the identification of a requested and missed block of information and the identification of a block to be swapped out of the cache memory and into die shared cache to make room for the requested block and also for selectively broadcasting the identification onto die CPU bus, a command/address input buffer stack for temporarily storing the identifications of all requested and missed blocks and blocks to be swapped from all the CPUs in the group, a first comparator for comparing the identifications in the command/address output buffer stack with the identifications in the command/address input buffer stack and control logic, responsive to the first comparator ascertaiiiing that the same identification is in both the output buffer stack and the input buffer stack, for inhibiting the swap requirement from being placed on the CPU bus and instead transfoπning it into a "siphon" operation for servicing the request from another CPU without unduly tying up system resources. To further enhance the efficiency of the system, each private cache controller also includes: a bus directory coupled to receive identification signal groups from the command/address input buffer stack when an acknowledgment signal has been received over the CPU bus, thereby updating the bus directory and a command/address transfer buffer stack coupled to receive identifications of such updates from the bus directory to provide for timely update, from the transfer buffer stack, of the processor directory at an opportune time. In addition, a second comparator monitors the identifications in the output buffer stack and those in the transfer buffer stack and, in the event of a compare, carries out the previously described inhibit of the swap and the conversion of the swap operation to a "siphon" operation to efficiently service the request of the other CPU. Description of the Drawing The subject matter of the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, may best be understood by reference to the following description taken in conjunction with the subjoined claims and the accompanying drawing of which: FIG. 1 is a high level block diagram of an exemplary multiprocessor computer system incorporating the invention; FIG. 2 is a slightly lower level block diagram showing additional details of a CPU board in the multiprocessor system of FIG. 1 ; and FIG. 3 is an intermediate level block diagram particularly showing additional details of a private cache representative of those associated with each CPU on the CPU board shown in HG.2. FIGs. 4 - 40 relate to an actual implementation of a presently preferred embodiment of the invention; more particularly: HG. 4 is a private cache coherency model; FIG.5 is a shared cache coherency model; FIG. 6 is a block diagram illustrating access conditions to a processor (CD) directory component; FIG.7 is a chart illustrating access priority to the CD directory; FIGs. 8 - 12 are tables showing certain "next" codes pertaining to CD directory access; FIGs. 13- 15 illustrate data flow through a CD AD block component of a private cache controller; FIGs. 16 - 19 illustrate data flow through a DDDP block component of the private cache controller; FIG. 20 is a timing chart of signals sent by a private cache to a CPU bus; FIG. 21 is a timing chart of signals sent by a shared cache to the CPU bus; FIG. 22 is a logic diagram of a priority determination network for prioritizing competing requests on the CPU bus; FIG. 23 is a block diagram of the CPU bus management logic; FIG. 24 is block diagram of data flow through a DDSB block shown in FIG. 21; FIGs. 25 - 29 are detailed logic diagrams of the DDSB block; FIG. 30 is a table summarizing the updating of processor (CD) and CPU bus (DD) directories in the private cache controller consequent to internal requests; FIGs. 31 and 32A/B are flow diagrams illustrating cache controller behavior on internal requests which require a CD directory access; FIG. 33 is a flow diagram illustrating cache controller behavior on a swap operation; FIG. 34 is a table summarizing the updating of processor (CD) and CPU bus (DD) directories in the private cache controller consequent to external requests; FIGs. 35A/B, 36, 37A B are flow diagrams illustrating cache controller behavior on external requests requiring a directory update; FIG. 38 is a timing diagram illustrating boundary of interference operation; HG. 39 is a timing diagram illustrating null command operation; HG. 40 is a chronogram of an external read request on a block swap; HG. 41 is a table summarizing DTCL coding; and HG. 42 is a timing diagram illustrating data transfer of a block. Description of the Preferred Embodiments Attention is first directed to HG. 1 which is a high level block diagram of an exemplary multiprocessor computer system incorporating the invention. A first CPU board (CPU Board "0") 1 includes four central processor units 2 (CPU "0"), 4 (CPU "1"), 6 (CPU "2"), 8 (CPU "3"). Each of the central processor units 2, 4, 6, 8 situated on the first CPU board 1 includes an integral private cache memory module, 3, 5, 7, 9, respectively. The private cache modules 3, 5, 7, 9 are each configured as "store into"; i.e„ the results of each coπφleted operation performed in the CPU are stored into the private cache. Each of the private cache modules 3, 5, 7, 9 on CPU Board "0" 1 interface with a CPU bus 10 for direct communications between the CPUs 2, 4, 6, 8. In the exemplary system, there are three additional CPU boards 12 (CPU Board "1"), 13 (CPU Board "2") and 14 (CPU Board "3"), each of which is substantially identical to CPU board 1 although those skilled in the multiprocessor art will understand that each CPU board and each CPU on each of the boards is assigned a unique identification number to facilitate communication and cooperation among the CPUs in the system. CPU board 1 (i.e., CPU Board "0") also includes a shared cache 11 disposed between ("bridging") the CPU bus 10 and a system bus 15. It will be understood that each of the CPU boards 12, 13, 14 also each includes a CPU bus and a shared cache, identically oriented. A system control unit 16 serves to couple the system bus 15 to a main memory unit 18 via a memory bus 17. In addition, one or more input/output units 19 interface the system bus 15 with various input/output subsystems, not shown, to achieve input/output functions on a system basis, all as well known to those skilled in the art Similarly, other subsystems 20, not otherwise specified or shown, may be connected to the system bus 15 to complete a given multiprocessor system, and other, complete, multiprocessor systems, represented by the block 21, may also communicate with the multiprocessor system depicted in HG. 1 via de system bus 15. System control unit 16 also conventionally provides a multi-phase clock to all the system units. HG. 2 is a slightly lower level block diagram of CPU "0" 2 of CPU board 1 (CPU Board "0") illustrating additional structure which is present in each CPU in the system. CPU "0" 2 includes a basic processing unit 22 and support circuitry 23 therefor. An example of a suitable basic processing unit 22 and support circuitry 23 may be found in U.S. Patent 5,195,101, entitled EFHCIENT ERROR DETECTION IN A VLSI CENTRAL PROCESSING UNIT, by Russell W. Guenthner et al, assigned to the same assignee as the present invention and incorporated by reference herein. As previously described, CPU "0" 2 also includes private cache module "0" 3 which constitutes a cache control unit 24 and a private cache 25 (which itself includes additional logic to be described below). Cache control unit 24 includes paging unit 26, cache management unit 27 and CPU bus unit 28. Paging unit 26 interfaces with basic processing unit "0" 22 and cache management unit 27. Cache management unit 27 also interfaces with private cache memory 25 and CPU bus unit 28. CPU bus unit also interfaces with CPU bus 10 and, via CPU bus 10, shared cache 11. Private cache 25 is also coupled directly to receive information from and send information to the CPU bus 10 and to receive information from and send information to basic processing unit "0" 22. As previously described, shared cache 11 also interfaces with system bus 15 and, via system bus 15, with system control unit 16 and other systems/subsystems shown in HG. 1. Main memory 17 may be accessed via the system control unit 16 and memory bus 18. It will be seen that there are numerous paths for information flow among the various blocks shown in HGs. 1 and 2. The types of information may include control, address, instructions and operands. A given CPU may direcdy access its own private cache module and indirectly access the private cache modules incorporated into the other CPUs on a shared CPU board. Thus, CPU "0" 2 can access, via the CPU bus 10, the shared cache 11 it shares with CPU "1" 4, CPU "2" 6 and CPU "3" 8. CPU "0" 2 can also, under defined conditions, access the private cache module of CPU "2" 6 (for example) via the CPU bus 10 to effect a local "siphon". Further, CPU "0" 2 can access (via CPU bus 10, shared cache 11 and system bus 15) the shared caches (not shown) on each of CPU Board "1" 12, CPU Board "2" 13 and CPU Board "3" 14. Still further, a given CPU may indirectiy access die private cache modules (not shown) of a CPU (not shown) on another CPU board; e.g„ CPU "0" on CPU board 1 (CPU Board "0") may, under defined conditions, access the private cache module of any one of the CPUs on CPU Board "2" 13 (HG. 1) via CPU bus 10, shared cache 11, system bus 15 and the shared cache on CPU Board "2" to effect a remote "siphon". Further yet for example, CPU "0" 2 can access main memory 17 via CPU bus 10, shared cache 11, system bus 15, SCU 16 and memory bus 18. Still further, for example, CPU "0" 2 can access, via CPU bus 10, shared cache 11 and system bus 15, any other block shown coupled to the system bus 15 in HG. 1 to achieve bilateral communication with input/output devices, other subsystem components and even other multiprocessor systems. Most of these accesses can be either read or write and can be in either direction. Therefore, those skilled in the art will understand that not only must access request conflicts be conclusively resolved, but coherence among the private caches modules, shared caches and main memory must be unerringly maintained. As notoriously well known in the art, achievement of absolute coherence is an extraordinarily difficult process which becomes geometrically more complex as the number of communicating entities increases. HG. 3 shows more details of die structure and information flow, particularly control and address information, within the cache management unit 27 and CPU bus unit 28 in conjunction with the private cache 25. In cache management unit 27, a private cache control block 30 communicates with paging unit 26, private cache 25, a processor directory 31 and various blocks in CPU bus unit 28. If a cache miss occurs, i.e., if basic processing unit 22 requests a block of information which is not (according to processor directory 31) resident in a private cache random access memory 40 (hereinafter, cache memory 40), private cache control block 30 carries out/directs several operations. First the fetching of the requested block is anticipated such that the processor directory 31 is updated to indicate its imminent presence. Second, inasmuch as room must be made in the cache memory 40 for the requested block, a suitable algorithm (typically, least recendy used) is run to select a block to be swapped out of the cache memory 40 and into the shared cache 11 via one of the swap buffers 50, 51 and the CPU bus 10. Third, private cache control 30 sends control and address information pertaining to the cache miss to CPU bus unit 28 where this information is placed into a command/address output buffer stack 32 which selectively delivers control and address information to the CPU bus 10. This information includes not only the identification of the block under request but also d e identification of the block being swapped out to make room for the expected block. The contents of output buffer stack 32 are also read by first inputs to two comparators 33, 34. Control and address information issued by the output buffer stacks 32 of all the CPUs on the board (including that from local output buffer stack 32) in response to private cache misses is received from the CPU bus 10 into a command/address input buffer stack 35. The output stage of input buffer stack 35 selectively delivers information to a bus directory 37 which selectively supplies address and control information to a command/address transfer buffer stack 36. In addition, the contents of input buffer stack 35 are also read by a second input to comparator 34, and the contents of transfer buffer stack 36 are also read by a second input to comparator 33. Transfer buffer stack 36 selectively delivers control and address information to the private cache control block 30 in cache management unit 27 for timely update of the processor directory required by servicing external requests such as siphons which remove a block from the cache memory 40. The outputs of each of the comparators 33 and 34 are coupled to the bus interface control block 38 which also communicates with private cache control block 30 in cache management unit 27. Data may be received directly into the private cache memory 25 from the CPU bus 10 and supplied to basic processing unit "0" 22. In addition, data may be transferred from the private cache memory 25 onto the CPU bus 10. CPU bus 10 is divided into two sections: data section 10A and control/address section 10B. CPU bus data section 10A communicates bilaterally with private cache memory 25 and shared cache 11 which includes a shared cache directory 53. Similarly, CPU bus control/address section 10B communicates bilaterally with both CPU bus unit 28 and shared cache 11. Data (operands/instructions) from CPU bus data section 10A is applied as a first input to multiplexer 46 which controls input to an operand buffer set of four operand buffers 56, 57, 58, 59. A second input to multiplexer 46 is from the basic processing unit 22 via a result bus 61, and a third input is from cache memory 40. Multiplexers 47, 48 respectively control input to cache memory 40 and to instruction buffers 54, 55 from cache memory 40. Thus, the instruction buffers and operand buffers selectively receive input (instructions and operands, respectively), via multiplexers 48, 46 from cache memory 40. In addition, the operand buffers selectively receive result information from the basic processing unit 22 via result bus 61 and from the CPU data section 10A. Similarly, cache memory 40 selectively receives input via multiplexers 41 and 47, from the operand buffers and from EDI buffer 59. Operand information read from cache memory 40 is available to one input of multiplexer 42 and at the input of multiplexer 43. A second input to multiplexer 42 is from the operand buffers via multiplexer 41. Operands are selectively sent to basic processing unit 22 via operand bus 63. Instructions received from cache memory 40 are selectively directed to basic processing unit 22 via multiplexer 48, instruction buffers 54, 55, multiplexer 45 and instruction bus 60. A block output buffer set selectively transfers information from the private cache 25 to the data section 10A of the CPU bus 10. The block output buffer set includes swapO buffer 50, swapl buffer 51 and siphon buffer 52. Input to multiplexer 43 from cache memory 40 is selectively steered to one of the swap buffers 51, 52 or, under certain conditions, to siphon buffer 52. Another input to siphon buffer 52 is from the operand buffers via multiplexer 41. Control signals from private cache control block 30 conventionally serve to dynamically select the appropriate inputs to the multiplexers 41, 42, 43, 44, 45, 46, 47, 48 for appropriate transfer to their respective outputs. Consider now the operation of the private cache module shown in HG. 3. During normal operation, the basic processing unit 22 sends successive requests for information blocks (instructions or operands) to the cache management unit 27 (via request bus 63 and paging unit 26) where the processor directory 31 is consulted to determine if the requested block is resident in cache memory 40. If the requested block is currendy in the cache memory 40, it is transferred to the instruction buffers (via multiplexer 48) or to the operand buffers (via multiplexer 46) or more direcdy to the basic processing unit (via multiplexer 42), depending upon the type of block requested and various other conditions, all under control of signals issued by the private cache control block 30. This is a normal private cache hit However, if the consultation with the processor directory 31 indicates that the requested block is not resident in the cache memory 40, various actions are taken as discussed above, including copying address and control signals (identifying both the missed and requested block and the block being swapped out to make room in the cache memory 40 for the expected block) to command address output buffer stack 32 in the CPU bus unit 28. Assuming that no action is taken to inhibit placing this information on the CPU bus address and control section 10B, the request is broadcast onto the CPU bus address/control section 10B in a relative priority among the current CPU bus operations of the several CPUs and the shared cache. All units on the CPU bus 10 (i.e., the private caches of all four of the CPUs on the board; e.g., the private caches 3, 5, 7, 9 in HG. 1 and the shared cache 11) receive the address and control information relating to a given request (including the identification of the block being swapped out of the requesting private cache) into their respective command/address input buffer stacks 35. In the service of a request the requested block is placed on the data section 10A of the CPU bus 10 in quarter-block increments by the unit (usually the shared cache 11) which owns the block, and an acknowledgment signal is sent by the shared cache 11. The data is selectively read into the operand buffers of the requesting unit Thus, if the cache memory 40 is busy, the requested block is still proπφdy available to the basic processing unit via the operand buffers which also update the cache memory when it becomes available. Therefore, the basic processing unit does not have to wait for the cache memory to be updated before the requested information received on the CPU bus 10 is available to it In this manner, the service of a routine private cache miss is carried out similarly to the process for a single processor system employing a store-into cache and a main memory for supplying blocks in the case of a cache miss. In the straightforward case of a request from another CPU to siphon a block from the local CPU shown in HG. 3 in which there is no compare condition in comparator 34, the request enters the command/address buffer stack 35 and routinely moves to the bus directory 35. If the local cache memory 40 does not have the requested block, the request goes not further, and the processor directory 31 need not be consulted. If the local cache 40 does own the block, the bus directory acknowledges this fact and a routine siphon operation is carried out to transfer the block from the cache memory 40 (or operand buffers) to the requesting CPU via the siphon buffer 52 and CPU bus data section 10A. However, in the case of a multiprocessor system configured as in HG. 1, interferences or conflicts can arise which must be definitively resolved with as little performance loss as possible. Therefore, consider now a second example in which there has been a private cache miss in the given CPU shown in HG. 3 and another unit on die CPU bus 10 has already placed a request on the CPU bus address/control section 10B for the block the given CPU has determined will be swapped out and sent to the shared cache 11. This block is therefore resident in one or the other of the swap buffers 50, 51. In that case, the identification of this interfered block is not only in the command/address output buffer 32 of die given CPU, but also is in the command/address input buffer 35, and the comparator 34 senses this condition by comparing the swap identifications in output buffer 32 with request identifications in input buffer 35. When this condition is sensed, comparator 34 accordingly signals the bus interface control 38 which notifies the private cache control 30 to send out the block temporarily stored in one of the swap buffers 50, 51 onto the CPU bus data section 10A to service the other requesting CPU as if a siphon had taken place. The bus interface control block 38 also nulls the swap aspect of the affected request in output buffer 32 so that no swap information will be placed on the control/address section 10B of the CPU bus as this would be an illegal operation that would slow down the system; further space has already been made for the block in one of the swap buffers 50, 51 in the other requesting CPU which has performed its own swap. In effect this particular swap operation is converted to a "siphon" operation on the fly. When the acknowledgment signal is received, indicating that die transfer of the block requested by the other CPU (or, rarely, by the shared cache) has been (or soon will be) completely transferred, the bus directory 37 is updated to indicate that the "siphoned" block is no longer in the cache memory 40. In addition, all the same address and control content from the input buffer 35 is passed along to the transfer buffer 36 which serves to permit updating the processor directory 31 (to reflect the serviced "siphon" request) when such action is not disruptive to the normal consultation of the processor directory in conjunction with ongoing requests from the basic processing unit 22. It has previously been mentioned that a comparison is also made between the contents of the buffers 32 and 36 by the comparator 33. If a compare condition is detected by the comparator 33, the swap aspect of servicing die locally requested block can be converted to a "siphon" as previously described. It will be understood that the bus directory 37 is updated prior to the updating of the processor directory 31 for serviced external requests. This permits the processor directory 31 to operate with the basic processing unit 22 without interruption, updating being carried out when the processor directory 31 is not otherwise occupied. However, as the bus directory 37 is already updated, external inquiries from other units on the CPU bus 10 can routinely interrogate the bus directory to determine if a copy of the externally requested block is resident in the private cache memory 40. If the block is there, a normal siphon operation transfers the requested block to the CPU bus 10 via the operand buffers and the siphon buffer EDO 52. This leaves a "vacancy" in the cache memory 40 such that it may not be necessary to carry out a swap consequent to a subsequent private cache miss, thereby further iπφroving the efficiency of the system. In this case, the control signals sent to the cache miss output buffer 32 from the private cache control block 30 reflect this condition. By providing two alternately used swap EDO buffers 50, 51 and the previously described logic, two local private cache miss requests can be outstanding simultaneously to the extent that a second miss procedure can be started as soon as the data from the first request is received, even if the first swap is not completed. (Additional simultaneous requests could be accommodated by providing corresponding additional swap buffers and the relevant support logic.) Another feature of the circuitry shown in HG. 3 is that not only can two outstanding requests be accommodated, but the quarter block segments of the two requested blocks sent to the private cache from the CPU bus 10 from the transmitting unit(s) can be received and handled intermixed. This is because each of the operand buffers 56, 57, 58, 59 are half-block in length and are divided into two quarter block addressable sections. Thus, a first full block can be loaded from the CPU bus data section 10A, in quarter block increments, into operand buffers OB00 56 and OB01 57 (the combination, strictly speaking, being a first full operand buffer); and a second full block can be loaded from the CPU bus data section 10A, in quarter block increments, into operand buffers OB 10 58 and OB 11 59 (the combination, strictly speaking, being a first full operand buffer) under the direction of the multiplexer 46. Since the identity of the arriving quarter-block segments is known to the private cache control block 30 from information on the CPU bus control/address section 10B, the appropriate gating signals can be supplied to the multiplexers 46, 48 to coordinate the intermixed arrival of the quarter- block segments of two internally requested blocks and the selective transfer of any quarter block in the operand buffers to the basic processing unit 22. The foregoing is a somewhat simplified, for clarity, disclosure of the invention, which, however, fully teaches the principles for its practice to those skilled in the art of computer system design. There follows a more detailed disclosure of a presentiy preferred embodiment of the invention. In the following exposition and in HGs. 4 - 40, certain convenient abbreviations are employed. Thus, for example, the CD directory corresponds to the processor directory 31, the DD directory corresponds to the CPU bus directory 35, ACB corresponds to the CPU bus 10, ASB corresponds to the system bus 15 and AX is the address and execution coπφonent of the basic processing unit 22 as set forth more fully in the aforementioned U.S. Patent 5,195,101. Referring to HG. 4, the coherency model for each private cache is shown. Arrowed solid lines denote a received request while dashed lines indicate a request sent. "Valid" indicates that memory is coherent and copies may exist in one or more other private caches; "invalid" indicates that no copy is present in the set; "dirty" indicates that menory is not coherent; no other copy exists in any cache other than this unique one. Referring to HG. 5, the coherency model for the shared cache is shown. From the shared cache view point: 'invalid" indicates that no copy is present in the set; "valid" indicates that the shared cache block is coherent with the memory; copies of the block may exist in private caches of the CU board and in caches of any other CPU board; "valid (with exclusive property)" means that the shared cache block is coherent with menory; copies may exist only in the private caches of this CPU board; "dirty (with updated shared cache copy)" means that the shared cache is the unique owner of the block; and "dirty (with CPUx owner of the line)" means that CPUx is the unique owner of the line in the system. A. CD DIRECTORY ACCESS AND BUFFER MANAGEMENT A.1 - BUFFERS OB-IB-EDI-EDO A.1.1 - Overview In order to minimize the number of private cache memory or directory accesses and to allow prefetch, the private cache (corresponding to the private cache 25 in HG. 3) provides: - two 32 byte instruction buffers (IB; i.e., IBO, IB1 - corresponding to the buffers54, 55 in HG. 3) with read-only access; and - four 32 byte operand buffers (OB; i.e., OB00, OB01, OB10, OB11 - corresponding to buffers 56, 57, 58, 59 in HG. 3) with read-only, write-only and read-alter-rewrite access. In order to overlap requests on the CPU bus 10, the cache provides CPU bus buffers: - one 64 byte external data-in (EDI) buffer (corresponding to buffer 59 in HG. 3) used for read requests; and - three 64 byte External Data Out (EDO) buffers (corresponding to buffers 50, 51, 52 in HG. 3) used for two swap requests and one siphon request The cache controller manages the transfer of data in the private cache memory 40 with several signals: B-IRB-T1N 0:5 - selects the word to be sent on the instruction bus from IB to the basic processing unit 22; B-ORB-TIN 0:8 - selects the source of data to be sent on the operand bus (from cache memory or OB) to the basic processing unit 22; B-RLB-T2N 0:4 - selects the eight-word buffer to be loaded with the cache memory output (into IB, OB or EDO); B-RAW-T1N0.4 - selects the OB or EDI to be written into cache memory (for codes lOxx, used to transfer OB to EDO without write in cache memory); B-WR-T1N 0:13 - selects the words coming from operand or result bus and the OB written; and B-SLB-T1N 0:6 - selects the OB/IB to be written with EDI from the CPU bus or the EDO to be sent onto CPU bus. The addresses of the sub-block contained in those buffers are temporarily stored: - in CDAD block for IB and OB and - in DDSB block for EDI and EDO. In CDAD, a temporary address latch (ITEMP-AD-L2) is used to hold the target address of a transfer or a conditional transfer until the TRGO cycle. The loading of these buffer address latches is controlled by DD. Address comparison is done on OB/IB address latches for each address received from the CPU. A.1.2 - CPU Bus Buffers EDI and EDO The cache. controller sends only one read miss onto the CPU bus at a time, so it needs only one EDI buffer 59 to stack data coming from the CPU bus. The EDI buffer is used only in case of EIS (extended instruction set) commands; in this case, the cache memory will be updated from EDL For general commands, the block is loaded directly into a pair of operand buffers (OBOO and OB01 or OB10 and OB11). The cache memory will be updated from the operand buffer (cache memory will write into EDI, but it will be never used). The CPU bus miss may be associated with a swap request so the cache controller needs one EDO buffer 50 to stack the 64 byte block to be written into the shared cache. A second EDO buffer 51 is provided for swap, in order to start a second miss as soon as the cache controller receives the data of the first miss, even if the first swap is not terminated Qower priority than miss). The shared cache (corresponding to the shared cache 11 in HG. 30 allows only one siphon at a time, so the cache controller needs only one EDO buffer (corresponding to buffer 52 in HG. 3) to stack the 64 Bytes block for delivering siphon data to the CPU bus. The address of the block in EDI is not temporarily stored because it is die same as in OB. As soon as all the data are received from the CPU bus, EDI is written back to cache memory. The addresses of the blocks in EDO are temporarily stored in DDSB, until the data is sent on the CPU bus. A.1.3 - Instruction Buffer A.l.3.1 - Overview The instruction fetching is done through the cache instruction buffer which consists of two 32 byte sub-block buffers (corresponding to buffers 54, 55 in HG. 3). Instructions are supplied in sequential order to the basic processing unit 22 (HG. 3) on the instruction bus (four bytes) following a transfer command and are continuously prefetched into the instruction buffer until a page crossing is reached or another transfer command is received. A.l.3.2 - Initial Fill of the Instruction Buffers The initial fill of the instruction buffers occurs during an unconditional transfer or a conditional transfer that goes. The basic processing unit supplies the address to the cache directory, and the addressed word is placed on the operand bus to be transferred to the basic processing unit If the cache receives the transfer go signal (A-I-TRGO-T2) from the basic processing unit, the addressed word is sent on the operand bus, and the addressed block is loaded into the instruction buffers, then instruction fetching is continued from that point If the transfer is no go, the load of the new block in the instruction buffer is cancelled, and the cache memory continues to send instructions from the current instruction sub-block buffer on the instruction bus. The instruction buffer page address latch is loaded from CPU address during unconditional transfer and from the temporary address latch during transfer go cycle of a conditional transfer. The sub-block address part is loaded from the same origin during a transfer and from B-L-Xl during prefetch. The private cache sends the instruction word along to the basic processing unit with the instruction available signal (B-I-AVAIL-T2) which indicates that a valid instruction word is on the instruction bus. The basic processing unit responds with the instruction request signal (A-INST-RQ-T2), notifying the cache that the last instruction word was accepted and the next instruction should be sent. If the cache does not receive A-INST-RQ-T2, the previous instruction word is sent every cycle until A-INST-RQ-T2 is received. A.1.3.3 - Instruction Buffer Refilling The next sub-block is prefetched with an unconditional prefetch if the transfer target is not the last word of the sub-block and this sub-block is not the last one of the page. If die prefetch is instituted and the block is not present in the cache or is still in transit from CPU bus, prefetching is held off until the instruction buffer is emptied An unconditional prefetch is made when the instruction buffer is emptied. The instruction buffer is continuously refilled in this manner until either a page boundary is reached or a transfer command is executed. A.l.3.4 - Page Crossing When the instruction buffer delivers the last instruction on a page, the cache control waits for the next instruction request and notifies the basic processing unit that the cache memory needs a new virtual address for the next instruction (B-STATUS-Tl 0). The basic processing unit will calculate the new virtual address and issue a transfer command to restart instmction transmissions. A.1.3.5 - Store Compares In the general command set the sub-block real addresses associated with all store commands must be compared with the instruction buffer real addresses in the event that the store command references the real address cuπrenuy in one of the instruction sub-block buffers. In the EIS command set the comparison is only done at the page level. (The comparison is done at the page level in CDAD and at the sub-block level in DDDP.) If a store command has a hit in the current mstruction buffer, the current instmction transfer will be aborted, die instruction buffer will be invalidated and the basic processing unit will be notified of the store compare (B-STR-CMP- FLT-T2). The basic processing unit will issue a transfer command to restart instruction transmissions. A.1.4 - Operand Buffers A.1.4.1 - Overview Two operand buffers, OBO and OBI (corresponding to buffers 54, 55 in HG. 3), are used to perform operand read or write transfers over the operand or result bus. The sub-block address latches are loaded from CPU address under DD control or from DD (B-L-Xl) during prefetch. A transfer on operand buffer is terminated: - by the cache, when the byte count (length) of the transfer is exhausted, - by the basic processing unit using A-CMD-CAN-T1 signal (cancel command), or - by the basic processing unit using a stop bit of a command or a CHG-GEN. A.l.4.2 - Prefetch In the general command set, there is no prefetch. In carrying out EIS commands, on LBUF1, the next sub-block is systematically prefetched. In 9- 9 move, 9-9 compare (i.e., move and compare of nine bit character strings) and EIS non-9-9 alphanumeric accesses, the next sub-block to be loaded into an operand buffer is fetched when all bytes or words of that sub-block have been read or written. In EIS decimal access, no prefetch is necessary if two sub-blocks are needed, and the three sub-blocks case is too rare to justify a prefetch implementation. A.l.4.3 - Write Back When an operation is ended, the written operand buffers are written back to cache memory when there is an opportunity (cache not busy) or when a buffer is needed and no buffer is free. The operand buffers are systematically written back on the CHG-EIS command and on reset reception. In a siphon, if the block is modified in an operand buffer, the data will be written directly from the operand buffer to EDO for one sub-block, then from cache memory to EDO for the second sub-block. If a write occurs on a read operand buffer, this buffer can still be read because there is software protection (the bytes read are not the same as the bytes written). A.1.4.4 - Use in the General Command Set With general commands, the four sub-block operand buffers are used independendy, except in case of a miss where the block is loaded in OBOO and OBOl or in OB10 and OBI 1. The sub-block containing the two, four or eight byte operand is loaded in the next operand buffer which is not-occupied, not-waiting-to-be-written, in the prioritized order: OB11, OBOl, 0B10, OBOO. A.1.4.5 - Use in the EIS Command Set With the EIS (extended instruction set) commands, a written operand is always loaded in OBOl. For alpha instructions, if there are two operands, the first operand is loaded in OBOO and the second one in OBOl. For decimal- numeric instructions, the operand is loaded into the even part of OBOO or OBOl for an even address and into the odd part of OBOO or OBOl for an odd address. A.2 - CD Directory Access A.2.1 - Overview The CD directory (corresponding to processor directory 31 in HG. 3) is accessed: - on a basic processing unit internal request, - on an external request when cache controller must be invalidated or changed from status modified to status valid, and - on a DD request to write back buffer or do prefetch. For an internal request, the swap is detected during the CD directory update, and the address is sent to DD. DD selects the operand buffer to write with B-LOB-X1N 0:2 and the operand buffer or instruction buffer to read with B-SOB-XIN 0:3. DD specifies to CD the next CD directory cycle to do on the B-NXT-X1 and B- CL-X2 lines coming from DDCS. A.2.2 - Access Types in CD Directory Refer to HG. 6 and the following discussion. A.2.2.1 - Consultation Cycle CD executes a consultation cycle only on basic processing unit requests, prefetch, internal invalidate or full-write requests in case of interference and reread after siphon. The high address used to access the processor directory comes from: - TED (the virtual to real, associative memory address translator in the paging unit) if TED hit or from CDPD if paging on basic processing unit requests, - an operand buffer or a temporary address register ITEMP for a prefetch, - DD on reread-c (B-H-Xl 0:18), or - SSP (i.e., service scan path). The low address used to access the processor directory comes from: - CDPD on basic processing unit requests, - DD on prefetch and on reread-ob or reread-c (B-L-Xl 18:9), or - .SSP. A.2.2.2 - Read Cycle The processor directory executes a read cycle on write back, CPU bus block change and reread-ob. The level used to access CD comes from: - DD on CPU bus block change (B-LV-X1 0:4), - an operand buffer for an OB write back, or - SSP. The low address used to access the directory comes from: - DD on CPU bus block change (B-L-Xl 18:9), - an operand buffer for an OB write back or a reread-ob, or - SSP. A.2.2.3 - Write Cycle CD executes a write cycle on write after a CPU bus block change. The low address used to access CD comes from DDSB (B-L-Xl 18:9) or from SSP. The high address written in the directory comes from DDSB (B-H-Xl 0:18) or from SSP. DDSB sends the status to be written in CD directory on the B-L-Xl 20:4 lines. A.2.2.4 - NOP Cycle CD executes some NOP (no operation) cycles without specific addresses when there is no operation. A.2.3 - Priority Tree As shown in HG. 7, 0 is the highest priority to access the CD Directory, 7 is the lowest The nxt (next) codes are described in HGs. 6 - 10. A.3 - Address Data Path CDAD is the real address data path for the access to the CD directory. DDDP is the byte address in the block and block in the page address. A.3.1 - CDAD Data Path The main functions of the CDAD data path, shown in HGs. 11, 12, 13, are: - to provide the real address to CD: - in operand access operations, - in prefetch, re-read, and write- OB, EDI buffers to RAM operations, and - in operations in which a directory location is written to change the status and/or address, and to load a new block or invalidate a block; - to temporarily store the instmction buffer and operand buffer sub- block addresses and detect if these addresses are equal to the current address; - to temporarily store the lower and upper limit address registers for memory and expanded memory unit (EMU) access and detecting if an address is out of the bounds; - to temporarily store the real address trap and detect a match with the current address; and - to transfer data between CDPD and DDSB. This data path is 30 bits (27 bits data and 3 bits parity) wide. The control part is in CDCC (the selection of the multiplexers is calculated in CDCC from the next codes). The main interfaces of the CDCC block are: - interface with CDPD: - reception of the real page address if TED hit TED-OUT-X2 4:18 (+2P), - reception of the real page address if TED miss: VIRTUAL- ADDER-OUTPUT-X20:18 (+2P), - reception of the address in the page: LOW-PAGE-ADDR-X2 0:9 (+P), - enission/reception of data exchanged with CDAD: CD-TRI- DATA-X1 0:18 (+2P), and - reception of data from CDAD: CD-LOW-DATA-X1 18:9 (+P); - interface with the CD directory and the data to access the cache, and read or write the directory addresses: - emission of the address in the page RSAL-X2 0:7 (block address for CD directory and sub-block address for data) for the access of a set, - emission of the page address CON-WRTTE-BUS-Xl 0:18 (+2P) for write or comparison, - reception of the page address read in the directory CDAM- OUT-RB-X1 1:18 (+status and 2P, used for swap or SSP); - interface with DDSB : - reception of data from DDSB: B-H-Xl 0:18 and B-L-Xl 18:9 (+2P) and - emission of data to DDSB: P-H-Xl 0_18 and P-L-Xl 18:9 (+2P). The real address trap register is loaded in CDAD with the basic processing unit command LD-TRAP, PL 6=1. The limit address registers (ULAR, LLAR, EULAR, ELLAR) are loaded in CDAD with the basic processing unit command LD-LARn, and sent back to the basic processing unit with the basic processing unit command ST-LARn. For main memory access in VMOS mode, the comparison of the current real address with the upper and lower limits is systematically done. For EMU access in VMOS mode, the comparison of the current real address with the EMU upper and lower limits is done with the basic processing unit command EMU-LIM. If CDAD detects that the address is out of bounds, CD will send a VW bound fault to the basic processing unit Integrity: the inputs of CDAD are protected with a 9 bit parity. They are systematically verified at the input CDAD emits its outputs with 9 bit parities. A.3.2 - DDDP Data Path Referring to HGs. 16 - 19, the main functions of the DDDP data path are: - the calculation of the next address for operand or instmction (prefetch), - the detection of sub-block, block and page crosses, and - the incrementation of the address for EIS. The control part is in DDEC. This data path is 12 bits wide, without parity. The main interfaces of the DDDP block are: - with the basic processing unit: - reception of the address in the page: A-AD-T1 24-35 and - reception of the parameter-length: A-PL-T 1 0_11 ; - with cache memory: - emission of the selection of the word to be sent on the instruction bus: B-IRB-T1N 0:5 (+P); - with DDSB: - emission of CTL-ADR-Ll 0:9 (+P) that will be temporarily stored in ESB or TSB registers in DDSB and - emission of CTL-TRAC-BUS-L-X1 18:9 (+P) that will be sent on B-L-Xl or temporarily stored in ESB or TSB registers in DDSB; and - interface with DDB8, DDCS, DDMF - transmission of the parameter-length PARM-LN-A-X2 to DD control blocks, - emission of CTL1-ADR-X2 6:6 and CTL2-ADR-X2 6:6 to DDDT, for B-ORB-TIN and B-WR-T1N generation (from DDDT to DRAM), and - emission of the crossing, size information to DD control blocks. DDDP is divided in three parts called controllers: - a general controUerG used for instruction target prefetch and target fetch, - a controller 1 used for main operand and EIS operandl address, count, and - a controlled used only as a multiplexer for general commands and for EIS operand2 address, count DDDP receives from the basic processing unit a byte address in the page (bits 24:35 of the virtual address). Bits 24:7 (sub-block address in the page) are used in the prefetch sub-block incrementer/decrementer processes to form the address of die next 32 byte sub-block of any sequentially accessed string of instructions or of bytes, words and decimal digits. This sub-block is to be prefetched and loaded into one instruction buffer or operand buffer. The prefetch address is transmitted to CD over the B-L-Tl 18:7 lines. Bits 31:5 (byte address in sub-block) are incremented or decremented in the controllers G, 1, 2 to form the address of the byte string within a 32 byte sub-block for each cache memory buffer access and instruction, operand, or result bus transmission. These addresses are transferred to the cache memory (direcdy or via DDDT) in control codes spedfying the buffer source of a sub-block and the word or byte address within the sub-block. B. SYSTEM DIRECTORY ACCESS AND CPU BUS REQUEST MANAGEMENT B.1 - Role of the Bus Directory The Bus Directory (corresponding to BDIR 37 in HG. 3) is also called d e DD directory. Two roles can be distinguished: to interface with the CPU bus and to filter CPU bus invalidate requests. B.l.l. CPU Bus Interface The DD directory (and its support logic circuitry) interfaces with the CPU bus in order to be able to access the shared cache 11, other private caches on the board and the main memory through the shared cache. The DD directory is accessed on every CPU bus request All the block state evolutions or changings are ordered by the shared cache, and this procedure takes place when the shared cache acknowledges the request (internal request or external request). So, updates are performed in the same order in the shared cache directory and in the private cache DD directory. B.1.2 - Filter on CPU Bus Invalidate Requests A second function of DD is to filter external invalidate, full- write and read-&-exclusivity requests not filtered by the shared cache. Indeed, when the shared cache receives a CPU bus (or system bus) invalidate request on a lock with the state valid or valid-exclusive, it acknowledges all the CPU's on the board because it doesn't know which private caches own a copy of this block. The same situation applies on a full-write or a read-&-exclusivity request Similar conditions apply each time a block with the state valid, valid-exclusive or valid-modify (dirty) has to be replaced in the shared cache. In this case, an invalidate command is sent to all the private caches on the board. So, if the DD directory doesn't have a copy of this block, it will not disturb the CD directory. B.2 - CPU Bus Requests The CPU bus requests can be separated into two classes: internal requests and external requests. All requests emitted by the private cache are deemed internal, and all requests coming from the CPU bus are deemed external There are two types of external requests: requests originating in the shared cache and requests originating in the other private caches connected on the board. B.2.1 Remind on CPU Bus Request Management B.2.1.1 - Request Sent by a Private Cache As shown in HG. 20, management of requests from a private cache requires three steps: 1st step: request emission (address and command), 2nd step: acknowledgment of that request and update of private cache directories (shared cache directory has previously been updated), and 3rd step: data transfer (not in FIFO mode versus request emission or acknowledgment) if needed. B.2.1.2 - Request Sent by the Shared Cache Referring to HG. 21, management of requests from the shared cache requires only two steps: 1st step: Request emission (address and command) with implicit acknowledgment => CD and DD directories are updated (shared cache directory has previously been updated). 2nd step: Data transfer (in HFO mode versus shared cache Request emission) if needed. B.2.2 - CPU Bus Commands The following commands are available on the CPU bus. All commands may be sent by cache controller. Only read, read-with-exclusivity, invalidate and interrupt commands may be sent by the shared cache. 0000 NOP no data 0001 INTERRUPT no data 0010 INVALIDATE no data 0011 WRITE REGISTER 16 byte data 0110 FULLWRITE no data 0111 READ REGISTER 16 byte data 1010 SWAPP 64 byte data 1110 READ WITH EXCLUSIVITY 64 byte data 1111 READ 64 byte data The missing binary codes are forbidden and not sent by the cache controller. B.3 - CPU Bus Address Allocation The cache controller sends a request onto the CPU bus, each time: - a miss occurs in the CD directory (read, read-&-exclusivity, full- write), - a write is performed on a block with the state valid (invalidate), and - a block with the state valid-modify has to be replaced in the cache (swap). The cache controller also sends interrupt requests to manage the interrupts and read register or write register requests to manage some specifics commands such as read-calendar-clock-from-SCU. When the cache controller needs to send a request on the CPU bus, it sets the CB-PCRQO signal in order to obtain the CPU bus allocation. This information is broadcast to all the units connected on the CPU bus. B.3.1. Priority Determination The priority is computed simultaneously in the five units (shared cache and four CPUs) on the board. Referring to HG. 20, this priority is done by sampling CB_PCRQi, and applies the following rules: - the shared cache request has the highest priority and does not enter into the sample such that when the shared cache sends a request, it is automatically elected, it is the responsibility of the shared cache to send its request at an appropriate time, and when the CB_SHRQ signal is present it means that the two next CPU bus cycles will be used for the servicing the shared cache address transfer; - the elected request depends on the CPU number; i.e.: inside a sample, first the CPU #0 request which is elected, then the CPU #1 request, then the CPU #2 request and then the CPU #3 request; - all the requests inside a sample are treated before the temporary storage of another sample. Each private cache samples the requests coming from the four private cache controllers. The CPU number is used to determine from which buffer the CB_PCRQi signal has to be sent The priority network inside the five units (four CPUs and the shared cache) on the CPU bus elects one request (CB-PCOiACl-Xl). Once elected, this request is reset and the cache controller for the elected unit drives the CPU bus address and command bus section during the following two cycles. Every two cycles, if there is no request from the shared cache, one request can be elected. When all requests inside a sample have been handled, the signal CB-PCSMPL-L2 is set and a new sample is temporarily stored. A request from the shared cache invalidates the priority network. B.3.2 - CPU Bus Address Emission When a given cache controller's request has been elected, it drives the CPU bus during two cycles during which it sends the address of the block, the associated command, the label (request number) and the type of the data (private or shared). The address transfer lasts two cycles: Cycle 1 : - low address bits 16:16 - data type on bit 0 of the CB.CMD field 0 stands for private data 1 stands for shared data - label of the request on bit 1 of the CB_CMD field (this label is used to qualify the responses on die CPU bus) Cycle 2 : - high address bits 00: 16 - command During the broadcast of an address, all the transferred information is temporarily stored in each unit in order to anticipate the request acknowledgment B.4 - CPU Bus Address Management B.4.1 - CPU Bus Blocks Overview The CPU bus is managed by several blocks as shown in HG. 23. The address and command data path is in DDSB. The control part is done in DDB3. The interrupts are treated in DDB8 and DDB9. The DDB4 block manages the errors and the SSP. B.4.2 - CPU Bus Address and Command Data Path Referring to HGs. 24 - 28, the CPU bus addresses and commands data path is in DDSB whose main functions are: - the address managemeit for the internal requests (output address register), - the address management for the external requests (input address register), - the address management for the DD directory update (low address and high address), - the address management for the CD directory update, - the address management for the temporary and EIS registers, and - the SSP. The main interfaces of DDSB are: - the CPU bus: - addresses and commands from the CPU bus: (CB-ADI-X1 0:16, CB-ADIP-X1 0:2, CB-CMDI-X1 0:4, CB-CMDIP-X1) and - addresses to the CPU bus: (CB-ADO-L1 0:16, CB-ADOP-L1 0:2, CB-CMDO-L1 0:4); - the DD directory RAM - low address to the DIR RAM (POAL-X20_5), - high address to the DIR RAM write bus (CWB-X2 1_17 & CWBPA-X20_l), and - output of the DD directory (RB-P-X1 1_17 & RBPA-P-X1 0_1); - the CDAD block - to CDAD: to access the CD directory, to transmit the interrupt word and for MPSC management: - low address (B-L-Xl 18_26) and - high address (B-H-Xl 0_17 & B-HP-X1 0_2); - from CDAD: to manage the internal CPU bus request the temporary and EIS Registers and MPSC: - low address (P-L-Xl 18_26 & P-LP-X1) and - high address (P-H-Xl 0_17 & P-HP-X1 0_1); and - DDB9 block for interrupt management the get-length-after-a-page- crossing and read TMSK results: (B9_B_H_X20_3 ! B9_B_H_X25 17). B.4.3 - Internal Address Management When a cache controller receives a request from its BPU via its paging unit, it consults the CD directory. The CD directory is addressed by the CDAD block through the lines P-ADR-X20:7. If a miss occurs, the DDMF block, informed via the CDCC block, sends the signal CB-PCRQ__LD_X1 to the DDB3 block in order to set the CB_PCRQ line on the CPU bus. The address miss is temporarily stored in the CB-ADO FIFO 32 until the CPU bus is granted to the cache controller. Then, when access to the bus is granted, the address, the command, the data type and the label of the request are broadcast onto the CPU bus. Each unit on the CPU bus owns a bank of eight registers (named CB- ADIN FIFO), corresponding to FIFO stack 35 in HG. 3, in which the commands and the addresses are temporarily stored, according to the CPU number and the label of the request In order to update the directories, all this information remains in the CB-ADIN register bank as long as the acknowledgment from the shared cache hasn't yet been received. (There can be at least two concurrent non-acknowledged requests per CPU). B.4.3.1 - Directory Updating As summarized in HG. 30, internal requests can be divided into three types : - miss requests for which data is expected, - invalidate requests with no data, and - other requests which don't require a directory access (read-register, write-register, interrupt). On a read miss, CD updates its own directory, and the DD directory is updated after receiving the acknowledgment for the read command. The address for the CD directory update is loaded in the TSB01 register and sent on the B-L and B-H lines. During the cycle read of the CD directory, a swap operation can be required (i.e., a block with the state valid-modify has to be replaced). In this case, the CD directory will issue a swap command with the associated address and the level chosen. Thus, there can be at most two internal requests pending: one read and one swap. The swaps can be detected before the corresponding read request has been sent onto the CPU bus. For internal read-&-exclusivity requests, the cache controller has the same behavior. On an internal invalidate, the CD directory is updated after the consultation and then waits for the ack (i.e., acknowledge) or nack (i.e., not acknowledge) indication from the DD directory before proceeding with the write. The DD directory is updated after receiving acknowledgment from the shared cache, taking the interferences into account The ack/nack signals from the DD directory also takes interferences into account Interferences management is described below. B.4.3.2 - Request Acknowledgement The request acknowledgment (CB_ACK) signal is sent by the shared cache to all the private caches on the board. This acknowledgment concerns the requests which are temporarily stored in the CB-ADIN HFO. On die CPU bus, three signals (CB.ACKID 0:2 & CB_ACKLB) serve to identify which request is being acknowledged. They will be used to read the CB- ADIN HFO. The CB_ACKPCi signals determine if the local private cache is concerned by the acknowledgment or not and so, if a directory update is necessary. The request acknowledgment has several roles including: - update the cache controller directories, - request complete for requests such as interrupts, read register, write register, and - boundary of interference for invalidate or swap requests. When the cache controller receives an acknowledgment it reads the CB-ADIN FIFO and: - updates the DD Directory: - the low address is sent on the POAL-X20_5 lines and - the high address is sent on the CWB-X21_17 lines; the DD directory update lasts two cycles: one read (or CONS for external requests) and one write cycle; these two cycles are called cycle 2 and cycle 3; - for external requests, writes the needed information in the CB_ADx FIFO to update the CD directory at a later point in time; and - for internal invalidate requests, sends the acknowledge (or not acknowledge in case of interference) signal. B.4.3.3 - Cache Controller Behaviour on Internal Requests HGs. 31, 32A/B and 33 describe the cache controller behavior on internal requests which need a directory access. B.4.4 - External Address Management There are two types of external (as to a given CPU) requests: requests originating in the shared cache and requests originating in the private cache of another CPU on the board. As discussed above in section B.4.3 and due to the fact that for the private cache requests, the acknowledgment from the shared cache can be sent "n" cycles after the address is broadcast on the CPU bus, the address and command are temporarily stored into the CB-ADIN FIFO. However, an address coming from the shared cache doesn't have to be temporarily stored in the CB-ADIN FIFO to maintain the correct timing between the CB- ACKPCi and the directory update. It is sent direcdy to the B1-DSAD-L2 latch to be able to access the directory RAM on the next cycle. B.4.4.1. Directory Updating As summarized in HG. 34, in handling external requests, the DD directory is consulted first then, if a hit occurs, the CD directory is accessed. B.4.4.2 - Request Acknowledgement For external requests, there are two types of acknowledgment: - one implicit acknowledgment for the requests sent by the shared cache (the directory update is done on the cycle following the transmission of the high address on the CPU bus) and - one explicit acknowledgment for all the requests sent by a private cache (reception of CB_ACK). The CPU bus acknowledgment is used to update the cache controller directories and to take into account the address temporarily stored in the CB- ADINFIFO. B.4.4.3. Cache Controller Behaviour on External Requests HGs. 35A B, 36, 37A/B describe the cache controller behavior on external requests which need a directory access: read, read-&-exclusivity, invalidate. B.5. Coherence Conflict Management There may be a coherence conflict (called "interference") when there are two requests not complete for the same block (one internal request and one external request). The requests that may generate coherence conflicts are invalidate and swap. The coherence conflict ceases when the internal request is acknowledged. B.5.1. Interference on Invalidate An internal invalidate request may be interfered by an external invalidate and an external read-&-exclusivity. If an interference is detected, the invalidate command is nulled, and the cache controller sequencer consults the CD directory again. The initial invalidate sequence is changed in a write miss (read-&-exclusivity) sequence on the second access. B.5.2 - Interference on Swap An internal swap request may be interfered by an external read and an external read-&-exclusivity. If an interference is detected, the swap command is nulled - the request is complete. The data will be transferred only once; i.e., during the siphon procedure. B.5.3 - Boundary of Interference Referring to HG. 38, three cycles after the reception of the explicit acknowledgment (CB.ACK), the cache controller guarantees it emits its request with the right command. Three cycles after the reception of the implicit acknowledgment (CB_ACKPCi) of a request from the shared cache, the cache controller guarantees it emits its request with the correct command. B.5.4. Null Command The cache controller has to null the command on any acknowledgment received two cycles before the broadcast of an address. If the acknowledgment is sent later, the shared cache will null the command and will acknowledge the emitter with the signal CB_RQIFR set Attention is directed to HG. 39. B.5.5 - Cache Controller Interference Management The interferences are detected at two levels in order to send the correct command to the shared cache: - first when a request is written into the CB-ADO FIFO (comparators on CB-ADx FIFO); (in HG. 3, this corresponds to the FIFO stack 32 and FIFO stack 36 compared by comparator 33) and - then, when an external request is acknowledged (comparators on CB- ADO FIFO); (in HG. 3, this corresponds to the FIFO stack 32 and FIFO stack 35 compared by comparator 34). B.5.5.1 - Comparators on CB-ADx FIFO To detect an interference, a comparison has to be performed (corresponding to comparator 34 in HG. 3) between the address in the CB- ADOx register bank (corresponding to HFO stack 32 in HG. 3) CD wants to send on the CPU bus and the address present in the CB-ADx FIFO (corresponding to FIFO stack 36 in HG. 3). Indeed, the content of the CD and DD directories is not always the same, and the difference is saved in the CB-ADx FIFO. If CD wants to send an invalidate onto the CPU bus, and an equality has been found between this request and another invalidate or read-&- exclusivity waiting for CD update in the CB-ADx HFO, the invalidate must be nulled. This comparison is done, when writing the CB-ADOx register bank, between the address in the CB-ADOMX2-L2 latch and the sixteen addresses temporarily stored in the CB-ADx FIFO. Then, the request written in the CB-ADOx FIFO is validated or not according to the result of the comparison. B.5.5.2 - Comparators on CB-ADOx FIFO To detect the other interferences and to be able to send the correct command to the shared cache, a comparison (corresponding to the comparator 33 in HG. 3) has also to be carried out between the address which is acknowledged (i.e., stored in the CBADx FIFO, corresponding to the FIFO 36 in HG. 3) and the addresses which are waiting for emission on the CPU bus in the CB-ADOx FIFO or waiting for acknowledgment B.5.5.3 - CB_RQBFR Signal from the Shared Cache Once the request has been sent onto the CPU bus, die cache controller takes the CB_RQIFR signal from the shared cache into account to determine if its request has been interfered. This signal indicates that the shared cache has computed the request as a NOP. This line is valid with CB_ACK and means that no directory updating has to be carried out B.5.5.4 - ACK/NACK from DD to CD The ack/nack network in DD takes into account: - a possible interference between d e address CD wants to send on die CPU bus and the addresses waiting for a CD update in the CB-ADx FIFO, - a possible interference between an acknowledged request and the address waiting for the CPU bus in the CB-ADOx HFO, and - the CB_RQIFR signal from the shared cache. If the DD sends a nack signal to CD, the cache controller sequencer then consults CD again and this should lead to the write miss sequence (read- &-exclusive). B.5.5.5 - External Read Request on a Block Swap A chronogram is given in HG.40. C. CPU BUS DATA MANAGEMENT C.1 - Data Transfer on CPU Bus Except for the read register or the write register operations which manage 16 bytes, all the memory operations on the CPU bus are on 64 byte blocks. 16 bytes are transferred every cycle on the CPU bus such that for a given block, the data exchanges are split into four transfers of 16 bytes each. On the CPU bus, data is not necessarily delivered in the same order as requests have been issued; i.e., for a 64 byte block, the four transfers may not and need not be consecutive. For instance, two blocks requested by the same CPU may be served multiplexed. However, when data blocks are delivered by a CPU (swap request or siphon), these blocks are not mixed themselves. The four data transfers of a swapped or siphoned block will not be mixed with the data transfers of another swapped or siphoned block, but they can be interrupted by data transfer from the shared cache. Two signals per private cache (CBJDTCLO i:l and CB_DTCL1 i:l), sent by the shared cache, identify the type of data transfer which will take place on the next CPU bus cycle and are used to select which buffer in the cache memory unit is affected by the transfer. DTCL coding is shown in HG. 41. The CB_DTLB is sent with the CB_DTCL0 i:l and CB_DTCL1 i:l to identify which request is affected by the transfer. When the shared cache sends data to a private cache, the address acknowledgment may be sent after, during or before the data transfer of the request In the case of a swap command, the acknowledgment appears at least three cycles before the first data transfer. In the case of a write register command, data are always transferred before the acknowledgment C.2 - Data Transfer Timing HG.42 shows a data transfer of a block; the four cycles may be split C.3 - Buffers Interfacing With the CPU Bus There are three types of buffers which interface with the CPU bus: - the EDI buffer (external data input buffer): - one EDI buffer of 64 bytes, used to load the data coming from the CPU bus for EIS commands; - the OB buffers (operand buffers): - two operand buffers of 64 bytes, used to load the data coming from the CPU bus (these buffers are also used to perform operand read or write transfers over the operand or result bus, and they can be used as four independent buffers of 32 bytes each; - the EDO buffers (external data output buffers): - three EDO buffers of 64 bytes, used to send data to the CPU bus (two swap buffers and one siphon buffer). When the cache controller receives the DTCLO and DTCLl signals, it selects which type of buffer is concerned: - DTCL = 01 , data reception (EDI or OB), - DTCL = 10, data emission for a swap (EDO0 or EDOl): In this case, the choice is done with the CBJDTLB signal: If DTLB = 0, EDOO is selected and If DTLB = 1, EDOl is selected; - DTCL = 11, data emission for a siphon (ED02): If the siphon has interfered a swap, the buffer for the siphon can be EDOO or EDOl. This information is sent to the cache memory through the B-SLB 0_5 lines. The cache controller has to select the number of the 16 byte sub-block which is concerned by the DTCL. This information is sent to the cache memory unit through the B_EDOi_QAD_T2N 0:2 lines, (i = 0, 1 and 2 for the three EDO buffers). These signals are sent one clock phase before the B_SLB_lines. Thus, while the principles of the invention have now been made clear in an illustrative embodiment thee will be immediately obvious to those skilled in the art many modifications of structure, arrangements, proportions, the elements, materials, and components, used in the practice of the invention which are particularly adapted for specific environments and operating requirements without departing from those principles.

Claims: WHAT IS CLAIMED IS: 1. A computer system comprising: A) a group of central processor units; B) a private cache module for each of said central processor units in said group, each said private cache module communicating bilaterally with its central processor unit to receive requests for instruction and operand information blocks and for servicing such requests; C) a CPU bus coupled to all said private cache modules in said group for bilateral communication therewith of address, control and information blocks; D) a shared cache unit coupled to said CPU bus for bilateral commumcation therewith of address, control and information blocks; E) each said private cache memory module including a cache menory and a cache controller, said cache controller comprising: 1) a processor directory for identifying information blocks resident in said cache memory; 2) cache miss detection means for detecting that a requested block of information requested by said private cache memory module's central processing unit is not resident in said cache memory; 3) a command/address output buffer stack; 4) means for entering the identification of said requested block into said command/address output buffer stack; 5) means for selectively sending the identifications of said requested block from said command/address output buffer stack onto said CPU bus; and 6) an operand buffer set coupled to: a) selectively receive requested information blocks and information block segments from said CPU bus; b) selectively send information blocks and information block segments to said central processing unit; and c) selectively send information blocks to said cache memory. 2. The coπφuter system of Claim 1 in which said operand buffer set comprises: module's central processing unit is not resident in said cache memory; 3) means for selecting a block resident in said cache memory to be swapped out in contemplation of a serviced request to bring said requested block into said cache memory and for instituting a swap operation; 4) a command/address output buffer stack; 5) means for entering the identification of said requested block and the identification of said block to be swapped out into said command/address output buffer stack; 6) means for selectively sending the identifications of said requested block and said block to be swapped out from said command/address output buffer stack onto said CPU bus; 7) a command/address input buffer stack; 8) means for copying the identifications of all requested blocks from all said central processor units in said group into said command/address input buffer stack; 9) a first comparator for comparing the identifications of blocks to be swapped out in said command/address output buffer stack A) a first half-block length operand buffer divided into two quarter- block length segments; B) a second half-block length operand buffer divided into two quarter- block length segments; C) a third half-block length operand buffer divided into two quarter- block length segments; D) a fourth half-block length operand buffer divided into two quarter- block length segments; and E) an input multiplexer selectively transferring quarter-block segments of a requested block received from said CPU bus into said quarter- block segments of said first second, third and fourth half-block length operand buffers. 3. The conφuter system of Claim 2 in which said operand buffer set further includes an output multiplexer selectively transferring quarter-block segments resident in said first second, third and fourth half-block length operand buffers to said central processing unit with the identifications of blocks in said command/address input buffer stack; and 10) means, responsive to said first comparator ascertaining that a swap identification in said ∞mmand/address output buffer stack and a request identification in said command/address input buffer stack identify the same block, for inhibiting the transfer of a swap command for said same block onto said CPU bus and for converting said swap operation into a siphon operation to thereby service the request for the same block. 2. The compute system of Claim 1 in which said cache controller further includes: A) a bus directory; B) a command/address transfer buffer stack; C) means for sending information from said command/address input buffer stack for selective transfer into said bus directory to update the contents thereof; D) means for selectively transferring information from said bus directory to said command/address buffer stack; 4. The coπφuter system of Claim 3 in which said output multiplexer selectively transfers full blocks resident in said first and second half-block length operand buffers to said cache menory. 5. The coπφuter system of Claim 3 in which said output multiplexer selectively transfers full blocks resident in said third and fourth half-block length operand buffers to said cache memory.

Inventors: BARYLA ROBERT J, INOSHITA MINORU, SHELLY WILLIAM A, BARYLA, ROBERT, J., INOSHITA, MINORU, SHELLY, WILLIAM, A.
Applicants: BULL HN INFORMATION SYST, BULL HN INFORMATION SYSTEMS INC.
Application year: 1997
Main classification: G06F
IPC codes: G06F12