Abstract:
Each processor in a distributed shared memory system has an associated memory and a coherence directory. The processor that controls a memory is the Home processor. Under certain conditions, another processor may obtain exclusive control of a data block by issuing a Load Lock instruction, and obtaining a writeable copy of the data block that is stored in the cache of the Owner processor. If the Owner processor does not complete operations on the writeable copy of the data prior to the time that the data block is displaced from the cache, it issues a Victim To Shared message, thereby indicating to the Home processor that it should remain a sharer of the data block. In the event that another processor seeks exclusive rights to the same data block, the Home processor issues an Invalidate message to the Owner processor. When the Owner processor is ready to resume operation on the data block, the Owner processor again obtains exclusive control of the data block by issuing a Read-with Modify Intent Store Conditional instruction to the Home processor. If the Owner processor is still a sharer, a writeable copy of the data block is sent to the Owner processor, who completes modification of the data block and returns it to the Home processor with a Store Conditional instruction.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    Not applicable.  
         STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    Not applicable.  
         BACKGROUND OF THE INVENTION  
         [0003]    1. Field of the Invention  
           [0004]    The present invention generally relates to a computer system with multiple processors. More preferably, the present invention generally relates to the sharing of data among processors in a Distributed Shared Memory (“DSM”) computer system. Still, more particularly, the invention relates to a high performance directory based cache coherence protocol that permits processors in a DSM system to make more efficient use of their cache memory resources.  
           [0005]    2. Background of the Invention  
           [0006]    Distributed computer systems typically comprise multiple processors connected to each other by a communications network. In some distributed computer systems, the processors can access shared data. Such systems are sometimes referred to as parallel computers. If a larger number of processors are networked, the distributed system is considered to be a “massively” parallel system. One advantage of a massively parallel computer system is that it can solve complex computational problems in a relatively short time period.  
           [0007]    In parallel and massively parallel computer systems, it has become common to distribute memory throughout the computer system, and to permit most or all of the processors to access data stored in the distributed memory, regardless of where the memory is located. Such a shared memory architecture is conventionally known as a Distributed Shared Memory (“DSM”) system. One of the challenges that faces a system designer of a DSM system is to ensure that the data is stored and retrieved in a coherent manner. In particular, the system designer must implement a protocol for handling data that ensures that two different processors don&#39;t concurrently modify the same piece of data, and try to save that data back to the same memory location. Thus, to guarantee the coherency of the data in memory, steps must be taken to permit only one processor to modify any particular memory location at any one time.  
           [0008]    Recently, DSM systems have been built as a cluster of Symmetric Multiprocessors (“SMP”). In SMP systems, shared memory can be implemented efficiently in hardware since the processors are symmetric (e.g., identical in construction and in operation). It has become desirable to construct large-scale DSM systems in which processors efficiently share memory resources. Thus, the system preferably supports having a program executing on a first processor fetch data stored in a section of system memory that operates under the control of a second processor.  
           [0009]    It has become commonplace to provide memory caches (or cache memory) with processors. The memory cache typically is located on the same semiconductor device as the processor (an L1 cache), or is located immediately adjacent the processor, and connected to the processor by a high-speed data bus (an L2 cache). Memory devices used for cache memory usually comprise very high-speed devices, thus permitting much faster access times than standard system memory. The drawback, however, is that cache memory is relatively expensive, and relatively large in size. Thus, cache memory typically has a small data capacity relative to the system memory. The processor uses these high-speed cache memory devices to store data that is either being used repeatedly by the processor, or to store data expected to be used by the processor. Because of the small size of the cache, data is displaced from the cache if it is not used. The displaced data is then re-stored in the system memory. Usually, data is read from the system memory into cache memory, where it is modified by the processor. When the processor has completed modification of data, the data is displaced, and written back to the system memory.  
           [0010]    During normal operation, when the processor needs a particular piece of data, it first looks in its associated cache memory. If the desired data is in the cache memory, that is referred to as a “cache hit”, and the processor then performs any necessary operations on the version of data in the cache. If the desired data is not in the associated memory cache(s), that is referred to as a “cache miss”, and the processor then retrieves the data from system memory and stores a copy in the cache for further operations. The fact that each processor in a multi-processor system has its own cache greatly complicates the problem of data coherency. In particular, a problem can arise if multiple processors have made a cache copy of the same data from system memory. The processors then may proceed to modify, in different ways, the copy of the data stored in the processors&#39; associated caches. The various modified copies of data stored in each cache memory raise the specter that multiple inconsistent versions may exist of what was originally the same data obtained from the same memory location. A coherency protocol thus must be implemented that mitigates these concerns to prevent inconsistent versions of the same data from being created, or more particularly, from being indeterminately being written back to system memory.  
           [0011]    Because the size of computer systems continues to increase at a rapid rate, and because massively parallel computer systems are being developed with an ever-increasing number of processors, it is desirable if the coherency protocol is scaleable to processor configurations of various sizes. One technique that has been adopted in large multi-processor systems is to implement a directory-based cache coherence system. In a directory based solution, a directory is maintained for all of the memory in the system. Typically, like the system memory, the directory is distributed throughout the computer system. According to conventions developed by the assignee of the present invention, each processor is assigned the responsibility of controlling some portion of the distributed system memory. With respect to that portion of system memory under the control of a given processor, that processor maintains a directory that identifies which portions of that memory have been copied by a processor. In this fashion, it is possible to determine if a particular block of data has already been copied. Various instructions are then used to determine which data is valid in the system, and what data must not be used or must be flushed to make sure that data is maintained coherently.  
           [0012]    Most reduced instruction set core (“RISC”) processors use “Load Lock” and “Store Conditional” instructions to ensure synchronization in multiprocessor distributed shared memory architectures. A processor uses the “Load Lock” and “Store Conditional” instructions when it seeks exclusive access to a block of data (which is located in a particular memory address range), so that it is the only processor that can manipulate that data during the period that Load Lock is asserted. Thus, the processor issues a “Load Lock” instruction for a block of data coincidentally with reading that data block and storing a copy in the cache memory of that processor. After the data is manipulated, the processor then issues the “Store Conditional” instruction, which causes the data to be re-written to the original memory location where the data resided, at which time the memory block is released. If that section of memory has been written to in the interval between when “Load Lock” and “Store Conditional” was asserted, the “Store Conditional” instruction fails. Thus, the “Store Conditional” instruction is a conditional instruction that only executes (i.e., that only stores) if the data in that memory location has not been modified since the Load Lock instruction was issued.  
           [0013]    As an example and referring to FIG. 1, assume four processors A, B, C and D have shared access to memory locations M, N, O and P. Memory M operates under control of processor A, memory N operates under control of processor B, and so on. If processor A seeks exclusive access to memory block X in memory O, it will issue a “Load Lock” instruction for that block of memory to the processor responsible for block X (which happens to be processor C), and that processor records in its directory (shown as “D”) that processor A has a copy of that memory block. If, while the “Load Lock” is active, processor B writes to that same memory block X, processor A must be informed of that event, so that processor A can determine that the “Load Lock” has failed. In operation, processor A learns of the write operation during “Load Lock” when “Store Conditional” is executed.  
           [0014]    A “Load Lock” instruction may be used in situations where a programmer desires serialized access to a location that is shared in a distributed memory, multi-processor system. Thus, “Load Lock” may be used in situations where multiple processors are instructed to modify a block of data in a specific order (processor A modifies the data, followed by processor B, followed by processor D, . . . ).  
           [0015]    As mentioned above, DSM systems typically include a directory that records which memory locations have had data read out and stored in the local cache of a processor. The directory may be located anywhere in the computer system, and essentially comprises a table with entries corresponding to each block of memory. Typically, the directory is fragmented, and multiple processors are responsible for certain portions of the directory. Typically, control of the memory is distributed among various processors, and each processor maintains the directory for the portion of memory for which it is responsible. In the example, processor C is responsible for controlling memory block X in memory O, and processor C includes a directory for memory O, with an entry that indicates that processor A has made a copy of block X. When processor B writes to that same memory block X, the directory causes an “Invalidate” or “Shared Invalidate” instruction to be sent to circuitry within processor A, informing processor A that the block of data copied from memory block X is now invalid due to the write operation by processor B. In response, processor A will flush that data from its cache memory, since it is no longer valid.  
           [0016]    A block of memory can be requested as a writeable copy (i.e., a copy of the memory block with authority to modify the block), or a read-only copy (which can be read, but not modified). If processor A requests a read-only copy of memory block Y in memory P, and does not issue a “Load Lock” instruction, the directory for block Y in processor D records that processor A has a read-only copy of memory block Y, which is commonly referred to as a “shared” state. If another processor were to write to memory block Y, then the directory for processor D would issue an instruction to processor A invalidating the read-only copy of memory block Y.  
           [0017]    If processor A instead requests a writeable copy of block Y, that is referred to as an “exclusive” state, because only processor A has access to memory block Y. If, however, processor A begins working on other data, and the writeable copy of block Y is displaced from the cache memory of processor A, the data is returned to memory block Y, together with a “victim” instruction. The “victim” instruction clears the exclusive state of that memory block Y, and the directory in processor D for memory P no longer shows any association with processor A. If another processor then obtains a writeable copy of block Y, and modifies block Y, than the directory in processor D will not notify processor A because there is no longer any association of block Y with processor A. If processor A then subsequently executes a Store Conditional on block Y, it will be unaware that block Y has been modified, and the directory will not detect this event. Thus, between the assertion of “Load Lock” and “Store Conditional” by processor A, a different processor may cause the data that was the subject of these instructions to be modified. As a result, data incoherency exists because the data was not handled in the proper order specified by the programmer.  
           [0018]    To prevent this situation from occurring, most conventional designs do not allow the writeable copy to be displaced or evicted from the cache until the “Store Conditional” instruction executes. Instead, conventional designs require that writeable copies of memory be locked in the cache until operation on that copy is completed. This prevents subsequent writes to the same block from going undetected. This approach of locking the cache, however, ties up cache resources. In a multithreaded system where multiple programs can execute simultaneously on the same processor, this can severely impact performance of the system. As an example, block Y may be copied by processor A for program 1, but program 2 begins executing on Processor A. Unfortunately, the cache of processor A may become full, because instructions have also been fetched for program 1. It would be desirable if processor A could free its cache to operate expeditiously on program 2, if necessary. Under conventional implementations, block Y could not be evicted from the cache associated with processor A until operations are complete on block Y and the data has been conditionally stored. Despite the apparent inefficiencies such an approach produces, to date no one has developed an efficient solution to this problem.  
         BRIEF SUMMARY OF THE INVENTION  
         [0019]    The problems noted above are solved in large part by a multiprocessor distributed shared memory system in which a processor may obtain exclusive control of a data block by asserting a Load Lock signal to the Home processor that controls that data block If the processor with exclusive control does not complete operations on the data block prior to the time that the data block is displaced from the cache of that processor, it issues a Victim To Shared message, thereby indicating to the Home processor that it should remain a sharer of the data block, and the data block is written back. In the event that another processor seeks exclusive rights to the same data block, the Home processor issues an Invalidate message to what was previously the Owner processor. This causes that processor to recognize that the Load Lock/Store Conditional operation pair has failed. If no Invalidate message is received, and when the processor executes the Store Conditional instruction, it seeks exclusive control of the data block by issuing a Read with Modify Intent Store Conditional message to the Home processor. If that processor is still a sharer, a writeable copy of the data block is sent to that processor, who completes modification of the data block. If the Home processor does not recognize that processor as a sharer of that data block, this indicates to the Home processor that an intervening write has occurred, and the Home processor returns a Store Conditional Failure message.  
           [0020]    According to one embodiment of the invention, each processor in a multiprocessor system includes a directory that maps the status of each data block in an associated memory. Each processor also includes a cache memory in which data is stored that is currently being used by the processor. If instructed by a program, one of the processors may assert a Load Lock instruction to the directory of another processor, thereby obtaining exclusive possession of a block of data from the memory associated with the other processor. The data block is then stored in the cache memory of the requesting processor. If the requesting processor does not complete operation on the data block prior to the time that the data block is displaced from its cache, it returns the data block to the memory location, and instructs the directory that it needs to be recognized as a sharer to that data block. As a result, if other processors or system resources seek to modify that data block, the directory sends an Invalidate message to the requesting processor to notify that processor that its shared data is now invalid. If the requesting processor does not receive an Invalidate message, that processor may seek to again obtain an exclusive copy of the data block by asserting a Read-with Modify Intent Store Conditional instruction.  
           [0021]    According to an exemplary embodiment, each processor preferably includes a Load Lock register associated with each program thread. The Load Lock register stores the address of any data block over which that processor has obtained exclusive control through the assertion of a Load Lock instruction. Each time that data is expelled from the cache, the address of the expelled data block is compared to the address value in the Load Lock register. If a match occurs, the processor returns the data with a Victim To Share message, instead of a Victim message, thereby informing the Home processor that the processor returning the Victim To Share message should remain as a sharer to the data block.  
           [0022]    These and other features of the present invention will become apparent upon reading the following description and claims and reviewing the appended drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    For a detailed description of the preferred embodiments of the invention, reference will now be made to the accompanying drawings in which:  
         [0024]    [0024]FIG. 1 shows a portion of a conventional distributed shared memory system for purposes of illustration;  
         [0025]    [0025]FIG. 2 is a block diagram of a multiprocessor distributed processing system constructed in accordance with the preferred embodiment;  
         [0026]    [0026]FIG. 3 shows an exemplary embodiment of a system to generate a Victim To Shared message if a Load Lock data block is displaced from a cache prior to executing a Store Conditional instruction; and  
         [0027]    [0027]FIG. 4 illustrates a portion of a memory coherence directory in accordance with the preferred embodiment of FIG. 2. 
     
    
     NOTATION AND NOMENCLATURE  
       [0028]    Certain terms are used throughout the following description and claims to refer to particular system components. As one skilled in the art will appreciate, processor and computer companies may refer to a component and sub-components by different names. This document does not intend to distinguish between components that differ in name but not function. In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . .”. Also, the term “couple” or “couples” is intended to mean either a direct or indirect electrical connection. Thus, if a first device couples to a second device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections. The term “CPU”, “processor”, and “microprocessor” are used synonymously to broadly refer to the device in a computer system that interprets and executes programmed instructions. The term “Home processor” refers to a processor that manages the memory and directory for a particular data block. The term “Owner processor” refers to a processor that manages an exclusive copy of a data block in its cache. The term “sharer” refers to a processor that manages a shared copy of a data block in its cache. The term “shared” refers to data that has been copied by one or more processors. To the extent that any term is not specially defined in this specification, the intent is that the term is to be given its plain and ordinary meaning.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]    Referring now to FIG. 2, an exemplary computer system  90  is shown to illustrate the principles of the present invention. As shown in FIG. 2, computer system  90  comprises a plurality of processors  100 , each of which has an associated memory  125  and an input/output (“I/O”) controller  135 . In FIG. 2, the computer system  90  includes twelve processors  100   a - l  for purpose of illustration. Each processor preferably includes four ports for connection to adjacent processors. As such, each processor  100  can be connected to four other processors. The processors on both ends of the system layout (e.g.,  100   a  and  100   d ) wrap around and connect to implement a 2D torus-type connection. Although twelve processors  100   a - l  are shown in the exemplary embodiment of FIG. 1, any desired non-prime number of processors (e.g., 64, 128, 256, etc.) can be included.  
         [0030]    As noted, each processor may have an associated I/O controller  135 . The I/O controller  135  provides an interface to various input/output devices such as disk drives  105  and  110 , as shown in the lower, left-hand corner of FIG. 2. Data from the I/O devices thus enters the 2D torus via the I/O controllers.  
         [0031]    The system memory in computer system  90  comprises a distributed memory architecture, with the memory dispersed throughout the system. According to the preferred embodiment, each processor  100  has an associated memory device  125 . In accordance with the preferred embodiment, the memory  125  preferably comprises some form of synchronous DRAM, including SDRAM, DDR DRAM, or RDRAM memory devices, but other types of memory devices can be used, if desired. The capacity of the memory device  125  can be any suitable size. According to the preferred embodiment, discrete memory devices  125   a - l,  in aggregate, form the system memory of the computer system  90 . Each processor preferably includes appropriate logic for controlling the operation of the portion of the distributed memory to which it connects. Thus, for example, processor  100   c  couples to and controls discrete memory  125   c.  As an alternative, memory may be associated only with select processors.  
         [0032]    In general, computer system  90  preferably are configured so that any processor  100  can access its own memory and I/O devices, as well as the memory and I/O devices of all other processors in the system. Thus, as an example, processor  100   g  can access memory device  125   b,  or any other portion of the distributed memory  125   a - l  in the computer system. Preferably, the computer system may have physical connections between each processor resulting in low inter-processor communication times and improved memory and I/O device access reliability. If physical connections are not present between each pair of processors, a pass-through or bypass path is preferably implemented in each processor that permits accesses to a processor&#39;s memory and I/O devices by another processor through one or more pass-through processors.  
         [0033]    Referring still to FIG. 2, and in accordance with normal convention, each processor  100   a - l  preferably includes at least one memory cache  115 . Thus, as shown in FIG. 2, each processor  100   a - l  has an associated memory cache  115   a - l.  More preferably, each processor includes more than one memory cache, and may for example include both a level one (“L1”) cache and a level two (“L2”) cache. While the cache  115  is shown integrated with the processor  100 , one skilled in the art will understand that one or more of the memory caches may be located off-chip. In accordance with normal convention, the L1 cache preferably is located on-chip while the L2 chip is located off-chip. As one skilled in the art will understand, the processor retrieves data from one or more of the distributed memory devices  125   a - l  during the course of executing one or more programs. Data that is likely to be used or re-used by the processor is stored in one of the memory caches  115  of the processor. In the event that the data is not used, the data may be displaced from the cache and stored back in the distributed memory system  125   a - l.  Storage of often-used data in the cache  115  reduces the time that would otherwise be required to retrieve data from the distributed memory system  105   a - l.  According to the preferred embodiment, the possible cache states include those listed in Table 1.  
                         TABLE 1                           CACHE STATES            STATE   MEANING               Invalid   Block has been invalidated and a correct copy must be           obtained from memory.       Exclusive-Clean   This processor has an exclusive copy, the value is the           same as system memory.       Exclusive-Dirty   This processor has an exclusive copy, the value may           be different than memory.       Shared   This processor has a read-only copy, the value is the           same as memory.                  
 
         [0034]    To write to a data block in system memory (other than the local memory of that processor), the processor preferably must have a data block with a Dirty cache state.  
         [0035]    To ensure data coherency, each processor  100  preferably includes a memory coherence directory  150  that reflects the status of each block of memory in the portion of the distributed memory system associated with and under control of that processor. Thus, for example, memory coherence directory  150   d  reflects the status of memory  125   d.  In particular, the directory  150  tracks which processor has obtained a copy of a particular memory block, and also whether the copy is a read only copy (and thus can be shared) or whether the copy is a writeable copy (and thus is exclusive).  
         [0036]    The directory may exist in registers or memory within the processor, or may be physically located in a memory external to the processor, including the portion of system memory associated with that processor. FIG. 4 is a conceptual illustration of one register that may be used to store status information for one block of memory. A plurality of similar registers would be provided, with one register associated with each memory block. In accordance with an exemplary embodiment, each register is dedicated to a particular memory block. As an alternative, the register may include additional bits to permit identification of a particular memory block. As shown in FIG. 4, the register includes two primary fields. The first field identifies the status of the memory block, and indicates whether a copy of the data in the memory block has been copied to a processor. Further, the status bits preferably also indicate if the copy of the data block is a read-only copy, or whether the copy is writeable. The second portion of the register identifies the processor or processors that have obtained copies of the data block. Thus, each processor in the computer system is identified by a binary code, to enable the directory to track processors that have access to a memory block. According to an exemplary embodiment, local memory accesses from the Home processor need not be tracked in the memory coherence directory. Any remote requests, however, preferably are used to update the directory as a mechanism to globally track the status of each data block.  
         [0037]    As shown in Table 2, there are at least six different states that may be used to identify the status of a data block in the memory coherence directory  150 :  
                         TABLE 2                           DIRECTORY STATES            STATE   MEANING               Local state   Implies that no remote processors have a copy of the           block.       Incoherent state   An error state. Error status (“ERRResp”) is returned in           response to a request to an incoherent block.       Exclusive state   Implies that exactly one remote processor holds the           current copy of the block. The block may be either in the           exclusive-clean or dirty states in the cache at that           processor.       Shared1   Implies that one remote processor may have shared           access to a block (as well as the local processor).       Shared2   Implies that two processors may have shared copies of           the block (as well as the local processor).       Shared3   Implies that three processors may have shared copies of           the block (as well as the local processor). To permit           encoding of the three processors, a fewer number of           processors may be required.                  
 
         [0038]    The number of bits used to encode the state of a data block may be arbitrarily chosen, depending on the number of states available, and the amount of detail that is to be tracked. Similarly, the number of bits used to identify the processor or processors with access to the data block may also be chosen arbitrarily, depending on the number of processors in the system, and the number of processors who can concurrently share data. An example of the type of data that be encoded in the memory coherence directory is shown in Table 3. According to this exemplary embodiment, the four least significant bits are used to identify status of the memory block, while the eighteen most significant bits may be used to identify the processor or processors that have copies of the memory block, as follows:  
                             TABLE 3                           DIRECTORY STATE ENCODING            STATE   ENCODING:   COMMENTS               Local   XXXXXXXXXXXXXXXXXX0000           Incoherent   XXXXXXXXXXXXXXXXXX1000       Exclusive   XXXXXXXXXXEEEEEEEEI100       Shared1   XXXXXXXXXXSSSSSSSS1110   Shared with list of one               share-S       Shared2   XXTTTTTTTTSSSSSSSS0110   Shared with list of two               shares-T,S       Shared3   UUUTTTTTTUUSSSSSSU0010   Shared with list of               three sharers-U,T,S                                                                          
 
         [0039]    The foregoing table is only for purposes of illustration, and is not intended to limit the manner in which status data is encoded in the memory coherence directory.  
         [0040]    One of the coherence protocol messages found in many DSM multi-processor systems is a “Victim To Shared” message. The present invention uses this message in a unique way to handle data that has been displaced from a cache prior to issuing a Store Conditional instruction. Thus, in accordance with the present invention, when an exclusive (or a writeable) cache block x is displaced from a cache, the Owner processor checks to see if it has previously executed a Load Lock instruction for that cache block x. If the Owner processor discovers that it has previously issued a Load Lock (but not a Store Conditional instruction), the Owner processor preferably sends a Victim To Shared message to the Home directory, rather than the standard Victim message. The Victim To Shared message indicates that the Owner processor is no longer the exclusive owner of the data block, but that it desires to continue to share the data block. Thus, by issuing the Victim To Shared message, the processor continues to be listed as a sharer of that data block in the associated directory of the Home processor for that data block. As a result, if any other processor subsequently attempts to write to that data block before the Store Conditional instruction is issued, the processor that issued the Victim To Shared instruction will receive an Invalidate message from the Home directory, thereby causing the failure of the Load Lock/Store Conditional operation, and thus maintaining coherency.  
         [0041]    The foregoing concept will now be illustrated with reference to FIG. 2. Assume processor  100   a  obtains exclusive control of a block X of data from memory  125   d  by issuing a Load Lock signal. Directory  150   d  in processor  100   d  will update its table to indicate that processor  100   a  has exclusive control of data block X. Block X is then loaded into the cache  115   a  of processor  100   a . Prior to the time that processor  100   a  completes operation on data block X, processor  100   a  may begin to execute another program, or otherwise begin processing some other instructions not pertinent to block X. As a result, cache  115   a  desires to expel block X to make room for other data that is currently being used by the processor  100   a , as dictated by the cache algorithms that control which data is stored in the cache  115   a . Before displacing block X from its cache, processor  100   a  determines if it has previously issued a Load Lock instruction for data block X. If so, then processor  100   a  returns the data to processor  100   d , and sends a Victim To Shared message to Directory  150   d . As a result, the directory  150   d  changes the status of data block X to note that processor  100   a  is no longer the exclusive owner of data block X. However, because of the Victim To Shared message (instead of the Victim message), directory  150   d  lists processor  100   a  as a sharer of data block X. In the event that that another processor subsequently attempts to write to data block X before processor  100   a  executes the Store Conditional instruction, processor  100   a  will receive an Invalidate message from directory  150   d . In response, processor  100   a  will fail the Load Lock/Store Conditional operation.  
         [0042]    If, instead, processor  100   a  executes the Store Conditional instruction for data block X, prior to receiving an Invalidate signal from directory  150   d , processor  100   a  sends a Read-with-Modify Intent Store Conditional (“RdModSTC”) instruction to the directory  150   d . Stated differently, when the Store Conditional half of the Load Lock/Store Conditional pair is executed and misses in the cache, the message RdModSTC is transmitted to the targeted directory.  
         [0043]    If the directory  150   d  finds that processor  100   a  is a sharer of block X, then the directory  150   d  sends a writeable copy of block X to processor  100   a  and invalidates all other sharers, if any exist. If directory  150   d  finds that processor  100   a  is no longer a sharer of block X, then the directory  150   d  knows an intervening write to block X has occurred after the Load Lock instruction from processor  100   a . Thus, the Load Lock/Store Conditional instruction pair of processor  100   a  is invalid, and the directory  150   d  sends a Store Conditional Failure to processor  100   a.    
         [0044]    As one skilled in the art will understand, the generation of the Victim To Shared message may be generated in many ways. One exemplary technique is depicted in FIG. 3. As shown in the exemplary embodiment of FIG. 3, a single register  205  is implemented for each program or thread, which records the address of the pending Load Lock/Store Conditional pair. Each register  205  preferably has an associated comparator  215 . The output of comparator  215  functions as a select input to a multiplexer  240 . The input signals to the multiplexer  240  are a Victim signal and a Victim To Shared signals. When an exclusive cache block  220  is displaced, the contents of the displaced data block register(s)  220  are compared to the address of the Load Lock register  205 . If the addresses are the same, the comparator output selects the Victim To Shared message in multiplexer  240 . As a result, the Victim To Shared message is delivered to the Home directory, rather than the Victim message that otherwise is transmitted with a displaced data block. As shown in FIG. 3, a Store Conditional signal causes the Load Lock register  205  to be reset.  
         [0045]    The above discussion is meant to be illustrative of the principles and various embodiments of the present invention. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. For example, the directory states and encodings shown in Table 1, Table 2, and Table 3 may be extended to other encodings and directory states (e.g., a Shared4 state, sharing with four sharers) in the preferred embodiment of the invention. It is intended that the following claims be interpreted to embrace all such variations and modifications.