Abstract:
A multi-processor system includes a system bus communicating between processors, and a bus arbiter. Responsive to a cache line invalidation command, a processor cache conditionally casts back the cache line to a transition cache. Based on the system response to the invalidation command, the transition cache either discards the cast back or writes it to main memory. The processor also converts an exclusive read command requiring a reservation to non-exclusive if the reservation has been lost before placing the command on the system bus. Furthermore, the transition cache may shift memory coherency image state for a non-exclusive command, which is waiting for data to return, if a command involving the same real address is snooped. Responsive to a cache line request, the cache copies that cache line to the transition cache and updates cache line state. The transition cache holds the cache line pending system response.

Description:
RELATED APPLICATIONS 
   This is a divisional application of application Ser. No. 08/999,961, filed Jan. 8, 1998 (now U.S. Pat. No. 6,260,117, issued Jul. 10, 2001), which is a continuation-in-part of application Ser. No. 08/932,826, filed Sep. 18, 1997 (now U.S. Pat. No. 6,065,098, issued Mar. 16, 2000), both of which applications are hereby incorporated by reference. This application is related to co-pending application Ser. No. 09/862,486, filed May 23, 2001. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method for increasing efficiency in a multi-processor system and a multi-processor system with increased efficiency. 
   2. Description of Related Art 
   The processors in conventional multi-processor systems include inclusive caches, which means that the higher level caches store the same cache lines stored in the lower level caches as well as cache lines not stored in the lower level caches. 
   For instance, in a processor having a level one or L1 cache and an external level two or L2 cache, the L1 cache, by design, is disposed closer to the execution units of the processor and has a lower storage capacity than the L2 cache such that the L1 cache has a lower access time. The L2 cache, however, stores a larger number of cache lines and includes all the cache lines stored in the L1 cache. Because of this inclusivity, only the L2 cache needs to monitor commands on the system bus, which provides communication between the processors, and generate responses thereto. This monitoring of commands on the system bus is referred to as snooping. 
   Commands on the system bus are snooped as part of a memory coherency protocol. Typically, such protocols require the caches in each processor to associate a memory coherency image state with each cache line. The memory coherency image state of a cache line indicates the status of the cache line. Through snooping, the caches of a processor continually update the memory coherency image state for each cache line stored therein. 
   A cache snoops a command by determining whether the real address associated with the snooped command matches the real address of a cache line stored therein. If a match is found, the cache updates the memory coherency image state for the cache line in a well-known manner, and outputs a snoop response based on the updated memory coherency image state in a well-known manner. If no match is found, the cache outputs a snoop response indicating no match found. 
   Using inclusive caches, however, requires higher level caches (1) to track the cache lines stored in the lower level caches, and (2) to constantly update the cache lines stored therein based on changes in the cache lines stored by the lower level caches. By using non-inclusive caches, both the tracking and updating functions can be eliminated. Because the caches are non-inclusive, each cache must be considered when snooping a command. Outputting multiple responses to one snooped command from a single processor, however, increases the complexity of maintaining data integrity. 
   Whether inclusive or non-inclusive caches are used, conventional multi-processor systems suffer from various inefficiencies when processing commands and updating memory coherency image states. For the purpose of discussing these inefficiencies, the MESI memory coherency image state protocol will be used. 
   As is well-known, the MESI states are Modified, Exclusive, Shared, Invalid, shared Owner, fill Pending, and various error states. Modified means that cache line associated therewith includes modified data. Exclusive means the cache storing the cache line has exclusive ownership of the cache line. Shared means that the associated cache line is also stored in another cache. Shared Owner means that the associated cache line is stored in another cache, but that the cache storing the cache line has had the last access to the cache line. Fill Pending means that a command associated with the cache line has not received a system response thereto. 
   In response to instructions from the associated processor, caches often generate commands. These commands are placed on a system bus providing communication between the processors of the multi-processor system. The processors connected to the system bus snoop these commands, or more properly the cache or caches associated with the processors snoop these commands, and generate snoop responses thereto. The snoop responses are generated based on the MESI states of cache lines having the same real address as the real address associated with the snooped command or the lack of such cache lines. The possible snoop responses include, in order of priority: retry, indicating that the snooped command should be retried at a later point in time; the MESI state Modified; the MESI state Shared; and null, indicating no real address match. 
   The caches also update the MESI state of the cache lines stored therein based on the type of command snooped. Generally, commands are either exclusive or non-exclusive. Exclusive means that the processor issuing the command intends to store, flush or invalidate the cache line in some way, while non-exclusive means that this cache line could be shared. 
   An arbiter connected to the system bus collects the snoop response from each processor and generates a system response. Typically the system response is the highest priority snoop response among those output by the processors. The system response notifies the processors whether the command can not be completed at this time because of, for example, a collision with another command, i.e., the retry response, or whether the command can complete and the effect on other processors, i.e., any response other than retry referred to as not retry. 
   A collision occurs, for instance, when a first processor issues a first command associated with a cache line and before that first command completes, a second processor issues a second command associated with the same cache line. In this event, the first processor will maintain memory coherency by always outputting a retry snoop response to the second command. Accordingly, having to process the retry response and reissue the second command at a later time reduces the efficiency of the multi-processor system. 
   An unwanted number of retries and resulting decrease in efficiency also occurs when a processor issues a kill command. A kill command is one that requests each cache storing the associated cache line to invalidate that cache line regardless of the MESI state therefor. But, when the MESI state for the cache line in the cache of a processor is modified, that processor, to preserve the modified data, will output the cache line for storage in the main memory of the multi-processor system. This is referred to as performing a castback of the cache line. As a result of needing to castback the cache line, the processor will output a retry response to the kill command. Again, the reason for responding to the kill command in this fashion is memory coherency. The castback causes the modified data to be stored in a main memory of the system; thus, preserving the modifications. 
   SUMMARY OF THE INVENTION 
   One object of the present invention is to provide a method for maintaining multi-level cache coherency in a processor with non-inclusive caches. 
   Another object of the present invention is to provide a processor with non-inclusive caches which maintains coherency therebetween. 
   A further object of the present invention is to provide a method and higher level cache which prevent collisions between two cache queries, one of which is the result of a snooped command. 
   Also an object of the present invention is to provide a method of increasing efficiency in a multi-processor system and a multi-processor system having increased efficiency. 
   Another object of the present invention is to provide a method of increasing efficiency in a multi-processor system and a multi-processor system having increased efficiency which reduce the number of retry responses. 
   A further object of the present invention is to provide a method of increasing efficiency in a multi-processor system and a multi-processor system having increased efficiency which update memory coherency image states more efficiently. 
   A still further object of the present invention is to provide a method of increasing efficiency in a multi-processor system and a multi-processor system having increased efficiency which prevent unwanted and undesirable invalidation of a cache line throughout the multi-processor system. 
   These and other objectives are achieved by providing a processor which includes at least a lower and a higher level non-inclusive cache, and a system bus controller. The system bus controller snoops commands on the system bus, and supplies the snooped commands to each level of cache. Additionally, the system bus controller receives the snoop responses to the snooped command from each level of cache, and generates a combined response thereto. 
   When generating responses to the snooped command, each lower level cache supplies its response to the next higher level cache. Higher level caches generate their response to the snooped command based in part upon the response of the lower level caches. Also, high level caches determine whether or not the cache address, to which the real address of the snooped command maps, matches the cache address of at least one previous high level cache query. If a match is found by a high level cache, then the high level cache generates a retry response to the snooped command, which indicates that the snooped command should be resent at a later point in time, in order to prevent a collision between cache queries. 
   The objectives are further achieved by providing a multi-processor system including at least first and second processors, a system bus providing communication between the first and second processors, and a bus arbiter generating system responses to commands on the system bus. The first processor generates a first command associated with a real address, and the second processor generates a second command associated with a real address. When the first processor snoops the second command on the system bus, the first processor delays generating a snoop response to the second command until the system response to the first command is received. Based on the system response to the first command, the first processor generates a snoop response to the second command. 
   The objectives are additionally achieved by providing a multi-processor system including at least first and second processors, a system bus providing communication between the first and second processors and a bus arbiter generating system responses to commands on the system bus. The first processor has at least one level of cache associated therewith, a system bus controller controlling communication between the first processor and the system bus, and a transition cache serving as an interface between each level of cache and the system bus controller. When the first processor snoops a first command on the system bus requesting invalidation of a cache line, each level of cache associated with the first processor invalidates the cache line prior to the first processor snooping a system response to the first command. Additionally, if the memory coherency image state for the cache line in the one of the caches indicates that modified data is included therein, then prior to invalidating the cache line, the cache generates a castback command and transfers the castback command and a copy of the cache line to the transition cache. If the system response to the first command is a retry, then the transition cache converts the castback command to a second command which requests that the cache line be stored in a main memory of the multi-processor system. If a non-retry system response to the first command is received, then the castback command is discarded by the transition cache. 
   The objects are also achieved by providing a multi-processor system having at least first and second processors, a system bus providing communication between the first and second processors, and a bus arbiter generating system responses to commands on the system bus. The first processor includes at least a level one cache, a system bus controller controlling communication between the first processor and the system bus, and a transition cache controlling and tracking communication between each level of cache and the system bus controller. The system bus controller checks a reservation of a first command, which requires a reservation, generated by the level one cache prior to placing the first command on the system bus. If the reservation has been lost, then the system bus controller converts the first command into a second command, which does not require a reservation, and places the second command on the system bus. 
   The objects of the present invention are still further achieved by providing a multiprocessor system including at least first and second processors, a system bus providing communication between the first and second processors, and a bus arbiter generating system responses to commands on the system bus. The first processor has at least one level of cache associated therewith, a system bus controller controlling communication between the first processor and the system bus, and a transition cache controlling and tracking communication between each level of cache and the system bus controller. The transition cache determines whether data has started to arrive at the transition cache in response to a non-exclusive command when the first processor snoops a command on the system bus associated with the same real address as the non-exclusive command. Based on whether data has started to arrive at the transition cache, the transition cache generates a snoop response to the snooped command. 
   Additional objects of the present invention are achieved providing a multi-processor system including at least first and second processors, a system bus providing communication between the first and second processors, and a bus arbiter generating system responses to commands on the system bus. The first processor has at least one cache associated therewith, a system bus controller controlling communication between the first processor and the system bus, and a transition cache controlling and tracking communication between each cache and the system bus controller. When the first processor receives a first command on the system bus requesting a cache line, one of the caches associated with the first processor that stores the requested cache lines copies the requested cache line to the transition cache as part of a response to the first command. Each cache associated with the first processor that stores the requested cache line then updates the memory coherency image state associated with the requested cache line prior to snooping a system response to the first command. Then, once the first processor snoops the system response on the system bus to the first command, the requested cache line is processed at the first processor based on the system response. 
   Other objects, features, and characteristics of the present invention; methods, operation and functions of the related elements of the structure; combination of parts; and economies of manufactural will become apparent from the following detailed description of the preferred embodiments and accompanying drawings, all of which form a part of the specification, wherein like reference numerals designate corresponding parts in the various figures. 

   
     DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
       FIG. 1  illustrates a multi-processor system including a processor with non-inclusive caches which employs the method for obtaining multi-level cache coherency according to the present invention; 
       FIG. 2  illustrates the adjustments to the times at which the snoop commands are supplied to each level of cache by the processor according to the present invention; 
       FIG. 3  illustrates a portion of the system bus controller according to the present invention; 
       FIG. 4  illustrates a table showing the generation of the combined response by the priority combining logic according to the present invention; 
       FIG. 5  illustrates the snoop pipeline in the system bus controller according to the present invention; 
       FIG. 6  illustrates the timing of overlapping snooped commands; 
       FIG. 7  illustrates a flow chart of the method for preventing collisions between two cache queries in high level caches where one of the cache queries is the result of a snooped command; 
       FIG. 8  illustrates a Table which shows a higher level cache&#39;s snoop response based in part on the response from the lower level cache; 
       FIG. 9  illustrates a table showing the generation of a conditional retry response by the transition cache and the possible combined responses; 
       FIG. 10  illustrates a table showing the possible MESI state changes at the caches of a processor in response to a kill command when the MESI state for one of the caches was initially modified and the possible resulting commands output from the processor; 
       FIG. 11  illustrates a table showing the transition cache snoop response and possible MESI state change when a collision occurs between a first command stored at the transition cache and a later snooped second command; and 
       FIG. 12  illustrates a table showing the possible MESI state changes at the caches of a processor receiving a first command requesting a cache line stored therein and the resulting second command output by the processor for each possible system response to the first command. 
   

   It is respectfully suggested that  FIG. 1  be printed on the face of the patent. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  illustrates a multi-processor system including a processor with non-inclusive caches which employs the method for maintaining multi-level cache coherency and the method for increasing efficiency in a multi-processor system according to the present invention. The system includes a system bus  30  interconnecting multiple processors  10  and at least one processor  20  with non-inclusive caches. The processor  20  may include inclusive or non-inclusive caches, however, when only employing the method for increasing efficiency in a multi-processor system according to the present invention. In a preferred embodiment of this method, and for the purposes of discussion, the processor  20  will be described with non-inclusive caches. 
   A main memory  40 , one or more input/output or I/O interfaces  50 , and a bus arbiter  60  are also connected to the system bus  30 . Disk drive storage devices, tape storage devices, etc.  70  are connected to the I/O interfaces  50 . 
   The multiple processors  10  and the processor  20  communicate over the system bus  30  with each other, the main memory  40 , and the I/O interfaces  50 . The I/O interfaces  50  control the operation of such input and output devices as disk drive storage device, tape storage devices, etc. The main memory  40  serves as central storage for the multi-processor system, and stores, for instance, all the cache lines in the system. The bus arbiter  60  decides which of the devices connected to the system bus  30  drives the system bus  30  based on any well-known protocol. Also, for each command placed on the system bus  30 , the bus arbiter  60  collects the snoop responses thereto, and generates as the system response the highest priority response collected. 
   Besides the processor  20 , one or more of the multiple processors  10  could also be a processor according to the present invention. As shown in  FIG. 1 , the processor  20  includes a system bus controller  100  which controls communication between the system bus  30  and the processor  20 . A transition cache  110 , level one or L1 data cache  120 , with controller included, and a level two or L2 cache controller  130  are connected to the system bus controller  100 . A level two or L2 cache  140 , external to the processor  20 , is connected directly to the L2 cache controller  130 . The L1 data cache  120 , the L2 cache controller  130  and L2 cache  140  are connected to the transition cache  110 . The L1 data cache  120  includes a reservation register  122  for storing the reservation bit set by the L1 data cache  120  with respect to a command, such as a store conditional, which requires a reservation to complete. As shown in  FIG. 1 , the system bus controller  100  can check the status of a reservation bit stored in the reservation register  122 .  FIG. 1  further illustrates that the snoop response of the transition cache  110  is supplied to the L1 data cache  120 , and that the snoop response of the L1 data cache  120  is supplied to the L2 cache controller  130 . 
   It should be understood that the various paths illustrated in  FIG. 1  have been represented in greatly simplified form for clarity. In reality, many separate data paths into and out of the various components are required. Furthermore, for the sake of clarity, many of the data and control lines have been eliminated entirely from the figures. It will also be appreciated that, for clarity, other components forming the processor  20 , such as execution units, have been eliminated from FIG.  1 . 
   The L1 data cache  120  has a lower storage capacity, but faster access time, than the L2 cache  140 . Also, unlike what is characteristic of conventional processors, the L2 cache  140  is non-inclusive. Specifically, the L2 cache  140  does not store all of the cache lines stored in the L1 data cache  120 . Consequently, the response to a snooped command provided by the L2 cache controller  130  does not represent the response to the same snooped command which could be issued by the L1 data cache  120 . 
   As implied above, the L2 cache controller  130  controls the communication of data and commands to the L2 cache  140 . As illustrated in  FIG. 1 , the transition cache  110  handles the communication of commands and data between the system bus controller  100 , the L1 data cache  120  and, via the L2 cache controller  130 , the L2 cache  140 . As mentioned above, for the purpose of clarity, the connections to effect such communication between the transition cache  110  and the other components of the processor  20  have not been illustrated in detail. Also, any direct connections between the L1 data cache  120  and the L2 cache  140  have also been eliminated for clarity. 
   The transition cache  110  is a transition cache such as disclosed in application Ser. Nos. 08/761,378; 08/761,379; and 08/761,380, all filed Dec. 9, 1996; the entire contents of all are hereby incorporated by reference. Alternatively, however, the transition cache  110  could be replaced by cache line buffers which maintain the memory coherency image state of each cache line stored therein. 
   For purposes of discussion, the present invention will be discussed using the well-known MESI state protocol as the memory coherency image state protocol employed by the multi-processor system. It should be understood, however, that any memory coherency image protocol could be used with the present invention. 
   Generating a Combined Response at Processor  20   
   Next, the operation of the processor  20  with respect to generating a single combined response to a snooped command will be discussed. As described in detail below, the processor  20  with non-inclusive caches according to the present invention includes a system bus controller  100  which snoops commands on a system bus  30 , and delays supplying those snooped commands to lower levels of cache. The delay times are set such that each lower level cache will output a snoop response at a known point in time. Based on the snoop response from each level of cache, the system bus controller  100  generates a single combined response, and outputs the combined response on the system bus  30 . Besides generating snoop responses based on the snooped commands, higher level caches also generate their snoop responses based on the snoop responses of lower level caches. In this manner, the memory coherency image states in the higher level caches are efficiently updated. 
   During operation, the system bus controller  100  snoops commands on the system bus  30 . The system bus controller  100  then supplies the snooped command to the transition cache  110 , the L1 data cache  120  and the L2 cache controller  130 . The system bus controller  100 , however, does not simultaneously supply the snooped command to the transition cache  110 , the L1 data cache  120 , and the L2 cache controller  130 . 
   As discussed above, the L1 data cache  120  has a faster access time then the L2 cache  140 . Similarly, the transition cache  110 , which has a smaller storage capacity than even the L1 data cache  120 , has a faster access time than the L1 data cache  120 . As shown in  FIG. 2 , the system bus controller  100  adjusts the time at which the snooped command is supplied to the transition cache  110 , the L1 cache  120  and the L2 cache controller  130  such that the responses to the snooped command generated by the transition cache  110 , the L1 data cache  120  and the L2 cache controller  130  are output at a known or a fixed time. More specifically, as shown in  FIG. 2 , the snooped command is supplied to the L2 cache controller  130  first, then after a predetermined period of time, the snooped command is supplied to the L1 data cache  120 . After a further predetermined period of time, the snooped command is supplied to the transition cache  110 . 
   Even in view of the delay, the transition cache  110  will generate its response prior to the L1 data cache  120  and the L2 cache controller  130  generating responses. Similarly, the L1 data cache  120  will generate a response to the snooped command prior to the L2 cache controller  130  generating a response. As stated above, the delays in supplying the snooped command to the transition cache  110  and the L1 data cache  120  are set such that the transition cache  110  and the L1 data cache  120  output their respective responses at known or fixed points in time. 
   When a cache such as the transition cache  110 , the L1 data cache  120 , and the L2 cache controller  130  receive a snooped command, the cache determines whether or not the real address associated with the snooped command matches the real address of any cache lines stored therein. If a match is found, the MESI state for that cache line is modified in a well-known manner based on the snooped command, and regardless of whether a match is found, a snoop response is generated. In the L1 data cache  120 , the MESI state and snoop response are further modified based on the snoop response of the transition cache  110 . Similarly, the MESI state and snoop response of the L2 cache controller  130  are further modified based on the snoop response of the L1 data cache  120 . 
     FIG. 8  illustrates the snoop responses generated by a higher level L(n+1) cache given the MESI state of the higher level L(n+1) cache and the snoop response from the lower level L(n) cache. In the figures, “M” stands for Modified, “E” stands for Exclusive, “S” stands for shared, “O” stands for shared owner and “I” stands for invalid. Also, “→” indicates a change in the MESI state based on the snooped command. 
   The system bus controller  100  receives the snoop responses from the transition cache  110 , the L1 data cache  120 , and the L2 cache controller  130 . The portion of the system bus controller  100  handling the snoop responses from the transition cache  110 , the L1 data cache  120 , and the L2 cache  130  is illustrated in greater detail in FIG.  3 . As shown in  FIG. 3 , the snoop responses from the transition cache  110 , the L1 data cache  120  and the L2 cache controller  130  are received by priority response combining logic  230 . The response from the transition cache  110 , however, is delayed by a first delay  200  and a second delay  210  prior to receipt by the priority response combining logic  230 . Also, the snoop response from the L1 data cache  120  is delayed by a third delay  220  prior to receipt by the priority response combining logic  230 . The first, second and third delays  200 ,  210  and  220  delay a signal by the same period of time in a preferred embodiment of the present invention. Also, the delay time of the first, second and third delays  200 ,  210  and  220  is set such that the priority response combining logic  230  receives the response from the transition cache  110 , the L1 data cache  120  and the L2 cache controller  130  substantially at the same time. While not forming a part of this method, as described in detail below, the priority response combining logic  230  also selectively receives the system response for a snooped command. The priority response combining logic  230  then generates a single combined response based on the responses from the transition cache  110 , the L1 data cache  120  and the L2 cache controller  130  in accordance with the Table shown in FIG.  4 . 
   As shown in  FIG. 3 , the priority response combining logic  230  outputs the combined response to a snoop pipeline  240 . The snoop pipeline  240  is illustrated in FIG.  5 . As shown in  FIG. 5 , when the system bus controller  100  snoops a command, the command is stored in the snoop pipeline  240 , i.e., memory in the system bus controller  100 . As time goes on, the snooped command flows through the snoop pipeline  240 . The operations taking place with respect to the snooped command are illustrated to the right of the snoop pipeline  240  in chronological order. As shown, the snooped command is sent to the L2 cache controller  130 , then sent to the L1 data cache  120 , and finally, sent to the transition cache  110 . At some point in time prior to the receipt of the combined response from the priority response combining logic  230 , portions of the snooped command are no longer needed, and thus, as shown in  FIG. 5 , the amount of information stored with respect to a snooped command decreases. Soon afterwards, the combined response output by the priority response combining logic  230  is stored with the snooped command. If a system response to the snooped command is not required to generate a combined response, the combined response to the snooped command is then output on the system bus  30  by the system bus controller  100 , and later a system response is received and associated with the snooped command. Otherwise, generation of a combined response is delayed until the system response is received. Eventually, because the snoop pipeline  240  has a finite length, the snooped command is removed therefrom. As shown in  FIG. 6 , pipelining of snooped commands allows overlap thereof. 
   Preventing Collision Between L2 Cache Queries 
   Because of the longer access time of the L2 cache  140 , the possibility of a collision between two L2 queries, the second of which is the result of a snooped command, exists. 
   A collision occurs when the two L2 queries map to the same cache address, i.e., have the same congruence class. Because of this collision, an improper response to the snooped command can be generated. For instance, suppose that the L2 cache controller  130  receives a fill request indicating a miss in the L1 data cache  120 , and in response the L2 cache  140  performs an aging castback, discussed below. 
   Because caches have limited storage space, more than one real address can map to the same cache address. When this occurs between a cache line currently stored and a cache line to be received, the currently stored cache line is sent to, for instance, the main memory  40 . Treating the currently stored cache line in this manner is called an aging castback. 
   When the L2 cache controller  130  performs an aging castback with respect to a cache line stored in the L2 cache  140 , the cache line being castback is first transferred to the transition cache  110 . The transition cache  110  then transfers the cache line to the main memory  40  via the system bus controller  100  and the system bus  30 . 
   Next, suppose that the L2 cache controller  130  snoops a command having a real address which maps to the same cache address as the cache line which was the subject of the aging castback. If this collision were not prevented, the L2 cache controller  130  would generate a null response because the cache line was castback. If, however, the cache line had not been completely transferred to the transition cache  110  yet, then the transition cache  110  would also output a null response. Absent the collision, the L2 cache controller  130 , or perhaps the transition cache  110 , would have generated a non-null response. Therefore, the combined response issued by the system bus controller  100  may be improper. 
   The present invention avoids this problem by preventing such collisions. Namely, when the real address for a snooped command maps to the same cache address as another L2 query, the snooped command is not processed. Instead, the L2 cache controller  130  generates a retry response with respect to the snooped command, which instructs the system to retry issuing this command at a later point in time. 
   The method for preventing collisions between a snooped command and another command will be described referring to the flow chart illustrated in FIG.  7 . In step S 10 , the L2 cache controller  130  receives a snooped command. Then, in step S 20 , the L2 cache controller  130  accesses the tag array or directory for the L2 cache  140 , and stores the snooped command. In response to the tag array access, the L2 cache controller  130  will determine if the real address of a cache line stored in L2 cache  140  matches the real address associated with the snooped command. If a match exists, the L2 cache controller  130  accesses, from the L2 cache  140 , the MESI state associated with the cache line, and, in step S 60  discussed below, generates a snoop response based on the MESI state. If no match exists, the null snoop response will be generated in step S 60 . 
   In step S 30 , the L2 cache controller  130  compares the cache address, to which the real address of the snooped command maps, to the cache addresses associated with the L2 queries made one cycle ago and two cycles ago. Then in step S 40 , the L2 cache controller  130  determines whether a cache address match exists. If a match is found, the L2 cache controller  130  will output a retry response with respect to the snooped command in step S 50 . As shown in  FIG. 4 , because the L2 cache controller  130  outputs a retry response, the system bus controller  100  outputs a retry response as the combined response. As mentioned above, the retry response instructs the system to retry issuing the command at a later point in time. If no match is found, the L2 cache controller  130  waits for, in step S 60 , the response to the tag array access made in step S 20 , and once that response is received, the L2 cache controller  130  outputs a snoop response to the snooped command. In this manner, collisions between a snooped command and other commands can be detected and prevented. 
   Conditional Retry 
   Besides the responses discussed above with respect to  FIG. 4 , the transition cache  110  also may generate a conditional retry snoop response. When the transition cache  110  snoops a command from another processor  10  that is associated with the same real address, i.e., cache line, as a command output by the processor  20 , the transition cache  110  may output a conditional retry snoop response. Depending on the system response to the command output by the processor  20 , the system bus controller  100  treats the conditional retry snoop response as a retry snoop response or a not retry snoop response, i.e., a response other than a retry snoop response. In this manner, depending on the system response to the command output by the processor  20 , the system bus controller  100  may not output a retry response as the combined response even though a collision between commands has occurred. 
     FIG. 9  illustrates a table, to be appended to the table of  FIG. 4 , that shows when a conditional retry snoop response is generated by the transition cache  110  and the possible combined responses output from the system bus controller  100 . When the processor  20  puts a first command on the system bus  30 , the status and a copy of the first command is maintained by the transition cache  110 . The transition cache  110  snoops the first command on the system bus  30 , and updates the MESI state for the first command. Namely, the transition cache  110  sets the MESI state for the first command to fill Pending. 
   Also, the MESI state of the cache line associated with the first command in the L1 data cache  120  and the L2 cache  140  will be known or unknown based on the type of the first command. For instance, if the first command is a cache line read, then the MESI state in the L1 data cache  120  and/or the L2 cache  140  is invalid. However, if the first command is a request for ownership of a cache line, the MESI state in the L1 cache  120  and/or L2 cache  140  is unknown. 
   If, while in the fill Pending state, the transition cache  110  snoops a second command associated with the same real address as the first command, then the transition cache  110  generates either a retry or conditional retry snoop response. As shown in  FIG. 9 , the transition cache  110  generates a retry snoop response if the second command is an exclusive command or if the L1 data cache  120  and/or L2 cache  140  MESI states for the cache line associated with the real address are unknown. The transition cache  110  generates a conditional retry snoop response if the second command is a non-exclusive command and the MESI states in the L1 data cache  120  and/or the L2 cache  140  for the cache line associated with the real address are known to be invalid. 
   Assuming that the transition cache  110  generates a conditional retry snoop response, the system bus controller  100  delays generating a combined response until a system response is received for the first command. If the system response to the first command is retry, then as shown in  FIG. 9 , the system bus controller  100  does not generate a combined response of retry. If, however, the system response to the first command is not retry, then the system bus controller  100  generates retry as the combined response. 
   As discussed above, conventionally the second command causing a collision always received a retry response regardless of the system response to the first command involved in the collision. The present invention, however, provides for instances where a retry response is not generated even in the event of a collision; thus, reducing the number of retries placed on the system bus and increasing efficiency of the multi-processor system. 
   Conditional Castback 
   Besides an aging castback, the L1 data cache  120  and L2 cache controller  130  according to the present invention perform conditional castbacks. A conditional castback allows the transition cache  110  to optionally perform a write to main memory  40  based on the system response to the command causing the generation of the conditional castback. In this manner, the number of writes to main memory  40  is reduced. 
   When snooping a kill command which requests that an identified cache line be invalidated without regard to the current MESI state, the L1 data cache  120  and L2 cache controller  130  generate a conditional castback command if the current MESI state is Modified. Namely, the modified data is copied to the transition cache  110  and marked as a conditional castback. The L1 data cache  120  and/or L2 cache controller  130  then set the MESI state for the cache line to invalid. 
   The transition cache  110  holds the modified data until a system response to the kill command is received. The transition cache  110  then processes the conditional castback command based on the system response to the kill command as shown by the table in FIG.  10 .  FIG. 10  also illustrates the MESI state change taking place at the L1 data cache  120  and the L2 cache  140 . 
   As shown in  FIG. 10 , if the system response to the kill command is not retry, then the transition cache  10  discards the conditional castback command because the kill command has completed normally. If, however, the system response to the kill command is retry, the transition cache  110  converts the conditional castback to a memory write command for writing the modified data in the main memory  40 . 
   Because the conditional castback is discarded in the event the kill command completes normally, the number of writes to main memory  40  is reduced as well as the number of retries on the system bus  30 . Accordingly, system resources are not tied up. 
   Converting Exclusive Atomic Reads to Non-exclusive, Non-atomic Reads 
   As is well known, the load and reserve instruction and the store conditional instruction, together, permit atomic update, i.e., performed in its entirety with no visible fragmentation, of a storage location. The load and reserve instruction, when completed, obtains a reservation for a memory location, and causes the L1 data cache  120  to store a bit indicating the obtained reservation in the reservation register  122 . The store conditional instruction can only complete if the reservation is valid when data returns to the L1 data cache  120 . 
   Conventionally, in response to a store conditional instruction from the processor, the system bus controller therefor outputs an exclusive read regardless of whether the reservation is still valid. This causes any cache storing the cache line associated with the exclusive read to invalidate that cache line, i.e., MESI state set to Invalid. By contrast, prior to outputting an exclusive read, the system bus controller  100  of the processor  20  according to the present invention checks the validity of the reservation for this command. If the reservation has been lost, then the system bus controller  100  converts the exclusive read command to a non-exclusive read command. 
   Because the exclusive read issued from the L1 data cache  120 , a copy thereof is stored in the transition cache  110 , which tracks the progress of the command. Namely, the transition cache  110  snoops the non-exclusive read on the system bus  30 , recognizes the non-exclusive read as being converted from the exclusive read command stored therein, and converts the exclusive read stored therein to a non-exclusive read. 
   When data returns in response to the non-exclusive read, the data is transferred to the L1 data cache  120  via the transition cache  110 , and the store conditional completes but the MESI state therefor is not updated because the reservation was lost. 
   Because the exclusive read is converted to a non-exclusive read, the present invention prevents the cache line associated with the non-exclusive read from being unnecessarily and undesirably invalidated in the caches of the other processors  10 . Accordingly, efficiency is increased by eliminating the need for those other processors  10  to re-validate the cache line. 
   Shifting Transition Cache State 
   With the transition cache implementation of present invention, a collision where another processor wants a copy of the cache line that is being filled on this processor can be detected. Most designs allow one processor to get the cache line when in transition MESI states. These designs retry all other processor accesses during this transitional period of time. The present invention allows for shifting the MESI state of the filling line at the transition cache  110 , i.e., without interacting with the L1 data cache  120  and L2 cache controller  130 . As a result, the present invention allows sharing with no performance impact during the time the transition cache  110  handles the cache line fill, and reduces the number of retries on the system bus  30 . 
   As discussed previously, when a first command such as a non-exclusive read command is output by the system bus controller  100 , the transition cache  110  snoops the non-exclusive read command on the system bus  30  and changes the MESI state for the non-exclusive read command to pending. Once a system response is received, the transition cache  110  updates the MESI state to one of Exclusive, Shared or shared Owner. The transition cache  110  then waits for the data associated with the non-exclusive read command to arrive. 
   If the transition cache  110  snoops a second command associated with the same real address as the non-exclusive read command, the transition cache  110  checks the status of the non-exclusive read command to determine if data has arrived at the transition cache  110  in response to the non-exclusive read command. As shown in  FIG. 11 , based on the determination of whether data has begun to arrive and whether the snooped second command is exclusive or non-exclusive, the transition cache  110  changes the MESI state for the first command and generates a snoop response to the second command. 
   Namely, if data has started to arrive, the transition cache  110  generates a retry snoop response and does not change the MESI state of the first command. If data has not started to arrive and the second command is an exclusive command, the transition cache  110  generates a null snoop response and changes the MESI state for the first command to Invalid. If the data has not started to arrive and the second command is a non-exclusive command, the transition cache  110  generates a shared snoop response and changes the MESI state for the first command to Shared. 
   As further shown in  FIG. 11 , if the first command is an exclusive command, then the transition cache  110  outputs a retry snoop response and the MESI state for the first command remains unchanged. 
   Accordingly, the present invention allows for shifting the MESI state of a non-exclusive command at the transition cache such that sharing with no performance impact is allowed and the number of retries on the system bus  30  is reduced. 
   Converting Cache-to-cache Transfer to a Memory Write 
   This technique of the present invention allows the L1 data cache and L2 cache controller  130  to immediately update the MESI state of a requested cache line even if the read command requesting that cache line receives a retry system response. Also, the response to the read command is converted into a write command if the system response to that read command is a retry and the response includes modified data. 
   When the processor  20  receives a read command requesting a cache line and the cache line is stored in the L1 data cache  120  and/or the L2 cache  140 , the one of the L1 data cache  120  and the L2 cache  140  having the highest MESI state, in the order of Modified, Exclusive shared Owner and Shared, copies the cache line to the transition cache  110 . The L1 data cache  120  and/or the L2 cache  140  which store the cache line then update the MESI state therefor as shown in the table of FIG.  12 . 
   The system bus controller  100  receives the cache line and holds the cache line until the system response to the read command is received.  FIG. 12  illustrates a table showing how the system bus controller  100  processes the cache line based on the system response. As shown in  FIG. 12 , if the system response to the read command is not retry, the system bus controller  100  sources the cache line to the system bus  30 . If, however, the system response is retry and the MESI state for the cache line in one of the L1 data cache and the L2 cache  140 , prior to being updated, was modified, then the system bus controller  100  converts the response to the read command to a memory write for writing the cache line in the main memory  40 . It should be noted that because the system bus receives the snoop response of the L1 data cache  120  and the L2 cache  140  to the read command, the system bus controller  100  is notified if the cache line includes modified data. 
   If the cache line does not include modified data and the system response to the read command is retry, then the system bus controller  100  discards the cache line. 
   By transferring the cache line to the transition cache  110 , the MESI state of the cache line in the L1 data cache  120  and/or the L2 cache  140  can be immediately updated. Also, by waiting for the system response to the read command, the cache line may be written to the main memory  40  to improve the efficiency with which the main memory  40  is updated with modified data. 
   It should be understood that the present invention is not limited to a processor with only two levels of cache. Instead, additional levels of non-inclusive cache could be included, with the method for preventing collisions according to the present invention being applied to the second and higher levels of cache. 
   While the invention has been described with what is presently considered the most practical and preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.