# Patent

## Title
Systems and methods for supporting a plurality of load and store accesses of a cache

## Abstract

Systems and methods for supporting a plurality of load and store accesses of a cache are disclosed. Responsive to a request of a plurality of requests to access a block of a plurality of blocks of a load cache, the block of the load cache and a logically and physically paired block of a store coalescing cache are accessed in parallel. The data that is accessed from the block of the load cache is overwritten by the data that is accessed from the block of the store coalescing cache by merging on a per byte basis. Access is provided to the merged data.

## Background
A cache in a central processing unit is a data storage structure that is used by the central processing unit of a computer to reduce the average time that it takes to access memory. It is a memory which stores copies of data that is located in the most frequently used main memory locations. Moreover, cache memory is memory that is smaller and that may be accessed more quickly than main memory. There are several different types of caches. These include physically indexed physically tagged (PIPT), virtually indexed virtually tagged (VIVT) and virtually indexed physically tagged (VIPT).
Caches that can accommodate multiple accesses in a single cycle provide performance advantages. In particular, such caches feature reduced access latencies. Conventional approaches to accommodating multiple accesses in a single cycle include the use of multi-ported caches and the provision of caches that include a plurality of tag and data banks.
A multi-ported cache is a cache which can serve more than one request at a time. In accessing some conventional caches a single memory address is used, whereas in a multi-ported cache, N memory addresses can be requested at a time, where N is the number of ports that is possessed by the multi-ported cache. An advantage of a multi ported cache is that greater throughput (e.g., a greater number of load and store requests) may be accommodated. However, the number of cache ports that are needed to accommodate increasingly high levels of throughput may not be practical.
Caches that include a plurality of tag and data banks can serve more than one request at a time as each bank can serve at least one request. However, when more than one request attempts to access the same bank, the request that will be allowed to access the bank must be determined. In one conventional approach, serial arbitrations are used to determine which request will be allowed to access the data bank and to access the tag bank for the tag that corresponds to the request. However, the time that it takes to execute such arbitrations can delay access to the bank and thus delay the triggering of the critical Load Hit signal. Moreover, because a plurality of both loads and stores may need to be accommodated, the additional banks, ports and arbitration that may be needed can exact unsatisfactory latency penalties.
SUMMARY
Conventional approaches to accommodating throughput that involves multiple accesses can result in unsatisfactory latency penalties as it relates to the timing of load hit signals. A method for supporting a plurality of load and store accesses of a cache is disclosed that addresses these shortcomings. However, the claimed embodiments are not limited to implementations that address any or all of the aforementioned shortcomings. As a part of the aforementioned method, responsive to a request of a plurality of requests to access a block of a plurality of blocks of a load cache, the block of the load cache and a logically and physically paired block of a store coalescing cache are accessed in parallel. The data that is accessed from the block of the load cache is overwritten by the data that is accessed from the block of the store coalescing cache by merging on a per byte basis. Thereafter, access is provided to the merged data. Because the data is merged locally and block access arbitrations involving a plurality of access requests to the load cache and the store coalescing cache are executed in parallel with the identification of associated tags, a plurality of loads and stores are provided quick access to the data maintained by the load cache and the store coalescing cache.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention, together with further advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:
FIG. 1A shows an exemplary operating environment of a system for supporting a plurality of load and store accesses of a cache in a single cycle based on an organization of load and store coalescing caches according to one embodiment.
FIG. 1B shows a diagram that illustrates the manner in which a plurality of data blocks facilitate the accessing of a load cache and a store coalescing cache by multiple requests in the same clock cycle according to one embodiment.
FIG. 1C shows a store coalescing cache and a load cache that are organized such that related entries are maintained in blocks of the respective caches that are positioned similarly within the caches according to one embodiment.
FIG. 1D shows blocks of a store coalescing cache and a load cache that provide data to a multiplexor as a part of merger operations according to one embodiment.
FIG. 1E illustrate operations performed by system for supporting a plurality of load and store accesses of a cache in a single cycle based on an organization of load and store coalescing caches according to one embodiment.
FIG. 2 shows components of a system for supporting a plurality of load and store accesses of a cache in a single cycle according to one embodiment.
FIG. 3 shows a flowchart of the steps performed in a method for supporting a plurality of load and store accesses of a cache in a single cycle according to one embodiment.



It should be noted that like reference numbers refer to like elements in the figures.
DETAILED DESCRIPTION
Although the present invention has been described in connection with one embodiment, the invention is not intended to be limited to the specific forms set forth herein. On the contrary, it is intended to cover such alternatives, modifications, and equivalents as can be reasonably included within the scope of the invention as defined by the appended claims.
In the following detailed description, numerous specific details such as specific method orders, structures, elements, and connections have been set forth. It is to be understood however that these and other specific details need not be utilized to practice embodiments of the present invention. In other circumstances, well-known structures, elements, or connections have been omitted, or have not been described in particular detail in order to avoid unnecessarily obscuring this description.
References within the specification to “one embodiment” or “an embodiment” are intended to indicate that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. The appearance of the phrase “in one embodiment” in various places within the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Moreover, various features are described which may be exhibited by some embodiments and not by others. Similarly, various requirements are described which may be requirements for some embodiments but not other embodiments.
Some portions of the detailed descriptions, which follow, are presented in terms of procedures, steps, logic blocks, processing, and other symbolic representations of operations on data bits within a computer memory. These descriptions and representations are the means used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. A procedure, computer executed step, logic block, process, etc., is here, and generally, conceived to be a self-consistent sequence of steps or instructions leading to a desired result. The steps are those requiring physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals of a computer readable storage medium and are capable of being stored, transferred, combined, compared, and otherwise manipulated in a computer system. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, or the like.
It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise as apparent from the following discussions, it is appreciated that throughout the present invention, discussions utilizing terms such as “accessing” or “comparing” or “merging” or “providing access” or the like, refer to the action and processes of a computer system, or similar electronic computing device that manipulates and transforms data represented as physical (electronic) quantities within the computer system's registers and memories and other computer readable media into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices.
Exemplary Operating Environment of a System for Supporting a Plurality of Load and Store Accesses of a Cache According to One Embodiment
FIG. 1A shows an exemplary operating environment of a system 101 for supporting a plurality of load and store accesses of a cache in a single cycle based on an organization of load and store coalescing caches according to one embodiment. System 101 directs, in response to each request of a plurality of requests to access a load cache, parallel accesses of the load cache and a store coalescing cache and a local comparing of their accessed content in a single cycle. Moreover, as a part of the operation of system 101, block access arbitrations involving a plurality of access requests to the load cache and the store coalescing cache are executed in parallel with the identification of associated tags. Consequently, a plurality of loads and stores are provided quicker access to data maintained by the load cache and the store coalescing cache. FIG. 1A shows system 101, level one (L1) cache 103, level two (L2) cache 107, CPU 105, main memory 109 and system interface 111.
Referring to FIG. 1A, L1 cache 103 is a level 1 cache and L2 cache 107 is a level 2 cache. In one embodiment, as shown in FIG. 1B, L1 cache 103 can include store coalescing cache 103a, load cache 103b, store coalescing cache tag memory 103c (e.g., SRAM or other type memory) and load cache tag memory 103d (e.g., SRAM or other type memory). In other embodiments, L1 cache 103 can include other components. As shown in FIG. 1B, load cache 103b can be divided into a plurality of data blocks. In one embodiment, load cache 103b can be divided into four 8 kilobyte data blocks. In other embodiments, load cache 103b can be divided into other numbers of data blocks that have the capacity to store other amounts of data. In one embodiment, store coalescing cache 103a, although smaller in size than load cache 103b, is organized similarly to load cache 103b. In one embodiment, as shown in FIG. 1B, the plurality of data blocks that are possessed by these caches facilitates the accessing of load cache 103b and store coalescing cache 103a by a throughput of multiple requests, 1-N, in the same clock cycle.
Referring to FIG. 1B, in one embodiment, as a part of the aforementioned configuration, store coalescing cache tag memory 103c and load cache tag memory 103d maintain a plurality of copies, 1-N, of the tags that correspond to cache line entries of store coalescing cache 103a and load cache 103b respectively. Accordingly, a dedicated tag memory copy for each access request 1-N is provided that facilitates the identification of tags that correspond to the requests and to the cache line entries within a single clock cycle. In one embodiment, arbitrations related to block accesses are executed in parallel with the identification of tags associated with the cache line entries (see FIG. 1B).
In one embodiment, store coalescing cache 103a and load cache 103b are organized such that related entries are maintained in blocks of the respective caches that are positioned similarly within the caches as is shown in FIG. 1C. Moreover, the blocks are logically and physically paired such that an access request to a block in load cache 103b results in an access request to the logically and physically paired block in store coalescing cache 103a (and a corresponding search of both store coalescing cache tag memory 103c and load cache tag memory 103d for the tags that correspond to the requests). Consequently, arbitrations related to block access as well as operations relating to the identification of tags corresponding to the requests are merged with regard to store coalescing cache 103a and load cache 103b. 
In one embodiment, the data maintained in store coalescing cache 103a is considered to be more up to date (e.g., is considered to be the latest version) than the data that is maintained in the logically and physically paired load cache 103b. Accordingly, the data that is maintained in store coalescing cache 103a is given priority over data that is maintained in load cache 103b in merge operations involving such data. This is shown in FIG. 1D where the input into a 2:1 multiplexor from a store coalescing cache block (store coalescing cache block 0) is given priority over an input from a load cache block (load cache block 0). The prioritization of the store coalescing cache contents over the load cache contents ensures the provision of the latest data to access requestors.
Referring again to FIG. 1A, system 101, responsive to a request to access load cache 103b, directs accesses of store coalescing cache 103a and load cache 103b in parallel. Thereafter, the data that is accessed from store coalescing cache 103a and the data that is accessed from load cache 103b is merged. The merged data is then made available to the access requestor. In one embodiment, the data can be merged using a 2:1 multiplexer as discussed above. In other embodiments, other manners of merging the data can be used. In one embodiment, as a part of the merger of the data, as discussed above, the data that is stored in store coalescing cache 103a is given priority over the data that is stored in load cache 103b. 
In one embodiment, system 101, is able to facilitate the execution of a plurality of loads and stores in a single cycle based on the organization of load cache 103b and store coalescing cache 103a. In one embodiment, the parallelism facilitated by system 101 operates to provide quicker load and store accesses. In one embodiment, system 101 can be located in a cache controller (not shown). In other embodiments, system 101 can be separate from a cache controller, but operate cooperatively therewith.
Main memory 111 includes physical addresses that store the information that is copied into cache memory. In one embodiment, when the information that is contained in the physical addresses of main memory that have been cached is changed, the corresponding cached information is updated to reflect the changes made to the information stored in main memory. Also shown in FIG. 1A is system interface 111.
Operation
FIG. 1E illustrates operations performed by system 101 for supporting a plurality of load and store accesses of a cache in a single cycle based on an organization of load and store coalescing caches according to one embodiment. These operations, which relate to supporting a plurality of loads and stores in a single cycle, are only exemplary. It should be appreciated that other operations not illustrated by FIG. 1E can be performed in accordance with one embodiment.
Referring to FIG. 1E, at A, a load request involving a specific block of a level one load cache is received.
At B, the block of the store coalescing cache that logically and physically corresponds to the specific block of the level one load cache is simultaneously identified. In one embodiment, because both the load cache and the store coalescing cache are similarly organized, the bits of the virtual address associated with the request that addresses the aforementioned specific block of the load cache, simultaneously addresses the similarly situated, logically and physically paired, block of the store coalescing cache.
At C, a read enable that corresponds to the block of the store coalescing cache and a read enable that corresponds to the block of the load cache are asserted in parallel (because arbitration for both blocks are the same) and data from each of the blocks is accessed.
At D, the data accessed from the accessed block of the store coalescing cache and the data accessed from the accessed block of the load cache is locally compared.
At E, the data accessed from the accessed block of the store coalescing cache and the data accessed from the accessed block of the load cache is merged. And, the merged data is made available to the load requestor.
In exemplary embodiments stores can access the store coalescing cache without a corresponding access of the load cache. However, as discussed herein, load requests trigger a parallel access of both the load cache and the store coalescing cache. Exemplary embodiments accommodate a throughput of many loads and stores, do not penalize the timing of the “load hit” signal and maintain consistency between loads and stores.
Components of System for Supporting a Plurality of Load and Store Accesses of a Cache According to One Embodiment
FIG. 2 shows components of a system 101 for supporting a plurality of load and store accesses of a cache in a single clock cycle based on an organization of load and store coalescing caches according to one embodiment. In one embodiment, components of system 101 implement an algorithm for supporting a plurality of load and store accesses in a single clock cycle. In the FIG. 3 embodiment, components of system 101 include load request accessor 201, block accessor 201, data comparer/merger 203 and data access provider 205.
Load request accessor 201 accesses requests to access blocks of a load cache (e.g., load cache 103b in FIG. 1B) and/or a store coalescing cache (e.g., store coalescing cache 103a in FIG. 1B). In one embodiment, the requests can include a plurality of load and store requests.
Block accessor 203, responsive to a request of a plurality of requests to access a block of a plurality of blocks in a load cache (e.g., 103b in FIG. 1B), accesses the block in the load cache and a logically and physically paired block in a store coalescing cache (e.g., 103a in FIG. 1B) in parallel. In one embodiment, the block in the load cache and the block in the store coalescing cache are logically paired and are situated similarly within the organizational structure of the respective caches.
Data comparer/merger 205 compares and merges data accessed from the accessed block of the load cache to data that is accessed from the accessed block of the store coalescing cache on a per byte basis. In one embodiment, the data that is maintained in the store coalescing cache is given priority over the data that is maintained in the load cache. As such, data that is inconsistent with that which is supplied by the store coalescing cache is replaced by the data supplied from the store coalescing cache. In one embodiment, data comparer/merger 203 can be implemented using a multiplexer that compares and merges the data supplied by the store coalescing cache and the data supplied by the load cache. In one embodiment, the multiplexer can be a 2:1 multiplexer. In one embodiment, the comparison is locally performed (carried out inside the level one data cache).
Data access provider 207 provides access to the merged data to the load requestor. In one embodiment, the herein described processes ensure that the data provided can be a part of a throughput of many loads and stores, is provided in a manner that does not penalize the timing of the “load hit” signal and maintains consistency between loads and stores.
It should be appreciated that the aforementioned components of system 101 can be implemented in hardware or software or in a combination of both. In one embodiment, components and operations of system 101 can be encompassed by components and operations of one or more computer components or programs (e.g., a cache controller). In another embodiment, components and operations of system 101 can be separate from the aforementioned one or more computer components or programs but can operate cooperatively with components and operations thereof.
Method for Supporting a Plurality of Load and Store Accesses of a Cache According to One Embodiment
FIG. 3 shows a flowchart 300 of the steps performed in a method for supporting a plurality of loads and stores in a single clock cycle based on an organization of load and store coalescing caches according to one embodiment. The flowchart includes processes that, in one embodiment can be carried out by processors and electrical components under the control of computer-readable and computer-executable instructions. Although specific steps are disclosed in the flowcharts, such steps are exemplary. That is the present embodiment is well suited to performing various other steps or variations of the steps recited in the flowchart. Although specific steps are disclosed in the flowcharts, such steps are exemplary. That is the present embodiments are well suited to performing various other steps or variations of the steps recited in the flowcharts.
Referring to FIG. 3, at 301, responsive to a request of a plurality of requests to access a block of a plurality of blocks of a load cache, the block of the load cache and a logically and physically paired block of a store coalescing cache are accessed in parallel. In one embodiment, the block of the load cache and the block of the store coalescing cache are logically and physically paired and are a part of similarly organized caches that facilitate their access in a single clock cycle.
At 303, data accessed from the block of the load cache is compared and merged with data that is accessed from the block of the store coalescing cache on a per byte basis. In one embodiment, as a part of the data comparison and merger process, the data that is maintained in the block of the store coalescing cache is given priority over the data that is maintained in the block of the load cache. Consequently, where the data in the block of the load cache and the data in the block of the store coalescing cache are different, the data from the block of the store coalescing cache is included in the merged data result.
At 305, access to the result of the data merger is provided to access requestors. In one embodiment, operations described herein ensure that access to the latest version of data that is associated with an address is provided to the requestor.
With regard to exemplary embodiments thereof, systems and methods for supporting a plurality of load and store accesses of a cache are disclosed. Responsive to a request of a plurality of requests to access a block of a plurality of blocks of a load cache, the block of the load cache and a logically paired block of the store coalescing cache are accessed in parallel. The data that is accessed from the block of the load cache is compared to the data that is accessed from the block of the store coalescing cache and merged on a per byte basis. Access is provided to the merged data.
Although many of the components and processes are described above in the singular for convenience, it will be appreciated by one of skill in the art that multiple components and repeated processes can also be used to practice the techniques of the present invention. Further, while the invention has been particularly shown and described with reference to specific embodiments thereof, it will be understood by those skilled in the art that changes in the form and details of the disclosed embodiments may be made without departing from the spirit or scope of the invention. For example, embodiments of the present invention may be employed with a variety of components and should not be restricted to the ones mentioned above. It is therefore intended that the invention be interpreted to include all variations and equivalents that fall within the true spirit and scope of the present invention.

## Claims

1. A method for supporting a plurality of load and store accesses, comprising:
responsive to a request of a plurality of requests to access a data block of a plurality of data blocks of said load cache, accessing said data block of said load cache and a logically and physically paired data block of a store coalescing cache in parallel;
comparing data accessed from said data block of said load cache to data that is accessed from said data block of said store coalescing cache and merging the data on a per byte basis; and
providing access to the merged data.

2. The method of claim 1 wherein said comparing and said merging is performed locally within a level one cache that comprises said load cache and said store coalescing cache.

3. The method of claim 1 wherein said data block of said load cache and said data block of said store coalescing cache are situated similarly within the organizational structure of said load cache and said store coalescing cache respectively.

4. The method of claim 1 wherein in said merging operation data from said store coalescing cache is given priority over data from said load cache.

5. The method of claim 1 wherein access arbitrations related to said data blocks, and searches for tags corresponding to said requests to access said data blocks, are merged with respect to said load cache and said logically paired store coalescing cache.

6. The method of claim 1 wherein said comparing and said merging is executed using a 2:1 multiplexer.

7. The method of claim 1 wherein a plurality of requests to access a same block of said plurality of blocks causes an access arbitration that is executed in parallel with an accessing of a tag memory for tags corresponding to said each request of said plurality of requests.

8. A cache system, comprising:
a level one cache comprising:
a store coalescing cache;
a load cache; and
a cache controller comprising:
a location accessing component responsive to a request to access a data block of said load cache, for accessing said data block of said load cache and a logically paired data block of a store coalescing cache in parallel;
a comparing and merging component for comparing data accessed from said data block of said load cache to data that is accessed from said data block of said store coalescing cache and merging the data on a per byte basis; and
an access providing component for providing access to the merged data.

9. The cache system of claim 8 wherein said comparing and said merging is performed locally within a level one cache that comprises said load cache and said store coalescing cache.

10. The cache system of claim 8 wherein said data block of said load cache and said data block of said store coalescing cache are situated similarly within the organizational structure of said load cache and said store coalescing cache respectively.

11. The cache system of claim 8 wherein, in said merging operation, data from said store coalescing cache is given priority over data from said load cache.

12. The cache system of claim 8 wherein access arbitrations related to data blocks, and searches for tags corresponding to access requests, are merged with respect to said load cache and said logically and physically paired store coalescing cache.

13. The cache system of claim 8 wherein said comparing and said merging is executed using a 2:1 multiplexer.

14. The cache system of claim 8 wherein a plurality of requests to access a same block of said plurality of blocks causes an access arbitration that is executed in parallel with an accessing of a tag storage memory for tags corresponding to said plurality of requests.

15. A processor, comprising:
a CPU; and
a cache system, comprising:
a level one cache system; and
a level two cache system;
said level one cache system comprising:
a store coalescing cache;
a load cache; and
a cache controller comprising:
a data block accessing component responsive to a request to access a data block of said load cache, for accessing said data block of said load cache and a logically and physically paired data block of a store coalescing cache in parallel;
a comparing and merging component for comparing data accessed from said data block of said load cache to data that is accessed from said data block of said store coalescing cache and merging the data on a per byte basis; and
an access providing component for providing access to said merged data.

16. The processor of claim 15 wherein said comparing and said merging is performed locally within a level one cache that comprises said load cache and said store coalescing cache.

17. The processor of claim 15 wherein said data block of said load cache and said data block of said store coalescing cache are situated similarly within the organizational structure of said load cache and said store coalescing cache respectively.

18. The processor of claim 15 wherein in said merging operation data from said store coalescing cache is given priority over data from said load cache.

19. The processor of claim 15 further comprising providing access to said merged data to a load request.

20. The processor of claim 15 wherein said comparing and said merging is executed using a 2:1 multiplexer.