Abstract: A method of operating a processing unit of a computer system, by issuing an instruction having an explicit prefetch request directly from an instruction sequence unit to a prefetch unit of the processing unit. The invention applies to values that are either operand data or instructions. In a preferred embodiment, two prefetch units are used, the first prefetch unit being hardware independent and dynamically monitoring one or more active streams associated with operations carried out by a core of the processing unit, and the second prefetch unit being aware of the lower level storage subsystem and sending with the prefetch request an indication that a prefetch value is to be loaded into a lower level cache of the processing unit. The invention may advantageously associate each prefetch request with a stream ID of an associated processor stream, or a processor ID of the requesting processing unit (the latter feature is particularly useful for caches which are shared by a processing unit cluster).

Abstract: A multiprocessor computer system in which snoop operations of the caches are synchronized to allow the issuance of a cache operation during a cycle which is selected based on the particular manner in which the caches have been synchronized. Each cache controller is aware of when these synchronized snoop tenures occur, and can target these cycles for certain types of requests that are sensitive to snooper retries, such as kill-type operations. The synchronization may set up a priority scheme for systems with multiple interconnect buses, or may synchronize the refresh cycles of the DRAM memory of the snooper's directory. In another aspect of the invention, windows are created during which a directory will not receive write operations (i.e., the directory is reserved for only read-type operations). The invention may be implemented in a cache hierarchy which provides memory arranged in banks, the banks being similarly synchronized.

Abstract: A data processing system includes a requester having a request queue and a recipient. The requester, which buffers a request in an entry of the request queue, transmits the request to the recipient for servicing. According to the request-and-forget protocol, the requester removes the request from the entry of the request queue without receipt of any indication that the request has been serviced.

Abstract: An apparatus for associating cache memories with processors within a multiprocessor data processing system is disclosed. The multiprocessor data processing system includes multiple processing units and multiple cache memories. Each of the cache memories includes a cache memory controller, and each cache memory controller includes a mode register. Each mode register has multiple processing unit fields, and each of the processing unit fields is associated with one of the processing units for indicating whether or not data from an associated processing unit should be cached by a cache memory associated to a corresponding cache memory controller.

Abstract: A data processing system includes a processing unit, a distributed memory including a local memory and a remote memory having differing access latencies, and a cache coupled to the processing unit and to the distributed memory. The cache includes a congruence class containing a plurality of cache lines and a plurality of latency indicators that each indicate an access latency to the distributed memory for a respective one of the cache lines. The cache further includes a cache controller that selects a cache line in the congruence class as a castout victim in response to the access latencies indicated by the plurality of latency indicators. In one preferred embodiment, the cache controller preferentially selects as castout victims cache lines having relatively short access latencies.

Abstract: A method of maintaining cache coherency, by designating one cache that owns a line as a highest point of coherency (HPC) for a particular memory block, and sending a snoop response from the cache indicating that it is currently the HPC for the memory block and can service a request. The designation may be performed in response to a particular coherency state assigned to the cache line, or based on the setting of a coherency token bit for the cache line. The processing units may be grouped into clusters, while the memory is distributed using memory arrays associated with respective clusters. One memory array is designated as the lowest point of coherency (LPC) for the memory block (i.e., a fixed assignment) while the cache designated as the HPC is dynamic (i.e., changes as different caches gain ownership of the line).

Abstract: A data processing system includes an interconnect, a system memory and a number of snoopers coupled to the interconnect, and response logic. In response to a requesting snooper issuing a data request on the interconnect specifying a memory address, the snoopers provide snoop responses. The response logic compiles the snoop responses to obtain a combined response including an indication of a demand-source snooper that will source requested data associated with the memory address to the requesting snooper and an indication of whether additional non-requested data will be supplied to the requesting snooper. This combined response is then transmitted to the snoopers on the interconnect to direct the provision of the requested data, and possibly unrequested prefetch data, to the requesting snooper.

Abstract: A multiprocessor data processing system includes an interconnect, a plurality of processing units coupled to the interconnect, and at least one system memory and a plurality of caches coupled to the plurality of processing units. A cache suitable for use in such a data processing system includes data storage containing multiple granules of data and a number of state fields associated with the granules of data. Each state field has a plurality of possible states including an O state indicating that an associated granule is consistent with corresponding data in the memory and has unknown coherency with respect to peer caches in the data processing system. Thus, a cache is permitted to store memory-consistent, but possibly non-coherent data in order to offer processing units in the data processing system lower latency to an image of system memory.

Abstract: A method of operating a computer system is disclosed in which an instruction having an explicit prefetch request is issued directly from an instruction sequence unit to a prefetch unit of a processing unit. In a preferred embodiment, two prefetch units are used, the first prefetch unit being hardware independent and dynamically monitoring one or more active streams associated with operations carried out by a core of the processing unit, and the second prefetch unit being aware of the lower level storage subsystem and sending with the prefetch request an indication that a prefetch value is to be loaded into a lower level cache of the processing unit. The invention may advantageously associate each prefetch request with a stream ID of an associated processor stream, or a processor ID of the requesting processing unit (the latter feature is particularly useful for caches which are shared by a processing unit cluster).

Abstract: A method and system for maintaining allocation information on data castout from an upper level cache provides a cache control with the ability to select victims based on whether a cache entry is present due to a read request from a higher level in the memory hierarchy or is present due to being modified in the higher level and then castout to the lower level. The information maintained may be a single bit indicating this status, may be a separate least-recently-used (LRU) array value indicating the order of allocation in the lower level for storage of cache entries castout from the higher level.

Abstract: A method and system for allocating lower level cache entries for data castout from an upper level cache provides improved computer system performance by adjusting the ordering of least-recently-used (LRU) information within a cache. Data that is castout from a higher level cache can be written after a read is satisfied and the castout entry will not be labeled as most-recently-used. This improves performance under certain operating conditions of a computing system, as castout data is often less important to keep in lower level cache than data that is also present in the higher level cache.

Abstract: A method of operating a computer system is disclosed in which an instruction having an explicit prefetch request is issued directly from an instruction sequence unit to a prefetch unit of a processing unit. In a preferred embodiment, two prefetch units are used, the first prefetch unit being hardware independent and dynamically monitoring one or more active streams associated with operations carried out by a core of the processing unit, and the second prefetch unit being aware of the lower level storage subsystem and sending with the prefetch request an indication that a prefetch value is to be loaded into a lower level cache of the processing unit. The invention may advantageously associate each prefetch request with a stream ID of an associated processor stream, or a processor ID of the requesting processing unit (the latter feature is particularly useful for caches which are shared by a processing unit cluster).

Abstract: A multiprocessor data processing system includes an interconnect, a plurality of processing units coupled to the interconnect, and at least one system memory and a plurality of caches coupled to the plurality of processing units. A cache suitable for use in such a data processing system includes data storage containing a data granule, a state field associated with the data granule, and a cache controller. The state field has a plurality of possible states including a first state that indicates that the data granule is consistent with corresponding data in the memory and has unknown coherency with respect to other peer caches among the plurality of caches. To update the state of the data granule from the first state, the cache controller issues on the interconnect a transaction specifying an address associated with the data granule. In response to receipt of a combined response of the plurality of caches, the cache controller updates the state field to a second state among the plurality of possible states.

Abstract: A method and system for bypassing cache levels when storing data castout from an upper level cache provides a memory hierarchy that can selectively skip one more more intermediate levels when writing castout entries from a higher level cache based on a number of detected conditions. The intermediate levels may be bypassed when an intermediate cache level is busy, has an entry with an address conflict with the castout value, or may skip levels based on program control. The control providing the skipping selection may be driven by a detector that analyzes load/store operations of a processor in order to produce efficient operation under changing memory use conditions.

Abstract: A multiprocessor data processing system includes an interconnect, a plurality of processing units coupled to the interconnect, and at least one system memory and a plurality of caches coupled to the plurality of processing units. A cache suitable for use in such a data processing system includes data storage containing multiple data granules and a number of state fields associated with the data granules. Each state field has a plurality of possible states including an OR state that indicates that an associated granule is consistent with corresponding data in the memory, that the associated data granule has unknown coherency with respect to other peer caches in the data processing system, and that the cache is responsible, among all of its peer caches that may store the associated data granule in a memory-consistent state with unknown coherency, for sourcing the data granule in response to a request.

Abstract: A multiprocessor data processing system includes an interconnect, a plurality of processing units coupled to the interconnect, and at least one system memory and a plurality of caches coupled to the plurality of processing units. A cache suitable for use in such a data processing system includes data storage containing multiple granules of data and a number of state fields associated with the granules of data. Each state field has a plurality of possible states including an O state indicating that an associated granule is consistent with corresponding data in the memory and has unknown coherency with respect to peer caches in the data processing system. The cache updates the state field from the O state to another of the plurality of states in response to a snooped transaction on the interconnect.

Abstract: A method and apparatus for casing out data within a cache memory hierarchy for a data processing system is disclosed. The data processing system has multiple processing units, each of the processing units having a multi-level cache memory hierarchy. The cache memory hierarchy includes a first cache and a second cache at a same cache level. Furthermore, the first cache and the second cache share a lower-level cache. In response to a castout write request from the first cache to the lower-level cache, the second cache aborts the data transfer for the castout write request if the second cache already has a copy of data of the castout write request. The coherency state of both the first and second caches are then updated.

Abstract: A method and apparatus for transmitting control signals within a hierarchial cache memory architecture of a data processing system is disclosed. The cache memory hierarchy includes multiple levels of cache memories, each level may have a different size and speed. In response to a processor request for information, a control command is sent to the cache memory hierarchy. The control command includes multiple control blocks. Beginning at the lowest possible cache level of the cache memory hierarchy, a determination is made whether or not there is a cache hit at a current level of the cache memory hierarchy. In response to a determination that there is not a cache hit at the current level, an abbreviated control command is sent to an upper cache level of the cache memory hierarchy, after a control block that corresponds to the current level is removed from the control command.

Abstract: A method and apparatus for casting out data within a cache memory hierarchy for a data processing system is disclosed. The data processing system has multiple processing units, each of the processing units having a multi-level cache memory hierarchy. In response to a castout write request from a cache memory to a non-inclusive lower-level cache memory within a cache memory hierarchy, the data transfer is aborted if the lower-level cache memory already has a copy of the data of the castout write. The coherency state of the lower-level cache memory is then updated, if necessary.

Abstract: A non-uniform memory access (NUMA) computer system includes a plurality of processing nodes coupled to a node interconnect. The plurality of processing nodes include at least a remote processing node, which contains a processor having an associated cache hierarchy, and a home processing node. The home processing node includes a shared system memory containing a plurality of memory granules and a coherence directory that indicates possible coherence states of copies of memory granules among the plurality of memory granules that are stored within at least one processing node other than the home processing node. If the processor within the remote processing node has a reservation for a memory granule among the plurality of memory granules that is not resident within the associated cache hierarchy, the coherence directory associates the memory granule with a coherence state indicating that the reserved memory granule may possibly be held non-exclusively at the remote processing node.