Abstract:
A data processing system includes a memory storing data to be retrieved and an I/O controller configured to request data stored in the memory at a plurality of addresses. The I/O may be responsive to an internal or external device requesting such data. A fetch machine provides or initiates retrieval of data stored at the requested address, while a prefetch machine predicts future requests and keeps track of memory requests already initiated and queued. Thus, the prefetch machine is responsive to the plurality addresses to predict others of the addresses and provide or initiate retrieval of data stored thereat. To avoid prefetching information already requested and in a fetch queue, the prefetch machine includes a memory storing a last one of the addresses subject to prefetching. Finally, to avoid conflicts between currently requested data and prefetch operation, an arbiter resolves memory accesses or data requests initiated by the fetch and prefetch machines.

Description:
BACKGROUND 
     In a multiprocessor system, problems arise when more than one processor attempts to access a memory location. While multiple processors have access to a common memory location, if one of those processors attempts to update the information in the memory location without informing the other processors also having access to the memory location, mismatches may occur resulting in a loss of data coherency. This is of particular concern in multiprocessor systems having memory cache associated with each processor, i.e., processor cache. The processor cache stores information most recently accessed by the processor. Processor cache is typically organized out into cache lines of, for example, 64 bits of data. Therefore, when a processor attempts to access a memory location, it first searches its cache to determine if it already has a copy of the information stored in that memory location. If the memory location is not currently stored in the processor cache, the processor attempts to obtain a copy of that memory location from the main memory. Alternatively, if the memory location is already available in the processor cache and is valid, the processor can immediately use the copy of data stored in the cache. Of course, conflict issues arise when multiple processors attempt to access the same memory location. 
     Regardless of the protocol used, the job of a cache coherency protocol is to make sure that if any caches in a system (especially cache between a processor and a memory or between system input and output in a memory and a processor) has exclusive use of the line, no other cache has a copy of the same exclusive line. Cache coherency protocol can be implemented by processors or by memory. 
     As previously described, when a processor requires access to a memory location, it first checks its processor cache to determine if the information is available locally. If the information is not present or is invalid, the processor must access the main memory to retrieve the data. This access methodology is not limited to processors. Other devices such as printers which require access to memory may also store data locally or access the main memory. Such a system and method is described, for example, in U.S. Pat. No. 5,829,042 entitled “Prefetch Operation for Network Peripheral Device Having Shared Memory” of Leung and incorporated herein by reference in its entirety. When devices use input/output (I/O) subsystems to access the memory, mismatches between device speed and I/O subsystem speeds can slow the device&#39;s access to the memory. The I/O subsystems often have longer memory latency times than that of processors in other devices and often have different access patterns. I/O subsystem transfers tend to be long bursts of data that are linear and sequential in fashion. Prefetch data techniques allow I/O subsystems to request information stored in memory prior to the device&#39;s need for that information. By prefetching data ahead of data consumption by the device, data can be continuously sent to the device without interruption thereby enhancing I/O system performance. For example, the amount of time necessary for I/O subsystems to access the memory is called the memory latency. To maximize the efficiency of the device the amount of information which is prefetched is determined by the speed of the device relative to the memory latency. Faster devices benefit from the larger amount of prefetched data (also called deeper prefetches) particularly when memory latency is high. Thus, the information required by the device is retrieved from memory before the device requires the information. This occurs because the prefetch operation requests the data before the device requests the data and allows more time for the memory information to be accessed and provide the data in anticipation of the upcoming request. 
     However, by their nature prefetch operations are speculative. In other words, since the device has not requested the specific memory location, a prefetch operation anticipates the data that the device will require before it has been requested. Typically a state machine is used to access the information requested by the device. This state machine is also used to prefetch data speculatively. Thus, the speculative prefetch activity consumes resources and time that could be used for fetching data that has been requested by the device. If the data which is speculatively accessed by the prefetched operation is not used, time has been wasted and overall input/output speed has been reduced. 
     Analogous to the processor&#39;s check for the processor cache for requested information, prefetches are checked for the availability of data once a device has requested the data. When the amount of data contained in the prefetch is increased, the overhead necessary to check the prefetch data for the availability of the requested data is increased. Accordingly, a need exists for a device and method of providing prefetch data that minimizes or eliminates the associated overhead required to check the information once a device has requested access to memory. Additionally, a need exists for a prefetch operation that does not compete or interfere with a device&#39;s request for access to a memory location. 
     SUMMARY OF THE INVENTION 
     These and other objects, features and technical advantages are achieved by a system and method which according to one aspect of the invention, a data processing system includes a memory storing data to be retrieved and an I/O controller configured to request data stored in the memory at a plurality of addresses. The I/O may be responsive to an internal or external device requesting such data. A fetch machine provides or initiates retrieval of data stored at the requested address, while a prefetch machine predicts future requests and keeps track of memory requests already initiated and queued. Thus, the prefetch machine is responsive to the plurality addresses to predict others of the addresses and provide or initiate retrieval of data stored thereat. To avoid prefetching information already requested and in a to fetch queue, the prefetch machine includes a memory storing a last one of the addresses subject to prefetching. Finally, to avoid conflicts between currently requested data and prefetch operation, an arbiter resolves memory accesses or data requests initiated by the fetch and prefetch machines. 
     According to a feature of the invention, the prefetch machine is responsive to addresses of data already supplied to inhibit an inclusion thereof in prefetching the data stored in the memory. 
     According to another aspect of the invention, a method of retrieving data from a memory is initiated upon receiving a first address of data to be retrieved. A fetch is initiated of data stored at the first address and a prefetch operation is initiated including predicting a first plurality of additional addresses corresponding to first data expected next to be requested. One or more of the predicted additional addresses are stored, for example the latest one of the addresses used to initiate a data request from memory. A prefetch is initiated of the first data expected next to be requested. The method accommodates a subsequent data request upon receiving a next address of data to be retrieved. If a prefetch of the next requested data has already been initiated, then this data is retrieved and provided to the requester. Otherwise, a new fetch is initiated when this second address is not included among the first data expected next to be addressed. The prefetch mechanism also adjusts to an erroneous prediction as indicated by the request not being within the range of data prefetched. Thus, the prefetch operations include selectively (i) continuing to initiate a prefetch of data stored at addresses predicted to follow the one of the additional addresses when the second address is included among the first data expected next to be requested, and (ii) predicting a revised second plurality of additional addresses corresponding to second data expected next to be requested when the second address is not included among the first data expected next. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
     FIG. 1 is a block diagram of a system bus/interconnect which establishes communications between CPUs, a memory controller, a host I/O bridge and a memory accessing device; 
     FIG. 2 is a block diagram of the interrelationship between the fetch machine, the prefetch machine and the DMA sequencer; 
     FIG. 3 depicts a relationship between fetch requests, fetch data, prefetch requester and cycles; 
     FIG. 4 depicts a relationship between fetches and prefetches when more than one I/O device is present. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates a system  100  in which a system bus  105  interconnects a number of CPUs  110  together. Also resident on the system bus  105  is a memory controller  115  and a host I/O bridge  120 . A device  125  which accesses memory is connected to the host I/O bridge  120  through links  130  and  135 . Device  125  may be part of system  100 , as shown, or may be external, such as peripheral. 
     When the device  125  requires access to memory, fetch subsystem  140  sends a message via link  130  to host I/O bridge  120 . Host I/O bridge  120  responds by forwarding the message across link  145  to the system bus  105 . The system bus  105  then communicates the request to memory controller  115 . In the absence of contentions with other devices for the memory location, memory controller  115  then accesses the appropriate cache line and sends the information across the system bus  105  across the link between the system bus interconnect and host I/O bridge  145 , through host I/O bridge  120 , across the link  130  to the fetch subsystem  140 . A copy of the information contained in the memory location is then stored in the fetch subsystem  140 . The device  125  can then accesses that information when necessary. 
     In a preferred embodiment of the present invention a prefetch subsystem  150  is also present in the device. When the device. 125  needs to access a specific location through the fetch subassembly  140  the prefetch subsystem  150  attempts to anticipate the next memory location that the device  125  will access. Accordingly, prefetch subsystem  150  sends a request to access memory via link  135  to the host I/O bridge  120 , which will forward the request via link  145  to the system bus  105 . From the system bus  105  the request will be passed to the memory controller  115  that will access the specific memory location required. The information stored in that memory location will be sent by the memory controller  115  via the system bus  105  and link  145  to host I/O bridge  120 . The information stored at the memory address will then be sent across link  130  to be stored in local memory. While the request for access to a memory location from the prefetch goes out across link  135 , the returning information which was contained at the memory location returns via link  130 . Numerous prefetch operations can be performed. 
     In a preferred embodiment of device  125  the fetch subassembly  140  has a higher priority than the prefetch subsystem  150 . This higher priority in the fetch subassembly  140  prevents the prefetch subassembly  150  from consuming too many system resources in anticipation of the memory requirements of device  125 . Since the fetch subassembly  140  has a higher priority, the actual memory requests coming from device  125  have a higher priority than the speculative memory accesses requested by prefetch subassembly  150 . 
     FIG. 2 illustrates system  200  as an alternative preferred embodiment of the present invention. The direct memory access or DMA sequencer  205  determines the memory address to be accessed. These memory access requesters can originate from multiple devices. The DMA sequencer  205  then sends this information to the fetch machine  210 . The fetch machine  210  sends this information both to the prefetch machine  215  and to the fetch first-in, first-out stack or FIFO  220 . The fetch FIFO  220  then accesses the memory to acquire the necessary data. Once the necessary data is accessed and retrieved, it is sent from memory to the fetch return  225  and then to the fetch machine  210 . The data is then stored for later access by the requesting device. The prefetch machine  215  then accesses additional memory locations via fetch FIFO  220  in anticipation of future memory requests. This prefetch requested data is also returned via fetch return  225  and fetch machine  210 . 
     FIG. 3 illustrates the coordination between the I/O card request, the fetch machine  210  and the prefetch machine  215 . In FIG. 3, the prefetch depth used is  3 , however any depth can be used. At cycle  1  the I/O card attempts to read memory location A 0 . In response to the I/O card&#39;s request to read memory location A 0  in cycle  1 , the fetch machine in cycle  2  fetches memory location A 0 . In response to fetch machine&#39;s request to fetch memory location A 0 , the prefetch machine at cycles  3 ,  4  and  5  sends control messages to prefetch memory locations A 1  A 2  and A 3 , respectively. Although the illustration in FIG. 3 shows a latency of four cycles, the latency can typically vary dynamically depending on various system factors. Assuming it takes four (4) system cycles to receive memory back after a fetch has been issued, A 0  data is available to the I/O card in cycle  6 . In cycle  7  the prefetch machine, in an attempt to stay four (4) steps ahead of the current I/O card request, requests prefetch of memory A 4 . In cycle  8  the information stored in memory location A 1  returns to the fetch machine and is available to the I/O card in cycle  9 . Since the data is currently available this saves four (4) cycles in the retrieval of information from memory location A 1 . Similarly, because of the prefetch&#39;s access to memory location A 2  in cycle  4 , the information stored in memory location A 2  is immediately available to the I/O card and connected device (not shown). 
     As previously described, the fetch machine has priority over the prefetch machine. Thus, in cycle  15  when the I/O card requests read memory location B 0 , the fetch machine&#39;s request for access to memory location B 0  at Step  16  overrides any access the prefetch machine is currently undertaking. In response to the fetch request at cycle  16 , the prefetch machine again attempts to anticipate the next block of information required by the I/O card. In this case, in cycle  17  the prefetch machine attempts to prefetch the information available in memory location B 1  and in cycle  18 , the prefetch machine predicts that the information contained in memory location B 2  will be necessary. If the prefetch machine is programmed for prefetch to a depth of  3 , in cycle  19  the prefetch machine accesses memory location B 3 , again in anticipation of the I/O card&#39;s need for that information. 
     Additionally, during Cycle  9  when the I/O card requests access to the information in memory, A 1  the prefetch machine responds by predicting that the information stored in memory location A 5  will also be required in the future and initiates retrieval of this additional information. Thus, the prefetch machine keeps track of the information that has already been requested and anticipates, based on the I/O card&#39;s current use of information, what additional information will be required. 
     FIG. 4 depicts a relationship between two I/O cards, the fetch return machine, the prefetch machine and the tag look-up. Ignoring for the moment the column labeled “I/O card No.  2 ”, in cycle  0  the I/O card No.  1  requests that memory location A 0  be read. In response to that request, in cycle  1 , the fetch return machine fetches memory location A 0 . Accordingly, also in cycle  1 , a tag lookup is set to lookup A 0 . In response to the fetch return machine&#39;s fetch of memory location A 0  at cycle  2 , the prefetch machine predicts the need for the information located in memory location A 1  and attempts to prefetch the information stored therein. In response to this prefetch operation, the tag lookup in cycle  2  is changed to lookup A 1 . The tag lookup is the check required to ensure that the data being fetched is not already present in the cache memory. 
     Now referring to the column labeled “I/O card No.  2 ” at cycle  2 , I/O card No.  2  requests the data stored at memory location  100 , which is read. At cycle  2  the I/O card No.  2  requests the data stored at memory location A 100  which it is also read. In response, at cycle  3 , the fetch return machine attempts to fetch the information stored at memory location A 100 . Simultaneously, the tag look up value is updated to look up A 100 . During cycle  4 , the fetch return machine receives the information stored in memory location A 0  and the prefetch machine attempts to prefetch the information located in memory location A 2 . Once the prefetch machine attempts to look up the information located in memory location A 2 , the lookup tag in cycle  4  is updated to lookup A 2 . In a preferred embodiment of the current invention, cycle  3 &#39;s fetch A 100 , resulting from the I/O card No.  2 &#39;s request to read memory location A 100 , takes priority over the prefetch machine&#39;s operation. Therefore, during cycle  3  the fetch machine&#39;s fetch of memory location A 100  precludes any operation from the prefetch machine during the same cycle. During cycle  4  the prefetch machine attempts to prefetch the information stored in memory location A 2  and the tag lookup is updated to lookup A 2 . Also in cycle  4  the information retained in memory location A 0  is returned to the fetch return machine. At cycle  5 , since the I/O card No.  1  requested A 0  data in cycle  0 , the A 0  data is returned to the I/O card. The prefetch machine in cycle  5  also attempts to anticipate the next set of data required by I/O card No.  2  by prefetching the information contained in memory location A 101 . In response, the tag lookup is adjusted to lookup A 101 . In cycle  6 , the information contained in memory location A 100  is returned to the fetch return machine and passed to the I/O card No.  2  in cycle  7 . In cycle  8 , in response to the prefetch operation which occurred in cycle  2  the A 1  data is returned to the fetch return machine and in cycle  9  that data is made available to I/O card No.  1 . 
     The use of separate fetch and prefetch machines in the present invention ensures that the prefetch process does not interfere with the fetch operations. The fetches prioritization over the prefetch process ensures that even when the two operations are queued, the time required for the fetch operation is not impacted by the presence of the prefetch process. The use of a register to track the last prefetch performed allows the prefetched data to be accessible sooner and ensures more efficient operation of the resources. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.