Patent Publication Number: US-6704820-B1

Title: Unified cache port consolidation

Description:
TECHNICAL FIELD 
     The invention relates to computer memory systems. More particularly, the invention relates to accessing cache memories. 
     BACKGROUND ART 
     In a computer system, the interface between a processor and memory is critically important to the performance of the system. Because fast memory is very expensive, memory in the amount needed to support a processor is generally much slower than the processor. In order to bridge the gap between fast processor cycle times and slow memory access times, cache memory is utilized. A cache is a small amount of very fast memory that is used to store a copy of frequently accessed data and instructions from main memory. A processor can operate out of this very fast memory and thereby reduce the number of wait states that must be interposed during memory accesses. When the processor requests data from memory and the data resides in the cache, then a cache read hit takes place, and the data from the memory access can be returned to the processor from the cache without incurring the latency penalty of accessing main memory. If the data is not in the cache, then a cache read miss takes place, and the memory request is forwarded to the main memory, as would normally be done if the cache did not exist. On a cache miss, the data that is retrieved from the main memory is provided to the processor and is also written into the cache due to the statistical likelihood that this data will be requested again by the processor in the near future. 
     The individual data elements stored in a cache memory are referred to as lines. Each line of a cache is meant to correspond to one addressable unit of data in the main memory. A cache line thus comprises data and is associated with a main memory address in some way. Schemes for associating a main memory address with a line of cache data include direct mapping, full association and set association, all of which are well known in the art. 
     The presence of a cache should be transparent to the overall system, and various protocols are implemented to achieve such transparency, including write-through and write-back protocols. In a write-through action, data to be stored is written to a cache line and to the main memory at the same time. In a write-back action, data to be stored is written to the cache and only written to the main memory later when the line in the cache needs to be displaced for a more recent line of data or when another processor requires the cached line. Because lines may be written to a cache exclusively in a write-back protocol, precautions must be taken to manage the status of data in a write-back cache so as to preserve coherency between the cache and the main memory. The preservation of cache coherency is especially challenging when there are several bus masters that can access memory independently. In such a case, well known techniques for maintaining cache coherency include snooping. 
     A cache may be designed independently of the microprocessor, in which case the cache is placed on the local bus of the microprocessor and interfaced between the processor and the system bus during the design of the computer system. However, as the density of transistors on a processor chip has increased, processors may be designed with one or more internal caches in order to decrease further memory access times. An internal cache is generally small, an exemplary size being 256Kb (262,144 bytes) in size. In computer systems that utilize processors with one or more internal caches, an external cache is often added to the system to further improve memory access time. The external cache is generally much larger than the internal cache(s), and, when used in conjunction with the internal cache(s), provides a greater overall hit rate than the internal cache(s) would provide alone. 
     In systems that incorporate multiple levels of caches, when the processor requests data from memory, the internal or first level cache is first checked to see if a copy of the data resides there. If so, then a first level cache hit occurs, and the first level cache provides the appropriate data to the processor. If a first level cache miss occurs, then the second level cache is then checked. If a second level cache hit occurs, then the data is provided from the second level cache to the processor. If a second level cache miss occurs, then the data is retrieved from main memory (or higher levels of caches, if present). Write operations are similar, with mixing and matching of the operations discussed above being possible. 
     Caches are also categorized on the basis of the type of information stored in their contents. For example, a data cache stores data (i.e., operands, integers, floating point values, packed representations and other formats of raw data). On the other hand, an instruction cache stores instructions (e.g., op codes or execution syllables with or without immediate data embedded in the instruction). If a single cache is utilized to store information of diverse types (e.g., data and instructions), then it is called a unified cache. A unified cache offers greater flexibility than one or more non-unified caches, in that the unified cache can flexibly store different types of information and therefore achieve a more efficient utilization of the valuable cache memory space. 
     A unified cache  100  is illustrated in FIG.  1 . The unified cache  100  comprises a memory array  105 , each element of which can store a unit of data or an instruction. The cache  100  also comprises a plurality of address ports. Each address port accepts an address bus. The address buses are shown alternately as data address buses DATA and instruction address buses INST. The width of each address bus is M bits. A given address uniquely identifies one cache line, or a subset of that one line, in the memory array  105 . A conflict resolution and address decoder bank  110  processes the addresses on the address buses DATA and INST. The conflict resolution processing is described in detail below. The address decoding processing for each address bus involves decoding the address and selectively asserting word lines that access the addressed word in the memory array  105 , such that the addressed word is connected to an I/O module  115 . The I/O module  115  comprises drivers and sense amplifiers to write and read the addressed memory word, respectively, as determined by cache control logic. One or more I/O buses, coupled to the I/O module  115 , accept or provide the addressed word(s). 
     Because a unified cache has a larger number of connections than a non-unified cache, a unified cache faces a substantially greater burden for resolving address conflicts. Address conflicts arise when two or more connections access the same memory cell at the same time. Resolving address conflicts in a rationale manner is important to avoid inconsistencies in the cache contents. For example, in FIG. 1, if the top address bus DATA accesses a particular cache line for writing, and the bottom instruction address bus INST attempts to read the same cache line, then it is important that the two operations proceed in the proper order. Otherwise, the wrong information would be read. To detect address conflicts, the conflict resolution and address decoder bank  110  contains logic that compares the address on each address bus to every other connection. Each comparison circuit requires M  2 -input exclusive OR (XOR) gates, if the address buses are M bits wide. The number of comparison circuits increases as the square of the number of address buses to the cache  100 . Specifically, N address buses to the cache  100  require “N choose 2” or (N 2 −N)/2 comparison circuits. Thus, a small increase in the number of address buses results in a significant increase in the necessary comparison circuits. For example, if there are four address buses to the cache  100 , then six comparison circuits are necessary; whereas, if there are eight address buses to the cache  100 , then  28  comparison circuits are necessary. 
     SUMMARY OF THE INVENTION 
     In one respect, the invention is an apparatus for using a plurality of address buses with a single port of a memory. The apparatus comprises a multiplexor and a logic circuit. The multiplexor is connected to the plurality of address buses. The multiplexor has a control input and a memory connection. The logic circuit produces an output signal tied to the control input. In another form, the apparatus comprises a means for selectively coupling a single one of the plurality of address buses to the memory, and a means for controlling the means for coupling. Preferably, the memory is a cache memory, and the plurality of address buses comprise a data address bus and an instruction address bus. 
     In another respect, the invention is a method for using a single memory address bus for a plurality of access types. The method accepts one or more memory addresses on one or more respective ones of a plurality of address buses. If there are memory addresses simultaneously active on two or more of the plurality of address buses, then the method selects one of the simultaneously active addresses and connects the selected address to the single memory access port. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a unified cache memory with separate address ports for data and instructions; 
     FIG. 2 is a block diagram of a unified cache memory with consolidated address ports for data and instructions, according to an embodiment of the invention; and 
     FIG. 3 is a block diagram of a port combination circuit and processor section, according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     FIG. 2 is a block diagram of a memory system  200 , according to an embodiment of the invention. The memory system  200  includes the cache  100 , which is a unified cache memory shown with its address port connections only, and port combination circuits. Each port combination circuit comprises a multiplexor  205  and an arbitration logic  210 . On the left side of each multiplexor  205 , as shown, are a data address bus DATA and an instruction address bus INST. On the right side of each multiplexor  205  is a memory connection. More specifically, the memory connection is a connection to an address port of the cache  100 . The multiplexor  205  selectively couples one of the data address bus DATA or the instruction address bus INST to the connection to the address port of the cache  100 . Each arbitration logic  210 produces one or more output signals, which are connected to a control input of a corresponding multiplexor  205 . In this way, each arbitration logic  210  controls the selection of the multiplexor  205  to which the arbitration logic  210  is connected. The arbitration logic  210  can operate according to any arbitration scheme, preferably giving preference to the instruction address bus INST whenever the an instruction address bus INST and an data address bus DATA both seek access to the cache  100 , because timely handling of instructions is generally more important to the overall performance of a computer. 
     Other inputs to the multiplexor are possible. For example, address buses for snoop operations and/or fill operations from a main system bus or higher level cache can be additional inputs to the multiplexors  205 . 
     In the memory system  200 , the number of address ports on the cache  100  is decreased, compared to the memory system  100  (FIG.  1 ). As a result, the cache  100  contains significantly less address conflict detection and resolution circuitry, which makes available more space, weight and power for valuable memory space. 
     FIG. 3 is a block diagram  300  of a port combination circuit and its surrounding circuitry in greater detail. In particular, latches  305  and  310  are included on the data address bus DATA and the instruction address bus INST, respectively. The data address bus DATA and the instruction address bus INST are connected to a section of a processor core  320 . More specifically, the data address bus DATA is connected to a general register file  325  of the processor core  320 , and the instruction address bus INST is connected to an instruction fetch engine  330  in the processor core  320 . The instruction fetch engine  330  supplies instructions to an execution pipeline  335 , which generally reads from, writes to and otherwise manipulates registers in the general register file  325  while executing instructions. 
     Various control signals are passed among the arbitration logic  210 , the latches  305  and  310 , the general register file  325 , the instruction fetch engine  330  and the execution pipeline  335 . Although a single line illustrates these connections, several physical lines may be involved. The execution pipeline  335  sends an enable signal to the latch  305  when the general register file transmits a data address to the latch  305 . The instruction fetch engine  330  sends an enable signal to the latch  310  when the instruction fetch engine  330  transmits an instruction address to the latch  310 . The arbitration logic  210  senses the enable signals and thereby knows when addresses are being supplied to the cache  100  from the processor core  320 . When the arbitration logic  210  suppresses one of the accesses, the arbitration logic notifies the appropriate source in the processor core  320 , so that the source can attempt the access at a later time. Most likely, the arbitration logic  210  suppresses a data access, in which case the arbitration logic  210  notifies the execution pipeline  325  of the suppression. Optionally, a latch (not shown) can be provided on the output of the multiplexor  205  for timing purposes. 
     Preferably, the arbitration logic  210  not only checks for address conflicts but also determines the type of operation to which the addresses pertains and checks for size conflicts. The type of operation is relevant because not all accesses to the same memory address are necessarily in conflict. For example, the same address can be read simultaneously by two observers, but the same address cannot be written by two sources simultaneously. Neither can the same address be reliably written and read simultaneously (unless perhaps special protocols for doing so are followed). The size of the piece of memory accessed is also relevant to conflicts, because the bandwidth of the I/O module  115  is limited. For example, assume that the width of the I/O bus coupled to the I/O module  115  is 128 bits. Then, if a read access to a first address returns a 96 bit result from the memory array  105  and a read access to a second address returns a 96 bit result form the memory array  105 , then both reads cannot occur simultaneously. The size of the memory piece being accessed can typically be determined on the basis of the operation (or op code) associated with the access (e.g., load vs. long load). 
     To perform op code compares and size compares, the individual arbitration logics  210  are connected and coordinated together. In one embodiment, the individual arbitration logics  210  are combined in a single arbitration logic block that jointly controls all of the several multiplexors  205 . Additional advantages can be realized in this way. First, size constraints among different data addresses or among different instruction addresses can be detected and arbitrated on a global basis. For instance, if the top data address bus DATA attempts a 96 bit write while the third address bus DATA attempts a 96 bit read, this size conflict can be detected and arbitrated. Second, unused address buses can be utilized more efficiently. Suppose for example that the top data address and the top instruction are in conflict but there is no access requested on the second multiplexor  205 . In this case, the arbitration logic  210  could request to the processor core  320  that either the data address or the instruction address be redirected to the second, unused multiplexor  205 . The benefit of this redirection is that it avoids stalling the processor core  320 . 
     Whatever arbitration functions are not performed by the individual arbitration logics  210  outside the cache  100  can be performed inside the cache  100  by traditional means. If all arbitration functions are performed outside of the cache  100 , then the cache  100  is internally simplified. An important advantage of reduced address conflict checking cost is realized so long as instruction and data accesses are merged outside the cache  100 . The arbitration logics  210  need only manage this merge to realize this advantage. The extent to which other conflict checking is shifted from the cache  100  to the arbitration logics  210  is a matter that one skilled in the art can balance in any particular situation. 
     The terms and descriptions used herein are set forth by way of illustration only and are not meant as limitations. Those skilled in the art will recognize that many variations are possible within the spirit and scope of the invention as defined in the following claims, and their equivalents, in which all terms and phrases are to be understood in their broadest reasonable sense.