A DATA ADDRESS PREDICTION STRUCTURE AND A METHOD FOR OPERATING THE SAME

Abstract: A data prediction structure for a superscalar microprocessor is provided. The data prediction structure predicts a data address that a group of instructions is going to access while that group of instructions is being fetched from the instruction cache. The data bytes associated with the predicted address are placed in a relatively small, fast buffer. The decode stages of instruction processing pipelines in the microprocessor access the buffer with addresses generated from the instructions, and if the associated data bytes are found in the buffer they are conveyed to the reservation station associated with the requesting decode stage. Therefore, the implicit memory read associated with an instruction is performed prior to the instruction arriving in a functional unit. The functional unit is occupied by the instruction for a fewer number of clock cycles, since it need not perform the implicit memory operation. Instead, the functional unit performs the explicit operation indicated by the instruction.

Description: TITLE: A DATA ADDRESS PREDICTION STRUCTURE AND A METHOD FOR OPERATING THE SAME BACKGROUND OF THE INVENTION field of the Invention This invention relates to the field of superscalar microprocessors and, more particularly, to data prediction mechanisms in superscalar microprocessors 2 Description of the Relevant Art Superscalar microprocessors achieve high performance by simultaneously executing multiple instructions in a clock cycle and by specifying the shortest possible clock cycle consistent with the design As used herein the term "clock cycle" refers to an interval of time during which the pipeline stages of a microprocessor preform their intended functions For example, superscalar microprocessors are typically configured with multiple instruction processing pipelines which process instructions The processing of instructions includes the actions of fetching dispatching, decoding executing and writing back results Each action may be implemented in one or more pipeline stages and an instruction flows through each of the pipeline stages where an action or portion of an action is performed At the end of a clock cycle, the instruction and the values resulting from performing the action of the current pipeline stage are moved to the next pipeline stage When an instruction reaches the end of an instruction processing pipeline, it is processed and the results of executing the instruction have been recorded Since superscalar microprocessors execute multiple instructions per clock cycle in their multiple instruction processing pipelines and the clock cycle is short, a high bandwidth memory system is required to provide instructions and data to the superscalar microprocessor (i e a memory system that can provide a large number of bytes in a short period of time) Without a high bandwidth memory system, the microprocessor would spend a large number of clock cycles waiting for instructions or data to be provided, then would execute the received instructions and/or instructions dependent upon the received data m a relatively small number of clock cycles Overall performance would be degraded by the large number of idle clock cycles Superscalar microprocessors are, however, ordinarily configured into computer systems with a relatively large mam memory composed of dynamic random access memory (DRAM) cells DRAM cells are characterized by access times which are significantly longer than the clock cycle of modern superscalar microprocessors Also, DRAM cells typically provide a relatively narrow output bus to convey the stored bytes to the superscalar microprocessor Therefore DRAM cells provide a memory system that provides a relativelv small number of bytes in a relatively long period of time, and do not form a high bandwidth memoπ system Because superscalar microprocessors are typically not configured into a computer system with a memory system having sufficient bandwidth to continuously provide instructions and data, superscalar microprocessors are often configured with caches Caches include multiple blocks of storage locations configured on the same silicon substrate as the microprocessor or coupled nearby The blocks of storage locations are used to hold previously fetched instruction or data bytes The bytes can be transferred from the cache to the destination (a register or an instruction processing pipeline) quickly, commonly one or two clock cycles are required as opposed to a large number of clock cycles to transfer bytes from a DRAM main memory Unfortunately, the latency of one or two clock cycles within a data cache is becoming a performance problem for superscalar microprocessors as they attempt to execute more instructions per clock cycle The problem is of particular importance to superscalar microprocessors configured to execute instructions from a complex instruction set architecture such as the x86 architecture As will be appreciated by one skilled in the art, x86 instructions allow for one of their "operands" (the values that the instruction operates on) to be stored in a memory location Instructions which have an operand stored in memory are said to have an implicit (memory read) operation and an explicit operation which is defined by the particular instruction being executed (i e an add instruction has an explicit operation of addition) Such an instruction therefore requires an implicit address calculation for the memory read to retrieve the operand, the implicit memory read, and the execution of the explicit operation of the instruction (for example, an addition) Typical superscalar microprocessors have required that these operations be performed by the execute stage of the instruction processing pipeline The execute stage of the instruction processing pipeline is therefore occupied for several clock cycles when executing such an instruction For example, a superscalar microprocessor might require one clock cycle for the address calculation, one to two clock cycles for the data cache access, and one clock cycle for the execution of the explicit operation of the instruction Conversel} instructions with operands stored in registers configured within the microprocessor retrieve the operands before they enter the execute stage of the instruction processing pipeline, since no address calculation is needed to locate the operands Instructions with operands stored in registers would therefore only require the one clock cycle for execution of the explicit operation A structure allowing the retrieval of memory operands for instructions before they enter the execute stage is therefore desired SUMMARY OF THE INVENTION The problems outlined above are in large part solved by a data prediction structure for a superscalar microprocessor in accordance with the present invention The data prediction structure predicts a data address that a group of instructions is going to access while that group of instructions is being fetched from the instruction cache The data bytes associated with the predicted address are placed in a relatively small, fast buffer Decode stages of instruction processing pipelines in the microprocessor access the buffer with addresses generated from the instructions If the associated data bytes are found in the buffer, the data bytes are conveyed to the reservation station associated with the requesting decode stage Therefore, an implicit memory read operation associated with an instruction is performed prior to the instruction arriving in a functional unit (which forms the execute stage of the instruction processing pipeline) The functional unit is occupied by the instruction for a fewer number of clock cycles since it need not perform the implicit memory operation The functional unit performs the explicit operation indicated by the instruction Performance may be advantageously increased by using the clock cycles that would have been used to execute the implicit operation in the functional unit to execute other instructions Broadly speaking, the present invention contemplates a method for predicting a data address which will be referenced by a plurality of instructions residing in a basic block when the basic block is fetched The method comprises several steps First, a data prediction address is generated The data prediction address is then fetched from a data cache into a data buffer The data buffer is later accessed for load data The present invention further contemplates a data address prediction structure comprising an array for storing data prediction addresses, a data buffer for storing data bytes associated with predicted data addresses, and a decode stage of an instruction processing pipeline configured to access the data buffer The data prediction addresses are stored into a particular storage location within the array according to the address of a branch instruction which begins a basic block containing an instruction which generates the data prediction address BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings in which Figure 1 is a block diagram of a superscalar microprocessor Figure 2 is a diagram depicting a branch prediction unit, a load/store unit, and a data cache of the superscalar microprocessor of Figure 1. along with several elements of one embodiment of the present invention Figure 3 A is a diagram of the branch prediction unit shown in Figure 2 depicting several elements of one embodiment of the present invention Figure 3B is a diagram of a decode unit of the superscalar microprocessor shown in Figure 1 showing several elements of one embodiment of the present invention Figure 4 is a diagram of the information stored in a storage location of a branch prediction array in accordance with the present invention While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims DETAILED DESCRIPTION OF THE INVENTION Turning now to Figure 1. a block diagram of a superscalar microprocessor 200 including decode units 208 and a branch prediction unit 220 in accordance with the present invention is shown As illustrated in the embodiment of Figure 1 , superscalar microprocessor 200 includes a prefetch/predecode unit 202 and a branch prediction unit 220 coupled to an instruction cache 204 Instruction alignment unit 206 is coupled between instruction cache 204 and a plurality of decode units 208A-208F (referred to collectively as decode units 208) Each decode unit 208 A-208F is coupled to respective reservation station units 21 OA-21 OF (referred to collectively as reservation stations 210), and each reservation station 210A-210r is coupled to a respective functional unit 212A-212F (referred to collectively as functional units 212) Decode units 208, reservation stations 210, and functional units 212 are further coupled to a reorder buffer 216, a register file 218 and a load/store unit 222 A data cache 224 is finally shown coupled to load/store unit 222, and an MROM unit 209 is shown coupled to instruction alignment unit 206 Generally speaking, instruction cache 204 is a high speed cache memory provided to temporarily store instructions prior to their dispatch to decode units 208 In one embodiment, instruction cache 204 is configured to cache up to 32 kilobytes of instruction code organized in lines of 16 bytes each (where each byte consists of 8 bits) During operation, instruction code is provided to instruction cache 204 by prefetching code from a mam memory (not shown) through prefetch/predecode unit 202 It is noted that instruction cache 204 could be implemented in a set-associative a full -associative or a direct-mapped configuration Prefetch/predecode unit 202 is provided to prefetch instruction code from the mam memory for storage within instruction cache 204 In one embodiment, prefetch/predecode unit 202 is configured to burst 64-bit wide code from the main memory into instruction cache 204 It is understood that a variety of specific code prefetching techniques and algorithms may be employed by prefetch/predecode unit 202 As prefetch/predecode unit 202 fetches instructions from the main memory, it generates three predecode bits associated with each byte of instruction code a start bit, an end bit, and a "functional" bit The predecode bits form tags indicative of the boundaries of each instruction The predecode tags may also convey additional information such as whether a given instruction can be decoded directly by decode units 208 or whether the instruction must be executed by invoking a microcode procedure controlled by MROM unit 209, as will be described in greater detail below Table 1 indicates one encoding of the predecode tags As indicated within the table, if a given byte is the first byte of an instruction, the start bit for that byte is set If the byte is the last byte of an instruction, the end bit for that byte is set If a particular instruction cannot be directly decoded by the decode units 208. the functional bit associated with the first byte of the instruction is set On the other hand, if the instruction can be directly decoded by the decode units 208, the functional bit associated with the first byte of the instruction is cleared The functional bit for the second byte of a particular instruction is cleared if the opcode is the first byte, and is set if the opcode is the second byte It is noted that in situations where the opcode is the second byte, the first byte is a prefix byte The functional bit values for instruction byte numbers 3-8 indicate whether the byte is a MODRM or an SIB byte or whether the byte contains displacement or immediate data Table 1 Encoding of Start. End and Functional Bits Instr Start End Functional Byte Bit Bit Bit Number Value Value Value Meaning 1 1 X 0 Fast decode 1 1 X 1 MROM instr 2 0 X 0 Opcode is first byte 2 0 X 1 Opcode is this byte, first byte is prefix 3-8 0 X 0 Mod R/M or SIB byte 3-8 0 X 1 Displacement or immediate data, the second functional bit set in bytes 3-8 indicates immediate data 1-8 X 0 X Not last byte of instruction 1 -8 X 1 X Last byte of instruction As stated previously, in one embodiment certain instructions within the x86 instruction set may be directly decoded by decode units 208 These instructions are referred to as "fast path" instructions The remaining instructions of the x86 instruction set are referred to as "MROM instructions" MROM instructions are executed by invoking MROM unit 209 More specifically, when an MROM instruction is encountered, MROM unit 209 parses and serializes the instruction into a subset of defined fast path instructions to effectuate a desired operation A listing of exemplary x86 instructions categorized as fast path instructions as well as a description of the manner of handling both fast path and MROM instructions will be provided further below Instruction alignment unit 206 is provided to channel variable byte length instructions from instruction cache 204 to fixed issue positions formed by decode units 208A-208F Instruction alignment unit 206 independently and parallel selects instructions from three groups of instruction bytes provided by instruction cache 204 and arranges these bytes into three groups of preliminary issue positions Each group of issue positions is associated with one of the three groups of instruction bytes The preliminary issue positions are then merged together to form the final issue positions each of which is coupled to one of decode units 208 Before proceeding with a detailed description of the data prediction structure embodied in decode units 208 and branch prediction unit 220, general aspects regarding other subsystems employed within the exemplary superscalar microprocessor 200 of Figure 1 will be described For the embodiment of Figure 1 each of the decode units 208 includes decoding circuitry for decoding the predetermined fast path instructions referred to above In addition, each decode unit 208A-208F routes displacement and immediate data to a corresponding reservation station unit 2 I 0A-210F Output signals from the decode units 208 include bit-encoded execution instructions for the functional units 212 as well as operand address information, immediate data and/or displacement data The superscalar microprocessor of Figure 1 supports out of order execution, and thus employs reorder buffer 216 to keep track of the original program sequence for register read and write operations, to implement register renaming, to allow for speculative instruction execution and branch misprediction recovery, and to facilitate precise exceptions As will be appreciated by those of skill in the art, a temporary storage location within reorder buffer 216 is reserved upon decode of an instruction that involves the update of a register to thereby store speculative register states Reorder buffer 216 may be implemented in a first-in- first-out configuration wherein speculative results move to the "bottom' of the buffer as they are validated and written to the register file, thus making room for new entries at the 'top' of the buffer Other specific configurations of reorder buffer 216 are also possible, as will be described further below If a branch prediction is incorrect, the results of speculatively-executed instructions along the mispredicted path can be invalidated in the buffer before they are written to register file 218 The bit-encoded execution instructions and immediate data provided at the outputs of decode units 208 A-208F are routed directly to respective reservation station units 21 OA-21 OF In one embodiment, each reservation station unit 21 OA-21 OF is capable of holding instruction information (l e , bit encoded execution bits as well as operand values, operand tags and/or immediate data) for up to three pending instructions awaiting issue to the corresponding functional unit It is noted that for the embodiment of Figure 1 , each decode unit 208A-208F is associated with a dedicated reservation station unit 21 OA-21 OF, and that each reservation station unit 21 OA-210F is similarly associated with a dedicated functional unit 212A-212F Accordingly, six dedicated "issue positions" are formed by decode units 208, reservation station units 210 and functional units 212 Instructions aligned and dispatched to issue position 0 through decode unit 208A are passed to reservation station unit 210A and subsequently to functional unit 212A for execution Similarly, instructions aligned and dispatched to decode unit 208B are passed to reservation station unit 21 OB and into functional unit 212B, and so on Upon decode of a particular instruction, if a required operand is a register location, register address information is routed to reorder buffer 216 and register file 218 simultaneously Those of skill in the art will appreciate that the x86 register file includes eight 32 bit real registers (I e , typically referred to as EAX, EBX, ECX, EDX, EBP, ESI, EDI and ESP) Reorder buffer 216 contains temporary storage locations for results which change the contents of these registers to thereby allow out of order execution A temporary storage location of reorder buffer 216 is reserved for each instruction which, upon decode, is determined to modify the contents of one of the real registers Therefore, at various points during execution of a particular program, reorder buffer 216 may have one or more locations which contain the speculatively executed contents of a given register If following decode of a given instruction it is determined that reorder buffer 216 has a previous location or locations assigned to a register used as an operand in the given instruction, the reorder buffer 216 forwards to the corresponding reservation station either 1 ) the value in the most recently assigned location, or 2) a tag for the most recently assigned location if the value has not yet been produced by the functional unit that will eventually execute the previous instruction If the reorder buffer has a location reserved for a given register, the operand value (or tag) is provided from reorder buffer 216 rather than from register file 218 If there is no location reserved for a required register in reorder buffer 216. the value is taken directly from register file 218 If the operand corresponds to a memory location, the operand value is provided to the reservation station unit through load/store unit 222 Details regarding suitable reorder buffer implementations may be found within the publication "Superscalar Microprocessor Design" by Mike Johnson, Prentice-Hall, Englewood Cliffs, New Jersey, 1991 , and within the co-pending, commonly assigned patent application entitled "High Performance Superscalar Microprocessor", Serial No 08/146,382. filed October 29, 1993 by Witt, et al These documents are incorporated herein by reference in their entirety Reservation station units 21 OA-21 OF are provided to temporarily store instruction information to be speculatively executed by the corresponding functional units 212A-212F As stated previously, each reservation station unit 210A-210F may store instruction information for up to three pending instructions Each of the six reservation stations 21 OA-21 OF contain locations to store bit-encoded execution instructions to be speculatively executed by the corresponding functional unit and the values of operands If a particular operand is not available, a tag for that operand is provided from reorder buffer 216 and is stored within the corresponding reservation station until the result has been generated (i e , by completion of the execution of a previous instruction) It is noted that when an instruction is executed by one of the functional units 212A- 212F, the result of that instruction is passed directly to any reservation station units 21 OA-21 OF that are waiting for that result at the same time the result is passed to update reorder buffer 216 (this technique is commonly referred to as "result forwarding") Instructions are issued to functional units for execution after the values of any required operand(s) are made available That is. if an operand associated with a pending instruction within one of the reservation station units 21 OA-21 OF has been tagged with a location of a previous result value within reorder buffer 216 which corresponds to an instruction which modifies the required operand, the instruction is not issued to the corresponding functional unit 212 until the operand result for the previous instruction has been obtained. Accordingly, the order in which instructions are executed may not be the same as the order of the original program instruction sequence. Reorder buffer 216 ensures that data coherency is maintained in situations where read-after-write dependencies occur. In one embodiment, each of the functional units 212 is configured to perform integer arithmetic operations of addition and subtraction, as well as shifts, rotates, logical operations, and branch operations. It is noted that a floating point unit (not shown) may also be employed to accommodate floating point operations. Each of the functional units 212 also provides information regarding the execution of conditional branch instructions to the branch prediction unit 220. If a branch prediction was incorrect, branch prediction unit 220 flushes instructions subsequent to the mispredicted branch that have entered the instruction processing pipeline, and causes prefetch/predecode unit 202 to fetch the required instructions from instruction cache 204 or main memory. It is noted that in such situations, results of instructions in the original program sequence which occur after the mispredicted branch instruction are discarded, including those which were speculatively executed and temporarily stored in load/store unit 222 and reorder buffer 216. Exemplary configurations of suitable branch prediction mechanisms are well known. Results produced by functional units 212 are sent to the reorder buffer 216 if a register value is being updated, and to the load/store unit 222 if the contents of a memory location is changed. If the result is to be stored in a register, the reorder buffer 216 stores the result in the location reserved for the value of the register when the instruction was decoded. As stated previously, results are also broadcast to reservation station units 21 OA-21 OF where pending instructions may be waiting for the results of previous instruction executions to obtain the required operand values. Generally speaking, load/store unit 222 provides an interface between functional units 212A-212F and data cache 224. In one embodiment, load/store unit 222 is configured with a load/store buffer with eight storage locations for data and address information for pending loads or stores. Decode units 208 arbitrate for access to the load/store unit 222. When the buffer is full, a decode unit must wait until the load/store unit 222 has room for the pending load or store request information. The load/store unit 222 also performs dependency checking for load instructions against pending store instructions to ensure that data coherency is maintained. Data cache 224 is a high speed cache memory provided to temporarily store data being transferred between load/store unit 222 and the main memory subsystem. In one embodiment, data cache 224 has a capacity of storing up to eight kilobytes of data. It is understood that data cache 224 may be implemented in a variety of specific memory configurations, including a set associative configuration. Turning now to Figure 2, several elements of one embodiment of a data prediction structure are shown Branch prediction unit 220 is included, along with load/store unit 222 and data cache 224 Branch prediction unit 220 is connected to a data prediction bus 253 which is coupled to an arbitration multiplexor 254 Also coupled to arbitration multiplexor 254 is a second request bus 255 and an arbitration select line 256 In this embodiment, signals on both second request bus 255 and arbitration select line 256 originate in load/store unit 222 The output bus of arbitration multiplexor 254 is coupled to an input port of data cache 224 A first request bus 257 is coupled between load/store unit 222 and an input port of data cache 224 Data cache 224 is configured with two output ports which are coupled to a first reply bus 258 and a second reply bus 259 Both first reply bus 258 and second reply bus 259 are coupled to load/store unit 222, and second reply bus 259 is coupled to a data buffer 260 Associated with data buffer 260 is a data buffer control unit 263 Data buffer 260 is also coupled to a plurality of decode address request buses 261 provided by decode units 208 Associated with the plurality of decode address request buses 261 is a plurality of data buses 262 Generally speaking, data buffer 260 is a relatively small, high speed cache provided to store data bytes associated with data prediction addresses provided by branch prediction unit 220 During a clock cycle in which branch prediction unit 220 predicts a taken branch instruction it also produces a data prediction address on data prediction bus 253 The data prediction address is a prediction of the data addresses that are used by instructions residing at the target address of the predicted branch instruction During clock cycles in which the data prediction address is selected by arbitration multiplexor 254, the data prediction address accesses data cache 224 The data cache line of data bytes associated with the data prediction address is transferred on second reply bus 259 to a write port on data buffer 260 Data buffer 260 stores the data bytes along with the data prediction address under the direction of control unit 263 Decode units 208 in a later clock cycle, may request data bytes associated with an implicit memory read operation of an instruction using the plurality of decode address request buses 261 The requested data bytes are provided to the respective reservation station 210 associated with a decode unit 208 which requested the data bytes Because data buffer 260 is relatively small, it is capable of providing data within the same clock cycle that an address request is made (as opposed to requiring the entire cycle or more, as a typical data cache does) Therefore, the implicit memory read associated with an instruction may be performed before the instruction enters a functional unit 212, and only the explicit operation of the instruction is executed in functional unit 212 Performance may be advantageously increased by allowing other instructions to execute duπng cycles that such an implicit memory read operation would have occupied functional unit 212 As stated previously, branch unit 220 produces a data prediction address during clock cycles in which a taken branch is predicted Instructions residing at the target address of the taken branch instruction are therefore in a new "basic block" Basic blocks are blocks of instructions having the property that if the first instruction in the block is executed, then each subsequent instruction in the block is executed Basic blocks end with a branch instruction One property often associated with the instructions in a basic block is "data locality" The property of data locality exists when the instructions of a basic block access data memory locations which are physically near each other (perhaps in the same data cache line) Therefore, if a data cache line that a basic block will access is fetched simultaneously with the instructions that reside in the basic block, the data cache latency can be endured before the instructions are prepared to access the data bytes Then, when the instructions access the data buffer the latency is shorter Data may be accessed in an amount of time similar to a register data access from register file 218 of Figure 1 When a data prediction address misprediction is detected (as described below with respect to Figures 3A and 3B), the branch instruction address associated with the data prediction address and the correct data prediction address are conveyed to branch prediction unit 220 Branch prediction unit 220 stores the correct prediction address for use in a subsequent prediction for the basic block The data prediction address is conveyed on data prediction bus 253 to arbitration multiplexor 254, which arbitrates between the data prediction address and a second request from load/store unit 222 The term arbitration, as used herein, means the selection of one request over another according to an arbitration scheme In one embodiment, the arbitration scheme is a priority scheme in which a load/store request conveyed on second request bus 255 is a higher priority that the data prediction address request Therefore, the arbitration select signal conveyed on arbitration select line 256 is an indication that a load/store request is being conveyed on second request bus 255 If a valid load/store request is being made during a clock cycle, arbitration multiplexor 254 selects the load/store request to be conveyed to data cache 224 If no valid load/store request is being made during a clock cycle, arbitration multiplexor 254 selects the data prediction address to be conveyed to data cache 224 In one embodiment, load/store unit 222 is configured to make up to two requests to data cache 224 in a clock cycle In clock cycles where load/store unit 222 is making one request, first request bus 257 is used to allow the data prediction address the maximum opportunity to access data cache 224 If a data prediction address request is valid during a clock cycle in which load/store unit 222 is making no requests or one request to data cache 224, then the data prediction address request will be given access to data cache 224 Load/store unit 222 receives the data bytes associated with requests on first request bus 257 and second request bus 255 on first reply bus 258 and second reply bus 259, respectively It is noted that other embodiments of load/store unit 222 may have different numbers of request buses for data cache 224 Second reply bus 259 is also coupled to a write port on data buffer 260 Data buffer control unit 263 detects cycles when the data prediction address is selected by arbitration multiplexor 254 and hits in data cache 224, and causes data buffer 260 to store the data conveyed on second reply bus 259 In one embodiment, the entire cache line associated with the data prediction address is conveyed on second reply bus 259 to data buffer 260 In this embodiment, data buffer 260 is configured with eight storage locations each of which store a data prediction address and the associated data bytes Data buffer control unit 263 operates the storage locations of this embodiment as a first-in, first-out stack It is noted that other replacement policies could be alternatively used Data buffer 260 is also configured with a number of read ports, one for each decode unit 208 Each of the plurality of decode address request buses 261 is coupled to a respective read port Decode units 208 each provide a request address on one of the plurality of decode address request buses 261 Each request address is compared to the addresses stored in data buffer 260 If a match is detected, data bytes associated with the address are conveyed on the associated one of the plurality of data buses 262 to the reservation station unit 21 OA-21 OF associated with the requesting decode unit 208A-208F The associated reservation station 21 OA-21 OF stores the data bytes in the same manner that it stores data bytes from a register If the data bytes for the memory operand of an instruction are stored with the instruction in a reservation station 210A-210F, then the associated functional unit 212A-212F does not perform the implicit memory read for that instruction Instead, it executes the explicit operation of the instruction If the data bytes of the memory operand are not stored with the instruction in a reservation station 21 OA-21 OF, then the implicit address calculation and memory read operation are performed by the associated functional unit 212A-212F prior to performing the explicit operation of the instruction Data buffer control unit 263 is further configured to monitor first request bus 257 and second request bus 255 for store memory accesses When data buffer control unit 263 detects a store memory access, it invalidates the associated cache line in data buffer 260 (if the cache line is currently stored in data buffer 260) Similarly, if data cache 224 replaces a line with data fetched from mam memory, the replaced line is invalidated in data buffer 260 Reorder buffer 216 implements dependency checking for reads that follow writes in cases where the reads receive their data from data buffer 260 before the write occurs to data cache 224 In this manner, read-after-write hazards and data coherency are correctly handled with respect to data buffer 260 Turning now to Figure 3 A, an embodiment of branch prediction unit 220 is shown The data prediction addresses are stored within a branch prediction array 270 along with branch prediction information In this embodiment, branch prediction array 270 is a linear array of storage locations The current fetch address is used to index into branch prediction array 270 and select a storage location, and the data stored within that storage location is used to form the aforementioned branch prediction It is noted that any branch prediction scheme may be implemented by branch prediction unit 220 Also stored within the selected storage location is a data prediction address The data prediction address is conveyed on data prediction bus 253 except during clock cycles in which an updated data prediction address associated with the current fetch address is stored in a data prediction holding register 271 Data prediction holding register 271 stores a data prediction address update until the next taken branch or branch misprediction is detected, and then the update is stored into branch prediction array 270 During the clock cycles between storing the update in data prediction holding register 271 and the subsequent update into branch prediction array 270, the instruction address associated with the update is compared to the current fetch PC If a match is found, then the data prediction address is selected from data prediction holding register 271 by output multiplexor 272 If a match is not found, then the data prediction address from branch prediction array 270 is selected as the output of multiplexor 272 In one embodiment, branch prediction unit 220 receives three sets of inputs from decode units 208 First, branch prediction unit 220 receives the plurality of decode address request buses 261 which convey the actual data addresses generated by decode units 208 as well as a valid indicator for each address indicating that the address is valid If a decode unit 208A-208F detects a mispredicted data prediction address, the associated one of decode address request buses 261 will convey the corrected address (as will be explained below with respect to Figure 3B) Each decode unit 208A-208F also provides one of a plurality of mispredicted lines 273 indicating whether or not the actual data address conveyed on the associated one of decode address request buses 261 matches the data prediction address provided with the instruction currently being decoded Finally, branch prediction unit 220 receives a plurality of branch address buses 274 from decode units 208 conveying branch instruction addresses associated with the data prediction addresses The branch address is used to index into branch prediction array 270 in order to store the updated data prediction address Decode address request buses 261 , mispredicted lines 273, and branch address buses 274 are received by a prediction validation and correction block 275 within branch prediction unit 220 If one of mispredicted lines 273 signals an incorrect prediction and the corresponding address valid bit from the associated one of decode address request buses 261 indicates the actual data address conveyed on that associated decode address request bus is valid, then prediction validation and correction block 275 causes the corresponding actual data address to be stored in data prediction holding register 271 as the new prediction address The corresponding branch instruction address is also stored in data prediction holding register 271 as the index for updating branch prediction array 270 In this manner a data prediction address is corrected if predicted incorrectly Additionally, a data prediction address is created if a valid data prediction address was not previously stored in branch prediction array 270 A particular one of mispredicted lines 273 indicates a misprediction if the data prediction address associated with the instruction currently being decoded is invalid Therefore, the address provided on the associated one of decode request address buses 261 is stored as the data prediction address If a particular one of mispredicted lines 273 signals a correct prediction and the corresponding address valid bit conveyed on the corresponding one of decode address request buses 261 is set. then prediction validation and correction block 275 causes the previous prediction to be left unchanged and nothing is stored in data holding register 271 For mispredicted lines and address valid bits in a given clock cycle, decode unit 208A's outputs are given highest priority, then decode unit 208B's outputs, etc In this manner, the first data address calculated by the instructions in a basic block is used to validate and create the data prediction addresses Once a data prediction address is validated or changed, the associated branch instruction address is saved by prediction validation and correction block 275 Subsequent signals are ignored by prediction validation and correction unit 275 until a branch address different from the saved branch instruction address is conveyed from a decode unit 208 to branch prediction unit 220 Turning now to Figure 3B, an embodiment of decode unit 208A is depicted Decode units 208B- 208F are configured similar to decode unit 208A The logic circuits shown in Figure 3B are exemplary circuits for implementing the data prediction structure Other circuits (not shown) decode x86 instructions into a format suitable for functional units 212 Therefore, decode units 208 are each a decode stage of an instruction processing pipeline In order to implement one embodiment of the data prediction structure, several logic elements are added to decode unit 208 A an adder circuit 300, a prediction address register 301. a comparator circuit 302, an address valid logic circuit 303, and a branch address register 304 Adder circuit 300 is used to add the contents of a base register and a displacement value to produce an actual data address if the instruction being decoded by decode unit 208A has a memory operand In one embodiment, the base register is the EBP or ESP register, and the displacement value is conveyed with the instruction bytes as part of the instruction This type of addressing is a common addressing mode used for x86 instructions If the base register is not available, or the instruction does not have an operand residing in a memory location, or the addressing mode is different then a base register and displacement value, then validation logic circuit 303 indicates that the actual data address formed by adder circuit 300 is invalid Reservation station unit 210A ignores any data bytes that may be transferred from data buffer 260 in clock cycles where the address formed by adder circuit 300 is invalid The output of address valid circuit 303 is conveyed with the address, as indicated in Figure 3B If the address formed by adder circuit 300 is invalid and the instruction requires a memory operand, the implicit load operation associated with the instruction will be executed by functional unit 212A The actual data address formed by adder circuit 300 is conveyed on decode address request bus 261 A to data buffer 260 and branch prediction unit 220 Decode address request bus 261 A is one of the plurality of decode address request buses 261 If the actual data address is a hit in data buffer 260, the associated data bytes are returned to reservation station 210A on one of the plurality of data buses 262 The actual data address is also conveyed to comparator circuit 302 which is coupled to prediction address register 301 Prediction address register 301 stores the data prediction address for the instruction currently being decoded The data prediction address is transferred to decode unit 208A with the instruction Also transferred is a valid bit indicative of whether or not a data address prediction was made for the block associated with the instructions (stored in prediction address register 301 ) and the instruction address of the branch instruction associated with the data prediction address (stored in branch address register 304) If both the data prediction address and the actual data address are valid and compare equal, then comparator circuit 302 conveys a value indicating that the data address prediction is correct to branch prediction unit 220 If either address is invalid or the addresses do not compare equal, then the output of comparator circuit 302 conveys a value indicating that the data prediction address is incorrect to branch prediction unit 220 The value is conveyed on a mispredicted line 273A Mispredicted line 273A is one of the plurality of mispredicted lines 273 The branch instruction address stored in branch address register 304 is conveyed on a branch address bus 274A Branch address bus 274A is one of the plurality of branch address buses 274 Turning now to Figure 4, a diagram of the fields within a storage location of branch prediction array 270 is shown A branch prediction address field 400 is included for storing a branch prediction address associated with a fetch address which indexes to the storage location In one embodiment, 12 bits of the address are stored The 12 bits that are stored are the index to instruction cache 204 that contains the target branch instructions Associated with the branch prediction address is a branch prediction information field 401 In one embodiment, the branch prediction information field includes counters associated with the branch prediction mechanism, the way value of the indexed line within instruction cache 204 which contains the branch target instructions, the byte position within the current line that contains the branch instruction that generated the associated prediction address, and a valid bit indicating whether or not the branch prediction address is valid Another field of information stored in the storage locations of branch prediction array 270 is the data prediction address field 402 The data prediction address is stored in this field Associated with the data prediction address is a data prediction address valid bit 403 which indicates whether or not the data prediction address is valid In another embodiment, multiple branch prediction address fields and branch prediction information fields are stored within each storage location of branch prediction array 270 In yet another embodiment multiple data prediction addresses are stored, along with a selection mechanism to determine which data address to select One particular embodiment stores two data prediction addresses and a two bit counter to select between the two The most significant bit of the counter selects the data prediction address to be used for a particular prediction If the most significant bit is set, then the second data prediction address is selected If the most significant bit is cleared, then the first data prediction address is selected The counter increments each time the second data prediction address is selected and is a correct prediction and increments each time the first data prediction prediction is selected and is an incorrect prediction The counter decrements each time the first data prediction address is selected and is a correct prediction and decrements each time the second data prediction address is selected and is an incorrect prediction In another embodiment of the data address prediction structure, data prediction address bus 253 is connected to an additional read port on data cache 224 In this embodiment, arbitration multiplexor 254 may be eliminated It is noted that, although the preceding discussion described an embodiment of the data prediction structure within a processor implementing the x86 architecture, any microprocessor architecture could benefit from the data prediction structure In accordance with the above disclosure, a data address prediction structure is described which allows an implicit memory operation of an instruction to be performed before the instruction reaches a functional unit Therefore, the number of cycles required in a functional unit to execute the instruction is advantageously reduced Performance may be increased by implementing such a data prediction structure Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated It is intended that the following claims be interpreted to embrace all such variations and modifications

Claims: WHAT IS CLAIMED IS: 1 A method for predicting a data address which will be referenced by a plurality of instructions residing in a basic block when said basic block is fetched, comprising generating a data prediction address, fetching said data prediction address from a data cache into a data buffer, and accessing said data buffer for load data 2 The method as recited in claim 1 further comprising arbitrating said data prediction address with a load/store address from a load/store unit for access to said data cache prior to said accessing 3 The method as recited in claim 2 wherein said arbitrating selects said load/store address for access to said data cache during a clock cycle in which said load/store address is valid 4 The method as recited in claim 2 wherein said arbitrating selects said data prediction address for access to said data cache during a clock cycle in which said load/store address is not valid 5 The method as recited in claim 1 further comprising updating a branch prediction array with a new data prediction address during a clock cycle in which said data prediction address is found to be mispredicted 6 The method as recited in claim 5 wherein said data prediction address is found to be mispredicted by comparing said data prediction address to a decode address generated by a decode stage of an instruction processing pipeline 7 The method as recited in claim 5 wherein said updating is delayed until a next basic block is fetched 8 The method as recited in claim 5 wherein said updating further comprises storing said new data prediction address into said branch prediction array in a storage location wherein said storage location previously stored said data prediction address 9 The method as recited in claim 1 further comprising generating an actual data address from a base register and a displacement data wherein said base register and said displacement data are specified by one of said plurality of instructions 10 The method as recited in claim 9 further comprising comparing said actual data address to said data prediction address 1 1. The method as recited in claim 10 further comprising transferring said data address and a result of said comparing to a branch prediction unit. 12. The method as recited in claim 9 wherein said accessing said data buffer further comprises determining if a plurality of bytes associated with said actual data address are stored within said data buffer. 13. The method as recited in claim 12 further comprising transferring said plurality of bytes to a decode stage of an instruction processing pipeline. 14. The method as recited in claim 1 wherein said generating a data prediction address is performed during a clock cycle in which said basic block is being fetched from an instruction cache 15. The method as recited in claim 1 wherein said accessing is performed from a decode stage of an instruction processing pipeline. 16. Λ data address prediction structure comprising: an array including a plurality of storage locations for storing a plurality of data prediction addresses wherein a particular one of said plurality of storage locations in which a particular one of said plurality of data prediction addresses is stored is determined by the instruction address of a branch instruction which begins a basic block containing an instruction which generates said data prediction address, a data buffer for storing a plurality of bytes associated with a data prediction address; and a decode stage of an instruction processing pipeline configured to access said data buffer. 17. The data address prediction structure as recited in claim 16 wherein said array is a linear array of said storage locations. 18. The data address prediction structure as recited in claim 16 wherein said buffer is further configured to simultaneously store a plurality of pluralities of data bytes wherein each one of said plurality of pluralities of data bytes is associated with a different data prediction address 19. The data address prediction structure as recited in claim 16 further comprising a data cache coupled to said data buffer and further coupled to said array, wherein said data prediction address is used to fetch data from said data cache into said data buffer. 20. The data address prediction structure as recited in claim 16 wherein said decode stage includes an adder circuit for producing a data address form a base register and a displacement value. 21. The data prediction structure as recited in claim 20 wherein said decode stage further includes a comparator circuit and a register wherein said comparator circuit is coupled to said adder circuit and said register. 22. The data prediction structure as recited in claim 21 wherein said register is configured to store said data prediction address. 23. The data prediction structure as recited in claim 22 wherein said comparator circuit is configured to produce a value indicative of the correctness of said data prediction address. 24. The data prediction structure as recited in claim 23 wherein said comparator circuit is configured to convey said value to said array.

Inventors: TRAN THANG M, TRAN, THANG, M.
Applicants: ADVANCED MICRO DEVICES INC, ADVANCED MICRO DEVICES, INC.
Application year: 1996
Main classification: G06F9
IPC codes: G06F9