INSTRUCTION PREDECODE AND MULTIPLE INSTRUCTION DECODE

Abstract: Variable-length instructions are prepared for simultaneous decoding and execution of a plurality of instructions in parallel by predecoding each byte of an instruction, assuming each byte to be an opcode byte since the actual opcode byte is not identified. The predecoding operation associates an instruction length to each instruction byte. The instruction length is found for some instructions by reading a single instruction byte. For other instructions require more information to determine the instruction length and two or three instruction bytes are read. Based on the instruction length determination, instructions are classified into a group of instructions in which multiple instructions are decoded in parallel and a group of instructions in which multiple instructions are not decoded in parallel. Predecode information including a designation of instruction length and a designation of classification group is stored for each instruction byte. The instruction bytes and associated predecode information are applied to a decoder that includes a plurality of first group instruction decoders for decoding a plurality of parallel-decodable instructions in parallel and a second group instruction decoder for decoding instructions that are not decodable in parallel.

Description: INSTRUCTION PREDECODE AND MULTIPLE INSTRUCTION DECODE TECHNICAL FIELD This invention relates to processors More specifically this invention relates to processors having an instruction decoder for decoding multiple instructions simultaneously and a predecoder for preparing instructions for rapid decoding BACKGROUND ART Advanced microprocessors, such as P6-class x86 processors are defined by a common set of features These features include a superscalar architecture and performance, decoding of multiple x86 instructions per cycle and conversion of the multiple x86 instructions into RISC-like operations The RISC-like operations are executed out-of-order in a RISC-type core that is decoupled from decoding These advanced microprocessors support large instruction windows for reordering instructions and for reordering memory references In a microprocessor having a superscalar architecture, multiple computer instructions are transferred from a memory . decoded and executed simultaneously so that the execution of instructions is accelerated A RISC-type processor typically has no problem decoding multiple instructions per cycle because all instructions are the same length When all instructions are the same length, multiple instructions are easily transferred from memory in parallel because the memory locations of all instructions is known or readily ascertainable, allowing multiple instructions to be simultaneously transferred to instruction registers inside the processor However, many microprocessors and computers, such as x86 processors that have a complex instruction set computer (CISC) type architecture, use a CISC-type variable-length instruction set in which different instructions have different instruction lengths varying from one byte (8 bits) to fifteen bytes (120 bits) in length Thus the locating of multiple instructions is a sequential process, and therefore slow process, because one instruction must be decoded to determine the length of the instruction before the starting location of the next decodable instruction is ascertained Decoding of multiple x86 instructions in parallel has additional difficulties For example, the actual length of various instructions depends not only on the first byte of the instruction, generally the opcode byte which defines the operation of the instruction, but also depends on additional bytes as a complicated function of the opcode and other instruction fields The minimum-size instruction in the x86 architecture is a single byte, usually containing an 8-bit opcode X86 instruction opcode fields range in length from 8 bits to 16 bits long For some instructions, a byte containing the opcode field (the "opcode" byte) also contains a register field X86 operations include register- to-register, register-to-memory, or memory-to-register (but not memory-to-memory) operations An optional modrm field followmg the opcode field contams the register specifier and specifies memory addressing of the instruction In some instructions, the modrm field, employed in a slightly different format, selects condition flags Some instructions contain an immediate data field that ranges from 0 to 4 bytes m length Some instruction fields specify whether or not other fields are present For example, modrm and sib fields indicate whether a displacement (DISP) field is present and the length of the DISP field The DISP field is further modified by a prefix byte that changes the address size of an instruction The modrm field has various possible formats The first byte of the modrm field always contains a modrm specifier of register and addressing mode. The modrm field also contains an optional displacement or offset field for address computation. The 8-bit sib field specifies the operative manner of address computation. Length of the displacement and immediate fields depends on the data-width mode of the instruction. Various instructions operate on 8-bit, 16-bit, or 32-bit data. Data width is determined primarily by a segment descriptor in the memory management architecture, but the default for the segment can be overridden by a bit in the instruction (a w-bit) or by a prefix byte which toggles the effect of the w-bit. Prefix bytes can appear before any instruction. A prefix byte changes the interpretation of the instruction. A prefix byte, for example, changes the memory address or operand size of the instruction, changes the default segment used in memory addressing, or indicates that the instruction should be executed with the external bus locked. In some instructions more than one prefix is included before an instruction. Each type of prefix byte is independent of the others, so that the maximum instruction length is 15 bytes for non-redundant prefixes. What is needed is a system and method for accessing a plurality of variable-length instructions and preparing the variable-length instructions for decoding of multiple instructions in parallel. One aspect of this system and method is a technique for determining the length of the variable-length instructions in parallel. DISCLOSURE OF THE INVENTION In accordance with the present invention, variable-length instructions are prepared for simultaneous decoding and execution of a plurality of instructions in parallel by predecoding each byte of an instruction, assuming each byte to be an opcode byte since the actual opcode byte is not identified. The predecoding operation associates an instruction length to each instruction byte. The instruction length is found for some instructions by reading a single instruction byte. For other instructions require more information to determine the instruction length and two or three instruction bytes are read. Based on die instruction length determination, instructions are classified into a group of instructions in which multiple instructions are decoded in parallel and a group of instructions in which multiple instructions are not decoded in parallel. Predecode information including a designation of instruction length and a designation of classification group is stored for each instruction byte. The instruction bytes and associated predecode information are applied to a decoder that includes a plurality of first group instruction decoders for decoding a plurality of parallel-decodable instructions in parallel and a second group instruction decoder for decoding instructions that are not decodable in parallel. Predecode information is useful for quickly identifying the next instructions to be decoded, facilitating the decoding of multiple instructions per cycle. Predecode information also determines whether an instruction is decodable in parallel with other instructions and aids quick calculation of the length of instructions that are decodable in parallel to determine whether a sufficient number of bytes of the instruction are available in an instruction register for a full instruction to be decoded. Predecode information includes a HASMODRM bit that is useful for determining whether predecode bits are valid for an instruction. In accordance with one aspect of the present invention, an apparatus and method determines and stores in a memory, for many instruction bytes, the location of a following instruction byte assuming that each of the instruction bytes is the first byte of an instruction The instruction bytes are subsequently transferred from the memory and a first byte of an actual instruction is located, followed by rapid determination of the location of first bytes of subsequent actual mstructions A plurality of subsequent instructions are identified, allowmg simultaneous loading of instructions into several instruction registers for simultaneously decodmg and executmg mstructions Many advantages are achieved using the described system and method Multiple mstructions are advantageously decoded per cycle in a processor architecture that resists decodmg in parallel An inherently serial decodmg operation is advantageously converted to a parallel operation Instructions that can execute in parallel and mstructions that cannot are recognized prior to decodmg so that decoding in parallel is advantageously accomplished only for those instructions that would benefit from decoding in parallel Predecode information is advantageously derived and stored as a combination of instruction length and program counter information so that data pointers are easily determined and instructions easily accessed It is advantageous that the predecode bits designate instruction location in the form of a list and that, in some embodiments, the list controller is implemented to avoid time-consuming calculations, thus allowing for fast traversal of a list while reducing storage space consumed to store each pointer It is advantageous that the predecode bits are compactly stored and easily expanded for usage to reduce the size of memory storage BRIEF DESCRIPTION OF THE DRAWINGS The features of the invention believed to be novel are specifically set forth in the appended claims However, the invention itself, both as to its structure and method of operation, may best be understood by referring to the followmg descπption and accompanying drawings Figure 1 is a block diagram which illustrates a computer system in accordance with one embodiment of the present invention Figure 2 is a block diagram illustrating one embodiment of processor for usage m the computer system shown in Figure 1 Figure 3 is a timing diagram which illustrates pipeline timing for an embodiment of the processor shown in Figure 2 Figure 4 is a schematic block diagram showing an embodiment of an instruction decoder used in the processor shown in Figure 2 Figure 5 is a block diagram of a memory including sixteen addressable memory locations holding sixteen mstruction bytes that make up, for example, three variable length instructions in combination with predecode bits indicative of following-instruction location information Figure 6 is a flow chart illustrating a method for efficiently storing a pointer and rapidly identifying the address of a next element in a list of a plurality of elements in accordance with one embodiment of the present invention Figure 7 is a pictorial diagram illustrating a 32-word memorv storage, such as an mstruction cache, for storing a plurality of memory elements or units and pointers in which the elements of the list are ordered in increasing sequential address locations Figure 8 is a block diagram of a memory including sixteen addressable memory locations holding sixteen mstruction byte values that make up. for example three variable length instruction codes, in combination with predecode bit codes indicative of following-instruction location information Figure 9 is a flow chart illustrating a method of preparing computer instructions for rapid decoding in accordance with an embodiment of the present invention Figure 10 is a schematic block diagram showing a circuit for preparing computer mstructions for rapid decodmg in accordance with the present invention Figure 11 is a schematic circuit diagram showing an embodiment of a circuit for preparing computer mstructions for rapid decoding m accordance with an embodiment of the present invention Figure 12 is a schematic pictorial diagram of an addressable memory location containing an mstruction code byte and a code for the location of the next mstruction assuming that the mstruction code byte is the startmg byte of an mstruction Figure 13 is a block diagram of a personal computer incorporating a processor having an mstruction decoder including emulation us g indirect specifiers in accordance with an embodiment of the present invention MODF.fS)FOR CARRYING OUTTHEINVENTION Referring to Figure 1, a computer system 100 is used in a vaπetv of applications, including a personal computer application The computer system 100 includes a computer motherboard 110 containing a processor 120 in accordance with an embodiment of the invention Processor 120 is a monolithic integrated circuit which executes a complex mstruction set so that the processor 120 may be termed a complex mstruction set computer (CISC) Examples of complex mstruction sets are the x86 mstruction sets implemented on the well known 8086 family of microprocessors The processor 120 is connected to a level 2 (L2) cache 122, a memory controller 124 and local bus controllers 126 and 128 The memory controller 124 is connected to a mam memory 130 so that the memory controller 124 forms an interface between the processor 120 and the main memory 130. The local bus controllers 126 and 128 are connected to buses including a PCI bus 132 and an ISA bus 134 so that the local bus controllers 126 and 128 form interfaces between the PCI bus 132 and the ISA bus 134. Referring to Figure 2, a block diagram of an embodiment of processor 120 is shown. The core of the processor 120 is a RISC superscalar processing engine. Common x86 instructions are converted by instruction decode hardware to operations in an internal RISC86 instruction set. Other x86 instructions, exception processing, and other miscellaneous functionality is implemented as RISC86 operation sequences stored in on-chip ROM. Processor 120 has interfaces including a system interface 210 and an L2 cache control logic 212. The system interface 210 connects the processor 120 to other blocks of the computer system 100. The processor 120 accesses the address space of the computer system 100, including the main memory 130 and devices on local buses 132 and 134 by read and write accesses via the system interface 210. The L2 cache control logic 212 forms an interface between an external cache, such as the L2 cache 122. and the processor 120. Specifically, the L2 cache control logic 212 interfaces the L2 cache 122 and to an instruction cache 214 and a data cache 216 in the processor 120. The instruction cache 214 and the data cache 216 are level 1 (LI ) caches which are connected through the L2 cache 122 to the address space of the computer system 100. The instruction cache 214 is a 16 KByte instruction cache with associated control logic. The instruction cache 214 is organized as a two-way set associative cache having of line size of 32 Bytes. The instruction cache 214 has a total physical storage of 24 Kbytes for storing 16 Kbytes of instruction and 8 Kbytes of predecode information. This physical storage is configured as three units, including two instruction units and one predecode information unit, of a 512 x 128 cache RAM structure. The instruction cache 214 is logically dual-ported including a read port and a write port, although the cache has only a single physical port. Instructions from main memory 130 are loaded into instruction cache 214 via a predecoder 270 for anticipated execution. As the instruction cache 214 is filled with instructions, the predecoder 270 predecodes the instruction bytes, generating predecode information that is also stored in the instruction cache 214. Data in the instruction cache 214 are subsequently transferred to begin decoding the instructions, using the predecode information to facilitate simultaneous decoding of multiple x86 instructions. A fundamental impediment to efficient decoding of x86 instructions is the variable-length nature of these instructions in which x86 instruction length varies in a range from one word to fifteen words. The predecode information facilitates decoding of variable-length instructions by specifying the length of each x86 instruction, thereby designating the instruction boundaries for typically a plurality of x86 instructions. Once the instruction boundaries are known, multiple instruction decoding in parallel is substantially simplified. The predecoder 270 generates predecode bits that are stored in combination with instruction bits in the instruction cache 214. The predecode bits, for example 3 bits, are fetched along with an associated instruction byte (8 bits) and used to facilitate multiple instruction decoding and reduce decode time. Instruction bytes are loaded into instruction cache 214 thirty-two bytes at a time in a burst transfer of four eight-byte quantities. Logic of the predecoder 270 is replicated sixteen times so that predecode bits for all sixteen instruction bytes are calculated simultaneously immediately before being written into the instruction cache 214. Instructions in instruction cache 214 are CISC instructions, referred to as macroinstructions. An instruction decoder 220 converts CISC instructions from instruction cache 214 into operations of a reduced instruction set computing (RISC) architecture instruction set for execution on an execution engine 222. A single macroinstruction from instruction cache 214 decodes into one or multiple operations for execution engine 222. Instruction decoder 220 has interface connections to the instruction cache 214 and an instruction fetch control circuit 218 (shown in Figure 4). Instruction decoder 220 includes a macroinstruction decoder 230 for decoding most x86 instructions, an instruction decoder emulation circuit 231 including an emulation ROM 232 for decoding instructions including complex instructions, and a branch unit 234 for branch prediction and handling. Several types of macroinstructions are defined relating to the general type of operations into which the macroinstructions are converted. The general types of operations are register operations (RegOps), load-store operations (LdStOps), load immediate value operations (LIMMOps), special operations (SpecOps) and floating point operations (FpOps). Execution engine 222 has a scheduler 260 and six execution units including a load unit 240, a store unit 242, a first register unit 244, a second register unit 246. a floating point unit 248 and a multimedia unit 250. The scheduler 260 distributes operations to appropriate execution units and the execution units operate in parallel. Each execution unit executes a particular type of operation. In particular, the load unit 240 and the store unit 242 respectively load (read) data or store (write) data to the data cache 216 (LI data cache), the L2 cache 122 and the main memory 130 while executing a load store operation (LdStOp). A store queue 262 temporarily stores data from store unit 242 so that store unit 242 and load unit 240 operate in parallel without conflicting accesses to data cache 216. Register units 244 and 246 execute register operations (RegOps) for accessing a register file 290. Floating point unit 248 executes floating point operations (FpOps). Multimedia unit 250 executes arithmetic operations for multimedia applications. Scheduler 260 is partitioned into a plurality of, for example, 24 entries where each entry contains storage and logic. The 24 entries are grouped into six groups of four entries, called Op quads. Information in the storage of an entry describes an operation for execution, whether or not the execution is pending or completed. The scheduler monitors the entries and dispatches information from the entries to information-designated execution units. Referring to Figure 3, processor 120 employs five and six stage basic pipeline timing. Instruction decoder 220 decodes two instructions in a single clock cycle. During a first stage 310, a fetch cycle stage, the instruction fetch control circuit 218 fetches CISC instructions into mstruction cache 214. Predecoding of the CISC instructions during stage 310 reduces subsequent decode time. During a second stage 320, a decode cycle stage, instruction decoder 220 decodes instructions from instruction cache 214 and loads an Op quad into scheduler 260. During a third stage 330, an issue cycle stage, scheduler 260 scans the entries and issues operations to corresponding execution units 240 to 252 if an operation for the respective types of execution units is available. Operands for the operations issued during stage 330 are forwarded to the execution units in a fourth stage 340 For a RegOp, the operation generally completes in the next clock cycle which is stage 350, but LdStOps require more time for address calculation 352, data access and transfer of the results 362 The decode cycle stage 320 includes two half cycles In a first decode half cycle, the mstruction cache 214 is accessed, the D-bit is checked to determine whether the predecode instruction length bits are to be adjusted for D- bit status, and the predecode bits are converted from an indication of length into a pointer In a second decode half cycle, it is determined whether the instruction cache 214 or a branch target buffer (BTB) is the source of mstructions for decodmg, and it is further determined whether this mstruction source is to provide the mstructions or if an mstruction buffer is to source the instructions Also in the a second decode half cycle, the instruction bytes and predecode information are presented to rotators, data is rotated b\ the rotators to find the startmg mstruction byte, additional bytes of the first mstruction and the predecode information associated with the startmg mstruction byte A predecode mdex is used to rotate the mstruction byte data to find a second mstruction byte of an mstruction and additional bytes of the second mstruction Specifically, low-order bits of a program counter control a first rotator to rotate a first mstruction Subsequent rotators are controlled to rotate subsequent mstructions under control of the predecode information of the unmediately precedmg mstruction Also in the a second decode half cycle, output mstruction data are rotated to determine various other fields within an instruction register, any OF prefix is stripped from an instruction, the mstruction and control bits are registered, and instruction buffers are updated and the updated data is presented to rotators For branch operations, mstruction decoder 220 performs a branch prediction 324 during an initial decodmg of a branch operation A branch unit 252 evaluates conditions for the branch at a later stage 364 to determine whether the branch prediction 324 was correct A t o level branch prediction algorithm predicts a direction of conditional branching, and fetching CISC instructions in stage 310 and decoding the CISC instructions m stage 320 contmues m the predicted branch direction Scheduler 260 determines when all condition codes required for branch evaluation are valid, and directs the branch unit 252 to evaluate the branch mstruction If a branch was incorrectly predicted, operations in the scheduler 260 which should not be executed are flushed and decoder 220 begms loadmg new Op quads from the correct address after the branch. A tune penalty is incurred as mstructions for the correct branching are fetched. Instruction decoder 220 either reads a previously-stored predicted address or calculates an address us g a set of parallel adders If a previously-predicted address is stored, the predicted address is fetched in stage 326 and mstructions located at the predicted address are fetched in stage 328 without a delay for adders Otherwise, parallel adders calculate the predicted address In branch evaluation stage 364, branch unit 252 determines whether the predicted branch direction is correct If a predicted branch is correct, the fetchmg. decodmg, and instruction-executing steps continue without interruption For an incorrect prediction, scheduler 260 is flushed and mstruction decoder 220 begms decodmg macroinstructions from the correct program counter subsequent to the branch. Referr g to Figure 4, a schematic block diagram illustrates an embodiment of an mstruction preparation circuit 400 which is connected to the mam memory 130 The mstruction preparation circuit 400 mcludes the mstruction cache 214 that is connected to the mam memory 130 via the predecoder 270 The mstruction decoder 220 is connected to receive instruction bytes and predecode bits from three alternative sources, the mstruction cache 214, a branch target buffer (BTB) 456 and an instruction buffer 408 The mstruction bytes and predecode bits are supplied to the mstruction decoder 220 through a plurality of rotators 430, 432 and 434 via mstruction registers 450, 452 and 454 The macroinstruction decoder 230 has mput connections to the mstruction cache 214 and mstruction fetch control circuit 218 for receiving mstruction bytes and associated predecode information The macroinstruction decoder 230 buffers fetched mstruction bytes in an instruction buffer 408 connected to the mstruction fetch control circuit 218 The mstruction buffer 408 is a sixteen bvte buffer which receives and buffers up to 16 bytes or four aligned words from the mstruction cache 214 loading as much data as allowed by the amount of free space in the instruction buffer 408 The instruction buffer 408 holds the next instruction bytes to be decoded and contmuously reloads with new instruction bvtes as old ones are processed bv the macroinstruction decoder 230 Instructions in both the mstruction cache 214 and the mstruction buffer 408 are held in 'extended" bytes, containing both memory bits (8) and predecode bits (5), and are held the same alignment The predecode bits assist the macroinstruction decoder 230 to perform multiple mstruction decodes within a single clock cycle Instruction bytes addressed using a decode program counter (PC) 420, 422, or 424 are transferred from the mstruction buffer 408 to the macroinstruction decoder 230 The instruction buffer 408 is accessed on a byte basis by decoders in the macroinstruction decoder 230 However on each decode cycle, the mstruction buffer 408 is managed on a word basis for tracking which of the bytes in the mstruction buffer 408 are valid and which are to be reloaded with new bytes from the mstruction cache 214 The designation of whether an mstruction byte is valid is maintained as the mstruction byte is decoded For an invalid mstruction byte, decoder invalidation logic (not shown), which is connected to the macroinstruction decoder 230, sets a "byte invalid" signal Control of updating of the current fetch PC 426 is synchronized closely with the validity of mstruction bytes in the mstruction buffer 408 and die consumption of the instruction bytes by the instruction decoder 220 The macroinstruction decoder 230 receives up to sixteen bvtes or four aligned words of mstruction bytes fetched from the mstruction fetch control circuit 218 at the end of a fetch cycle Instruction bytes from the mstruction cache 214 are loaded to a 16-byte mstruction buffer 408 The instruction buffer 408 buffers mstruction bytes, plus predecode information associated with each of the mstruction bytes, as the mstruction bytes are fetched and/or decoded The mstruction buffer 408 receives as many mstruction bytes as can be accommodated by the mstruction buffer 408 free space, holds the next mstruction bvtes to be decoded and continually reloads with new mstruction bytes as previous mstruction bytes are transferred to individual decoders within the macroinstruction decoder 230 The mstruction predecoder 270 adds predecode information bits to the mstruction bytes as the mstruction bytes are transferred to the mstruction cache 214 Therefore, the mstruction bytes stored and transferred by the mstruction cache 214 are called extended bytes Each extended byte mcludes eight memory bits plus five predecode bits The five predecode bits include three bits that encode mstruction length, one D-bit that designates whether the instruction length is D-bit dependent, and a HasModRM bit that indicates whether an instruction code includes a modrm field. The thirteen bits are stored in the instruction buffer 408 and passed on to the macroinstruction decoder 230 decoders. The instruction buffer 408 expands each set of five predecode bits into six predecode bits. Predecode bits enable the decoders to quickly perform multiple instruction decodes within one clock cycle. The instruction buffer 408 receives instruction bytes from the instruction cache 214 in the memory-aligned word basis of instruction cache 214 storage so that instructions are loaded and replaced with word granularity. Thus, the instruction buffer 408 byte location 0 always holds bytes that are addressed in memory at an address of 0 (mod 16). Instruction bytes are transferred from the instruction buffer 408 to the macroinstruction decoder 230 with byte granularity. During each decode cycle, the sixteen extended instruction bytes within the instruction buffer 408 , including associated implicit word valid bits, are transferred to the plurality of decoders within the macroinstruction decoder 230. This method of transferring instruction bytes from the instruction cache 214 to the macroinstruction decoder 230 via the instruction buffer 408 is repeated with each decode cycle as long as instructions are sequentially decoded. When a control transfer occurs, for example due to a taken branch operation, the instruction buffer 408 is flushed and the method is restarted. The current decode PC has an arbitrary byte alignment in that the instruction buffer 408 has a capacity of sixteen bytes but is managed on a four-byte word basis in which all four bytes of a word are consumed before removal and replacement or the word with four new bytes in the instruction buffer 408. An instruction has a length of one to eleven bytes and multiple bytes are decoded so that the alignment of an instruction in the instruction buffer 408 is arbitrary. As instruction bytes are transferred from the instruction buffer 408 to the macroinstruction decoder 230, the instruction buffer 408 is reloaded from the instruction cache 214. Instruction bytes are stored in the instruction buffer 408 with memory alignment rather than a sequential byte alignment that is suitable for application of consecutive instruction bytes to the macroinstruction decoder 230. Therefore, a set of byte rotators 430, 432 and 434 are interposed between the instruction buffer 408 and each of the decoders of the macroinstruction decoder 230. Four instruction decoders, including three short decoders SDecO 410, SDecl 412 or SDec2 414, and one combined long and vectoring decoder 418, share the byte rotators 430, 432 and 434. In particular, the short decoder SDecO 410 and the combined long and vectoring decoder 418 share byte rotator 430. Short decoder SDecl 412 is associated with byte rotator 432 and short decoder SDec2 414 is associated with byte rotator 434. A plurality of pipeline registers, specifically instruction registers 450, 452 and 454, are interposed between the byte rotators 430, 432 and 434 and the instruction decoder 220 to temporarily hold the instruction bytes, predecode bits and other information, thereby shortening the decode timing cycle. The other information held in the instruction registers 450, 452 and 454 mcludes various mformation for assisting mstruction decodmg, mcludmg prefix (e g OF) status, immediate size (8-bit or 32-bit), displacement and long decodable length designations Although a circuit is shown utilizing three rotators and three short decoders, in other embodiments, different numbers of circuit elements may be employed For example, one circuit mcludes two rotators and two short decoders Instructions are stored m memory alignment, not instruction alignment, in the instruction cache 214, the branch target buffer (BTB) 456 and the instruction buffer 408 so that the location of the first mstruction byte is not known The byte rotators 430, 432 and 434 find the first byte of an instruction The macroinstruction decoder 230 also performs various instruction decode and exception decode operations, mcludmg validation of decode operations and selection between different types of decode operations Functions performed during decode operations clude prefix byte handling, support for vectoring to the emulation code ROM 232 for emulation of instructions and for branch unit 234 operations, branch unit interfacing and return address prediction Based on the mstruction bvtes and associated information, the macroinstruction decoder 230 generates operation mformation in groups of four operations corresponding to Op quads The macroinstruction decoder 230 also generates mstruction vectoπng control mformation and emulation code control mformation The macroinstruction decoder 230 also has output connections to the scheduler 260 and to the emulation ROM 232 for outputting the Op quad mformation. mstruction vectoring control mformation and emulation code control mformation The macroinstruction decoder 230 does not decode instructions when the scheduler 260 is unable to accept Op quads or is acceptmg Op quads from emulation code ROM 232 The macroinstruction decoder 230 has five distinct and separate decoders, including three "short" decoders SDecO 410, SDecl 412 and SDec2 414 that function in combination to decode up to three "short" decode operations of mstructions that are defined within a subset of simple instructions of the x86 mstruction set Generall> a simple mstruction is an mstruction that translates to fewer than three operations The short decoders SDecO 410, SDecl 412 and SDec2 414 each typically generate one or two operations, although zero operations are generated in certain cases such as prefix decodes Accord gly for three short decode operations, from two to six operations are generated m one decode cycle The two to six operations from the three short decoders are subsequently packed together by operation pack g logic 438 into an Op quad smce a maximum of four of the six operations are valid Specifically, the three short decoders SDecO 410, SDecl 412 and SDec2 414 each attempt to decode two operations, potentially generatmg six operations Only four operations may be produced at one time so that if more than four operations are produced, the operations from the short decoder SDec2 414 are mvalidated The five decoders also mclude a single "long" decoder 416 and a single "vectoring" decoder 418 The long decoder 416 decodes mstructions or forms of mstructions havmg a more complex address mode form so mat more than two operations are generated and short decode handling is not available The vectormg decoder 418 handles mstructions that cannot be handled by operation of the short decoders SDecO 410, SDecl 412 and SDec2 414 or by the long decoder 416 The vectormg decoder 418 does not actually decode an instruction but rather vectors to a location of emulation ROM 232 for emulation of the instruction. Various exception conditions that are detected by the macroinstruction decoder 230 are also handled as a special form of vectoring decode operation. When activated, the long decoder 416 and the vectoring decoder 418 each generates a full Op quad. An Op quad generated by short decoders SDecO 410, SDecl 412 and SDec2 414 has the same format as an Op quad generated by the long and vectoring decoders 416 and 418. The short decoder and long decoder Op quads do not include an OpSeq field. The macroinstruction decoder 230 selects either the Op quad generated by the short decoders 410, 412 and 414 or the Op quad generated by the long decoder 416 or vectoring decoder 418 as an Op quad result of the macroinstruction decoder 230 are each decode cycle. Short decoder operation, long decoder operation and vectoring decoder operation function in parallel and independently of one another, although the results of only one decoder are used at one time. Each of the short decoders 410, 412 and 414 decodes up to seven instruction bytes, assuming the first byte to be an operation code (opcode) byte and the instruction to be a short decode instruction. Two operations (Ops) are generated with corresponding valid bits. Appropriate values for effective address size, effective data size, the current x86-standard B-bit, and any override operand segment register are supplied for the generation of operations dependent on these parameters. The logical address of the next "sequential" instruction to be decoded is supplied for use in generating the operations for a CALL instruction. Note that the word sequential is placed in quotation marks to indicate that, although the "sequential" address generally points to an instruction which immediately precedes the present instruction, the "sequential" address may be set to any addressed location. The current branch prediction is supplied for use in generating the operations for conditional transfer control instructions. A short decode generates control signals including indications of a transfer control instruction (for example, Jcc, LOOP, JMP, CALL), an unconditional transfer control instruction (for example, JMP, CALL), a CALL instruction, a prefix byte, a cc-dependent RegOp, and a designation of whether the instruction length is address or data size dependent. Typically one or both operations are valid, but prefix byte and JMP decodes do not generate a valid op. Invalid operations appear as valid NOOP operations to pad an Op quad. The first short decoder 410 generates operations based on more than decoding of the instruction bytes. The first short decoder 410 also determines the presence of any prefix bytes decoded during preceding decode cycles. Various prefix bytes include OF. address size override, operand size override, six segment override bytes, REP/REPE, REPNE and LOCK bytes. Each prefix byte affects a subsequent instruction decode in a defined way. A count of prefix bytes and a count of consecutive prefix bytes are accumulated during decoding and furnished to the first short decoder SDecO 410 and the long decoder 416. The consecutive prefix byte count is used to check whether an instruction being decoded is too long. Prefix byte count information is also used to control subsequent decode cycles, including checking for certain types of instruction-specific exception conditions. Prefix counts are reset or initialized at the end of each successful non-prefix decode cycle in preparation for decoding the prefix and opcode bytes of a next instruction. Prefix counts are also reinitialized when the macroinstruction decoder 230 decodes branch condition and write instruction pointer (WRIP) operations. Prefix bytes are processed by the first short decoder 410 in the manner of one-byte short decode mstructions At most, one prefix byte is decoded m a decode cycle, a condition that is enforced through mvalidation of all short decodes followmg the decode of a prefix byte Effective address size, data size, operand segment register values, and the current B-bit, are supplied to the first short decoder 410 but can decode along with precedmg opcodes The address size prefix affects a decode of a subsequent instruction both for decodmg of mstructions for which the generated operation depends on effective address size and for decoding of the address mode and mstruction length of modr/m instructions The default address size is specified by a currently-specified D-bit, which is effectively toggled by the occurrence of one or more address size prefixes The operand size prefix also affects the decode of a subsequent instruction both for decodmg of mstructions for which the generated operation depends on effective data size and for decodmg of the mstruction length The default operand size is specified by a currently-specified x86-standard D-bit, which is effectively toggled by the occurrence of one or more operand size prefixes The segment override prefixes affect the decode of a subsequent instruction only in a case when the generation of a load-store operation (LdStOps) is dependent on the effective operand segment of the mstruction The default segment is DS or SS, depend g on the associated general address mode, and is replaced by the segment specified by the last segment override prefix The REP/REPE and REPNE prefixes do not affect the decode of a subsequent instruction If the mstruction is decoded by the macroinstruction decoder 230. rather than the emulation code ROM 232, then any precedmg REP prefixes are ignored However, if the instruction is vectored, then the generation of the vector address is modified some cases Specifically, if a string mstruction or particular neighbormg opcode is vectored, then an indication of the occurrence of one or more of the REP prefixes and designation of the last REP prefix encountered are included in the vector address For all other instructions the vector address is not modified and the REP prefix is ignored A LOCK prefix inhibits all short and long decodmg except the decodmg of prefix bytes, forcmg the subsequent mstruction to be vectored When the vector decode cycle of this subsequent mstruction occurs, so long as the subsequent mstruction is not a prefix, the opcode byte is checked to ensure that the mstruction is within a "lockable" subset of the mstructions If the instruction is not a lockable mstruction, an exception condition is recognized and the vector address generated by the vectormg decoder 418 is replaced by an exception entry point address Instructions decoded by the second and third short decoders 412 and 414 do not have prefix bytes so that decoders 412 and 414 assume fixed default values for address size data size, and operand segment register values Typically, the three short decoders generate four or fewer operations because three consecutive short decodes are not always performed and mstructions often short decode mto only a smgle operation However, for the rare occurrence when more than four valid operations are generated, operation packmg logic 438 inhibits or invalidates the third short decoder 414 so that only two mstructions are successfully decoded and at most four operations are generated for packmg mto an Op quad When the first short decoder 410 is unsuccessful, the action of the second and third short decoders 412 and 414 are mvalidated When the second short decoder 412 is unsuccessful, the action of the third short decoder 414 is mvalidated When even the first short decode is invalid, the decode cycle becomes a long or vectormg decode cycle In general, the macroinstruction decoder 230 attempts one or more short decodes and, if such short decodes are unsuccessful, attempts one long decode If the long decode is unsuccessful, the macroinstruction decoder 230 performs a vectoring decode Multiple conditions cause the short decoders 410 412 and 414 to be mvalidated Most generally, short decodes are mvalidated when the mstruction operation code (opcode) or the designated address mode of a modr/m mstruction does not fall widun a defined short decode or "simple" subset of mstructions This condition typically restricts short decode mstructions to those operations that generate two or fewer operations Short decodes are also invalidated when not all of the bytes m the mstruction buffer 408 for a decoded mstruction are valid Also, "cc-dependent" operations, operations that are dependent on status flags, are only generated by the first short decoder 410 to ensure that these operations are not preceded by and " cc" RegOps A short decode is mvalidated for a second of two consecutive short decodes when the unmediately precedmg short decode was a decode of a transfer control mstruction, regardless of the direction taken A short decode is mvalidated for a second of two consecutive short decodes when the first short decode was a decode of a prefix byte In general, a prefix code or a transfer control code inhibits further decodes m a cycle Furthermore, no more than sixteen mstruction bytes are consumed by the macroinstruction decoder 230 smce the mstruction buffer 408 only holds sixteen bytes at one time Also, at most four operations can be packed mto an Op quad These constraints only affect the third short decoder 414 since the length of each short decoded mstruction is at most seven bytes and operations in excess of four only arise in the third short decoder 414 In a related constraint, if the current D-bit value specifies a 16-bit address and data size default, then an mstruction havmg a length that is address and or data dependent can only be handled by the first short decoder 410 smce the predecode mformation is probably incorrect Also, when multiple mstruction decodmg is disabled, only the first short decoder 410 is allowed to successfully decode mstructions and prefix bytes Validation tests are controlled by short decoder validation logic (not shown) m the macroinstruction decoder 230 and are mdependent of the operation of short decoders 410, 412 and 414 However, each of the short decoders 410, 412 and 414 does set zero, one or two valid bits dependmg on the number of operations decoded These valid bits, a total of six for the three short decoders 410, 412 and 1 , are used by the operation packmg logic 438 to determine which operations to pack mto an Op quad and to force invalid operations to appear as NOOP (no operation) operations The operation packmg logic 438 operates without short decoder validation mformation smce valid short decodes and associated operations are preceded only by other valid short decodes and associated operations The short decoders 410, 412 and 414 also generate a plurality of signals representing various special opcode or modr/m address mode decodes These signals mdicate whether a certain form of mstruction is currently being decoded by the mstruction decoder 220 These signals are used by short decode validation logic to handle short decode validation situations The mstruction bytes, which are stored unaligned m the mstruction buffer 408, are aligned by byte rotators 430, 432 and 434 as the mstruction bytes are transferred to the decoders 410-418 The first short decoder SDecO 410, the long decoder 416 and the vectormg decoder 418 share a first byte rotator 430 The second and thu-d short decoders SDec 1 412 and SDec2 414 use respective second and third byte rotators 432 and 434 Durmg each decode cycle, the three short decoders SDecO 410, SDec l 412 and SDec2 414 attempt to decode what are, most efficiently, three short decode operations using three independently-operating and parallel byte rotators 430. 432 and 434 Although the multiplexing by the byte rotators 430. 432 and 434 of appropriate bytes m the instruction buffer 408 to each respective decoder SDecO 410, SDecl 412 and SDec2 414 is conceptually dependent on the precedmg mstruction decode operation, mstruction length lookahead logic 436 uses the predecode bits to enable the decoders to operate substantially in parallel The long and vectormg decoders 416 and 418, in combination, perform two parallel decodes of eleven mstruction bytes, taking the first byte to be an opcode byte and generatmg either a long mstruction decode Op quad or a vectormg decode Op quad Information analyzed by the long and vectormg decoders 416 and 418 mcludes effective address size, effective data size, the current B-bit and DF-bit, any override operand segment register, and logical addresses of the next sequential and target instructions to be decoded The long and vectormg decoders 416 and 418 generate decode signals mcludmg an mstruction length excludmg precedmg prefix bits, a designation of whether the mstruction is within the long decode subset of mstructions, a RET mstruction, and an effective operand segment register, based on a default implied by the modr/m address mode plus any segment override Durmg a decode cycle m which none of the short decoders SDecO 410, SDecl 412 and SDec2 414 successfully decodes a short mstruction, the macroinstruction decoder 230 attempts to perform a long decode using the long decoder 416 If a long decode cannot be performed, a vectormg decode is performed In some embodiments, the long and vectormg decoders 416 and 418 are conceptually separate and mdependent decoders, just as the long and vectoring decoders 416 and 418 are separate and mdependent of the short decoders 410, 412 and 414 Physically, however, the long and vectormg decoders 416 and 418 share much logic and generate similar Op quad outputs Instructions decoded by the long decoder 416 are generally mcluded within the short decode subset of mstructions except for an address mode constraint such as that the mstruction cannot be decoded by a short decoder because the mstruction length is greater than seven bytes or because the address has a large displacement that would require generation of a third operation to handle to displacement The long decoder 41 also decodes certain additional modr/m mstructions that are not the short decode subset but are sufficiently common to warrant hardware decodmg Instruction bytes for usage or decodmg by the long decoder 416 are supplied from the mstruction buffer 408 by the first byte rotator 430. the same mstruction multiplexer that supplies mstruction bytes to the first short decoder SDecO 410 However, while the first short decoder SDecO 410 receives only seven bytes, the long decoder 416 receives up to eleven consecutive instruction bytes, corresponding to the maximum length of a modr/m instruction excluding prefix bytes Thus the first byte rotator 430 is eleven bytes wide although only the first seven bytes are connected to the first short decoder SDecO 410 The long decoder 416 only decodes one mstruction at a time so that associated predecode information within the mstruction buffer 408 is not used and is typically invalid The first byte of the first byte rotator 430 is fully decoded as an opcode byte and, m the case of a modr/m mstruction, the second mstruction byte and possibly the third are fully decoded as modr/m and sib bytes, respectively The existence of a OF prefix is considered m decoding of the opcode byte The OF prefix byte inhibits all short decodmg since all short decode instructions are non-OF or ' one-byte" opcodes Because all prefix bytes are located within the "one-byte' opcode space, decoding of a OF prefix forces the next decode cycle to be a two-byte opcode mstruction, such as a long or vectormg decode mstruction In addition to generatmg operations based on the decoding of modr/m and sib bytes, the first byte rotator 430 also determmes the length of the mstruction for usage by various program counters, whether the mstruction is a modr/m mstruction for inhibiting or invalidating the long decoder, and whether the mstruction is an mstruction within the long decode subset of operation codes (opcodes) The long decoder 416 always generates four operations and, like the short decoders 410, 412 and 141, presents the operations in the form of an emulation code-like Op quad, excluding an OpSeq field The long decoder 416 handles only relatively simple modr/m instructions A long decode Op quad has two possible forms that differ only m whether the third operation is a load operation (LdOp) or a stoi e operation (StOp) and whether the fourth operation is a RegOp or a NOOP A first long decode Op quad has the form LIMM t2.<ιmm32> LIMM tl,<dιsp32> LD b/d t8L/t8,@(<gam>)OS.a <RegOp> . A second long decode Op quad has the form LIMM t2,<ιmm32> LIMM tl,<dιsp32> ST b/d @(<gam>),t2L/t2,OS a NOOP The @(<gam>) address mode specification represents an address calculation corresponding to that specified by the modr/m and/or sib bytes of the mstruction, for example @(AX + BX*4 + LD) The <unm32> and <dιsp32> values are four byte values containing the immediate and displacement mstruction bytes when the decoded mstruction contams such values The long decoder 416, like the first short decoder 410, generates operations taking mto account the presence of any prefix bytes decoded by the short decoders during preceding decode cycles Effective address size, data size, operand segment register values, and the current B-bit are supplied to the long decoder 416 and are used to generate operations. No indirect size or segment register specifiers are included within the final operations generated by the long decoder 416. Only a few conditions inhibit or invalidate an otherwise successful long decode. One such condition is an instruction operation code (opcode) that is not included in the long decode subset of instructions. A second condition is that not all of the instruction buffer 408 bytes for the decoded instruction are valid. The vectoring decoder 418 handles instructions that are not decoded by either the short decoders or the long decoder 416. Vectoring decodes are a default case when no short or long decoding is possible and sufficient valid bytes are available. Typically, the instructions handled by the vectoring decoder 418 are not included in the short decode or long decode subsets but also result from other conditions such as decoding being disabled or the detection of an exception condition. During normal operation, only non-short and non-long instructions are vectored. However, all instructions may be vectored. Undefined opcodes are always vectored. Only prefix bytes are always decoded. Prefix bytes are always decoded by the short decoders 410, 412 and 414. When an exception condition is detected during a decode cycle, a vectoring decode is forced, generally overriding any other form of decode without regard for instruction byte validity of the decoded instruction. When a detected exception condition forces a vectoring decode cycle, the generated Op quad is undefined and the Op quad valid bit for presentation to the scheduler 260 is forced to zero. The Op quad valid bit informs the scheduler 260 that no operations are to be loaded to the scheduler 260. As a result, no Op quad is loaded into the scheduler 260 during an exception vectoring decode cycle. Few conditions inhibit or invalidate a vectoring decode. One such condition is that not all of the bytes in the instruction buffer 408 are valid. When an instruction is vectored, control is transferred to an emulation code entry point. An emulation code entry point is either in internal emulation code ROM 232 or in external emulation code RAM 236. The emulation code starting from the entry point address either emulates an instruction or initiates appropriate exception processing. A vectoring decode cycle is properly considered a macroinstruction decoder 230 decode cycle. In the case of a vectoring decode, the macroinstruction decoder 230 generate the vectoring quad and generate the emulation code address into the emulation code ROM 232. Following the initial vectoring decode cycle, the macroinstruction decoder 230 remains inactive while instructions are generated by the emulation code ROM 232 or emulation code RAM 236 until a return from emulation (ERET) OpSeq is encountered. The return from emulation (ERET) sequencing action transitions back to macroinstruction decoder 230 decoding. During the decode cycles following the initial vectoring decode cycle, the macroinstruction decoder 230 remains inactive, continually attempting to decode the next "sequential" instruction but having decode cycles repeatedly invalidated until after the ERET is encountered, thus waiting by default to decode the next "sequential" instruction. Instruction bytes for usage or decodmg by the vectoring decoder 418 are supplied from the instruction buffer 408 by the first byte rotator 430, the same instruction multiplexer that supplies instruction bytes to the first short decoder SDecO 410 and to the long decoder 416 The vectoring decoder 418 receives up to eleven consecutive mstruction bytes, corresponding to the maximum length of a modr/m mstruction excludmg prefix bytes Thus, the full eleven byte width of the first byte rotator 430 is distributed to both the long decoder 416 and the vectoπng decoder 418 The predecode information withm the instruction buffer 408 is not used by the vectoπng decoder 418 As in the case of the long decoder 416. the first byte of the first byte rotator 430 is fully decoded as an opcode byte and, in the case of a modr/m mstruction, the second instruction bvte and possibly the third are fully decoded as modr/m and sib bytes, respectively The vectoring decoder 418 generates operations takmg mto account the presence of any prefix bytes decoded by the short decoders during preceding decode cycles The existence of a OF prefix is considered in decodmg of the opcode byte In addition to generating operations based on the decodmg of modr/m and sib bytes, the first byte rotator 430 also determines the length of the mstruction for usage by various program counters whether the instruction is a odr m mstruction for inhibiting or invalidating the long decoder, and whether the instruction is an instruction within the long decode subset of operation codes (opcodes) If not, a vectormg decode is initiated Effective address size data size and operand segment register values are supplied to the vectoπng decoder 418 and are used to generate operations No indirect size or segment register specifiers are included within the final operations generated by the vectoring decoder 418 During a vectoπng decode cycle, the vectormg decoder 418 generates a vectormg Op quad, generates an emulation code entry pomt or vector address, and mitializes an emulation environment The vectormg Op quad is specified to pass various mformation to initialize emulation environment scratch registers The value of the emulation code entry pomt or vector address is based on a decode of the first and second mstruction bytes, for example the opcode and modr/m bytes, plus other mformation such as the presence of an OF prefix, a REP prefix or the like In the case of vectoπng caused by an exception condition, the entry point or vector address is based on a simple encoded exception identifier The emulation environment is stored for resolvmg environment dependencies All of the short decoders 410, 412 and 414 and long decoder 416 directly resolve environmental dependencies, such as dependencies upon effective address and data sizes, as operations are generated so that these operations never contam indirect size or register specifiers However, emulation code operations do refer to such effective address and data size values for a particular instance of the instruction being emulated The emulation environment is used to store this additional mformation relatmg to the particular mstruction that is vectored This mformation mcludes general register numbers, effective address and data sizes, an effective operand segment register number, the prefix byte count, and a record of the existence of a LOCK prefix The emulation environment also loads a modr/m reg field and a modr/m reg field are loaded mto Reg and Regm registers The emulation environment is initialized at the end of a successful vectoring decode cycle and remains at the initial state for substantially the duration of the emulation of an instruction by emulation code, until an ERET code is encountered. The vectoring decoder 418 generates four operations of an Op quad in one of four forms. All four forms include three LIMM operations. The four forms differ only in the immediate values of the LIMM operations and in whether the third operation is an LEA operation or a NOOP operation. A first vectoring decode Op quad has the form: LIMM t2,<imm32> LIMM tl,<disp32> LEA t6,@(<gam>),_.a LIMM t7,LogSeqDecPC[31..0]//logical seq. next //instr. PC A second vectoring decode Op quad has the form: LIMM t2,<imm32> LIMM tl ,<disp32> NOOP LIMM t7,LogSeqDecPC[31..0] A third vectoring decode Op quad has the form: LIMM t2,<+/- l/2/4> //equiv to "LDK(D)S t2,+ l/+2" LIMM tl,<+/- 2/4/8> //equiv to "LDK(D)S tl ,+2/+4" NOOP LIMM t7,LogSeqDecPC[31..0] A fourth vectoring decode Op quad has the form: LIMM t2,<+2/4> //equiv to "LDKD t2,+2" LIMM tl ,<disp32> LD t6,@(SP),SS.s LIMM t7,LogSeqDecPC[31..0] //predicted RET target adr //from Return Address Stack The first two forms of vectoring Op quads apply for most opcodes. The first form is used for memory- referencing modr/m instructions for which the LEA operation is used to compute and load a general address mode effective operand address into a treg. The second form is used for non-modr/m and register-referencing modr/m instructions. For instructions having the second form no address is necessarily computed, although the <imm32> and <disp32> values remain useful insofar as they contain instruction bytes following the opcode byte. The third form of vectoring Op quad is used for all string mstructions plus some neighboring non-modr/m instructions. A fourth form of vectoring Op quad supports special vectoring and emulation requirements for near RET instructions. The macroinstruction decoder 230 has four program counters, including three decode program counters 420, 422 and 424, and one fetch program counter 426. A first decode program counter, called an instruction PC 420, is the logical address of the first byte, including any prefix bytes, of either the current instruction being decoded or, if no instruction is currently decoding, the next instruction to be decoded. If the decode operation is a multiple instruction decode, instruction PC 420 points to the first instruction of the multiple instructions to be decoded. The instruction PC 420 corresponds to the architectural address of an instruction and is used to generate instruction fault program counters for handling of exceptions. The instruction PC 420 is passed down the scheduler 260 with corresponding Op quads and is used by an operation commit unit (OCU) (not shown) of the scheduler 260 to produce instruction fault program counters to be saved during exception processing. When an Op quad is generated by the macroinstruction decoder 230. the current instruction PC 420 value is tagged to the Op quad and loaded into the Scheduler 260 Op quad entry along with the Op quad. A second decode program counter, called a logical decode PC 422, is the logical address of the next instruction byte to be decoded and addresses either an opcode byte or a prefix byte. A third decode program counter, called a linear decode PC 424, is the linear address of the next instruction byte to be decoded and addresses either an opcode byte or a prefix byte. The logical decode PC 422 and the linear decode PC 424 point to the same instruction byte. The linear decode PC 424 designates the address of the instruction byte currently at the first byte rotator 430. The various decoders in the macroinstruction decoder 230 function on the basis of decoding or consuming either prefix bytes or whole instructions minus any prefix bytes so that prefixes are generally handled as one-byte instructions. Therefore, the address boundaries between instruction and prefix byte decodes are more important than instruction boundaries alone. Consequently, at the beginning of each decode cycle, the next instruction byte to be decoded is not necessarily the true beginning of an instruction. At the beginning of a decode cycle the logical decode PC 422 and the linear decode PC 424 contain the logical and linear addresses of the next instruction to be decoded, either an instruction or a prefix byte. The linear decode PC 424 is a primary program counter value that is used during the decoding process to access the instruction buffer 408. The linear decode PC 424 represents the starting point for the decode of a cycie and specifically controls the byte rotator feeding bytes from the instruction buffer 408 to the first short decoder 410 and to the long and vectoring decoders 416 and 418. The iinear decode PC 424 also is the reference point for determining the instruction addresses of any further short decode instructions or prefix bytes, thus generating control signals for the byte rotators feeding the second and third short decoders 412 and 414. The linear decode PC 424 also acts secondarily to check for breakpoint matches during the first decode cycles of new instructions, before prefix bytes are decoded, and to check for code segment overruns by the macroinstruction decoder 230 during successful instruction decode cycles. The logical decode PC 422 is used for program counter-related transfer control instructions, including CALL instructions. The logical decode PC 422 is supplied to the branch unit 234 to be summed with the displacement value of a PC-relative transfer control instruction to calculate a branch target address. The logical decode PC 422 also supports emulation code emulation of instructions. The next sequential logical decode program counter (PC) 422 is available in emulation code from storage in a temporary register by the vectoring Op quad for general usage. For example, the next sequential logical decode PC 422 is used to supply a return address that a CALL instruction pushes on a stack. A next logical decode PC 428 is set to the next sequential logical decode program counter value and has functional utility beyond that of the logical decode PC 422 The next logical decode PC 428 directly furnishes the return address for CALL mstructions decoded by the macroinstruction decoder 230 The next logical decode PC 428 also is passed to emulation code logic durmg vectormg decode cycles via one of the operations within the vectormg Op quad During a decode cycle, the linear decode PC 424 points to the next mstruction bytes to be decoded The four least significant bits of linear decode PC 424 pomt to the first instruction byte within the mstruction buffer 408 and thereby directly indicate the amount of byte rotation necessary to align the first and subsequent mstruction bytes in the instruction cache 214 The first byte rotator 430 is an instruction multiplexer, specifically a 16 1 byte multiplexer, for accessing bytes in the instruction buffer 408 that are offset by the lmear decode PC 424 amount The first byte rotator 430 is seven bytes wide for the first short decoder SDecO 410 and eleven bytes wide for the long decoder 416 and the vectoring decoder 418 in combination Shared logic in the first short decoder SDecO 410, the long decoder 416 and the vectoring decoder 418 generate a first instruction length value ILenO for the first mstruction The second and third byte rotators 432 and 434 are seven bvte-wide instruction multiplexers, specifically 16 1 byte multiplexers The second b\te rotator 432 accesses bytes m the mstruction buffer 408 that are offset by the sum of the lmear decode PC 424 amount and the first instruction length ILenO Logic in the second short decoder SDecO 412 generate a second instruction length value ILen 1 for the second mstruction The third byte rotator 434 accesses bytes m the mstruction buffer 408 that are offset by the sum of the lmear decode PC 424 amount and the first and second instruction lengths ILenO and ILen 1 The byte rotators 430, 432 and 434 multiplex mstruction bytes but not predecode bits The byte rotators 430. 432 and 434 are controlled using predecode mformation in which the predecode bits associated with the first opcode byte or the first byte of the first mstruction directly controls the second rotator 432 The first bvte of the second mstruction directly controls the third rotator 434 Each predecode code implies an mstruction length but what is applied to the next rotator is a pointer The pointer is derived bv taking the four least significant bits of the program counter at the present mstruction plus the length to attain the program counter to the next instruction The first and second short decoders SDecO 410 and SDecl 412 do generate the mstruction byte lengths ILenO and ILen 1 as a matter of course so that the process of decoding multiple short decode mstructions occurs seπally However, such serial processmg is unacceptable for achieving a desired decodmg speed To hasten the decodmg process the three short decoders SDecO 410. SDecl 412 or SDec2 414 are operated in parallel by mcludmg separate mstruction length lookahead logic that quickly, although serially, determmes the mstruction lengths ILenO and ILenl using the predecode bits associated v\ ith the mstruction bytes in each mstruction buffer 408 The predecode bits associated with each instruction byte in the mstruction buffer 408 are the ILen bits specifying the length of an mstruction startmg with that byte A parameter more useful than ILen for computation by lookahead logic is the value ILenO plus lmear decode PC 424 (mod 16) and the value ILenO plus ILenl plus lmear decode PC 424 (mod 16) Consequently, the four mstruction length predecode bits of the six predecode bits associated with each mstruction byte in the mstruction buffer 408 are set to pomt to the first byte of the next mstruction assuming that this byte is the opcode byte which begins an instruction. As a result, the predecode bits associated with the first opcode byte multiplexed by the first byte rotator 430 and applied to the first short decoder SDecO 410 directly specify the instruction buffer 408 byte index of the first opcode byte for the second short decoder SDecl 412 The predecode bits associated with the second opcode byte multiplexed by the second byte rotator 432 and applied to the second short decoder SDecl 412 directly specify the instruction buffer 408 byte mdex of the first opcode byte for the third short decoder SDec2 414 The instruction lookahead logic mcludes two four- bit-wide 16: 1 multiplexers which produce instruction buffer 408 index bits for the multiplexers of second and third byte rotators 432 and 434 Structurally, all of the instruction buffer 408 predecode bits are connected separately from the instruction buffer 408 instruction bytes The predecode bits are used solely by the lookahead logic. The mstruction bytes are applied to the instruction multiplexers of the short decoders 410, 412 and 414. The instruction lookahead logic also includes logic for determining whether sets of predecode bits used durmg decode cycle are valid Specifically, the instruction lookahead logic determmes whether an mstruction byte is the first byte of a valid short decode mstruction If the predecode bits for a particular byte m the mstruction buffer 408 point to the particular byte, implying a zero mstruction length for an instruction starting at the location of the particular byte, then that byte is not the start of a short decode instruction and no further short decoding is possible Otherwise, a short decode operation is possible and the predecode bits point to the beginning of the next mstruction The following pseudo-RTL description summarizes the structure and functionality of the instruction buffer 408 (IBuf), the mstruction multiplexers (IMux), the instruction lookahead and multiplexer control logic, and the multiplexer connections to the different instruction decoders The instruction buffer 408 is IBuf, mcludmg mstruction bytes IByte[8] and predecode bits PreDec[4] The lmear decode PC 424 is DecPC[3..0]. struct ExtIByte{ lByte[8] PreDec[4] } IBuftlό] IBIndex0[3..0] = DecPC[3..0] IBIndexl [3..0] = IBuf[IBIndex0[]].PreDec[] IBIndex2[3..0] = IBuf[IBlndexl []].PreDec[] IBIndex3[3..0] = IBuf[IBIndex2[]].PreDec[] IndexOV = ~(IBIndexl[2..0] = IBIndexO[2..0J) Index IV = ~(IBIndex2[2..0] = IBIndexl [2..0]) Index2V = ~(IBIndex3[2..0] = IBIndex2[2..0]) for (ι=0..10) IMux0[ι][] = IBufI(IBIndexO[]+i) mod 16] IByte[] LVOpQuad. = Lon VecDecode(IMux0[ 10 0][]) SDecOp0[].SDecOpl [], = ShortDecode(IMux0[6. 0][]) for (ι=0..6) IMuxl[i][] = IBuf[(IBIndexl[]+ι) mod 16].IByte[] SDecOp2[],SDecOp3[],.. = ShortDecode(lMuxl [6..0][]) for (i=0..6) IMux2[i][] = IBufI(IBIndex2[]+i) mod 16].IByte[] SDecOp4[],SDecOp5[],... = ShortDecode(lMux2[6..0][]) The short decoders SDecO 410, SDecl 412 or SDec2 414 each generate up to two Ops. The long and vectoring decoder 418 generates a full Op quad. The six Ops from the three short decoders SDecO 410, SDecl 412 or SDec2 414 are subsequently packed together by Op packing logic into a proper Op quad having at most four of the Ops which are guaranteed to be valid. Either the Op quad generated by the short decoders or the Op quad generated by the long/vectoring decoder is chosen as the Op quad result of the macroinstruction decoder 230 for a decode cycle. The instruction buffer 408 is accessed on a byte basis by instruction multiplexers of the macroinstruction decoder 230, but the instruction buffer 408 is managed on an instruction word basis each decode cycle for tracking validity of instruction bytes in the instruction buffer 408 and for determining which instruction bytes of the instruction buffer 408 are to be reloaded with new instruction bytes from the instruction cache 214. The predecode information is also used to determine whether the instruction bytes that are decoded and used by each decoder within the macroinstruction decoder 230 decoder are valid. When an instruction byte is not valid, an invalid valid signal is asserted and applied to decoder validation logic (not shown) associated with that decoder. Control of updating of the current fetch PC 426, which is used to address the next instruction cache 214 access, is synchronized closely with the consumption of valid instruction bytes by the various decoders within the instruction buffer 270. In this manner, control of updating of the current fetch PC 426 is also managed by the control logic (not shown) within the instruction buffer 408. Control logic within the instruction buffer 408 is described in pseudo-RTL as follows: struct ExtlByte { IByte[8] PreDec[4] } IBuf[16] Instruction buffer 408 word validity signals are described as follows: IBufEmpty = -JFetchPC[4] Λ DecPC[4]) + ~DEC_ValidIF FullFetch = ('b00 = FetchPC[3..2]) IB0V = -IBufEmpty (-FullFetch + ('b00 >= DecPC[3..2])) = -IBufEmpty (-FullFetch + ('b00 = DecPC[3..2])) IB1 V = -IBufEmpty (-FullFetch + ('b01 >= DecPC[3..2])) = -IBufEmpty (-FullFetch + ~DecPC[3]) IB2V = -IBufEmpty (-FullFetch + ('blO >= DecPC[3..2])) = -IBufEmpty (-FullFetch + ~('bl 1 = DecPC[3..2]» IB3V = -IBufEmpty (-FullFetch + ('bl 1 >= DecPC[3..2])) = -IBufEmpty Wraparound is detected within the mstruction buffer 408 in response to collective instruction byte consumption of the short decoders in logic functionally described as follows in pseudo-RTL' DecPCWrap = (SeqDecPC[3..2] < DecPC[3..2]) Instruction buffer 408 word load control signals are described as follows' LoadlBO = -IB0V + DecTakenXC + Declnstr majoπty(('b00 >= DecPC[3..2]), ('b00 < SeqDecPC[3. 2]),DecPCWrap) = -IB0V + DecTakenXC + Declnstr majoπty(('b00 = DecPC[3..2]), ~('b00 = SeqDecPC[3..2]),DecPCWrap) LoadlB 1 = -IB 1 V + DecTakenXC + Declnstr majoπty(('b01 >= DecPC[3..2]), ('bOl < SeqDecPC[3 2]),DecPCWrap) = -IB1 V + DecTakenXC + Declnstr majoπty(~DecPC[3].SeqDecPC[3],DecPCWrap) LoadIB2 = -IB2V + DecTakenXC + Declnstr majoπty(('b l O >= DecPC[3..2J), Cbl 0 < SeqDecPC[3 2]).DecPCWrap) = ~1B2V 1- DecTakenXC - Declnstr majoπty 'b l 1 = DecPC[3..2]), fbl I = SeqDecPC[3 2]).DecPCWrap) LoadIB3 = -IB3V + DecTakenXC + Declnstr majonty(('bl 1 >= DecPC[3..2]), ('bl l < SeqDecPC[3 2]),DecPCWrap) = -IB3 V + DecTakenXC + Declnstr DecPCWrap Loading of the mstruction buffer 408 is described by the following pseudo-RTL: @clk: if (LoadlBO) IBuf[3..0] = IBufln[3..0] @clk: if (LoadlBl ) IBuf[7. 4] = IBufln[7..4] @clk- if (LoadIB2) IBuf[11..8] = IBufln[l 1..8] @clk: if (LoadIB3) IBuf[15..12] = IBufln[15..12] Valid instruction byte signals that are indicative of valid bytes received from the mstruction buffer 408 are described for each decoder in the instruction decoder 270 as follows IBufSOV = -IBufEmpty ( (IBIndexl[3..2] < FetchPC[3..2]) + (IBIndexl [3..2] >= IBIndexO[3..2]) ) IBufSI V = -IBufEmpty majority( IBlndexl [3..2] >= FetchPC[3..2], IBIndex2[3..2] < FetchPC[3 .2], IBlndex2[3..2] >= IBlndex 1 [3. 2] ) IBufS2V = -IBufEmpty majorιty( IBlndex2[3..2] >= FetchPC[3. 2], IBIndex3[3..2] < FetchPC[3..2], IBlndex3[3..2] >= IBlndex2[3..2] ) IBufLVV = -IBufEmpty ( (LVSeqDecPC[3..2] >= DecPC[3..2]) + (LVSeqDecPC[3..2] < FetchPC[3..2]) ) A valid bit relating to requests of the instruction fetch control cσcuit 218 is evaluated as follows: @clk: if (SC_Vec2Dec + RUX_WrIP + ERM_StopIF + IC_StopIF + SI_Reset) DEC_ValidIF = (SC_Vec2Dec + RUX_WrIP) ~SI_Reset A valid bit for mstruction bytes received from the instruction cache 214 is evaluated as follows: IFetchV = DEC_ValidIF ~IC_HoldIF ~ITB_HoldIF ~ITB_PgViol & -(E cMode DEC_ExtEmc) A next sequential fetch PC value is evaluated as follows SeqFetchPC[31 4] = (IBufEmpty + DecPCWrap Declnstr) * > FetchPC[31 4]+ 1 FetchPC[31 4] SeqFetchPC[3 2] = if ( (IBufEmpty + DecPCWrap Declnstr) FetchPC[4] ) 'bOO else (IBufEmpty + -Declnstr) ■ > DecPC[3 2] SeqDecPC[3 2] A new fetch PC value having two least significant bits invariably set to 'bOO is evaluated as follows DEC_NextFetchAddr[31 2] = if (SC_Vec2Dec + RUX_WrIP + DecTakenXC -BtbHit + DecRetXC) NewDecPC[31 2] elseif (DecTakenXC BtbHit) BfbFetchAddr l 2] elseif (IFetchV) SeqFetchPC[31 2] else FetchPC[31. 2] @clk FetchPC[31 2] = DEC_NextFetchAddr[31 2] A processor typically operates by transferring program instructions from a memory mto mstruction registers, then decodmg and executing the instructions Execution speed of a processor is improved by transferring, decodmg and executing more than one instruction at a time In some processors, all instructions are the same length and transferring of data from memory for decoding is simple because the memory locations of all mstructions are readily determined, allowing several instructions to be simultaneously transferred into several mstruction registers However, x86 mstructions have variable lengths so that each instruction must typically be decoded to determine the mstruction length before the starting location of the next decodable mstruction is ascertainable, making the locat g of mstructions a slow, sequential process To allow simultaneous decodmg of multiple variable-length instructions, the predecoder 270 includes logic for preparing mstructions for rapid decodmg Predecoder 270 determmes and stores the instruction cache 214, for many mstruction bytes, the location of the next succeeding mstruction by assummg that each mstruction byte is the first byte of an mstruction When mstructions are transferred from the mstruction cache 214, logic for preparing mstructions for rapid decodmg locates the actual first byte of an instruction so that several subsequent instructions are rapidly located and allowmg simultaneous loadmg of mstructions into a plurality of instruction registers for simultaneous decodmg and execution When mstruction bytes are loaded from mam memory 130 into the instruction cache 214, a preliminary decode operation of these mstruction bytes is performed to generate additional bits that are stored with each mstruction byte m the mstruction cache 214 The predecode bits are fetched in combmation with the associated mstruction bytes, loaded into the instruction buffer 408, and subsequently used to facilitate the processmg of simultaneous decodmg of multiple instruction bytes A pseudo-RTL declaration summarizes this aspect of the mstruction cache 214 tag RAM and data RAM entries as follows struct ExtlCByte { lByte[8] PreDec[3] } IDataEntryfoJ] Predecode bits associated with each instruction byte serve to locate the next instruction boundary relative to an opcode byte. Prefix bytes are decoded in the form of single-byte long "instructions". Bits of the prefix bytes are used directly by instruction lookahead logic to generate the control signals for instruction multiplexers feeding the instruction decoders. More specifically, the following two values are quickly computed by the lookahead logic: (DecPC[3..0]+ILenO) mod 16 (DecPC[3..0]+ILenO+ILen I ) mod 16 where DecPC[3:0] is the linear decode PC 424, ILenO is the instruction length of the instruction applied to the first short decoder SDecO 410 and ILenl is the instruction length of the instruction applied to the second short decoder SDecl 412. In some embodiments, both ILenO and ILenl are calculated for every instruction byte in the instruction buffer 408 when mstruction bytes are first loaded into the instruction buffer 408. Because instruction bytes are stored in the mstruction buffer 408 with memory-alignment, the calculation of ILenO and ILenl are possible very early in the instruction processing path. In particular, the ILenO and ILenl are performed when the associated instruction bytes are loaded into the mstruction cache 14 and the resulting ILenO and ILenl values are stored in the instruction cache 214 as predecode bits. The instruction byte in the mstruction buffer 408 is indexed by Linear decode PC 424 and has a fixed value for the low four bits of the mstruction byte memory address which is equal to linear decode PC 424. The instruction length ILen of an instruction starting with an opcode byte equal to linear decode PC 424 is calculated solely on the basis of the opcode byte or, in some cases, on the basis of the opcode byte and the next byte following the opcode byte. The instruction buffer 408 index of the first byte of the next instruction is ILen plus a constant (mod 16), specifically (DecPC[3..0]+ILen0) mod 16. The computation of (DecPC[3 .0]+lLen0+ILen l ) mod 16 results simply from taking the value (DecPC[3..0]+ILenO) mod 16. which is generated by the first computation indexed by (DecPC[3..0]+ILen0) mod 16. In mis manner, only (DecPC[3..0]+ILen0) mod 16 is computed for each instruction byte and becomes the predecode value for the instruction byte. Instruction bytes are loaded into the instruction cache 214 during cache fills sixteen bytes at a time so that predecode logic is replicated sixteen times and predecode bits for all sixteen bytes are calculated simultaneously immediately before the sixteen bytes are written into the data RAM of the mstruction cache 214. Predecode logic effectively adds an extra half cycle to the miss/fill latency of the instruction cache 214. Each of the sixteen sets of predecode logic, one set for each instruction byte loaded into the instruction cache 214, examines the associated instruction byte plus, for instruction bytes equal to some opcodes, the following one or two instruction bytes. The first instruction byte is decoded as an opcode byte, regardless of whether the byte actually is an opcode. If the first instruction byte is a modr/m opcode, the second and third instruction bytes are decoded as modr/m and sib bytes. Based on these three bytes alone, the length ILen of the instruction is determined and the instruction is assigned to be inside or outside the subset of short decode instructions of the instruction set. For some opcodes, the short decode instruction subset is defined bv the opcode byte alone For modr/m opcodes, the short decode instruction subset is specified by the opcode byte in combination with the modr/m and sib bytes A fixed value of the four memory byte address least significant bits corresponding to a particular set of predecode logic (mod 16) is added to the computed length ILen to determine the predecode expression, (DecPC[3 0]+ILen0) mod 16 If the opcode is, m fact, a short decode mstruction opcode, then the resultmg four-bit value is assigned as the predecode bits that are loaded mto the instruction cache 214 Otherwise, the fixed four-bit memory address least significant bit value for the mstruction byte is used as the predecode bits as though the mstruction length was zero Predecode information is only used to facilitate the decoding of multiple short decode mstructions, therefore instruction length is determined accurately only for short decode instructions The length of all short decode instructions ranges from one to seven bytes For instructions that are not short decode mstructions, the predecoder 270 specifies an effective instruction length of zero to enable the instruction lookahead logic to quickly and easily determine the validity of the predecode information that is calculated during each decode cycle In particular, instruction lookahead logic compares the values of the predecode bits and the fixed address least significant bits for an instruction byte in the instruction buffer 408 If the values match, then the mstruction byte is not the starting byte of a short decode instruction Short decode instructions are never longer than seven bytes so that the difference between the four predecode bits and the fixed byte address least significant bit values is never more than seven bytes Accordingly only three mstruction length predecode bits of six total predecode bits are stored the mstruction cache 214 because the most significant bit is readily reconstructed from the least significant three bits plus the associated fixed byte address least significant bit value The reconstruction or expansion of the three mstruction length predecode bits mto four bits is performed as instruction bytes are fetched from the mstruction cache 214 and loaded mto the mstruction buffer 408 More specifically, the instruction buffer 408 includes mput circuitry with sixteen sets of predecode expansion logic which independently and simultaneously expand the predecode bit values associated with all sixteen instruction bytes transferred from the mstruction cache 214 For the final two bytes of the sixteen bytes that are predecoded and loaded mto the mstruction cache 214, only one and two bytes are available for examination by the predecode logic For non-modr/m opcodes a full predecode analysis is possible However, for modr/m opcodes the full instruction length cannot be determined Consequently, predecode logic for bytes 1 and 15 having address least significant bits of 1 1 10 and 1 1 1 1 respectively, are modified versions of the predecode logic associated with the byte 0 through 13 For byte 15, an effective instruction length of zero is forced for all modr/m opcodes as well as for non-short decode mstructions For byte 14, an effective mstruction length of zero is forced for non-short decode instructions and for modr/m opcodes havmg an address mode which requires examination of a sib byte to reliably determine mstruction length Various uncertainties aπse in the predecoding process For example, a prefix byte hav g a hexidecimal value of OF may be mcluded m the instruction byte stream Also uncertainty concemmg the effective address and operand sizes arises due to the possible presence of a prefix byte and a possible difference between the D bit value in effect at cache fill time and the D bit value in effect when the predecode information is used at decode time. Predecode logic addresses these uncertainties by handling the instruction bytes under the assumption that the opcode byte is, in fact, a one byte opcode, and that the effective address and operand sizes are both 32 bits. This assumption results in appropriate handling of a prefix byte in the same manner as handling of a long and vectoring decode so that no predecode information is used. With regard to the variable D-bit value, the assumption that the instruction byte is actually a one byte opcode leads to appropriate handling because, when the D-bit value in effect during a decode cycle indicates a sixteen-bit address, decoding of multiple instructions by the short decoder is inhibited. The proper effective address and operand size are used by the first short decoder SDecO 410 but associated predecode information is not used. The effective address sizes for the second and third short decoders SDecl 412 and SDec2 414 always match the current D-bit value. The following pseudo-RTL description summarizes logic within the predecoder 270 logic for one instruction byte channel of the sixteen instruction byte channels. Logic in the predecoder 270 examines each of sixteen instruction bytes prior to loading into the instruction cache 214 and determines, for each instruction byte independently of the other instruction bytes, a three-bit predecode value. The three-bit predecode value is determined on the basis of a calculation of two values, ShortI and SILen[2..0], from a predecode operation performed on each instruction byte and the immediately following one or two instruction bytes. For the final two instruction bytes, bytes 14 and 15, of a group of sixteen instruction bytes, a reduced predecode operation employing analysis of only one or two instruction bytes is performed. The values ShortI and SILen[2..0] specify whether the instruction byte, as an opcode byte, is included within the short decode subset of instructions. If the instruction is a short decode instruction, then the values ShortI and SILen[2..0] also specify the length, in bytes, of the short decode instruction. In this description a ShortI function applies to fully decode opcode, modr/m, and sib bytes for all short decode cases, excluding the determination of sib address modes for instruction byte fourteen of the sixteen instruction byte channels and excluding all modr/m opcodes for instruction byte fifteen. In contrast, the SILen[2..0] function applies to partially decode opcode, modr/m and sib bytes using non-short decode opcode and address mode cases as don't cares. Instruction length is determined using the SILen function. if (ShortI) PreDec[2..0] = FixedAddr[2..0] + SILen[2..0] else PreDec[2..0] = FixedAddr[2..0] Here, FixedAddr is equal to i for instruction byte i. The following pseudo-RTL description summarizes the pre-decode expansion logic: if (PreDec[2..0] < FixedAddr[2..0]) PreDec[3] = ~FixedAddr[3] else PreDec[3] = FixedAddr[3] The predecoder 270 may be implemented using various circuits, methods and techniques, mcludmg conventional logic circuits To most simply express the function of the predecoder 270 a functional definition is given in terms of a lookup table and a small amount of additional logic The lookup table includes 12 entries and is indexed by the first opcode byte plus an additional signal that is decoded from part of a byte followmg the first opcode byte Each entry contains the followmg fields ILen[2 0], ShortIV, Modrm, NoLrgDisp, and OnlySubOpc7 The ILen field directly furnishes the instruction length value for non-modr/m instructions For modr/m instructions, the ILen field in combination with an address mode-dependent value, furnishes the mstruction length value Final SILen[2 0] values range from I to 7 bvtes for valid short decode instructions The ShortIV field largely indicates whether the byte is a short decode instruction The Modrm field is used to gate or mask ShortIV for the sixteenth byte Other fields represent additional constraints, in the case of modr/m opcodes relating to whether the particular address mode or sub-opcode of the instruction is acceptable in conjunction with that opcode The following table designates table entries potentiallv corresponding to the short decode instructions Non-short decode instruction table entries have undefined field values except for ShortIV=0 Equations following the table define the computation of the final, effecm e ShortI signal in response to input signals including three predecoded bvtes, Opc0[7 0], Ope l [7 0], and Opc2[7 0] For the fifteenth and sixteenth bytes the third byte Opc2 or the second and third bytes Opel and Opc2 respectively correspond to existmg bytes havmg values that are not previously defined 15,x 1A,0 1A,1 1B,0 1B,1 lC,x lD,x 22,0 22,1 23,0 23,1 24,x 25,x 26,x 2A,0 2A,1 2B,0 2B,1 2C,x Entry _# 2D,x 2E,x 32,0 32,1 33,0 33,1 34,x 35,x 36,x 3A,0 3A,1 3B,0 3B,1 3C,x 3D,x 3E,x 40,x 41,x 42,x 43,x 44,x 45,x 46,x 47,x 48,x 49,x 4A,x B,x C,x D,x E,x F,x 50,x 51,x 52,x 53,x 54,x 55,x 56,x 57,x 58,x 59,x 5A,x 5B,x 5C,x 5D,x Entry _U 5E,x 5F,x 64,x 65,x 66,x 67,x 70,x 71 ,x 72,x 73,x 74,x 75,x 76,x 77,x 78,x 79,x 7A,x 7B,x 7C,x 7D,x 7E,x 7F,x 80,0 80,1 81 ,1 82,0 82, 1 83,0 83,1 88,0 88,1 89,0 89,1 8A,0 8A,1 8B,0 8B,1 8D,0 90,x A0,x Al,x A2,x A3,x B0,x Bl,x B2,x 1 0 x MOV reg8,imm8 Entry Short Modr NoLrg Instruction _# ILen IV m Ei≤E Mnemonic B3.x 2 0 x MOV reg8,imm8 B4,x 2 0 x MOV reg8,imm8 B5,x 2 0 x MOV reg8,imm8 B6,x 2 0 x MOV reg8,imm8 B7,x 2 0 x MOV reg8,imm8 B8,x 5 0 x MOV reg32,imm32 B9,x 5 0 X MOV reg32,imm32 BA,x 5 0 X MOV reg32,imm32 BB,x 5 0 X MOV reg32,imm32 BC,x 5 0 X MOV reg32,imm32 BD,x 5 0 X MOV reg32,imm32 BE,x 5 0 X MOV reg32,imm32 BF,x 5 0 X MOV reg32,imm32 C0,1 3 0 Shift r8,imm8 Cl ,l 3 0 Shift r32,imm8 C6,0 3+ MOV m8,imm8 C7,0 6+ MOV m32,imm32 D0,1 2 0 Shift r8, l D1J 2 0 Shift r32,l D2, l 2 0 Shift r8,CL D3,l 2 0 Shift r32,CL E2,x 2 0 X LOOP E8,x 5 0 X CALL disp32 E9,x 5 0 X JMP disp32 EB,x 2 0 X JMP dispδ FO.x 0 X LOCK prefix F2,x 1 0 X REPNE prefix F3,x 1 0 X REP REPE prefix F6J 2 1 0 NOT r8 F7, l 2 1 0 NOT r32 default x 0 x X all other table entries Entry ILen Short Modrm NoLrg Instruction # IV Disp Mnemonic Logic for calculating the effective ShortI value is described by the following pseudo-RTL, as follows: LrgDisp = "disp32" + "da32" = (Opcl [7..6] = 'blO) + (Opc l [7..6,2..0] = 'b00101 ) -<- ((Opc l[7..6,2..0] = 'b00100) (Opc2[2..0] = 'bl 01 )) DA16 = (Opcl[7..6,2..0] = 'bOOl 10) Logic for ensuring that only NOT r/m is recognized as a short decode for this opcode byte value is described as follows: SubOpc2 = (Opcl [5..3] = 'b010) OnlySubOpc2 = (Opc0[7..0] = 'bl 11101 lx) Logic for ensuring that only CMP m,imm is recognized as a short decode for this opcode byte value and not for other "op"s is described as follows: SubOpc7 = (Opcl [5..3] = 'bl l l) OnlySubOpc7 = (Opc0[7..0] = 'blOOOOxxx) -RegModr Logic for ensuring that only non-rotate shift mstructions are recognized as short decodes is described as follows RotShift = (Opc0[7 0] = 'bl lOxOOxx) (Opel [5] = 'b0) Logic for determining the ShortI value for the first fourteen bvtes is described as follows ShortI = ShortIV & -(Modrm (NoLrgDisp (LrgDisp - DA 16) + Onl\ SubOpc7 ~SubOpc7 ■ > ■ RotShift)) Logic for determining the ShortI value for the fifteen byte is described as follows ShortI = ShortIV & -(Modrm (NoLrgDisp (LrgDisp + DA 16) + SubOpc7 ~SubOpc7 + RotShift + (Opel [7 6.2 0] = 'bOO l OO)) ) Logic for determining the ShortI value for the sixteen byte is described as follows ShortI = ShortIV & -Modrm Logic for determining the ILen value is described as follow s Sib = (Opel [7 6,2 0] = (~'bl lxxx 'bxxlOO) ) Dιsp8 = (Opcl [7 6] = 'b01 ) ALen[2 0] = 3'bO , if (Sib) ALen[] = ALen[] + 1 , if (Dιsp8) ALen[] = ALen[] + 1 , ιf (LrgDιsp) ALen[] = ALen[] -t- 4 SILen[2 0] = (Modrm) " (ILen[2 0] + ALen[2 0]) mod 8 ILen[2 0] For the instruction MOV m32,ιmm32 with (Dιsp8 Sib), then final length ALen=8 which ultimately results causes this instruction byte to be treated as a non-short decode opcode despite determining a ShortI value of 1 During each decode cycle, the macroinstruction decoder 230 checks for several exception conditions, including an instruction breakpoint, a pending nonmaskable interrupt (NMI), a pending interrupt (INTR), a code segment overrun, an instruction fetch page fault, an instruction length greater than sixteen bytes, a nonlockable mstruction with a LOCK prefix, a floating point not available condition and a pendmg floating point error condition Some conditions are evaluated only durmg an otherwise successful decode cycle, other conditions are evaluated irrespective of any decoding actions during the cycle When an active exception condition is detected, all instruction decode cycles including short, long and decode cycles, are inhibited and an "exception" vectoring decode is forced in the decode cycle follow ing exception detection The recognition of an exception condition is only overridden or inhibited by inactivity of the macroinstruction decoder 230, for example, when emulation code Op quads are accepted by the scheduler 260, rather than short and long or vector decoder Op quads In effect, recognition and handling of any exception conditions are dela\ ed until an ERET Op seq returns control to the macroinstruction decoder 230 During the decode cycle that forces exception vectoring, a special emulation code vector address is generated in place of a normal instruction vector address The vectoπng Op quad that is generated by the long and vectoring decoders 416 and 418 is undefined. The exception vector address is a fixed value except for low-order bits for identifying the particular exception condition that is recognized and handled. When multiple exception conditions are detected simultaneously, the exceptions are ordered in a priority order and the highest priority exception is recognized. The instruction breakpoint exception, the highest priority exception condition, is recognized when the linear decode PC 424 points to the first byte of an instruction including prefixes, the linear decode PC 424 matches a breakpoint address that is enabled as an instruction breakpoint, and none of the instruction breakpoint mask flags are clear. One mask flag (RF) specifically masks recognition of instruction breakpoints. Another mask flag (BNTF) temporarily masks NM1 requests and instruction breakpoints. The pending NMI exception, the penultimate priority exception, is recognized when an NMI request is pending and none of the NMI mask flags are clear. One mask NF) specifically masks nonmaskable interrupts. Another mask flag (BNTF) temporarily masks NMI requests and instruction breakpoints. The pending INTR exception, the next exception in priority following the pending NMI exception, is recognized when an INTR request is pending and the interrupt flag (IF) and temporary interrupt flag (ITF) are clear. The code segment overrun exception, the next exception in priority following the pending INTR exception, is recognized when the macroinstruction decoder 230 attempts to successfully decode a set of instructions beyond a current code segment limit. The instruction fetch page fault exception, having a priority immediately lower than the code segment overrun exception, is recognized when the macroinstruction decoder 230 requires additional valid instruction bytes from the instruction buffer 408 before decoding of another instruction or prefix byte is possible and the instruction translation lookaside buffer (ITB) signals that a page fault has occurred on the current instruction fetch. A faulting condition of the instruction fetch control circuit 218 is repeatedly retried so that the ITB continually reports a page fault until the page fault is recognized by the macroinstruction decoder 230 and subsequent exception handling processing stops and redirects instruction fetching to a new address. The fault indication from the ITB has the same timing as instructions loaded from the instruction cache 214 and, therefore, is registered in the subsequent decode cycle. The ITB does not necessarily signal a fault on consecutive instruction fetch attempts so that the macroinstruction decoder 230 holds the fault indication until fetching is redirected to a new instruction address. Upon recognition of a page fault, additional fault information is loaded into a special register field. The instruction length greater than sixteen exception, which has a priority just below the instruction fetch page fault exception, is recognized when the macroinstruction decoder 230 attempts to successfully decode an instruction having a total length including prefix bytes of greater than fifteen bytes. The instruction length greater than sixteen bytes exception is detected by counting the number of prefix bytes before an actual instruction is decoded and computing the length of the rest of the instruction when it is decoded. If the sum of the prefix bytes and the remaining instruction length is greater than sixteen bytes, an error is recognized. The nonlockable instruction with a LOCK prefix exception, having a priority below the instruction length exception, is recognized when the macroinstruction decoder 230 attempts to successfully decode an instruction having a LOCK prefix, in which the instruction is not included in the lockable instruction subset The nonlockable LOCK instruction exception is detected based on decode of the opcode byte and existence of a OF prefix The nonlockable LOCK instruction exception only occurs during vectoring decode cycles since the LOCK prefix inhibits short and long decodes The floating point not available exception, having a next to lowest priority, is recognized when the macroinstruction decoder 230 attempts to successfully decode a WAIT instruction or an ESC instruction that is not a processor control ESC, and the reporting of a floating point error is pending Macroinstruction decoder 230 detects the floating point not available exception based on decoding of an opcode and modr/m byte, in addition to the existence of a OF prefix During each decode cycle, the macroinstruction decoder 230 attempts to perform some form of instruction decode of one or more instructions Typically, the macroinstruction decoder 230 succeeds in performing either one or multiple short decodes, one long decode or an instruction vectoring decode Occasionally no decode is successful for three types of conditions including detection of an active exception condition lack of a sufficient number of valid bytes in the instruction buffer 408, or the macroinstruction decoder 230 does not advance due to an external reason When an active exception condition is detected all forms of instruction decode are inhibited and, during the second decode cycle after detection of the exception condition, an exception vectoring decode cycle is forced, producing an invalid Op quad When an insufficient number of valid bytes are available in the instruction buffer 408 either no valid bytes are held in the instruction buffer 408 or at least the first opcode is valid and one of the decoders decodes the instruction but the decoded instruction length requires further valid bytes in the instruction buffer 408, not all of which are currently available When an external reason prevents macroinstruction decoder 230 advancement either the scheduler 260 is full and unable to accept an additional Op quad durmg a decode cycle or the scheduler 260 is currently accepting emulation code Op quads so that the macroinstruction decoder 230 is inactive awaiting a return to decoding In the latter two cases, the decode state of the macroinstruction decoder 230 is inhibited from advancing and the macroinstruction decoder 230 simply retries the same decodes in the next decode cycle Control of macroinstruction decoder 230 inhibition is based on the generation of a set of decode valid signals with a signal corresponding to each of the decoders For each decoder there are multiple reasons which are combined into decoder valid signals to determine whether that decoder is able to successfully perform a decode The decoder valid signals for all of the decoders are then monitored, in combination, to determine the type of decode cycle to perform The type of decode cycle is indicative of the particular decoder to perform the decode The external considerations are also appraised to determine whether the selected decode cycle type is to succeed. Signals indicative of the selected type of decode cycle select between various signals internal to the macroinstruction decoder 230 generated by the different decoders, such as alternative next decode PC values, and also are applied to control an Op quad multiplexer 444 which selects the input Op quad applied to the scheduler 260 from the Op quads generated by the short decoders, the long decoder 416 and the vectoring decoder 418. In the case of vectoring decode cycles, the macroinstruction decoder 230 also generates signals that initiate vectoring to an entry point in either internal emulation code ROM 232 or external emulation code RAM 236. The macroinstruction decoder 230 then monitors the active duration of emulation code fetching and loading into the scheduler 260. The branch target buffer (BTB) 456 is a circuit employed for usage in a two-level branch prediction algorithm that is disclosed in detail in U.S. Patent No. 5,454,1 17, entitled CONFIGURABLE BRANCH PREDICTION FOR A PROCESSOR PERFORMING SPECULATIVE EXECUTION (Puziol et al., issued September 26, 1995), U.S. Patent No. 5,327,547, entitled TWO-LEVEL BRANCH PREDICTION CACHE (Stiles et al., issued July 5, 1994), U.S. Patent No. 5,163.140, entitled TWO-LEVEL BRANCH PREDICTION CACHE (Stiles et al.. issued November 10, 1992), and U.S. Patent No. 5, 093,778, entitled INTEGRATED SINGLE STRUCTURE BRANCH PREDICTION CACHE (Favor et al., issued March 3, 1993). The BTB 456 stores quadword data from the first successful fetch of the instruction cache 214 following an event such as a cache miss. The BTB 456 is used to avoid a one cycle delay that would normally occur between the decode of a transfer instruction and a decode of the target instructions. The BTB 456 includes sixteen entries, each entry having sixteen instruction bytes with associated predecode bits. The BTB 456 is indexed by the branch address and is accessed during the decode cycle. Instructions from the BTB 456 are sent to the instruction decoder 220, eliminating the taken-branch penalty, for a cache hit of the BTB 456 when a branch history table (not shown) predicts a taken branch. During each decode cycle, the linear decode PC 424 is used in a direct-mapped manner to address the BTB 456. If a hit, which is realized before the end of the decode cycle, occurs with a BTB entry, a PC-relative conditional transfer control instruction is decoded by a short decoder and the control transfer is predicted taken, then two actions occur. First, the initial target linear fetch address directed to the instruction cache 214 is changed from the actual target address to a value which points to an instruction byte immediately following the valid target bytes contained in the BTB entry. This modified fetch address is contained in the BTB entry and directly accessed from the BTB entry. Second, the instruction byte and predecode information from the entry is loaded into the instruction buffer 408 at the end of the decode cycle. If a PC-relative conditional transfer control instruction is decoded by a short decoder and the control transfer is predicted taken, but a miss occurs, then a new BTB entry is created with the results of the target instruction fetch. Specifically, simultaneously with the first successful load of target instruction bytes into the instruction buffer 408 from the instruction cache 214, the same information is loaded into a chosen BTB entry, replacing the previous contents. The target fetch and instruction buffer 408 load otherwise proceed normally. Each entry includes a tag part and a data part The data part holds sixteen extended mstruction bytes mcludmg a memory byte and three associated predecode bits The correspondence of the memory byte is memory-aligned with the corresponding mstruction buffer 408 location The tag part of a BTB entry holds a 30-bit tag mcludmg the 32-bit linear decode PC 424 associated with the transfer control mstruction havmg a cached target, less bits [4 1], an entry valid bit and the 30-bit modified initial target lmear mstruction fetch address No explicit mstruction word valid bits are used since the distance between the true target address and the modified target address directly implies the number and designation of valid mstruction words within the BTB 456 The purpose of the BTB 456 is to capture branch targets within small to medium sized loops for the time period a loop and nested loops are actively executed In accordance with this purpose, at detection of a slightest possibility of an inconsistency, the entire BTB is invalidated and flushed The BTB 456 is invalidated and flushed upon a miss of the mstruction cache 214, any form of mvalidation of instruction cache 214, an ITB miss or any form of ITB invalidation Branch targets outside temporal or spatial locality are not effectively cached Typically the BTB 456 contains only a small number of entries so that complexity is reduced while the majority of performance benefit of ideal branch target caching is achieved The instruction cache 214 stores information in the form of a list of a plurality of instruction bytes in a predetermined order This predetermined order is specified by associating a pointer with each element m the list that points to the starting point of the next element in the list The pointers for the instruction cache 214 list are the predecode bits One aspect of predecoder 270 operations is the generation of these pointers and storage of the pointers as predecode bits in the instruction cache 214 Referring to Figure 5, a list 500 is shown having three instructions 510, 520 and 530 in a sequence Each mstruction 510, 520 and 530 includes respective instruction byte data 512, 522 and 532 and respective pointers 514. 524 and 534 where the first two pointers 514 and 524 point to the respective instructions 520 and 530 in the list 500 Typically, the instruction byte data and pointers are stored in a binary format with the number of instruction bytes in an instruction byte data group being variable In this example, the first instruction byte data group 512 mcludes four instruction bytes, the second instruction byte data group 522 includes six instruction bytes and the third instruction byte data group 532 includes three instruction bytes The first byte of each instruction is an opcode byte and the address of the opcode byte is the address of the instruction For lists in which the opcode byte of successive instructions are separated by a known maximum number of bytes, the pointers are defined as an offset between the current address and the next address so that the bit width of the pointers is limited to correspond to the maximum number of bytes Accordingly, for a maximum number 2 m of bytes that separate any two instructions, the pointer is m bits wide The size of memory storage is advantageously reduced When the address of the next instruction is sought, the pointer value is easily expanded into a full storage address for usage by an addressable storage device In some embodiments of the predecoder 270, various predefined pointer values represent special events, such as the end of the list or instructions that exceed a specified mstruction byte length Consequently, the maximum number of storage locations from one mstruction to the next is reduced to 2 m less the number of special event instances The list 500 includes pomters 514, 524 and 534 that are stored as 3-bit values and the pointer 534 of the last instruction 530 is set to a code that indicates the end of A pointer that points to a next instruction is equal to the m least significant bits of the address of the next instruction Thus, pointers 514 and 524 are equal to the m least significant bits of the addresses of instructions 520 and 530, respectively Pointer 534 of the third instruction 530 is set to a code, for example zero, that indicates the Referring to Figure 6, a flow chart sho s a method for generating a next instruction address from a pointer In a first step 610, the associated pointer and starting address of an instruction are read The value of the pointer is compared to the value of the m least significant bits of the instruction starting address in logical step 612 If the value of the pointer is greater than the value of the m least significant bits of the instruction starting address, the next instruction is located within the same page as the current instruction For an instruction buffer memory 408 having 2" memory locations, the remaining n-m most significant bits of the current instruction is located In step 616, the storage location of the next instruction is determined by appending the n-m most significant bits of the current instruction starting address to the pointer, the pointer bits being used as the m least significant bits If the pointer value is less than the value of the m least significant bits of the address of the current instruction the next instruction is located on the page following the cuπent instruction The n-m most significant bits of the storage address of the current instruction is adjusted, for example by incrementing the bits, in step 614 prior to being appended to the pointer in step 616 Referring to Figure 7, a memory 700 is shown to illustrate operation of the predecoder 270 for handl g the instruction cache 214 as a list In this example, the memory 700 has a storage capacity of 32 instruction bytes 702 which is extended by three bits of pointer information 704 per instruction byte In various embodiments, the pointer bits may be stored with the mstruction bytes or contained in a separate memory Also in this example, three mstructions 710, 720 and 730 having instruction sizes ranging from one to seven mstruction bytes are stored in memory 700 The first byte of each of the instructions 710, 720 and 730 is an opcode mstruction byte 712, 722 and 732, respectively The opcode mstruction byte 712 is stored in memory location 0000 and the opcode instruction byte 722 is stored in memory location 00100 so that a pomter 714 from the first instruction 710 to the second instruction 720 is 100, which is equal to the three least significant bits of the address of the second instruction 720 The opcode instruction byte 732 is stored in memory location 0101 1 so that a pointer 724 associated with the second instruction 720 is 011 To traverse the memory 700, the pointer 714 (100) associated with the first instruction 710 in memory location 00000 is read and compared to the three least significant bits of the first instruction 720 address (000) The pointer 714 (100) is greater than the first instruction 720 address (000) so that the instruction address of the second instruction 720 is determined by appending the pomter 714 (100), which represents the least significant bits of the address of the second instruction 720, to the two most significant bits (00) of the address of the first mstruction 710 to produce the address of the second instruction 720 at memory location 00100 The location of the third instruction 730 is identified by reading the pomter 724 and comparing the pomter value to the three least significant bits (100) of the address of the opcode instruction byte 722 The pointer 724 (01 1 ) is smaller than the three least significant bits of the address of the opcode instruction byte 722 (100) so that the third instruction 730 is located on the subsequent page (01 ) after page (00) of the second mstruction 720 The remaining most significant bits (00) of the address of the second instruction 720 are incremented by one and appended to the pointer 724 to generate the address of the third instruction 730 (0101 1) The third instruction 730 is the last instruction in the list so that the three least significant bits of the third instruction address are stored in the third pointer 734 (01 1), indicating the end One special event that is represented by a pointer is the end of a list The pointer indicates the end of a list when the pointer is equal to the least significant bits of the current instruction Another special event is a number of instruction bytes within an instruction that exceeds the maximum allowed instruction length The pointer also indicates an illegal instruction length when the pointer is equal to the least significant bits of the current instruction In this manner, only an occasional increment of a few bits furnishes memory space conservation of a typical offset list method without performing a time-consuming addition operation This technique advantageously allows for fast traversal of a list while reducing the space consumed to store each pointer Instruction codes are composed of bytes of binary ones and zeros Nearly every combination of bits are used as opcodes, the first instruction bytes, so that nearly any opcode value could appear to be the first instruction byte This concept is illustrated by the pictorial memory 800 representation shown in Figure 8, which shows three instructions 810, 830 and 850 The first instruction 810 is a four-byte instruction including instruction bytes 812, 814, 816 and 818 In the four-byte instruction 810, any of the instruction bytes 812, 814, 816 and 818 could be the actual first instruction byte To decode the instruction 810, a rotator first ascertains which instruction byte is the first To ascertain which byte of instruction 810 is first, each of instruction bytes 812, 814, 816 and 818 is assumed to be the starting byte of the instruction For every instruction byte 812, 814, 816 and 818, the byte plus, for some instructions, subsequent instruction bytes identify bytes that could possibly be an instruction Once an instruction byte is identified as an actual opcode that is the beginning byte of an instruction, subsequent instruction locations are readily identified from the information associated with the first actual byte 812 and information with the first bytes of subsequent instructions 832 and 852 since the predecode bits indicate the number of mstruction bytes in each instruction For example, if instructions starting with byte 812 (011 10100) were four bytes long and instructions starting with byte 814 (10110100) were five bytes long, byte 812 could represent the start of an instruction with a four-byte length and byte 814 could represent the start of an instruction with a five-byte length While only one of these bytes could represent the start of an instruction, prior to the actual knowledge of which byte is the starting byte, both bytes could be assumed to be starting bytes for the purpose of associating the location of the start of the next instruction with each byte 812 and 814 Thus, pointer information 822 and 824 furnish the location of the next instruction byte if either respective byte 812 and 814 is the first byte of an instruction When byte 812 is identified as the actual starting byte of the instruction 810, the location of the following instruction 830 is immediately available using the pointer 822 Pointer 824 is ignored because byte 814 is not the first byte of an instruction The pointers associated with bytes that are not actual first bytes are always ignored, resulting in wasted analysis However, this wasted analysis is greatly overshadowed by the benefit of rapid knowledge of the start of all actual instructions and simultaneous decodmg of multiple instructions Certain instructions, such as complex instructions or instructions that execute over many cycles, are not executed simultaneously with other instructions even if decoded simultaneously The analysis to determine the first byte of an instruction is simplified if no attempt is made to decode these complex instructions in parallel Accordingly, some instructions that generally execute rapidly are classified as "short decodable" instructions For short decodable instructions, performance is improved by decoding a plurality of instructions in parallel Other instructions that are not short decodable take longer to execute so that no benefit is obtained by rapid decoding Accordingly, the location of a subsequent instruction is only determined in advance if the instruction is short decodable Referring to the flow chart shown in Figure 9, a predecoding method 900 of preparing computer instructions for rapid decoding is shown In step 910, an instruction byte is selected and assumed to be the starting byte of an instruction in step 912 The instruction byte is read and a determination is made whether the instruction byte belongs to the set of short decodable instructions in step 91 For some instruction bytes, subsequent adjacent instruction bytes are also read to determine short decodable status If the instruction byte is not within the set of short decodable instructions, the instruction is indicated to be of a "non-short-decodable" class in step 916 Although this indication may be made in various ways m different embodiments, in one embodiment an instruction is identified to be non-short-decodable by specifying an instruction length of zero Typically no valid instructions have an instruction length of zero If the instruction is a short decodable instruction, step 918 determines whether the instruction length of the instruction is ascertainable Step 918 typically prohibits predecoding of instructions that do not contain all information to allow a determination of instruction length If the instruction length is not ascertainable, the instruction is indicated to be of a "non-short-decodable" class in step 916 Otherwise, the instruction length is ascertainable and determined in step 920 in accordance with the instruction set definition of the processor The location of the next instruction is then determined in step 922 using the memory location of the first instruction and the instruction length furnished by the pointer The location of the following instruction, referenced to the first byte is stored in step 924 These steps are typically repeated in accordance with looping step 926, which determines whether additional bytes are in the memory, and the select next byte step 928 When the final instruction byte in the memory is analyzed, the method terminates in step 930 To determine the length of an instruction in step 922, information that is not in the opcode mstruction byte but which is instead contained in subsequent bytes is used for some instructions In some mstruction sets, mcludmg the x86 instruction set, certain instructions cause the interpretation of following instructions to change For example, in the x86 instruction set, instruction lengths sometimes vary depending upon the D-bit For some instructions, the predecode mstruction length is not affected by the data and address size For other instructions, the D-bit sets a 2X instruction length For still other mstructions, the D-bit designates a special instruction length that is not 2X Thus a D-bit interpretation change can result in a change in instruction length Tracking the interpretation of an instruction prior to actual execution of the instruction is often difficult or impossible In some cases, some interpretations of the instruction are short decodable interpretations and other interpretations are non-short decodable In these cases, the instruction is assumed to be short decodable The method, of course, performs correctly when the assumption is correct and the location of the following instruction is correct If the assumption is incorrect, the location of the following instruction is incorrect, but irrelevant, because the location information is only used for short decodable instructions At execution time, the selection of a different processor operating mode causes no search for short decodable instructions to be performed so that the incorrect assumption does not affect performance Figure 10 is a schematic diagram showing the operation of one embodiment of predecode logic 270 for transferring instructions from the instruction cache 214 to the macroinstruction decoder 230 The cache memory 214 holds a group of instruction bytes 990 One instruction length computation module 996 per cache memory location is connected to the cache memory 214 to read instruction bytes contained in the cache memory 214 In other embodiments, fewer mstruction length computation modules 996 are used However, one module 996 per instruction byte furnishes the fastest computation The output signal of the instruction length computation module 996 indicates a binary length of an instruction If the instruction is not predecodable, either because the instruction is not short decodable or inadequate information is contained in the cache memory 214 to determine an instruction length, the instruction length computation module 996 indicates this status, for example, by setting the instruction length to zero A next instruction computation module 998 adds the output signal of the instruction length computation module 996 to a location code of the corresponding byte in the cache memory 214 to produce the location code of the instruction following the instruction containing the corresponding byte This location code is stored in the cache memory 214 to be read when the macroinstruction decoder 230 decodes the instruction bytes 990 stored in the cache memory 214 This predecode information is determined and stored soon after the instruction byte is placed in the cache memory 214 This computation is performed regardless of whether the mstruction byte is a first mstruction byte, or opcode Only the predecode mformation for an actual first mstruction is ultimately used for decoding instructions However, the operation of computing predecode information for all mstruction bytes furnishes the location of a true next instruction most rapidly Referring to Figure 11 , a schematic circuit diagram shows an embodiment of a next instruction computation module 998 for connecting output signals of two instruction length computation modules 996 that are connected to cache memory locations 0000 and 0001 For each input signal 1110 and 1112, the next instruction computation module 998 adds the input signals 1110 and 1112 to the memory location code corresponding to the input signals The input signal 1110 connects to an instruction length computation module 996 that computes the instruction length for the byte or bytes starting at cache memory location 0000 The next instruction computation module 998 adds this input signal to binary 0000 to produce an output signal 1114 For cache memory location 0000, this sum is equal to the mput signal The next mstruction location computation module 998 transfers the input signal 1110 to the output signal 1114 via a bus 1126 Input signal 1112 are connected to receive the output signal of the instruction length computation module 996 for the byte or bytes beginning at cache memory location 0001 Output signals 1116 are equal to a binary representation of the input signals 1112 added to 0001 Inverters 1120, AND gates 1122 and OR gates 1124 furnish this next instruction computation function Output signals 1114 and 1116 are typically connected to the corresponding cache memory locations which hold the location of the next instruction for future potential decoding Referring to Figure 12, a schematic pictorial illustration of an arrangement of bits in the cache memory 214 is shown Instruction bits 1210 are stored adjacent to a predecode code 1212 which indicates the next location Eight instruction bits 1210 and five predecode bits 1212 including three next instruction location code bits 1212, a D-bit 1214 and a HASMODRM bit 1215 are illustrated In other embodiments different numbers of bits are used to encode instruction data and predecode information SYSTEM DISCLOSURE TEXT A wide variety of computer system configurations are envisioned, each embodying an instruction predecode and multiple instruction decode in accordance with the present invention For example, such a computer system (e g , computer system 1000) includes a processor 100 providing instruction predecode and multiple instruction decode in accordance with the present invention, a memory subsystem (e g , RAM 1020), a display adapter 1010, disk controller/adapter 1030, various input output interfaces and adapters (e g , parallel interface 1009, serial interface 1008, LAN adapter 1007, etc ), and corresponding external devices (e g , display device 1001, printer 1002, modem 1003, keyboard 1006, and data storage) Data storage includes such devices as hard disk 1032, floppy disk 1031, a tape unit, a CD-ROM, a jukebox, a redundant array of inexpensive disks (RAID), a flash memory, etc While the invention has been described with reference to various embodiments, it will be understood that these embodiments are illustrative and that the scope of the invention is not limited to them Many variations, modifications, additions, and improvements of the embodiments described are possible Additionally, structures and functionality presented as hardware in the exemplary embodiment may be implemented as software, firmware, or microcode in alternative embodiments For example, the description depicts a macroinstruction decoder having short decode pathways including three rotators 430, 432 and 434, three instruction registers 450, 452 and 454 and three short decoders SDecO 410, SDecl 412 and SDec2 414 In other embodiments, different numbers of short decoder pathways are employed A decoder that employs two decoding pathways is highly suitable These and other variations, modifications, additions, and improvements may fall within the scope of the invention as defined in the claims which follow

Claims: WE CLAIM; 1 An apparatus for accessing a plurality of variable-length instructions from an instruction source and preparing the instructions for execution, the variable length instructions including one or more instruction units, an instruction unit being a multiple-bit instruction portion, the apparatus comprising an instruction cache coupled to the instruction source and having a plurality of instruction storage elements for storing instruction units and having a plurality of code storage elements for storing code units, a code unit bemg a multiple-bit code portion, the instruction storage elements being associated with corresponding code storage elements and storing instruction units being associated with corresponding code units, a predecoder coupled to the mstruction cache for reading instruction units, assigning a code indicative of instruction length based on the read instruction units assuming each instruction unit is a first instruction of a variable-length instruction, a decoder coupled to the instruction cache to receive a pluralin of instruction units and a plurality of corresponding code units, the decoder including a plurality of first instruction type decoders and a second instruction type decoder, the decoder for decoding a plurality of first instruction type instructions in parallel using the plurality of first instruction type decoders when a first instruction type instruction unit and a corresponding code unit is received from the instruction cache and otherwise for decoding an instruction using the second instruction type decoder 2 An apparatus according to Claim 1 wherein the instruction unit is an eight-bit byte, and the code unit is a plurality of bits designating a next instruction pointer and an instruction type 3 An apparatus according to Claim 1 wherein the decoder decodes a plurality of first instruction type instructions in parallel using the plurality of first instruction type decoders when a plurality of consecutive first instruction type mstruction units and corresponding plurality of code units are received from the instruction cache 4 An apparatus according to Claim 1 further comprising a plurality of rotators coupled between respective first instruction type decoders and the instruction cache, the plurality of rotators for aligning the first instruction type instructions for decoding 5 An apparatus according to Claim 1 further comprising a plurality of instruction registers coupled between respective first instruction type decoders and the instruction cache, the plurality of instruction registers for temporarily holding the first instruction type instruction units and corresponding plurality of code units so that a decode timing cycle is reduced 6. An apparatus according to Claim 1 wherein the predecoder includes a circuit for generating an address of a next instruction in a list of a plurality of instructions, the instruction cache for storing the list of instructions at addressed locations, the circuit including: a signal generator having a plurality of output terminals coupled to the instruction cache and having an input terminal coupled to receive a current address pointer wherein, in response to receipt of the current instruction pointer, the signal generator activates one of the plurality of output terminals coupled to the next instruction at an addressed location. 7. An apparatus according to Claim 1 wherein the predecoder includes a circuit for generating a next address of a next instruction after a current instruction in an increasing sequential list of a plurality of instructions in the instruction cache, the instruction cache being addressable and storing the list of instructions, the current instruction including a current instruction pointer and a current instruction address having a first portion that is a number of most significant bits of the current instruction address and having a second portion that is a number of least significant bits of the current instruction address, the circuit for generating a next address including: a next instruction address computation unit coupled to the instruction cache and receiving from the instruction cache the current instruction pointer and the current instruction address, wherein: in response to a value of the current instruction address pointer being greater than a value of the second portion of the current instruction address, the next instruction address computation unit generates the next instruction address having a first portion equal to the first portion of the cuπent instruction address and having a second portion equal to the current instruction address pointer; and responsive to the value of the current instruction address pointer being less than the value of the second portion of the current instruction address, the next instruction address computation unit generates the next instruction address having a first portion of the current instruction address and having a second portion equal to the current instruction pointer. 8. An apparatus according to Claim 1 wherein the predecoder includes a circuit for generating a next address of a next instruction after a current instruction in a decreasing sequential list of a plurality of instructions in the instruction cache, the instruction cache being addressable for storing the list of instructions, the current instruction including a current instruction pointer and a current instruction address having a first portion that is a number of most significant bits of the current instruction address and having a second portion that is a number of least significant bits of the current instruction address, the circuit including: a next instruction address computation unit coupled to the instruction cache and receiving the current instruction pointer and the current instruction address from the instruction cache, wherein: responsive to a value of the current instruction pointer being less than a value of the second portion of the current instruction address, the next instruction address computation unit generates the next I 1 instruction address having a first portion equal to the first portion of the current mstruction 12 address and having a second portion equal to the current mstruction pointer, and 13 responsive to the value of the current instruction pointer being greater than the value of the second portion 14 of the current mstruction address, the next instruction address computation unit generates the next 15 instruction address having a first portion equal to an adjusted value of the first portion of the 16 current instruction address and having a second portion equal to the current instruction pointer 1 9 An apparatus for receiving a plurality of variable-length mstructions from an instruction source and 2 preparing the variable-length instructions for execution, the variable-length instructions including a variable number 3 of instruction units, an instruction unit being a multiple-bit instruction portion, the apparatus comprising 4 a decoder including a plurality of first instruction type decoders for decoding a plurality of instructions in 5 parallel and including a second instruction type decoder, 6 a memory coupled to the instruction source to receive a plurality of instruction units in parallel and coupled 7 to the decoder to supply a plurality of instruction units mutually associated into instructions, the 8 memory including a plurality of instruction storage units for storing instruction units and a 9 plurality of respective instruction length storage units associated to respective instruction storage 10 units, and I I a predecoder coupled to the memory and coupled to the decoder for reading one or more instruction units, 12 determining an instruction length associated to a first instruction unit assuming that the instruction 13 unit is a first unit of an instruction, storing the instruction length associated to the first instruction 14 unit, associating instruction units into instructions, determining whether a plurality of instruction 15 units are decodable by the first instruction type decoder, and applying a plurality of instruction 16 units to the decoder, the instruction units being applied to the first instruction type decoders for a 17 plurality of instructions decodable by the first instruction type decoders 1 10 An apparatus according to Claim 9 wherein 2 the instruction unit is an eight-bit byte 1 1 1 A method of receiving a plurality of variable-length instructions from an instruction source and 2 preparing the variable-length instructions for execution, the variable-length instructions including a variable number J of instruction units, an instruction unit being a multiple-bit instruction portion, the method comprising the steps of 4 receiving a plurality of instruction units in parallel, 5 storing the plurality of instruction units. 6 reading one or more instruction units, 7 determining an instruction length associated to a first mstruction unit assuming that the instruction unit is a 8 first unit of an instruction, 9 storing the instruction length associated to the first instruction unit, associating stored instruction units into instructions, determining whether a plurality of instruction units are decodable by a plurality of first instruction type decoders for decoding a plurality of instructions in parallel or are decodable by a second instruction type decoder serially, and applying a plurality of instruction units to the first instruction type decoders and the second instruction type decoder, the instruction units being applied to the first instruction type decoders for a plurality of instructions decodable by the first instruction type decoders 12 An apparatus according to Claim 1 1 wherein the instruction unit is an eight-bit byte 13 An apparatus for receiving a plurality of variable-length instructions from an mstruction source and preparing the variable-length instructions for execution, the variable-length instructions including a variable number of instruction units, the apparatus comprising a decoder including a plurality of first instruction type decoders for decoding a plurality of instructions in parallel and including a second instruction type decoder, a memory coupled to the instruction source to receive a plurality of instruction units in parallel and coupled to the decoder to supply a plurality of instruction units mutually associated into instructions, the memory including a plurality of instruction storage units for storing instruction units, and a predecoder coupled to the memory and coupled to the decoder for reading one or more instruction units, determining an instruction length associated to a first instruction unit assuming that the instruction unit is a first unit of an instruction, associating instruction units into instructions, determining whether a plurality of instruction units are decodable by the first instruction type decoder, and applying a plurality of instruction units to the decoder, the instruction units being applied to the first instruction type decoders for a plurality of instructions decodable by the first instruction type decoders 14 An apparatus for receiving a plurality of instruction units from an instruction source and furnishing one or more instructions to an instruction decoder, a variable number of instruction units being associated into a variable length instruction, the apparatus comprising an instruction length encoder for accessing a plurality of instruction units from the instruction source in parallel and assigning an instruction length to a designated instruction unit, the instruction length being indicative of the number of instruction units for forming an instruction characterized by the designated instruction unit, and an instruction type selector coupled to the instruction length encoder for determining an instruction type of a plurality of instructions, a first type of instruction being a type in which a plurality of instructions are decodable in parallel - 4o- 15 An apparatus for preparing processor instructions received from an instruction source for parallel accessing comprising an addressable storage having an input terminal coupled to the instruction source and having an output terminal, the addressable storage for storing a plurality of instruction units, one or more instruction units combining to form one processor instruction, and a plurality of location codes, the location codes respectively corresponding to the plurality of instruction units, ones of the location codes for indicating a location of a subsequent processor instruction following the ones of location codes, and one or more next instruction location computation modules ones of the next instruction location modules having one or more input terminals and one or more output terminals, the output terminal being coupled to the addressable storage, one or more input terminals for accepting an instruction length, the next instruction location computation module for generating and presenting at the one or more output terminals a code representing a sum of the instruction length coupled to the input terminal and an addressable storage address corresponding to the instruction unit representing a portion of a processor instruction having a length presented at the input terminal 16 An apparatus according to Claim 15 wherein the location of the subsequent processor instruction includes an addressable storage location of an instruction unit which immediately follows a processor instruction including the instruction unit representing a portion of a processor instruction having a length presented at the input terminal of the next instruction location computation module 17 An apparatus according to Claim 16 wherein ones of the plurality of instruction units are stored in a different location m the addressable storage, and the output terminal of ones of the next instruction computation modules are coupled to a different location in the addressable storage module 18 An apparatus according to Claim 17 wherein ones of the locations in the addressable storage are coupled to a different next instruction location computation module 19 An apparatus according to Claim 18 wherein the next instruction computation module includes a plurality of logic gates 20 An apparatus according to Claim 15 wherein an instruction unit is a byte 21 A computer system comprising a memory subsystem which stores data and instructions, and a processor operably coupled to access the data and instructions stored in the memory subsystem, wherem the processor includes an apparatus for accessing a plurality of variable-length instructions from an instruction source and preparing the instructions for execution, the vaπable length instructions including one or more instruction units, an instruction unit being a multiple-bit instruction portion, the apparatus mcludmg an mstruction cache coupled to the instruction source and having a plurality of mstruction storage elements for storing instruction units and having a plurality of code storage elements for storing code units, a code unit being a multiple-bit code portion, the instruction storage elements being associated with corresponding code storage elements and stormg instruction units being associated with corresponding code units, a predecoder coupled to the instruction cache for reading instruction units, assigning a code indicative of instruction length based on the read instruction units assummg each instruction unit is a first instruction of a variable-length instruction, a decoder coupled to the instruction cache to receive a plurality of instruction units and a plurality of corresponding code units, the decoder including a plurality of first instruction type decoders and a second instruction type decoder, the decoder for decodmg a plurality of first instruction type instructions in parallel using the plurality of first instruction type decoders when a first instruction type instruction unit and a corresponding code unit is received from the instruction cache and otherwise for decoding an mstruction usmg the second instruction type decoder 22 In a computer system having a memory subsystem which stores data and mstructions, and a processor operably coupled to access the data and instructions stored in the memory subsystem, the processor mcludmg an apparatus for accessmg a plurality of variable-length instructions from an instruction source and preparmg the instructions for execution, the variable length instructions including one or more instruction units, an mstruction unit bemg a multiple-bit mstruction portion, the apparatus characterized as comprising an instruction cache coupled to the instruction source and having a plurality of mstruction storage elements for storing instruction units and having a plurality of code storage elements for storing code units, a code unit being a multiple-bit code portion, the instruction storage elements being associated with corresponding code storage elements and storing instruction units being associated with corresponding code units, a predecoder coupled to the instruction cache for reading instruction units, assigning a code indicative of instruction length based on the read instruction units assuming each mstruction unit is a first mstruction of a variable-length instruction, a decoder coupled to the instruction cache to receive a plurality of instruction units and a plurality of corresponding code units, the decoder including a plurality of first instruction type decoders and a second instruction type decoder, the decoder for decodmg a plurality of first mstruction type instructions in parallel using the plurality of first instruction type decoders when a first instruction type instruction unit and a corresponding code unit is received from the instruction cache and otherwise for decoding an instruction using the second instruction type decoder.

Inventors: BEN-MEIR AMOS, FAVOR JOHN G, BEN-MEIR, AMOS, FAVOR, JOHN, G.
Applicants: ADVANCED MICRO DEVICES INC, ADVANCED MICRO DEVICES, INC.
Application year: 1996
Main classification: G06F9
IPC codes: G06F9