Patent Publication Number: US-11048507-B2

Title: Compressed instruction format

Description:
FIELD OF THE INVENTION 
     Embodiments of the invention relate to microprocessors and microprocessor systems. In particular, at least one embodiment of the invention relates to an instruction encoding technique resulting in a compressed instruction format. 
     BACKGROUND OF THE DISCLOSURE 
     Microprocessor technology has evolved over the years at a fast rate. Advances in computer architecture and semiconductor technology have created many opportunities to design new processors. There are typically two options for designing new processors: (1) defining a completely new architecture, and (2) extending the current architecture to accommodate new features. 
     New processors involve new features in both hardware and software. A new processor based on existing design typically has an additional set of instructions that can take advantage of the new hardware design. However, extending an instruction set by adding a new set of instructions is a challenging problem because of the constraints in the encoding of the instructions and in storing instruction opcode maps. Therefore there is a need in the industry to provide an efficient method for extending an instruction set without increasing hardware complexity and without significantly expanding the number and size of opcode maps. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and advantages of the invention will become apparent from the following detailed description of the invention in which: 
         FIG. 1  is a block diagram illustrating a processing system, in which at least one embodiment may be used. 
         FIG. 2  is a block diagram illustrating an instruction format which may include fields represented by one embodiment. 
         FIG. 3  is a block diagram illustrating logic to decode an instruction, in accordance with at least one embodiment. 
         FIG. 4  is a diagram illustrating a first phase decoder, according to one embodiment. 
         FIG. 5  is a diagram illustrating a second phase decoder, according to one embodiment. 
         FIG. 6  is a flow diagram illustrating operations to perform one embodiment. 
         FIG. 7  illustrates an embodiment of an instruction format. 
         FIG. 8  illustrates an embodiment of an instruction format. 
     
    
    
     DETAILED DESCRIPTION 
     At least one method, apparatus and system for extending an instruction set using embodiments of a unique instruction attribute encoding. Disclosed embodiments further enable an instruction set architecture to expand while conserving storage area for storing opcode maps associated with the instruction set architecture. In at least one embodiment, a set of existing instruction fields are mapped, compressed, or replaced by instruction attribute fields that may retain information associated with prior art instruction fields, but may result in a more compact instruction requiring less storage area for storing the instructions and instruction opcode maps. In one embodiment, instruction attribute encodings may enable further instruction and micro-architectural features, including an extra source operand to be used by some instructions as well as an increased data size for instructions, such as vector instructions (e.g., single-instruction-multiple-data (SIMD) instructions). 
     In the following description, for purposes of explanation, numerous specific details such as processor types, instruction formats, logic gate types, and escape opcode values are set forth in order to provide a thorough understanding of embodiments of the invention. However, it will be apparent to one skilled in the art that these specific details are not required in order to practice embodiments the invention. In other instances, well-known electrical structures and circuits are shown in block diagram form in order not to obscure embodiments of the invention. In the following description, the notation ‘0x’ indicates the number that follows is in hexadecimal format. 
     Reference to  FIG. 1  illustrates a processing system  100 , in which at least one embodiment of the invention may be used. System  100  may be used, for example, to decode and execute new instruction types  118   b  or existing instructions  118   a  in instruction set  118 . In one embodiment, a processing system includes any system that has a processor  110 , such as, for example; a digital signal processor (DSP), a microcontroller, an application specific integrated circuit (ASIC), or a microprocessor. The processor  110  may be of any type of architecture, such as complex instruction set computers (CISC), reduced instruction set computers (RISC), very long instruction word (VLIW), multi-threaded or hybrid architecture. Processing systems may include personal computers (PCs), engineering workstations, personal digital assistants and other hand-held devices, set-top boxes and the like). 
       FIG. 1  illustrates that the processor  110  includes a decode unit  116 , a set of registers  114 , at least one execution unit  112 , and at least one internal bus  111  for executing instructions. Of course, the processor  110  contains additional circuitry, which is not necessary to understanding the invention. The decode unit  116 , registers  114  and execution unit  112  are coupled together by one or more internal bus  111 . The decode unit  116  is used for decoding instructions received by processor  110  into control signals and/or microcode entry points. The instructions may be issued to the decode unit  116  by an instruction buffer (such as, e.g.,  310  in  FIG. 3 ). In response to these control signals and/or microcode entry points, the execution unit  112  performs the appropriate operations. The decode unit  116  may be implemented using any number of different mechanisms (e.g., a look-up table, a hardware implementation, a programmable logic array (“PLA”), etc.). 
     The decode unit  116  is shown to be capable of decoding instructions  106  that follow formats defined by an extended instruction set  118 . The instruction set  118  includes an existing instruction set  118   a  and a new instruction set  118   b , which may represent any new instructions that may be introduced to the architecture. The instruction set  118  includes instructions for performing operations on scalar and packed (or vector) data. The format for these operations can be any convenient format, including single-precision, double-precision, and extended floating-point numbers, signed and unsigned integers, and non-numeric data. For at least one embodiment, the instructions defined in the instruction set  118  may vary in length from one another. 
     Instructions  106 , which follow the formats set forth by the instruction set  118 , may be stored in a memory system  102 . Memory system  102  is intended as a generalized representation of memory or memory hierarchies and may include a variety of forms of memory, such as a hard drive, CD-ROM, random access memory (RAM), dynamic random access memory (DRAM), static random access memory (SRAM), flash memory and related circuitry. Memory system  102  may store, in addition to instructions  106 , data  104  represented by data signals that may be executed by processor  110 . 
     Instruction Format 
       FIG. 2  is a diagram illustrating a format of an instruction  200  which may include fields represented by one embodiment of the invention. The instruction format  200  includes a prefix field  210 , an opcode field  220 , and operand specifier fields (e.g., modR/M, scale-index-base, displacement, immediate, etc.). The operand specifier fields may be optional and include a modR/M field  230 , an SIB field  240 , a displacement field  250 , and an immediate field  260 . 
     In one embodiment, the term “escape opcode” refers to a two-byte value that indicates to decoder logic that an opcode to which an instruction corresponds may have various attributes, such as a three-byte opcode, in which two bytes correspond to the escape opcode and a third byte corresponds to an opcode indicating the function of the instruction. For example, in one embodiment, the two-byte value in the three-byte escape field may be one of the following values: 0x0F38, 0x0F39, 0x0F3A or 0x0F3B. The contents of the modR/M field  230  indicate an addressing-form. The modR/M field may specify registers and addressing modes. 
     Certain encodings of information in the modR/M field  230  may indicate that a second byte of addressing information is present in the SIB (Scale/Index/Base) field  240  to fully specify the addressing form of the instruction code. For instance, a base-plus-index addressing form and a scale-plus-index addressing form may each include information, such as scale factor, register number for the index, and/or register number of the base register, in the SIB field  240 . 
     One skilled in the art will recognize that the format  200  set forth in  FIG. 2  is illustrative, and that other organizations of data within an instruction code may be utilized with disclosed embodiments. For example, the fields  210 ,  220 ,  230 ,  240 ,  250 ,  260  need not be organized in the order shown, but may be re-organized into other locations with respect to each other and need not be contiguous. Also, the field lengths discussed herein should not be taken to be limiting. A field discussed as being a particular member of bytes may, in alternative embodiments, be implemented as a larger or smaller field. Also, the term “byte,” while used herein to refer to an eight-bit grouping, may in other embodiments be implemented as a grouping of any other size, including 4 bits, 16 bits, and 32 bits. 
     As used herein, an instruction (such as one of the instructions  106  illustrated in  FIG. 1 ) includes certain values in the fields of the instruction format  200  shown in  FIG. 2 . Such an instruction is sometimes referred to as “an actual instruction.” The bit values for an actual instruction are sometimes referred to collectively herein as an “instruction code.” 
     The acceptable byte values for an actual instruction are specified in the instruction set  118  ( FIG. 1 ). Acceptable byte values for each of the fields of the instruction format  200  are those values that decode logic, such as instruction length decoder  306  ( FIG. 3 ) and decode unit  116  ( FIG. 1 ), recognize and operate upon to generate decoded instruction code. For each instruction code, the corresponding decoded instruction code uniquely represents an operation to be performed by the execution unit  112  ( FIG. 1 ) responsive to the instruction code. The decoded instruction code may include one or more micro-operations. 
     The prefix field  210  illustrated in  FIG. 2  may include a number of prefixes. In one embodiment, the prefix field  210  includes up to four prefixes, with each prefix being one byte. In some applications, such as for an extended instruction set, the prefix field may be used to extend the opcode instruction space. Furthermore, in at least one embodiment, the prefix may be encoded to provide as much or more information about the instruction as in the prior art, but requiring less storage area than instructions using some prior art prefix encodings. 
     The contents of the opcode field  220  specify one or more operations to be performed by a processor executing the instruction. For at least one embodiment, the opcode field may include three bytes. In other embodiments, the opcode may include one, two or more than three bytes. For some instructions, the three-byte escape opcode value in the two-byte field  118   c  of the opcode field  220  is combined with the contents of a third byte  225  of the opcode field  220  to specify an operation. This third byte  225  is referenced to herein as an instruction-specific opcode. For some instructions, the three-byte escape opcode value in the two-byte field  118   c  of the opcode field  220  is combined with the contents of the prefix field  210  and the contents of the instruction-specific opcode field  225  of the opcode field  220  to specify an operation. 
     In general, the combination of the prefix field  210  and the opcode field  220  creates a number of different types of instructions. For illustrative purposes,  FIG. 2  shows only seven types of instructions: a regular one-byte instruction  212 , a regular instruction with prefix as qualifier  214 , a regular escape instruction  216 , a first extended instruction type  222 , a second extended instruction type  224 , a first new instruction type  226  and a second new instruction type  228 . As is known by one skilled in the art, other types of instruction can be similarly defined. In one extended instruction type  226 , a REX field is included to indicate the registers to be used with the instruction. For example, one instruction type indicated by the REX enables additional registers in 64-bit mode. 
     The regular one-byte instruction  212  includes regular instructions with one-byte instruction-specific opcodes in the opcode field  220 . The regular instruction with prefix as qualifier  214  includes regular instructions that use the prefix as a qualifier for the opcode. For example, a string instruction may use a REPEAT prefix value to repeat the string instruction by a number of times specified in the count register or until a certain condition is met. The prefix value used in instruction  214  does not add a completely new meaning to the opcode value that follows in the opcode field  220 . Rather, the prefix value is merely used as a qualifier to qualify the opcode with additional conditions. The use of the prefix in the instruction  214  may be different for different instruction types. 
     In one embodiment, the regular escape instruction  216  is a regular instruction that utilizes an escape opcode in a first field  215  of the opcode field  220  to indicate to decoder hardware that an instruction-specific opcode in a second field  217  of the opcode field  220  is used to define the instruction. For example, in one embodiment, a floating-point coprocessor escape opcode value 0xD8 through 0xDF in the first byte  215  of the opcode field  220  indicates that the opcode value that follows in the second byte  217  of the opcode field  220  should be interpreted as a coprocessor instruction and should be directed to coprocessor hardware for execution. 
     The first extended instruction type  222  is a particular type of escape instruction that is defined to contain a predefined escape opcode value, 0x0F, in a first field  221  of the opcode field  220 . The escape opcode 0x0F indicates to decoder hardware that an instruction-specific opcode value in a second field  223  of the opcode field  220  is used to define the instruction. Instructions of the first extended instruction type  222  may, depending on the value of the second opcode byte (and, in some cases, the value of three bits of the modR/M field  230 ), be of varying lengths. For example, two instructions (Packed Shift Left Logical) of extended instruction type  222  may include the following instruction field values, respectively: 
     PSLLW (shift value in register): 0F:F1:1b“11xxxyyy”, where xxx defines a first register and yyy defines a second register 
     PSLLW (shift by immed. value): 0F:71:1b“11110xxx”: immed data (8 bits), where xxx defines a register 
     In each of the PSSLW instructions set forth, the first word of the opcode field  220  includes the escape value 0x0F. The first instruction is three bytes long but the second instruction is four bytes because it includes a byte of immediate data. Accordingly, for extended instruction type  222  decoder hardware (such as, for example, instruction length decoder  306  in  FIG. 3 ) utilizes the escape opcode value 0x0F in the first field  221  of the opcode field  220  as well as the value in the second byte  223  of the two-byte opcode field  220  and the value of modR/M field  230  to decode the length of the instruction. 
     Examples of Instruction Prefixes and Escape Opcodes 
     Instruction prefixes were originally developed to enhance a set of instructions. For example, the repeat prefix is developed to repeat a string instruction. The repeat prefix codes are 0xF3 (REP, REPE) and 0xF2 (REPNE). The prefix used as such does not define a new meaning for the opcode that follows. It merely defines additional operational conditions for the opcode. 
     Escape opcodes provide an extension to the instruction set. For example, the escape opcodes 0xD8 through 0xDF are used to indicate that the second opcode byte  217  contains a value defining an instruction for the floating-point unit. The decoder passes the opcode to the floating-point decoder. 
     For at least one\instruction set, a 3-byte escape opcode is a two-byte entity having a value of 0x0F3y, where y is 0x8, 0x9, 0xA or 0xB. The 3-byte escape opcode value in the 3-byte escape code field  118   c  indicates to the decoder that the instruction-specific opcode value in the third byte  225  of the opcode field  200  indicates an instruction in a new instruction set. A new instruction set may be any new instruction set added to the architecture. 
     In contrast to a 2-byte escape opcode, the value in the 3-byte escape opcode field  118   c  indicates to the decoder the method to be used to determine the length of the instruction. That is, each value for the 3-byte escape opcode is associated with a particular method to be used to determine the instruction length for every instruction in the map corresponding to the particular 3-byte escape code. For instance, the value 0x0F38 in the 3-byte escape opcode field  118   c  is associated with an associated opcode map. The length for each instruction in the 0x0F38 opcode map may be calculated using the same length-determination method used to determine the length of the other instructions in the 0x0F38 opcode map. 
     Similarly, the length of each instruction of the respective opcode maps associated with the remaining 3-byte escape opcode values (0x0F39, 0x0F3A, 0x0F3B) may be calculated with the same length-determination logic used to determine the length of the other instructions in the respective opcode map. 
     The length-determination logic used to determine the length of instructions for each instruction in one of the new opcode maps is simplified in that the same set of input terms is evaluated to determine the length of each instruction in the opcode map. Such length-determination logic is referred to herein as a “fixed-input” logic or method. That is, each input term evaluated to determine the length of one instruction in the map is also relevant to determination of the length of every other instruction in the map. The fixed set of terms to be evaluated may differ from opcode map to opcode map. While the set of inputs to be evaluated may differ among opcode maps, the inputs evaluated to determine instruction length are the same across all instructions in a give 3-byte opcode map. 
     The combination of a prefix and an escape opcode provides a significant enlargement of a processor&#39;s opcode table to allow additional new instruction sets. This combination uses the existing prefix codes to define a new set of instructions, in addition to the instruction set created by the escape opcodes. By using the existing prefix codes, the decoding circuitry for the existing instruction set may remain relatively unmodified to support decoding of the new instructions  118   c  ( FIG. 1 ). 
     The instruction-specific opcode values (in the third byte  225  of the opcode field  220 ) of some or all of the new instructions may be the same as the opcodes of the existing instructions. By using the same opcodes with the prefix and escape opcodes to define a new set of instructions, the decoding circuitry may be less complex than having a completely new set of opcodes for the new instruction set. 
     In one embodiment, the prefix value is used to define new instructions. Furthermore, prefixes can still be used in the traditional role of enhancing the opcode or qualifying the opcode under some operational condition. 
     Table 1, below, illustrates some prior art examples of an instruction set using prefixes and three-byte escape opcodes. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 (Prefix)/Escape 
                   
               
               
                   
                 Opcode/Instruction- 
               
               
                   
                 specific opcode 
               
               
                 Instruction 
                 (in hex) 
                 Definition 
               
               
                   
               
             
            
               
                 PHADDW 
                 0F 38 01/r 
                 Add horizontally packed 
               
               
                   
                   
                 numbers from 64-bit 
               
               
                   
                   
                 register or memory to 64-bit 
               
               
                   
                   
                 register 
               
               
                 PHADDW 
                 (66)0F 38 01/r 
                 Add horizontally packed 
               
               
                   
                   
                 numbers from 128-bit 
               
               
                   
                   
                 register or memory to 128- 
               
               
                   
                   
                 bit register 
               
               
                 PHADDD 
                 0F 38 02/r 
                 Add horizontally packed 
               
               
                   
                   
                 numbers from 64-bit 
               
               
                   
                   
                 register or memory to 64-bit 
               
               
                   
                   
                 register 
               
               
                 PHADDD 
                 (66)0F 38 02/r 
                 Add horizontally packed 
               
               
                   
                   
                 numbers from 128-bit 
               
               
                   
                   
                 register or memory to 128- 
               
               
                   
                   
                 bit register 
               
               
                 PHADDSW 
                 0F 38 03/r 
                 Add horizontally packed 
               
               
                   
                   
                 numbers with saturation 
               
               
                   
                   
                 from 64-bit register or 
               
               
                   
                   
                 memory to 64-bit register 
               
               
                 PHADDSW 
                 (66)0F 38 03/r 
                 Add horizontally packed 
               
               
                   
                   
                 numbers with saturation 
               
               
                   
                   
                 from 128-bit register or 
               
               
                   
                   
                 memory to 128-bit register 
               
               
                 PHSUBW 
                 0F 38 05/r 
                 Subtract horizontally 
               
               
                   
                   
                 packed signed words in 64- 
               
               
                   
                   
                 bit register or memory to 
               
               
                   
                   
                 64-bit register 
               
               
                 PHSUBW 
                 (66)0F 38 05/r 
                 Subtract horizontally 
               
               
                   
                   
                 packed signed words in 
               
               
                   
                   
                 128-bit register or memory 
               
               
                   
                   
                 to 128-bit register 
               
               
                 PHSUBD 
                 0F 38 06/r 
                 Subtract horizontally 
               
               
                   
                   
                 packed signed double words 
               
               
                   
                   
                 in 64-bit register or memory 
               
               
                   
                   
                 to 64-bit register 
               
               
                 PHSUBD 
                 (66)0F 38 06/r 
                 Subtract horizontally 
               
               
                   
                   
                 packed signed double words 
               
               
                   
                   
                 in 128-bit register or 
               
               
                   
                   
                 memory to 128-bit register 
               
               
                 PHSUBSW 
                 0F 38 07/r 
                 Subtract horizontally 
               
               
                   
                   
                 packed signed words in 64- 
               
               
                   
                   
                 bit register or memory to 
               
               
                   
                   
                 64-bit register as saturated 
               
               
                   
                   
                 result 
               
               
                 PHSUBSW 
                 (66)0F 38 07/r 
                 Subtract horizontally 
               
               
                   
                   
                 packed signed words in 
               
               
                   
                   
                 128-bit register or memory 
               
               
                   
                   
                 to 128-bit register as 
               
               
                   
                   
                 saturated result 
               
               
                 PMADDUBSW 
                 0F 38 04/r 
                 Multiply and add packed 
               
               
                   
                   
                 signed and unsigned 
               
               
                   
                   
                 number in 64-bit register or 
               
               
                   
                   
                 memory to 64-bit register 
               
               
                 PMADDUBSW 
                 (66)0F 38 04/r 
                 Multiply and add packed 
               
               
                   
                   
                 signed and unsigned 
               
               
                   
                   
                 number in 128-bit register 
               
               
                   
                   
                 or memory to 128-bit 
               
               
                   
                   
                 register 
               
               
                 PMULHRSW 
                 0F 38 0B/r 
                 Packed multiply high with 
               
               
                   
                   
                 round and scaling from 64- 
               
               
                   
                   
                 bit register or memory to 
               
               
                   
                   
                 64-bit register 
               
               
                 PMULHRSW 
                 (66)0F 38 0B/r 
                 Packed multiply high with 
               
               
                   
                   
                 round and scaling from 128- 
               
               
                   
                   
                 bit register or memory to 
               
               
                   
                   
                 128-bit register 
               
               
                 PSHUFB 
                 0F 38 050/r 
                 Packed shuffle bytes in 64- 
               
               
                   
                   
                 bit register or memory to 
               
               
                   
                   
                 64-bit register 
               
               
                 PSHUFB 
                 (66)0F 38 00/r 
                 Packed shuffle bytes in 128- 
               
               
                   
                   
                 bit register or memory to 
               
               
                   
                   
                 128-bit register 
               
               
                 PSIGNB 
                 0F 38 08/r 
                 Packed sign byte 64-bit 
               
               
                   
                   
                 register or memory to 64-bit 
               
               
                   
                   
                 register 
               
               
                 PSIGNB 
                 (66)0F 38 08/r 
                 Packed sign byte 128-bit 
               
               
                   
                   
                 register or memory to 128- 
               
               
                   
                   
                 bit register 
               
               
                 PSIGNW 
                 0F 38 09/r 
                 Packed sign word 64-bit 
               
               
                   
                   
                 register or memory to 64-bit 
               
               
                   
                   
                 register 
               
               
                 PSIGNW 
                 (66)0F 38 09/r 
                 Packed sign word 128-bit 
               
               
                   
                   
                 register or memory to 128- 
               
               
                   
                   
                 bit register 
               
               
                 PSIGND 
                 0F 38 0A/r 
                 Packed sign double word 
               
               
                   
                   
                 64-bit register or memory to 
               
               
                   
                   
                 64-bit register 
               
               
                 PSIGND 
                 (66)0F 38 0A/r 
                 Packed sign double word 
               
               
                   
                   
                 128-bit register or memory 
               
               
                   
                   
                 to 128-bit register 
               
               
                 PSRMRG 
                 0F 3A 0F/r 
                 Pack shifted right and 
               
               
                   
                   
                 merge contents of 64-bit 
               
               
                   
                   
                 register or memory to 64-bit 
               
               
                   
                   
                 register 
               
               
                 PSRMRG 
                 (66)0F 3A 0F/r 
                 Pack shifted right and 
               
               
                   
                   
                 merge contents of 128-bit 
               
               
                   
                   
                 register or memory to 128- 
               
               
                   
                   
                 bit register 
               
               
                 PABSB 
                 0F 38 1C/r 
                 Packed byte absolute value 
               
               
                   
                   
                 of value in 64-bit register or 
               
               
                   
                   
                 memory to 64-bit register as 
               
               
                   
                   
                 unsigned result 
               
               
                 PABSB 
                 (66)0F 38 1C/r 
                 Packed byte absolute value 
               
               
                   
                   
                 of value in 128-bit register 
               
               
                   
                   
                 or memory to 128-bit 
               
               
                   
                   
                 register as unsigned result 
               
               
                 PABSW 
                 0F 38 1D/r 
                 Packed word absolute value 
               
               
                   
                   
                 of value in 64-bit register or 
               
               
                   
                   
                 memory to 64-bit register as 
               
               
                   
                   
                 unsigned result 
               
               
                 PABSW 
                 (66)0F 38 1D/r 
                 Packed word absolute value 
               
               
                   
                   
                 of value in 128-bit register 
               
               
                   
                   
                 or memory to 128-bit 
               
               
                   
                   
                 register as unsigned result 
               
               
                 PASBSD 
                 0F 38 1E/r 
                 Packed double word 
               
               
                   
                   
                 absolute value of value in 
               
               
                   
                   
                 64-bit register or memory to 
               
               
                   
                   
                 64-bit register as unsigned 
               
               
                   
                   
                 result 
               
               
                 PABSD 
                 (66)0F 38 1E/r 
                 Packed double word 
               
               
                   
                   
                 absolute value of value in 
               
               
                   
                   
                 128-bit register or memory 
               
               
                   
                   
                 to 128-bit register as 
               
               
                   
                   
                 unsigned result 
               
               
                   
               
            
           
         
       
     
     In the above examples, the instructions with the prefix 0x66 relate to instructions that utilize one or more extended-size registers (such as 128-bit register size), while the instructions without the prefix 0x66 relate to instructions that utilize one or more smaller-size registers (such as 64-bit register size). The smaller-size registers are referred to herein as “regular length” registers. As is known by one skilled in the art, the exact codes for prefixes are implementation-dependent and the 0x66 prefix value discussed above is merely for illustrative purposes. 
     The original x86 instruction set provided for a 1-byte opcode with various formats of address syllable and immediate operand contained in additional bytes whose presence was known from the first “opcode” byte. Additionally, there were certain byte values that were reserved as modifiers to the opcode (called prefixes, as they had to be placed before the instruction). When the original palette of 256 opcode bytes (including these special prefix values) was exhausted, a single byte was dedicated as an escape to a new set of 256 opcodes. As vector instructions (e.g., SIMD) were added, a need for more opcodes was generated, and the “two byte” opcode map also was insufficient, even when expanded through the use of prefixes. To this end, new instructions were added in additional maps which use 2 bytes plus an optional prefix as an identifier. 
     Additionally, in order to facilitate additional registers in 64-bit mode, an additional prefix may be used (called “REX”) in between the prefixes and the opcode (and any escape bytes necessary to determine the opcode). In one embodiment, the REX may have 4 “payload” bits to indicate use of additional registers in 64-bit mode. In other embodiments it may have fewer or more than 4 bits. The general format of at least one instruction set is illustrated generically by the following:
         [prefix] [rex] escape [escape2] opcode (etc.)       

     In this example “etc.” represents the address syllable (e.g., one to six bytes) and perhaps additional immediate information for the instruction. 
     As new instruction sets are added, it can be seen that the size of instructions, including prefix, data-size indicator, escape codes, opcodes, and intermediate, address fields, etc., are causing the code size to expand, which can be undesirable for a number of reasons. 
     In order to enable new instruction set extensions, while remaining backward compatible with existing and legacy instruction sets, a new instruction encoding technique may be desirable. Accordingly, at least one embodiment of the invention includes an instruction encoding that enables backward compatibility, enhanced functionality, while reducing code size. In one embodiment, the instruction encoding uses a new byte to indicate whether the instruction requires a second extension field or not. The following illustration shows a legacy instruction and how at least one embodiment of the invention at least maintains if not improves the code size to which the instruction contributes. In the example illustrated In  FIG. 7 , the legacy escape field is replaced with a new escape field, legacy prefixes are partially compressed as part of a “payload” byte, legacy prefixes may be reclaimed and made available for future instruction expansion, and new features are added (e.g., increased vector length increase and additional source register specifier. 
     As can be seen above, the non-fixed part of the instruction (prefix, REX, escape code) has gone from a range of 1 to 3 bytes to 2 bytes. In the worst case, the instruction encoding according to one embodiment adds only one byte the legacy encoding, and in the best case, it saves one byte of encoding over the legacy encoding. In the above example, the byte-long prefix, “[66, F2, F3]”, is represented by two bits “pp”, a field of the REX byte is represented by one bit, “R”, and the escape byte is re-coded from “0F” to “C5” (which will be discussed below). Furthermore, a 4-bit field, “VVVV” has been added to specify an extra source operand, and an increased vector size has been provided for through vector size bit “L”, which may designate either a 128 bit or a 256 bit vector size. 
     The following illustrates an embodiment using two bytes to encode an instruction set instruction, which may be used when a second escape code is present in the original instruction, or when extra bits (e.g, the XB and W fields) in the REX field need to be used. In the embodiment illustrated in  FIG. 8 , legacy Escape is represented by a new escape value, legacy prefixes are fully compressed as part of the “payload” bytes, legacy prefixes are reclaimed and available for future expansion, the second escape code is compressed in a “map” field, with future map or feature space available, and new features are added (e.g., increased vector length and an additional source register specifier). 
     In the above illustration, the original instruction is assumed to use either the REX or the “escape2” fields (or both), and as a result the length of the non-fixed part of the instruction has gone from a range of 2 to 4 bytes to 3 bytes. In the worst case, one embodiment of the invention increases the original instruction encoding by 1 byte, in the best case, the embodiment decreases the original instruction encoding by 1 byte. In contrast to the single escape code example mentioned before, the REX byte is represented by 4 bits (RXB and W) and the second escape field is present and encoded in 5 bits (“mmmmm”) instead of using a discrete byte as in the prior art encoding. 
     Although embodiments disclosed use 5 bits to compress up to 32 different values of escape2 code bytes, in other embodiments, bits from the escape2 code bits may be used for other functions, such as extending the vector length to 4 combinations (illustrated below, where the “mmmmm” bits are renamed as ‘L’) and maintaining the ability to encode 16 different escapes. 
     
       
         
           
               
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 C4 
                 RXB.mmmmm 
                 W.VVVV.Lpp 
               
               
                   
                 C4 
                 RXB.L′.mmmm 
                 W.VVVV.Lpp 
               
               
                   
                   
               
            
           
         
       
     
     Similarly, if only 4 values of escape2 code are needed, then the upper “mmmmm” bits could be renamed to indicate the QRS feature, which may assume seven values other than the original “default behavior” value compatible with the original implementation. 
     
       
         
           
               
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 C4 
                 RXB.QRS.mm 
                 W.VVVV.Lpp 
               
               
                   
                   
               
            
           
         
       
     
     In the above examples, the escape code is “C4”, indicating that the instruction is represented by the second encoding rather than one mentioned before, indicated by “C5”. It should be noted that the choice of values “C5” and “C4” values and the encoding of the payloads is chosen so that it will be backwards compatible with unimplemented opcodes on current processors of at least one architecture (e.g., x86). One of ordinary skill in the art will recognize that C5 and C4 correspond to the “LDS” and “LES” instructions, respectively, which are legal under opcodes in an x86 architecture. 
     In some embodiments, instructions in the format, “[prefixes] [REX] [escape bytes] opcode [additional bytes to end of instruction]”, may be translated into a more compact and efficient format, “[Attributes] [opcode] [additional bytes to end of instruction]”, where the “Attributes” field may include fewer bits than the original instruction from which it was translated. 
     In addition to instruction sets mentioned herein, other instructions from other instruction sets may also benefit from embodiments of the invention. 
     Instruction Decoding 
       FIG. 3  is a diagram illustrating a logic  300  to decode variable-length instructions. The logic  300  may include an instruction length decoder  306 , an instruction rotator  308 , an instruction buffer  310 , a first pre-decode logic  320 , a second pre-decode logic  330 , and an opcode decoder  340 . The first phase pre-decode logic  320 , the second phase pre-decode logic  330 , and the opcode decoder  340  form all or part of the decode unit  116  illustrated in  FIG. 1 . While illustrated as a single entity, any or each of first phase pre-decode logic  320 , the second phase pre-decode logic  330 , and the opcode decoder  340  may be implemented as separate logic blocks. 
     The instruction length decoder  306  determines the length of an actual instruction code that has been fetched from external memory (such as, e.g., memory  102 ,  FIG. 2 ). For illustrative purposes, an instruction code is assumed to include up to five bytes: the first byte corresponds to IN to I N+7 , the second byte corresponds to I K  to I K+7 , the third byte corresponds to I L  to I L+7 , the fourth byte corresponds to I M  to I M+7 , and the fifth byte corresponds to I P  to I P+7 , where I N  to I N+7 , I K  to I K+7 , I M  to I M+7  to I L+7 , and I P  to I P+7  refer to the bit positions of the instruction code. In practice, however, an actual instruction may include more than five bytes in its instruction code. Similarly, an actual instruction may include less than five bytes in its instruction code. 
     For at least one embodiment, the five illustrates bytes of the instruction code are contiguous, such that K=N+8, L=K+8 and L=N+16, and M=L+8, M=K+16 and M=K+24, and so on. However, as is discussed above in connection with  FIG. 2 , the fields of the format  200  illustrated in  FIG. 2  need not occupy the positions shown. Accordingly, the illustrative five bytes of an instruction code that are discussed herein may be in any order and need not be contiguous. 
     One of skill in the art will recognize that logic of the instruction length decoder  306  may implement fairly complex length decode methods in a system that supports variable-length instructions. This is especially true in systems that require different methods, that evaluate different inputs, to determine instruction length for instructions within the same opcode map. In some embodiments, the length of each instruction within an opcode map is determined by a single fixed-input length-determination logic. 
     The rotator  308  rotates the raw instruction bytes such that the first byte to be decoded is in an initial position. The rotator  308  thus identifies the beginning of the instruction bytes to be decoded. It should be noted that, although the rotator  308  may identify the first byte of an instruction, such as a prefix byte, the first byte need not be identified. 
     The instruction buffer  310  receives and stores the instructions that have been fetched from the external memory. For at least one embodiment, the instructions are length-decoded and rotated before being received by the instruction buffer  310 . For at least one embodiment, the instruction buffer  310  is implemented as an instruction cache. 
     The first phase instruction pre-decoder  320  receives the instruction bytes and detects the presence of one or more of a set of predefined prefixes and/or escape codes and/or opcodes used as part of an instruction set. The second phase pre-decode logic  330  utilizes the results of the pre-decoder  320  to translate the instruction information received from a prior art instruction format to one in accordance with one embodiment. The translated instruction is then sent to the opcode decoder  340  to generate instruction signals (e.g., micro-operations) consistent with the desired instruction operation. 
     In one embodiment, the translation logic  340  translates an instruction of format, “[prefixes] [REX] [escape bytes] opcode [additional bytes to end of instruction]”, into a more compact and efficient format, “[Attributes] [opcode] [additional bytes to end of instruction]”. In other embodiments, other instructions having other instruction formats are translated into the above compact format. 
       FIG. 4  is a block diagram illustrating at least one embodiment of a first phase pre-decoder logic  420 . In one embodiment, the pre-decoder  420  receives a fetched cacheline and produces information for bytes within the instruction. For instance, if a byte of the received instruction contains 0x66 the pre-decoder logic  420  may realize the following algorithm: If 0x66 is located within the prefix portion of the instruction, otherwise, if 0x66 is located in the opcode byte and preceded by 0x0F then the 0x0F is an escape code and the opcode corresponds to a first opcode map entry, otherwise if 0x66 is located in the opcode byte and not preceded by 0x0F then there is no escape code in the instruction and opcode corresponds to a second opcode map entry, if 0x66 is in the modr/m (address specifier) field in 32-bit mode, then 0x66 has a four byte displacement, etc. 
     As illustrated in  FIG. 4 , the pre-decoder logic  420  may include parallel logic, implemented in programmable logic arrays  421 ,  423   425 , etc., enabled to decode corresponding portions of an instruction, such as the prefix, escape codes, opcodes, etc. Each portion  421 - 425 , in one embodiment may interpret its respective instruction portion as meaning various things, according to the value in the corresponding instruction portion, as illustrated for logic array  421  in  FIG. 4 . After each portion of the instruction has been pre-decoded by logic  420  to determine what each portion means, the instruction may then be passed to a second pre-decode phase (illustrated as  320  in  FIG. 3 ) to translate the instruction into a more efficient and compact encoding scheme, according to one embodiment. 
       FIG. 5  illustrates a second phase pre-decode logic  430  to translate an instruction into a format consistent with at least one embodiment of the invention. For each opcode pre-decoded by the first pre-decode logic illustrated in  FIG. 4 , an attribute field  431 , opcode field  433 , and address/data/intermediate/etc field  435  is decoded and sent to opcode decoder logic (illustrated as  340  in  FIG. 3 ) to decode the opcode into signals to carry out the operations prescribed by the opcode. In one embodiment, attribute filed  431  may include bits to indicate the information described previously, including prior art opcode map information (e.g., escape codes), REX information, and prefix information, as well as new information according to one embodiment, such as an extra source register address and an expanded vector size. 
       FIG. 6  is a flowchart illustrating operations to perform an instruction decoding and translation technique according to at least one embodiment. At operation  601 , instruction bytes are rotated such that the first byte to be decoded is in an initial position. At operation  605 , an instruction buffer receives and stores the instructions that have been fetched from the external memory. At operation  610 , the instruction is pre-decoded in a first phase, in which the presence of one or more of a set of predefined prefixes and/or escape codes and/or opcodes used as part of an instruction set is detected. At operation  615 , a second phase pre-decoding of the instruction translates the instruction information received from a prior art instruction format to a format in accordance with one embodiment of the invention. At operation  620 , the translated instruction is then sent to the opcode decoder to generate instruction signals consistent with the desired instruction operation. 
     While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications can be made without departing from the present invention in its broader aspects. The appended claims are to encompass within their scope all such changes and modifications that fall within the true scope of the present invention.