Volume 3: General-Purpose and System Instructions - Stanford ...

24594 Rev. 3.10 February 2005 AMD64 Technology1 Instruction FormatsThe format of an instruction encodes its operation, as well asthe locations of the instruction’s initial operands and the resultof the operation. This section describes the general format andparameters used by all instructions. For information on thespecific format(s) for each instruction, see:• Chapter 3, “General-Purpose Instruction Reference.”• Chapter 4, “System Instruction Reference.”• “128-Bit Media Instruction Reference” in Volume 4.• “64-Bit Media Instruction Reference” in Volume 5.• “x87 Floating-Point Instruction Reference” in Volume 5.1.1 Instruction Byte OrderAn instruction can be between one and 15 bytes in length.Figure 1-1 shows the byte order of the instruction format.LegacyPrefixREXPrefixOpcode(1 or 2 bytes) ModRM SIBDisplacement(1, 2, or 4 bytes)Immediate(1, 2, or 4 bytes)Instruction Length ≤ 15 BytesFigure 1-1.Instruction Byte-OrderInstructions are stored in memory in little-endian order. Theleast-significant byte of an instruction is stored at its lowestmemory address, as shown in Figure 1-2 on page 2.Chapter 1: Instruction Formats 1