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AMD64 Technology 24594 Rev. 3.10 February 2005Case 1: Register-Register Addressing (No Memory Operand)ModRM ByteREX PrefixOpcode mod reg r/m4WRXB11 rrr bbbREX.X is not used44Rrrr BbbbCase 2: Memory Addressing Without an SIB ByteModRM ByteREX PrefixOpcode mod reg r/m4WRXB!11 rrr bbbREX.X is not usedModRM reg field != 1004Rrrr4BbbbCase 3: Memory Addressing With an SIB ByteModRM ByteREX PrefixOpcode mod reg r/m4WRXB!11 rrr 100SIB Bytescale index basebb xxx bbb4Rrrr4Xxxx4BbbbCase 4: Register Operand Coded in Opcode ByteREX Prefix4WRXBOpcode Byteop regbbbREX.R is not usedREX.X is not used4Bbbb513-302.epsFigure 1-3.Encoding Examples of REX-Prefix R, X, and B Bits18 Chapter 1: Instruction Formats
24594 Rev. 3.10 February 2005 AMD64 TechnologyTable 1-12.Special REX Encodings for RegistersModRM and SIBEncodings 2Meaning in Legacy andCompatibility ModesImplications in Legacyand Compatibility ModesAdditional REXImplicationsModRM Byte:• mod≠ 11• r/m 1 = 100 (ESP)SIB byte is present.SIB byte is required forESP-based addressing.REX prefix adds a fourth bit(b), which is decoded andmodifies the base registerin the SIB byte. Therefore,the SIB byte is alsorequired for R12-basedaddressing.ModRM Byte:• mod =00• r/m 1 =x101 (EBP)Base register is not used.Using EBP without adisplacement must bedone by setting mod = 01with a displacement of 0(with or without an indexregister).REX prefix adds a fourth bit(x), which is not decoded(don’t care). Therefore,using RBP or R13 without adisplacement must bedone via mod = 01 with adisplacement of 0.SIB Byte:• index 1 = x100 (ESP)Index register is not used.ESP cannot be used as anindex register.REX prefix adds a fourth bit(x), which is decoded.Therefore, there are noadditional implications.The expanded index field isused to distinguish RSPfrom R12, allowing R12 tobe used as an index.SIB Byte:• base = b101 (EBP)• ModRM.mod = 00Base register is not used ifModRM.mod = 00.Base register depends onmod encoding. Using EBPwith a scaled index andwithout a displacementmust be done by settingmod = 01 with adisplacement of 0.REX prefix adds a fourth bit(b), which is not decoded(don’t care). Therefore,using RBP or R13 without adisplacement must bedone via mod = 01 with adisplacement of 0 (with orwithout an index register).Notes:1. The REX-prefix bit is shown in the fourth (most-significant) bit position of the encodings for the ModRM r/m, SIB index, and SIBbase fields. The lower-case “x” for ModRM r/m (rather than the upper-case “B” shown in Figure 1-3 on page 18) indicates thatthe REX-prefix bit is not decoded (don’t care).2. For a description of the ModRM and SIB bytes, see “ModRM and SIB Bytes” on page 20.Chapter 1: Instruction Formats 19
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