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X86-64 Instruction Encoding: Difference between revisions
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| MODRM.reg||3 bits||This field can have one of two values:
* A 3-bit opcode extension, which is used by some instructions but has no further meaning other than distincting the instruction from other instructions. When ''/0'' through ''/7'' is specified at the opcode syntax, use that value as the value for the ''MODRM.reg'' field.
* A 3-bit register reference, which can be used as the source or the destination of an instruction (depending on the instruction). This is the case when ''/r'' is specified at the opcode syntax. Which register is meant depends on the [[#Operand-size and address-size override prefix|operand-size]] of the instruction and the instruction itself. See [[#Registers|Registers]] for the values to use for each of the registers. The REX.R, VEX.~R or XOP.~R field
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| MODRM.rm||3 bits||Specifies a direct or indirect register operand, optionally with a displacement. The REX.B, VEX.~B or XOP.~B field
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