X86-64 Instruction Encoding: Difference between revisions
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→VEX/XOP opcodes: Better layout |
→ModR/M: Better layout |
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{| {{wikitable}} |
{| {{wikitable|background-color: white}} |
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!style="background-color:#f9f9f9;"| Field |
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! Field |
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!style="background-color:#f9f9f9;"| Length |
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! Length |
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! Description |
!style="background-color:#f9f9f9;"| Description |
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!style="background-color:#f9f9f9; text-align: left; vertical-align: top;"| MODRM.mod |
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|style="vertical-align: top;"|2 bits||In general, when this field is b11, then register-direct addressing mode is used; otherwise register-indirect addressing mode is used. |
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!style="background-color:#f9f9f9; text-align: left; vertical-align: top;"| MODRM.reg |
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|style="vertical-align: top;"|3 bits||This field can have one of two values: |
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* A 3-bit opcode extension, which is used by some instructions but has no further meaning other than distinguishing the instruction from other instructions. |
* A 3-bit opcode extension, which is used by some instructions but has no further meaning other than distinguishing the instruction from other instructions. |
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* A 3-bit register reference, which can be used as the source or the destination of an instruction (depending on the instruction). The referenced register 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 can extend this field with 1 most-significant bit to 4 bits total. |
* A 3-bit register reference, which can be used as the source or the destination of an instruction (depending on the instruction). The referenced register 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 can extend this field with 1 most-significant bit to 4 bits total. |
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!style="background-color:#f9f9f9; text-align: left; vertical-align: top;"| MODRM.rm |
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|style="vertical-align: top;"|3 bits||Specifies a direct or indirect register operand, optionally with a displacement. The REX.B, VEX.~B or XOP.~B field can extend this field with 1 most-significant bit to 4 bits total. |
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