CODEWITHSUNDEEP
assemblyx86machine-codeopcodeinstruction-encoding
Poperton
Poperton

Reputation: 1746

How to read x86 instruction tables from this site

I'm reading https://www.felixcloutier.com/x86/mov (mov as example) and there are these opcodes in the tables like 88 /r and REX + 88 /r and even REX.W + 8C /r. I couldn't find an explanation at the website. What does r, REX, REX.W, and 8C mean? How do I come up with the hex value for an entire mov instruction based on this table?

Upvotes: 1

Views: 504

Answers (1)

Maxim Masiutin
Maxim Masiutin

Reputation: 4812

Intel® 64 and IA-32 Architectures Developer's Manual: Vol. 2A, section 2.2.1 gives the answer to your question.

REX prefixes are instruction-prefix bytes used in 64-bit mode. They do the following:

  • Specify GPRs and SSE registers.
  • Specify 64-bit operand size.
  • Specify extended control registers.

Particularities are given in Section 2.2.1.2:

  • Setting REX.W can be used to determine the operand size but does not solely determine operand width.
  • REX.R modifies the ModR/M reg field when that field encodes a GPR, SSE, control or debug register.
  • REX.X bit modifies the SIB index field.
  • REX.B either modifies the base in the ModR/M r/m field or SIB base field; or it modifies the opcode reg field used for accessing GPRs.

Please further refer to the Intel’s manual on how exactly they are encoded.

According to Section 3.1.1.1,

/r — Indicates that the ModR/M byte of the instruction contains a register operand and an r/m operand.

The ModR/M byte is explaned in section 2.1 of the Manual. 88, 89, 8A, 8B, 8C, etc. are just opcode bytes also explained in this section of the Manual.

Upvotes: 4

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