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

AMD64 Technology 24594 Rev. 3.10 February 2005REX prefixes are a set of sixteen values that span one row ofthe main opcode map and occupy entries 40h through 4Fh.Table 1-11 and Figure 1-3 on page 18 show the prefix fields andtheir uses.Table 1-11.REX Prefix-Byte FieldsMnemonic Bit Position Definition— 7–4 0100REX.W 3REX.R 2REX.X 1REX.B 00 = Default operand size1 = 64-bit operand size1-bit (high) extension of the ModRM reg field 1 ,thus permitting access to 16 registers.1-bit (high) extension of the SIB index field 1 ,thus permitting access to 16 registers.1-bit (high) extension of the ModRM r/m field 1 ,SIB base field 1 , or opcode reg field, thuspermitting access to 16 registers.Note:1. For a description of the ModRM and SIB bytes, see “ModRM and SIB Bytes” on page 20.REX.W: Operand Width. Setting the REX.W bit to 1 specifies a 64-bit operand size. Like the existing 66h operand-size prefix, theREX 64-bit operand-size override has no effect on byteoperations. For non-byte operations, the REX operand-sizeoverride takes precedence over the 66h prefix. If a 66h prefix isused together with a REX prefix that has the REX.W bit set to1, the 66h prefix is ignored. However, if a 66h prefix is usedtogether with a REX prefix that has the REX.W bit cleared to 0,the 66h prefix is not ignored and the operand size becomes16 bits.REX.R: Register. The REX.R bit adds a 1-bit (high) extension tothe ModRM reg field (page 20) when that field encodes a GPR,XMM, control, or debug register. REX.R does not modifyModRM reg when that field specifies other registers or opcodes.REX.R is ignored in such cases.REX.X: Index. The REX.X bit adds a 1-bit (high) extension to theSIB index field (page 20).16 Chapter 1: Instruction Formats