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AMD64 Technology 24594 Rev. 3.10 February 2005Depending on the hardware implementation, string moves withthe direction flag (DF) cleared to 0 (up) may be faster thanstring moves with DF set to 1 (down). DF = 1 is only needed forcertain cases of overlapping REP MOVS, such as when thesource and the destination overlap.1.2.7 REX Prefixes REX prefixes are a group of instruction-prefix bytes that can beused only in 64-bit mode. They enable access to the AMD64register extensions. Figure 1-1 on page 1 and Figure 1-2 onpage 2 show how a REX prefix fits within the byte order ofinstructions. REX prefixes enable the following features in 64-bit mode:• Use of the extended GPR (Figure 2-3 on page 31) or XMMregisters (Figure 2-8 on page 36).• Use of the 64-bit operand size when accessing GPRs.• Use of the extended control and debug registers, asdescribed in “64-Bit-Mode Extended Control Registers” inVolume 2 and “64-Bit-Mode Extended Debug Registers” inVolume 2.• Use of the uniform byte registers (AL–R15).Table 1-9 shows the REX prefixes. The value of a REX prefix isin the range 40h through 4Fh, depending on the particularcombination of AMD64 register extensions desired.Table 1-9.REX Instruction PrefixesPrefix TypeMnemonicPrefix Code(Hex)DescriptionRegister ExtensionsREX.WREX.RREX.XREX.B40 1through4F 1Access an AMD64 registerextension.Note:1. See Table 1-11 for encoding of REX prefixes.A REX prefix is normally required with an instruction thataccesses a 64-bit GPR or one of the extended GPR or XMMregisters. Only a few instructions have an operand size thatdefaults to (or is fixed at) 64 bits in 64-bit mode, and thus do not14 Chapter 1: Instruction Formats
24594 Rev. 3.10 February 2005 AMD64 Technologyneed a REX prefix. These exceptions to the normal rule arelisted in Table 1-10.An instruction can have only one REX prefix, although theprefix can express several extension features. If a REX prefix isused, it must immediately precede the first opcode byte in theinstruction format. Any other placement of a REX prefix, or anyuse of a REX prefix in an instruction that does not access anextended register, is ignored. The legacy instruction-size limitof 15 bytes still applies to instructions that contain a REXprefix.Table 1-10.CALL (Near)ENTERJccJrCXZJMP (Near)LEAVELGDTLIDTLLDTLOOPLOOPccLTRMOV CR(n)Instructions Not Requiring REX Size Prefix in 64-Bit ModePOP reg/memPOP regPOP FSPOP GSPOPFQPUSH imm8PUSH imm32PUSH reg/memPUSH regPUSH FSPUSH GSPUSHFQRET (Near)MOV DR(n)Chapter 1: Instruction Formats 15
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