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AMD64 Technology 24594 Rev. 3.10 February 2005In 64-bit mode, most instructions default to a 32-bit operandsize. For these instructions, a REX prefix (page 16) can specifya 64-bit operand size, and a 66h prefix specifies a 16-bit operandsize. The REX prefix takes precedence over the 66h prefix.However, if an instruction defaults to a 64-bit operand size, itdoes not need a REX prefix and it can only be overridden to a16-bit operand size. It cannot be overridden to a 32-bit operandsize, because there is no 32-bit operand-size override prefix in64-bit mode. Two groups of instructions have a default 64-bitoperand size in 64-bit mode:• Near branches. For details, see “Near Branches in 64-BitMode” in Volume 1.• All instructions, except far branches, that implicitlyreference the RSP. For details, see “Stack Operation” inVolume 1.Instructions that Cannot Use the Operand-Size Prefix. The operand-sizeprefix should be used only with general-purpose instructionsand the x87 FLDENV, FNSTENV, FNSAVE, and FRSTORinstructions, in which the prefix selects between 16-bit and 32-bit operand size. The prefix is ignored by all other x87instructions and by 64-bit media floating-point (3DNow!)instructions.When used with 64-bit media integer instructions, the 66h prefixacts in a special way to modify the opcode. This modificationtypically causes an access to an XMM register or 128-bitmemory operand and thereby converts the 64-bit mediainstruction into its comparable 128-bit media instruction. Theresult of using an F2h or F3h repeat prefix along with a 66hprefix in 128-bit or 64-bit media instructions is unpredictable.Operand-Size and REX Prefixes. The REX operand-size prefix takesprecedence over the 66h prefix. See “REX.W: Operand Width”on page 16 for details.1.2.3 Address-SizeOverride PrefixThe default address size for instructions that access non-stackmemory is determined by the current operating mode, as shownin Table 1-3. The address-size override prefix (67h) selects thenon-default address size. Depending on the operating mode,this prefix allows mixing of 16-bit and 32-bit, or of 32-bit and 64-bit addresses, on an instruction-by-instruction basis. The prefixchanges the address size for memory operands. It also changes6 Chapter 1: Instruction Formats
24594 Rev. 3.10 February 2005 AMD64 Technologythe size of the RCX register for instructions that use RCXimplicitly.For instructions that implicitly access the stack segment (SS),the address size for stack accesses is determined by the D(default) bit in the stack-segment descriptor. In 64-bit mode,the D bit is ignored, and all stack references have a 64-bitaddress size. However, if an instruction accesses both stack andnon-stack memory, the address size of the non-stack access isdetermined as shown in Table 1-3.Table 1-3.Address-Size OverridesLong ModeOperating Mode64-BitModeCompatibilityModeLegacy Mode(Protected, Virtual-8086, or RealMode)DefaultAddress Size(Bits)6432163216EffectiveAddress Size(Bits)Address-Size Prefix(67h) 1Required?64 no32 yes32 no16 yes32 yes16 no32 no16 yes32 yes16 noNote:1. A “no” indicates that the default address size is used.As Table 1-3 shows, the default address size is 64 bits in 64-bitmode. The size can be overridden to 32 bits, but 16-bitaddresses are not supported in 64-bit mode. In compatibilityand legacy modes, the default address size is 16 bits or 32 bits,depending on the operating mode (see “Processor Initializationand Long-Mode Activation” in Volume 2 for details). In thesemodes, the address-size prefix selects the non-default size, butthe 64-bit address size is not available.Chapter 1: Instruction Formats 7
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