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

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24594 Rev. 3.10 February 2005 AMD64 TechnologyAppendix CDifferences Between Long Mode <strong>and</strong> LegacyModeTable C-1 summarizes the major differences between 64-bitmode <strong>and</strong> legacy protected mode. The third column indicatesdifferences between 64-bit mode <strong>and</strong> legacy mode. The fourthcolumn indicates whether that difference also applies tocompatibility mode.Table C-1.Differences Between Long Mode <strong>and</strong> Legacy ModeType Subject 64-Bit Mode DifferenceApplies ToCompatibilityMode?ApplicationProgrammingAddressingData <strong>and</strong> AddressSizesInstructionDifferencesRIP-relative addressing availableDefault data size is 32 bitsREX Prefix toggles data size to 64 bitsDefault address size is 64 bitsAddress size prefix toggles address size to 32 bitsVarious opcodes are invalid or changed in 64-bit mode(see Table B-2 on page 445 <strong>and</strong> Table B-3 on page 446)Various opcodes are invalid in long mode (see Table B-4on page 446)MOV reg,imm32 becomes MOV reg,imm64 (with REXoper<strong>and</strong> size prefix)REX is always enabledDirect-offset forms of MOV to or from accumulatorbecome 64-bit offsetsMOVD extended to MOV 64 bits between MMX registers<strong>and</strong> long GPRs (with REX oper<strong>and</strong>-size prefix)noyesnoAppendix C: Differences Between Long Mode <strong>and</strong> Legacy Mode 451

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