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

24594 Rev. 3.10 February 2005 AMD64 TechnologyTable 1-8.MnemonicREPNE and REPNZ Prefix OpcodesOpcodeREPNx CMPS mem8, mem8REPNx CMPSBREPNx CMPS mem16/32/64, mem16/32/64REPNx CMPSWREPNx CMPSDREPNx CMPSQREPNx SCAS mem8REPNx SCASBREPNx SCAS mem16/32/64REPNx SCASWREPNx SCASDREPNx SCASQF2 A6F2 A7F2 AEF2 AFInstructions that Cannot Use Repeat Prefixes. In general, the repeatprefixes should only be used in the string instructions listed intables 1-6, 1-7, and 1-8, and in 128-bit or 64-bit mediainstructions. When used in media instructions, the F2h and F3hprefixes act in a special way to modify the opcode rather thancause a repeat operation. The result of using a 66h operand-sizeprefix along with an F2h or F3h prefix in 128-bit or 64-bit mediainstructions is unpredictable.Optimization of Repeats. Depending on the hardware implementation,the repeat prefixes can have a setup overhead. If therepeated count is variable, the overhead can sometimes beavoided by substituting a simple loop to move or store the data.Repeated string instructions can be expanded into equivalentsequences of inline loads and stores or a sequence of stores canbe used to emulate a REP STOS.For repeated string moves, performance can be maximized bymoving the largest possible operand size. For example, use REPMOVSD rather than REP MOVSW and REP MOVSW ratherthan REP MOVSB. Use REP STOSD rather than REP STOSWand REP STOSW rather than REP MOVSB.Chapter 1: Instruction Formats 13