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

AMD64 Technology 24594 Rev. 3.10 February 2005Segment Overrides in 64-Bit Mode. In 64-bit mode, the CS, DS, ES,and SS segment-override prefixes have no effect. These fourprefixes are not treated as segment-override prefixes for thepurposes of multiple-prefix rules. Instead, they are treated asnull prefixes.The FS and GS segment-override prefixes are treated as truesegment-override prefixes in 64-bit mode. Use of the FS or GSprefix causes their respective segment bases to be added to theeffective address calculation. See “FS and GS Registers in 64-Bit Mode” in Volume 2 for details.1.2.5 Lock Prefix The LOCK prefix causes certain kinds of memory read-modifywriteinstructions to occur atomically. The mechanism for doingso is implementation-dependent (for example, the mechanismmay involve bus signaling or packet messaging between theprocessor and a memory controller). The prefix is intended togive the processor exclusive use of shared memory in amultiprocessor system.The LOCK prefix can only be used with forms of the followinginstructions that write a memory operand: ADC, ADD, AND,BTC, BTR, BTS, CMPXCHG, CMPXCHG8B, DEC, INC, NEG,NOT, OR, SBB, SUB, XADD, XCHG, and XOR. An invalidopcodeexception occurs if the LOCK prefix is used with anyother instruction.1.2.6 Repeat Prefixes The repeat prefixes cause repetition of certain instructions thatload, store, move, input, or output strings. The prefixes shouldonly be used with such string instructions. Two pairs of repeatprefixes, REPE/REPZ and REPNE/REPNZ, perform the samerepeat functions for certain compare-string and scan-stringinstructions. The repeat function uses rCX as a count register.The size of rCX is based on address size, as shown in Table 1-4on page 8.REP. The REP prefix repeats its associated string instruction thenumber of times specified in the counter register (rCX). Itterminates the repetition when the value in rCX reaches 0. Theprefix can only be used with the INS, LODS, MOVS, OUTS, andSTOS instructions. Table 1-6 shows the valid REP prefixopcodes.10 Chapter 1: Instruction Formats