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

24594 Rev. 3.10 February 2005 AMD64 TechnologyREX.B: Base. The REX.B bit either adds a 1-bit (high) extensionto the base in the ModRM r/m field or SIB base field, or it adds a1-bit (high) extension to the opcode reg field used for accessingGPRs. (See Table 2-2 on page 47 for more about the REX.B bit.)Encoding Examples. Figure 1-3 on page 18 shows four examples ofhow the R, X, and B bits of REX prefixes are concatenated withfields from the ModRM byte, SIB byte, and opcode to specifyregister and memory addressing. The R, X, and B bits aredescribed in Table 1-11 on page 16.Byte-Register Addressing. In the legacy architecture, the byteregisters (AH, AL, BH, BL, CH, CL, DH, and DL, shown inFigure 2-2 on page 30) are encoded in the ModRM reg or r/mfield or in the opcode reg field as registers 0 through 7. The REXprefix provides an additional byte-register addressingcapability that makes the least-significant byte of any GPRavailable for byte operations (Figure 2-3 on page 31). Thisprovides a uniform set of byte, word, doubleword, andquadword registers better suited for register allocation bycompilers.Special Encodings for Registers. Readers who need to know thedetails of instruction encodings should be aware that certaincombinations of the ModRM and SIB fields have specialmeaning for register encodings. For some of thesecombinations, the instruction fields expanded by the REXprefix are not decoded (treated as don’t cares), thereby creatingaliases of these encodings in the extended registers. Table 1-12on page 19 describes how each of these cases behaves.Implications for INC and DEC Instructions. The REX prefix values aretaken from the 16 single-byte INC and DEC instructions, one foreach of the eight GPRs. Therefore, these single-byte opcodes forINC and DEC are not available in 64-bit mode, although theyare available in legacy and compatibility modes. Thefunctionality of these INC and DEC instructions is stillavailable in 64-bit mode, however, using the ModRM forms ofthose instructions (opcodes FF /0 and FF /1).Chapter 1: Instruction Formats 17