Or none immediate immediate data of 1 2 or 4 bytes or

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: base) byte Instructions without ModR/M: the reg field of the opcode In 64-bit mode, these formats do not change. Bits needed to define fields in the 64-bit context are provided by the addition of REX prefixes. More on REX Prefix Fields REX prefixes are a set of 16 opcodes that span one row of the opcode map and occupy entries 40H to 4FH. These opcodes represent valid instructions (INC or DEC) in IA-32 operating modes and in compatibility mode. In 64-bit mode, the same opcodes represent the instruction prefix REX and are not treated as individual instructions. The single-byte-opcode form of INC/DEC instruction not available in 64-bit mode. INC/DEC functionality is still available using ModR/M forms of the same instructions (opcodes FF/0 and FF/1). See Table 2-4 for a summary of the REX prefix format. Figure 2-4 though Figure 2-7 show examples of REX prefix fields in use. Some combinations of REX prefix fields are invalid. In such cases, the prefix is ignored. Some additional information follows: Setting REX.W can be used to determine the operand size but does not solely determine operand width. Like the 66H size prefix, 64-bit operand size override has no effect on byte-specific operations. For non-byte operations: if a 66H prefix is used with prefix (REX.W = 1), 66H is ignored. If a 66H override is used with REX and REX.W = 0, the operand size is 16 bits. REX.R modifies the ModR/M reg field when that field encodes a GPR, SSE, control or debug register. REX.R is ignored when ModR/M specifies other registers or defines an extended opcode. REX.X bit modifies the SIB index field. REX.B either modifies the base in the ModR/M r/m field or SIB base field; or it modifies the opcode reg field used for accessing GPRs. 2-10 Vol. 2 INSTRUCTION FORMAT Table 2-4. REX Prefix Fields [BITS: 0100WRXB] Field Name W R X B Bit Position 7:4 3 2 1 0 Definition 0100 0 = Operand size determined by CS.D 1 = 64 Bit Operand Size Extension of the ModR/M reg field Extension of the SIB index field Extension of the ModR/M r/m field, SIB base field, or Opcode reg field Figure 2-4. Memory Addressing Without an SIB Byte; REX.X Not Used Figure 2-5. Register-Register Addressing (No Memory Operand); REX.X Not Used Vol. 2 2-11 INSTRUCTION FORMAT Figure 2-6. Memory Addressing With a SIB Byte Figure 2-7. Register Operand Coded in Opcode Byte; REX.X & REX.R Not Used In the IA-32 architecture, byte registers (AH, AL, BH, BL, CH, CL, DH, and DL) are encoded in the ModR/M byte's reg field, the r/m field or the opcode reg field as registers 0 through 7. REX prefixes provide an additional addressing capability for byteregisters that makes the least-significant byte of GPRs available for byte operations. Certain combinations of the fields of the ModR/M byte and the SIB byte have special meaning for register encodings. For some combinations, fields expanded by the REX prefix are not decoded. Table 2-5 describes how each case behaves. 2-12 Vol. 2 INSTRUCTION FORMAT Table 2-5. Special...
View Full Document

This note was uploaded on 10/01/2013 for the course CPE 103 taught by Professor Watlins during the Winter '11 term at Mississippi State.

Ask a homework question - tutors are online