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Unformatted text preview: ModR/M byte contains three fields of information: The mod field combines with the r/m field to form 32 possible values: eight registers and 24 addressing modes. The reg/opcode field specifies either a register number or three more bits of opcode information. The purpose of the reg/opcode field is specified in the primary opcode. The r/m field can specify a register as an operand or it can be combined with the mod field to encode an addressing mode. Sometimes, certain combinations of the mod field and the r/m field is used to express opcode information for some instructions. Certain encodings of the ModR/M byte require a second addressing byte (the SIB byte). The base-plus-index and scale-plus-index forms of 32-bit addressing require the SIB byte. The SIB byte includes the following fields: The scale field specifies the scale factor. The index field specifies the register number of the index register. The base field specifies the register number of the base register. See Section 2.1.5 for the encodings of the ModR/M and SIB bytes. 2.1.4 Displacement and Immediate Bytes Some addressing forms include a displacement immediately following the ModR/M byte (or the SIB byte if one is present). If a displacement is required; it be 1, 2, or 4 bytes. If an instruction specifies an immediate operand, the operand always follows any displacement bytes. An immediate operand can be 1, 2 or 4 bytes. 2.1.5 Addressing-Mode Encoding of ModR/M and SIB Bytes The values and corresponding addressing forms of the ModR/M and SIB bytes are shown in Table 2-1 through Table 2-3: 16-bit addressing forms specified by the 2-4 Vol. 2 INSTRUCTION FORMAT ModR/M byte are in Table 2-1 and 32-bit addressing forms are in Table 2-2. Table 2-3 shows 32-bit addressing forms specified by the SIB byte. In cases where the reg/opcode field in the ModR/M byte represents an extended opcode, valid encodings are shown in Appendix B. In Table 2-1 and Table 2-2, the Effective Address column lists 32 effective addresses that can be assigned to the first operand of an instruction by using the Mod and R/M fields of the ModR/M byte. The first 24 options provide ways of specifying a memory location; the last eight (Mod = 11B) provide ways of specifying general-purpose, MMX technology and XMM registers. The Mod and R/M columns in Table 2-1 and Table 2-2 give the binary encodings of the Mod and R/M fields required to obtain the effective address listed in the first column. For example: see the row indicated by Mod = 11B, R/M = 000B. The row identifies the general-purpose registers EAX, AX or AL; MMX technology register MM0; or XMM register XMM0. The register used is determined by the opcode byte and the operandsize attribute. Now look at the seventh row in either table (labeled "REG ="). This row specifies the use of the 3-bit Reg/Opcode field when the field is used to give the location of a second operand. The second operand must be a general-purpose, MMX technology, or XMM register. Rows one through...
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- Winter '11