A-10
OPCODE MAP
A.2.5.Opcode Extensions For One- And Two-byte Opcodes
Some of the 1-byte and 2-byte opcodes use bits 5, 4, and 3 of the ModR/M byte (the nnn field
in Figure A-1) as an extension of the opcode. Those opcodes that have opcode extensions are
indicated in Table A-6 with group numbers (Group 1, Group 2, etc.). The group numbers (rang-
ing from 1 to A) provide an entry point into Table A-6 where the encoding of the opcode exten-
sion field can be found. For example, the ADD instruction with a 1-byte opcode of 80H is a
Group 1 instruction. Table A-6 indicates that the opcode extension that must be encoded in the
ModR/M byte for this instruction is 000B.
mod
nnn
R/M
Figure A-1. ModR/M Byte nnn Field (Bits 5, 4, and 3)
