2.4 Instruction Format

80386 instructions are composed of various elements and have various formats. The exact format of instructions is shown in Appendix B; the elements of instructions are described below. Of these instruction elements, only one, the opcode, is always present. The other elements may or may not be present, depending on the particular operation involved and on the location and type of the operands. The elements of an instruction, in order of occurrence are as follows:

• Prefixes -- one or more bytes preceding an instruction that modify the operation of the instruction. The following types of prefixes can be used by applications programs:
  1. Segment override -- explicitly specifies which segment register an instruction should use, thereby overriding the default segment-register selection used by the 80386 for that instruction.
  2. Address size -- switches between 32-bit and 16-bit address generation.
  3. Operand size -- switches between 32-bit and 16-bit operands.
  4. Repeat -- used with a string instruction to cause the instruction to act on each element of the string.
• Opcode -- specifies the operation performed by the instruction. Some operations have several different opcodes, each specifying a different variant of the operation.
• Register specifier -- an instruction may specify one or two register operands. Register specifiers may occur either in the same byte as the opcode or in the same byte as the addressing-mode specifier.
• Addressing-mode specifier -- when present, specifies whether an operand is a register or memory location; if in memory, specifies whether a displacement, a base register, an index register, and scaling are to be used.
• SIB (scale, index, base) byte -- when the addressing-mode specifier indicates that an index register will be used to compute the address of an operand, an SIB byte is included in the instruction to encode the base register, the index register, and a scaling factor.
• Displacement -- when the addressing-mode specifier indicates that a displacement will be used to compute the address of an operand, the displacement is encoded in the instruction. A displacement is a signed integer of 32, 16, or eight bits. The eight-bit form is used in the common case when the displacement is sufficiently small. The processor extends an eight-bit displacement to 16 or 32 bits, taking into account the sign.
• Immediate operand -- when present, directly provides the value of an operand of the instruction. Immediate operands may be 8, 16, or 32 bits wide. In cases where an eight-bit immediate operand is combined in some way with a 16- or 32-bit operand, the processor automatically extends the size of the eight-bit operand, taking into account the sign.

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