Intel 80386 manual Overview, ~~~

inter80386

8.2 INSTRUCTION ENCODING

8.2.1 Overview

All instruction encodings are subsets of the general instruction format shown in Figure 8-1. Instructions consist of one or two primary opcode bytes, possibly an address specifier consisting of the "mod rim" byte and "scaled index" byte, a displacement if re- quired, and an immediate data field if required.

Within the primary opcode or opcodes, smaller en- coding fields may be defined. These fields vary ac- cording to the class of operation. The fields define such information as direction of the operation, size of the displacements, register encoding, or sign ex- tension.

Almost all instructions referring to an operand in memory have an addressing mode byte following the primary opcode byte(s). This byte, the mod rim byte, specifies the address mode to be used. Certain

encodings of the mod rim byte indicate a second addressing byte, the scale-index-base byte, follows the mod rim byte to fully specify the addressing mode.

Addressing modes can include a displacement im- mediately following the mod rim byte, or scaled in- dex byte. If a displacement is present, the possible sizes are 8, 16 or 32 bits.

If the instruction specifies an immediate operand, the immediate operand follows any displacement bytes. The immediate operand, if specified, is always the last field of the instruction.

Figure 8-1 illustrates several of the fields that can appear in an instruction, such as the mod field and the rim field, but the Figure does not show all fields. Several smaller fields also appear in certain instruc- tions, sometimes within the opcode bytes them- selves. Table 8-2 is a complete list of all fields ap- pearing in the 80386 instruction set. Further ahead, following Table 8-2, are detailed tables for each field.

ITTTTTTT T I TTT TTTTT I mod TTT rim I ss index base Id32i16i8i none data32i16i8i none

:z 07

opcode

(one or two bytes) (T represents an opcode bit.)

o)~~~\ )

"mod rim"	"s-i-b"	address	immediate
byte	byte	displacement	data
\~--------~--------~)		(4, 2, 1 bytes	(4, 2, 1 bytes
register and address		or none)	or none)
mode specifier

Figure 8-1. General Instruction Format

Table 8-2. Fields within 80386 Instructions

Field Name	Description	Number of Bits
w	Specifies if Data is Byte or Full Size (Full Size is either 16 or 32 Bits	1
d	Specifies Direction of Data Operation	1
s	Specifies if an Immediate Data Field Must be Sign-Extended	1
reg	General Register Specifier	3
mod rim	Address Mode Specifier (Effective Address can be a General Register)	2 for mod;
		3 for rim
ss	Scale Factor for Scaled Index Address Mode	2
index	General Register to be used as Index Register	3
base	General Register to be used as Base Register	3
sreg2	Segment Register Specifier for CS, SS, OS, ES	2
sreg3	Segment Register Specifier for CS, SS, OS, ES, FS, GS	3
tttn	For Conditional Instructions, Specifies a Condition Asserted
	or a Condition Negated	4

Note: Table 8-1 shows encoding of individual instructions.

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