Справочник Пользователя для AMD LX 800@0.9W

33234H

Prefix bytes can be placed in front of any instruction to modify the operation of that instruction. When more than one prefix
is used, the order is not important. There are five types of prefixes that can be used:

1)

Segment Override explicitly specifies which segment register the instruction will use for effective address calculation.

2)

Address Size switches between 16-bit and 32-bit addressing by selecting the non-default address size.

3)

Operand Size switches between 16-bit and 32-bit operand size by selecting the non-default operand size.

4)

Repeat is used with a string instruction to cause the instruction to be repeated for each element of the string.

Table 8-3 lists the encoding for different types of prefix bytes.

Prefix (optional)

Prefix Field(s): One or more optional fields that are used to specify segment register over-
ride, address and operand size, repeat elements in string instruction, and LOCK# assertion. 

Opcode

Opcode Field: Identifies instruction operation.

mod 

Address Mode Specifier: Used with the r/m field to select addressing mode.

reg

General Register Specifier: Uses reg, sreg3, or sreg2 encoding depending on opcode field.

r/m

Address Mode Specifier: Used with mod field to select addressing mode.

ss

Scale factor: Determines scaled-index address mode.

index

Index: Determines general register to be used as index register.

base

Base: Determines general register to be used as base register.

Address Displacement

Displacement: Determines address displacement.

Immediate Data

Immediate Data: Immediate data operand used by instruction.

ES:

26h

Override segment default, use ES for memory operand.

CS:

2Eh

Override segment default, use CS for memory operand.

SS:

36h

Override segment default, use SS for memory operand.

DS:

3Eh

Override segment default, use DS for memory operand.

FS:

64h

Override segment default, use FS for memory operand.

GS:

65h

Override segment default, use GS for memory operand.

Operand Size

66h

Make operand size attribute the inverse of the default.

Address Size

67h

Make address size attribute the inverse of the default.

LOCK

F0h

Assert LOCK# internal hardware signal.

REPNE

F2h

Repeat the following string instruction.

REP/REPE

F3h

Repeat the following string instruction.