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Instruction Set (Continued)

8.1.1

Prefix (Optional)

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:

Segment Override explicitly specifies which segment

calculation.

Address Size switches between 16-bit and 32-bit

addressing by selecting the non-default address size.

Operand Size switches between 16-bit and 32-bit

operand size by selecting the non-default operand

size.

Repeat is used with a string instruction to cause the

instruction to be repeated for each element of the

string.

Lock is used to assert the hardware LOCK# signal

during execution of the instruction.

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

bytes.

8.1.2

Opcode

The opcode field specifies the operation to be performed

by the instruction. The opcode field is either one or two

bytes in length and may be further defined by additional

bits in the mod r/m byte. Some operations have more than

one opcode, each specifying a different form of the opera-

tion. Certain opcodes name instruction groups. For exam-

ple, opcode 80h names a group of operations that have an

immediate operand and a register or memory operand. The

reg field may appear in the second opcode byte or in the

mod r/m byte.

The opcode may contain w, d, s and eee opcode fields, for

example, as shown in Table 8-27 on page 223.

8.1.2.1

w Field (Operand Size)

When used, the 1-bit w field selects the operand size dur-

ing 16-bit and 32-bit data operations. See Table 8-4.

8.1.2.2

d Field (Operand Direction)

When used, the 1-bit d field determines which operand is

taken as the source operand and which operand is taken

as the destination. See Table 8-5.

Table 8-3. Instruction Prefix Summary

Prefix

Encoding

Description

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# hardware signal.

REPNE

F2h

Repeat the following string

instruction.

REP/REPE

F3h

Repeat the following string

instruction.

Table 8-4. w Field Encoding

Field

Operand Size

16-Bit Data

Operations

32-Bit Data

Operations

08 bits

8 bits

16 bits

32 bits

Table 8-5. d Field Encoding

Field

Direction of

Operation

Source

Operand

Destination

Operand

reg

mod r/m

mod ss-index-

base

Memory-to-Register

mod r/m

mod ss-

index-base

reg

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