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Instruction Set (Continued)

9.1

GENERAL INSTRUCTION SET FORMAT

Depending on the instruction, the GXm processor core

instructions follow the general instruction format shown in

Table 9-1.

These instructions vary in length and can start at any byte

address. An instruction consists of one or more bytes that

can include prefix bytes, at least one opcode byte, a mod

r/m byte, an s-i-b byte, address displacement, and imme-

diate data. An instruction can be as short as one byte and

as long as 15 bytes. If there are more than 15 bytes in the

instruction, a general protection fault (error code 0) is gen-

erated.

The fields in the general instruction format at the byte

level are summarized in Table 9-2 and detailed in the fol-

lowing subsections.

Table 9-1. General Instruction Set Format

Prefix (optional)

Opcode

Address

Displacement

Immediate

Data

mod r/m Byte

s-i-b Byte

mod

reg

r/m

index

base

0 or More Bytes

1 or 2 Bytes

7:6

5:3

2:0

7:6

5:3

2:0

0, 8, 16, or 32 Bits

Table 9-2. Instruction Fields

Field Name

Description

Prefix (optional)

Prefix Field(s): One or more optional fields that are used to specify segment register override, address

and operand size, repeat elements in string instruction, LOCK# assertion.

Opcode

Opcode Field: Identifies instruction operation.

mod

Address Mode Specifier: Used with 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.

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.

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