GXM Datasheet, PDF (202/244 Pages) National Semiconductor (TI) – Geode™ GXm Processor Integrated x86 Solution with MMX Support

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.
Prefix (optional)
0 or More Bytes
Opcode
1 or 2 Bytes
Table 9-1. General Instruction Set Format
Register and Address Mode Specifier
mod r/m Byte
mod reg r/m
7:6
5:3
2:0
s-i-b Byte
ss index base
7:6
5:3
2:0
Address
Displacement
0, 8, 16, or 32 Bits
Immediate
Data
0, 8, 16, or 32 Bits
Field Name
Prefix (optional)
Opcode
mod
reg
r/m
ss
index
base
Address Displacement
Immediate Data
Table 9-2. Instruction Fields
Description
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 Field: Identifies instruction operation.
Address Mode Specifier: Used with r/m field to select addressing mode.
General Register Specifier: Uses reg, sreg3 or sreg2 encoding depending on opcode field.
Address Mode Specifier: Used with mod field to select addressing mode.
Scale factor: Determines scaled-index address mode.
Index: Determines general register to be used as index register.
Base: Determines general register to be used as base register.
Displacement: Determines address displacement.
Immediate Data: Immediate-data operand used by instruction.
www.national.com
202
Revision 3.1