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80386
Table 2-1. Register Usage
Use in Use in
Use in
Register
Real Mode Protected Mode Virtual 8086 Mode
Load Store
Load
Store Load
Store
General Registers Yes Yes Yes Yes Yes
Yes
Segment Registers Yes Yes
Yes
Yes Yes
Yes
Flag
Register Yes Yes
Yes Yes
IOPL
IOPL'
Control Registers
Yes Yes
PL
= 0
PL
= 0 No Yes
GDTR
Yes Yes
PL
= 0 Yes
No
Yes
IDTR
Yes Yes
PL
= 0 Yes
No
Yes
LDTR
No No
PL
= 0 Yes
No No
TR
No
No
PL
= 0 Yes No
No
Debug
Control Yes Yes
PL
= 0
PL
= 0 No
No
Test Registers
Yes Yes
PL
= 0
PL
= 0
No
No
NOTES:
PL = 0: The registers can be accessed only when the current privilege level is zero.
'IOPL: The PUSHF and POPF instructions are made
I/O
Privilege Level sensitive in Virtual 8086 Mode.
5)
However, registers which have been previ-
ously
stored
may be reloaded
without
mask-
ing.
Depending upon
the
values
of
undefined regis-
ter
bits will make
your
software
dependent
upon
the
unspecified 80386 handling
of
these bits. De-
pending
on
undefined values risks making
your
software
incompatible
with
future
processors
that
define usages
for
the
80386-undefined bits.
AVOID ANY SOFTWARE DEPENDENCE UPON
THE STATE
OF
UNDEFINED 80386 REGISTER
BITS.
2.4
INSTRUCTION
SET
2.4.1
Instruction Set Overview
The instruction set is divided into nine categories of
operations:
Data Transfer
Arithmetic
Shift/Rotate
String Manipulation
Bit Manipulation
Control Transfer
High
Level Language Support
Operating System Support
Processor Control
These 80386 instructions are listed
in
Table 2·2.
14
All 80386 instructions operate
on
either
0,
1,
2,
or 3
operands; where
an
operand resides
in
a register,
in
the instruction itself, or
in
memory. Most zero oper·
and instructions (e.g.
CLI,
STI)
take only one byte.
One operand instructions generally are two bytes
long. The average instruction is
3.2
bytes long.
Since the 80386 has a 16·byte instruction queue,
an
average of 5 instructions will be prefetched. The
use
of two operands permits the following types of com-
mon instructions:
Register to Register
Memory to Register
Immediate to Register
Register to Memory
Immediate to Memory.
The operands can be either
8,
16, or
32
bits long. As
a
general rule, when executing code written for the
80386 (32-bit code), operands are 8 or
32
bits; when
executing existing
80286 or 8086 code (16-bit code),
operands are 8 or 16 bits. Prefixes can be added to
all instructions which override the default length of
the operands,
(Le.
use
32-bit operands for 16-bit
code, or 16-bit operands for 32-bit code).