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Chapter 21. 80386 Dependent Features

The bus lock prefix

lock

inhibits interrupts during execution of the instruction it precedes. (This is

only valid with certain instructions; see a 80386 manual for details).

The wait for coprocessor prefix

wait

waits for the coprocessor to complete the current instruction.

This should never be needed for the 80386/80387 combination.

The

rep

,

repe

, and

repne

prefixes are added to string instructions to make them repeat

%ecx

times (

%cx

times if the current address size is 16 bits).

The

rex

family of prefixes is used by x86 64 to encode extensions to i386 instruction set. The

rex

prefix has four bits    an operand size overwrite (

64

) used to change operand size from 32 bit to

64 bit and X, Y and Z extensions bits used to extend the register set.

You may write the

rex

prefixes directly. The

rex64xyz

instruction emits

rex

prefix with all the

bits set. By omitting the

64

,

x

,

y

or

z

you may write other prefixes as well. Normally, there is

no need to write the prefixes explicitly, since gas will automatically generate them based on the

instruction operands.

21.6. Memory References

An Intel syntax indirect memory reference of the form

section:[base + index*scale + disp]

is translated into the AT&T syntax

section:disp(base, index, scale)

where

base

and

index

are the optional 32 bit base and index registers,

disp

is the optional displace 

ment, and

scale

, taking the values 1, 2, 4, and 8, multiplies

index

to calculate the address of the

operand. If no

scale

is specified,

scale

is taken to be 1.

section

specifies the optional section

register for the memory operand, and may override the default section register (see a 80386 manual

for section register defaults). Note that section overrides in AT&T syntax must be preceded by a

%

. If

you specify a section override which coincides with the default section register,

as

does not output

any section register override prefixes to assemble the given instruction. Thus, section overrides can be

specified to emphasize which section register is used for a given memory operand.

Here are some examples of Intel and AT&T style memory references:

AT&T:

 4(%ebp)

, Intel:

[ebp   4]

base

is

%ebp

;

disp

is

 4

.

section

is missing, and the default section is used (

%ss

for address 

ing with

%ebp

as the base register).

index

,

scale

are both missing.

AT&T:

foo(,%eax,4)

, Intel:

[foo + eax*4]

index

is

%eax

(scaled by a

scale

4);

disp

is

foo

. All other fields are missing. The section

register here defaults to

%ds

.

AT&T:

foo(,1)

; Intel

[foo]

This uses the value pointed to by

foo

as a memory operand. Note that

base

and

index

are both

missing, but there is only one

,

. This is a syntactic exception.