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Chapter 21. 80386 Dependent Features

Immediate form long jumps and calls are

lcall/ljmp $section, $offset

in AT&T syntax;

the Intel syntax is

call/jmp far section:offset

. Also, the far return instruction is

lret

$stack adjust

in AT&T syntax; Intel syntax is

ret far stack adjust

.

The AT&T assembler does not provide support for multiple section programs. Unix style systems

expect all programs to be single sections.

21.3. Instruction Naming

Instruction mnemonics are suffixed with one character modifiers which specify the size of operands.

The letters

b

,

w

,

l

and

q

specify byte, word, long and quadruple word operands. If no suffix is specified

by an instruction then

as

tries to fill in the missing suffix based on the destination register operand

(the last one by convention). Thus,

mov %ax, %bx

is equivalent to

movw %ax, %bx

; also,

mov $1,

%bx

is equivalent to

movw $1, bx

. Note that this is incompatible with the AT&T Unix assembler

which assumes that a missing mnemonic suffix implies long operand size. (This incompatibility does

not affect compiler output since compilers always explicitly specify the mnemonic suffix.)

Almost all instructions have the same names in AT&T and Intel format. There are a few exceptions.

The sign extend and zero extend instructions need two sizes to specify them. They need a size to

sign/zero extend from and a size to zero extend to. This is accomplished by using two instruction

mnemonic suffixes in AT&T syntax. Base names for sign extend and zero extend are

movs...

and

movz...

in AT&T syntax (

movsx

and

movzx

in Intel syntax). The instruction mnemonic suffixes are

tacked on to this base name, the from suffix before the to suffix. Thus,

movsbl %al, %edx

is AT&T

syntax for "move sign extend from %al to %edx." Possible suffixes, thus, are

bl

(from byte to long),

bw

(from byte to word),

wl

(from word to long),

bq

(from byte to quadruple word),

wq

(from word to

quadruple word), and

lq

(from long to quadruple word).

The Intel syntax conversion instructions

  cbw

   sign extend byte in

%al

to word in

%ax

,

  cwde

   sign extend word in

%ax

to long in

%eax

,

  cwd

   sign extend word in

%ax

to long in

%dx:%ax

,

  cdq

   sign extend dword in

%eax

to quad in

%edx:%eax

,

  cdqe

   sign extend dword in

%eax

to quad in

%rax

(x86 64 only),

  cdo

   sign extend quad in

%rax

to octuple in

%rdx:%rax

(x86 64 only),

are called

cbtw

,

cwtl

,

cwtd

,

cltd

,

cltq

, and

cqto

in AT&T naming.

as

accepts either naming for

these instructions.

Far call/jump instructions are

lcall

and

ljmp

in AT&T syntax, but are

call far

and

jump far

in

Intel convention.

21.4. Register Naming

Register operands are always prefixed with

%

. The 80386 registers consist of

the 8 32 bit registers

%eax

(the accumulator),

%ebx

,

%ecx

,

%edx

,

%edi

,

%esi

,

%ebp

(the frame

pointer), and

%esp

(the stack pointer).

the 8 16 bit low ends of these:

%ax

,

%bx

,

%cx

,

%dx

,

%di

,

%si

,

%bp

, and

%sp

.

the 8 8 bit registers:

%ah

,

%al

,

%bh

,

%bl

,

%ch

,

%cl

,

%dh

, and

%dl

(These are the high bytes and

low bytes of

%ax

,

%bx

,

%cx

, and

%dx

)