Chapter 21. 80386 Dependent Features

131

AT&T:

%gs:foo

; Intel

gs:foo

This selects the contents of the variable

foo

with section register

section

being

%gs

.

Absolute (as opposed to PC relative) call and jump operands must be prefixed with

*

. If no

*

is

specified,

as

always chooses PC relative addressing for jump/call labels.

Any instruction that has a memory operand, but no register operand, must specify its size (byte, word,

long, or quadruple) with an instruction mnemonic suffix (

b

,

w

,

l

or

q

, respectively).

The x86 64 architecture adds an RIP (instruction pointer relative) addressing. This addressing mode

is specified by using

rip

as a base register. Only constant offsets are valid. For example:

AT&T:

1234(%rip)

, Intel:

[rip + 1234]

Points to the address 1234 bytes past the end of the current instruction.

AT&T:

symbol(%rip)

, Intel:

[rip + symbol]

Points to the

symbol

in RIP relative way, this is shorter than the default absolute addressing.

Other addressing modes remain unchanged in x86 64 architecture, except registers used are 64 bit

instead of 32 bit.

21.7. Handling of Jump Instructions

Jump instructions are always optimized to use the smallest possible displacements. This is accom 

plished by using byte (8 bit) displacement jumps whenever the target is sufficiently close. If a byte

displacement is insufficient a long displacement is used. We do not support word (16 bit) displace 

ment jumps in 32 bit mode (i.e. prefixing the jump instruction with the

data16

instruction prefix),

since the 80386 insists upon masking

%eip

to 16 bits after the word displacement is added. (See also

Section 21.12 Specifying CPU Architecture)

Note that the

jcxz

,

jecxz

,

loop

,

loopz

,

loope

,

loopnz

and

loopne

instructions only come in

byte displacements, so that if you use these instructions (

gcc

does not use them) you may get an

error message (and incorrect code). The AT&T 80386 assembler tries to get around this problem by

expanding

jcxz foo

to

jcxz cx_zero

jmp cx_nonzero

cx_zero: jmp foo

cx_nonzero:

21.8. Floating Point

All 80387 floating point types except packed BCD are supported. (BCD support may be added without

much difficulty). These data types are 16 , 32 , and 64  bit integers, and single (32 bit), double (64 

bit), and extended (80 bit) precision floating point. Each supported type has an instruction mnemonic

suffix and a constructor associated with it. Instruction mnemonic suffixes specify the operand's data

type. Constructors build these data types into memory.

Floating point constructors are

.float

or

.single

,

.double

, and

.tfloat

for 32 , 64 , and

80 bit formats. These correspond to instruction mnemonic suffixes

s

,

l

, and

t

.

t

stands for 80 bit

(ten byte) real. The 80387 only supports this format via the

fldt

(load 80 bit real to stack top) and

fstpt

(store 80 bit real and pop stack) instructions.