Chapter 21. 80386 Dependent Features

129

the 6 section registers

%cs

(code section),

%ds

(data section),

%ss

(stack section),

%es

,

%fs

, and

%gs

.

the 3 processor control registers

%cr0

,

%cr2

, and

%cr3

.

the 6 debug registers

%db0

,

%db1

,

%db2

,

%db3

,

%db6

, and

%db7

.

the 2 test registers

%tr6

and

%tr7

.

the 8 floating point register stack

%st

or equivalently

%st(0)

,

%st(1)

,

%st(2)

,

%st(3)

,

%st(4)

,

%st(5)

,

%st(6)

, and

%st(7)

. These registers are overloaded by 8 MMX registers

%mm0

,

%mm1

,

%mm2

,

%mm3

,

%mm4

,

%mm5

,

%mm6

and

%mm7

.

the 8 SSE registers registers

%xmm0

,

%xmm1

,

%xmm2

,

%xmm3

,

%xmm4

,

%xmm5

,

%xmm6

and

%xmm7

.

The AMD x86 64 architecture extends the register set by:

enhancing the 8 32 bit registers to 64 bit:

%rax

(the accumulator),

%rbx

,

%rcx

,

%rdx

,

%rdi

,

%rsi

,

%rbp

(the frame pointer),

%rsp

(the stack pointer)

the 8 extended registers

%r8

%r15

.

the 8 32 bit low ends of the extended registers:

%r8d

%r15d

the 8 16 bit low ends of the extended registers:

%r8w

%r15w

the 8 8 bit low ends of the extended registers:

%r8b

%r15b

the 4 8 bit registers:

%sil

,

%dil

,

%bpl

,

%spl

.

the 8 debug registers:

%db8

%db15

.

the 8 SSE registers:

%xmm8

%xmm15

.

21.5. Instruction Prefixes

Instruction prefixes are used to modify the following instruction. They are used to repeat string in 

structions, to provide section overrides, to perform bus lock operations, and to change operand and

address sizes. (Most instructions that normally operate on 32 bit operands will use 16 bit operands if

the instruction has an "operand size" prefix.) Instruction prefixes are best written on the same line as

the instruction they act upon. For example, the

scas

(scan string) instruction is repeated with:

repne scas %es:(%edi),%al

You may also place prefixes on the lines immediately preceding the instruction, but this circumvents

checks that

as

does with prefixes, and will not work with all prefixes.

Here is a list of instruction prefixes:

Section override prefixes

cs

,

ds

,

ss

,

es

,

fs

,

gs

. These are automatically added by specifying using

the

section

:

memory operand

form for memory references.

Operand/Address size prefixes

data16

and

addr16

change 32 bit operands/addresses into 16 bit

operands/addresses, while

data32

and

addr32

change 16 bit ones (in a

.code16

section) into

32 bit operands/addresses. These prefixes must appear on the same line of code as the instruction

they modify. For example, in a 16 bit

.code16

section, you might write:

addr32 jmpl *(%ebx)