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This is, produced by makeinfo version 4.0 from as.texinfo.
* As: (as). The GNU assembler.
This file documents the GNU Assembler "as".
Copyright (C) 1991, 92, 93, 94, 95, 96, 97, 98, 99, 2000, 2001 Free
Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1
or any later version published by the Free Software Foundation;
with no Invariant Sections, with no Front-Cover Texts, and with no
Back-Cover Texts. A copy of the license is included in the
section entitled "GNU Free Documentation License".

File:, Node: HPPA-Dependent, Next: ESA/390-Dependent, Prev: H8/500-Dependent, Up: Machine Dependencies
HPPA Dependent Features
* Menu:
* HPPA Notes:: Notes
* HPPA Options:: Options
* HPPA Syntax:: Syntax
* HPPA Floating Point:: Floating Point
* HPPA Directives:: HPPA Machine Directives
* HPPA Opcodes:: Opcodes

File:, Node: HPPA Notes, Next: HPPA Options, Up: HPPA-Dependent
As a back end for GNU CC `as' has been throughly tested and should
work extremely well. We have tested it only minimally on hand written
assembly code and no one has tested it much on the assembly output from
the HP compilers.
The format of the debugging sections has changed since the original
`as' port (version 1.3X) was released; therefore, you must rebuild all
HPPA objects and libraries with the new assembler so that you can debug
the final executable.
The HPPA `as' port generates a small subset of the relocations
available in the SOM and ELF object file formats. Additional relocation
support will be added as it becomes necessary.

File:, Node: HPPA Options, Next: HPPA Syntax, Prev: HPPA Notes, Up: HPPA-Dependent
`as' has no machine-dependent command-line options for the HPPA.

File:, Node: HPPA Syntax, Next: HPPA Floating Point, Prev: HPPA Options, Up: HPPA-Dependent
The assembler syntax closely follows the HPPA instruction set
reference manual; assembler directives and general syntax closely
follow the HPPA assembly language reference manual, with a few
noteworthy differences.
First, a colon may immediately follow a label definition. This is
simply for compatibility with how most assembly language programmers
write code.
Some obscure expression parsing problems may affect hand written
code which uses the `spop' instructions, or code which makes significant
use of the `!' line separator.
`as' is much less forgiving about missing arguments and other
similar oversights than the HP assembler. `as' notifies you of missing
arguments as syntax errors; this is regarded as a feature, not a bug.
Finally, `as' allows you to use an external symbol without
explicitly importing the symbol. _Warning:_ in the future this will be
an error for HPPA targets.
Special characters for HPPA targets include:
`;' is the line comment character.
`!' can be used instead of a newline to separate statements.
Since `$' has no special meaning, you may use it in symbol names.

File:, Node: HPPA Floating Point, Next: HPPA Directives, Prev: HPPA Syntax, Up: HPPA-Dependent
Floating Point
The HPPA family uses IEEE floating-point numbers.

File:, Node: HPPA Directives, Next: HPPA Opcodes, Prev: HPPA Floating Point, Up: HPPA-Dependent
HPPA Assembler Directives
`as' for the HPPA supports many additional directives for
compatibility with the native assembler. This section describes them
only briefly. For detailed information on HPPA-specific assembler
directives, see `HP9000 Series 800 Assembly Language Reference Manual'
(HP 92432-90001).
`as' does _not_ support the following assembler directives described
in the HP manual:
.endm .liston
.enter .locct
.leave .macro
Beyond those implemented for compatibility, `as' supports one
additional assembler directive for the HPPA: `.param'. It conveys
register argument locations for static functions. Its syntax closely
follows the `.export' directive.
These are the additional directives in `as' for the HPPA:
`.block N'
`.blockz N'
Reserve N bytes of storage, and initialize them to zero.
Mark the beginning of a procedure call. Only the special case
with _no arguments_ is allowed.
`.callinfo [ PARAM=VALUE, ... ] [ FLAG, ... ]'
Specify a number of parameters and flags that define the
environment for a procedure.
PARAM may be any of `frame' (frame size), `entry_gr' (end of
general register range), `entry_fr' (end of float register range),
`entry_sr' (end of space register range).
The values for FLAG are `calls' or `caller' (proc has
subroutines), `no_calls' (proc does not call subroutines),
`save_rp' (preserve return pointer), `save_sp' (proc preserves
stack pointer), `no_unwind' (do not unwind this proc), `hpux_int'
(proc is interrupt routine).
Assemble into the standard section called `$TEXT$', subsection
`.copyright "STRING"'
In the SOM object format, insert STRING into the object code,
marked as a copyright string.
`.copyright "STRING"'
In the ELF object format, insert STRING into the object code,
marked as a version string.
Not yet supported; the assembler rejects programs containing this
Mark the beginning of a procedure.
Mark the end of a procedure.
`.export NAME [ ,TYP ] [ ,PARAM=R ]'
Make a procedure NAME available to callers. TYP, if present, must
be one of `absolute', `code' (ELF only, not SOM), `data', `entry',
`data', `entry', `millicode', `plabel', `pri_prog', or `sec_prog'.
PARAM, if present, provides either relocation information for the
procedure arguments and result, or a privilege level. PARAM may be
`argwN' (where N ranges from `0' to `3', and indicates one of four
one-word arguments); `rtnval' (the procedure's result); or
`priv_lev' (privilege level). For arguments or the result, R
specifies how to relocate, and must be one of `no' (not
relocatable), `gr' (argument is in general register), `fr' (in
floating point register), or `fu' (upper half of float register).
For `priv_lev', R is an integer.
`.half N'
Define a two-byte integer constant N; synonym for the portable
`as' directive `.short'.
`.import NAME [ ,TYP ]'
Converse of `.export'; make a procedure available to call. The
arguments use the same conventions as the first two arguments for
`.label NAME'
Define NAME as a label for the current assembly location.
Not yet supported; the assembler rejects programs containing this
`.origin LC'
Advance location counter to LC. Synonym for the `{No value for
`as'}' portable directive `.org'.
`.param NAME [ ,TYP ] [ ,PARAM=R ]'
Similar to `.export', but used for static procedures.
Use preceding the first statement of a procedure.
Use following the last statement of a procedure.
Synonym for `.equ'; define LABEL with the absolute expression EXPR
as its value.
`.space SECNAME [ ,PARAMS ]'
Switch to section SECNAME, creating a new section by that name if
necessary. You may only use PARAMS when creating a new section,
not when switching to an existing one. SECNAME may identify a
section by number rather than by name.
If specified, the list PARAMS declares attributes of the section,
identified by keywords. The keywords recognized are `spnum=EXP'
(identify this section by the number EXP, an absolute expression),
`sort=EXP' (order sections according to this sort key when linking;
EXP is an absolute expression), `unloadable' (section contains no
loadable data), `notdefined' (this section defined elsewhere), and
`private' (data in this section not available to other programs).
`.spnum SECNAM'
Allocate four bytes of storage, and initialize them with the
section number of the section named SECNAM. (You can define the
section number with the HPPA `.space' directive.)
`.string "STR"'
Copy the characters in the string STR to the object file. *Note
Strings: Strings, for information on escape sequences you can use
in `as' strings.
_Warning!_ The HPPA version of `.string' differs from the usual
`as' definition: it does _not_ write a zero byte after copying STR.
`.stringz "STR"'
Like `.string', but appends a zero byte after copying STR to object
`.subspa NAME [ ,PARAMS ]'
`.nsubspa NAME [ ,PARAMS ]'
Similar to `.space', but selects a subsection NAME within the
current section. You may only specify PARAMS when you create a
subsection (in the first instance of `.subspa' for this NAME).
If specified, the list PARAMS declares attributes of the
subsection, identified by keywords. The keywords recognized are
`quad=EXPR' ("quadrant" for this subsection), `align=EXPR'
(alignment for beginning of this subsection; a power of two),
`access=EXPR' (value for "access rights" field), `sort=EXPR'
(sorting order for this subspace in link), `code_only' (subsection
contains only code), `unloadable' (subsection cannot be loaded
into memory), `common' (subsection is common block), `dup_comm'
(initialized data may have duplicate names), or `zero' (subsection
is all zeros, do not write in object file).
`.nsubspa' always creates a new subspace with the given name, even
if one with the same name already exists.
`.version "STR"'
Write STR as version identifier in object code.

File:, Node: HPPA Opcodes, Prev: HPPA Directives, Up: HPPA-Dependent
For detailed information on the HPPA machine instruction set, see
`PA-RISC Architecture and Instruction Set Reference Manual' (HP

File:, Node: ESA/390-Dependent, Next: i386-Dependent, Prev: HPPA-Dependent, Up: Machine Dependencies
ESA/390 Dependent Features
* Menu:
* ESA/390 Notes:: Notes
* ESA/390 Options:: Options
* ESA/390 Syntax:: Syntax
* ESA/390 Floating Point:: Floating Point
* ESA/390 Directives:: ESA/390 Machine Directives
* ESA/390 Opcodes:: Opcodes

File:, Node: ESA/390 Notes, Next: ESA/390 Options, Up: ESA/390-Dependent
The ESA/390 `as' port is currently intended to be a back-end for the
GNU CC compiler. It is not HLASM compatible, although it does support
a subset of some of the HLASM directives. The only supported binary
file format is ELF; none of the usual MVS/VM/OE/USS object file
formats, such as ESD or XSD, are supported.
When used with the GNU CC compiler, the ESA/390 `as' will produce
correct, fully relocated, functional binaries, and has been used to
compile and execute large projects. However, many aspects should still
be considered experimental; these include shared library support,
dynamically loadable objects, and any relocation other than the 31-bit

File:, Node: ESA/390 Options, Next: ESA/390 Syntax, Prev: ESA/390 Notes, Up: ESA/390-Dependent
`as' has no machine-dependent command-line options for the ESA/390.

File:, Node: ESA/390 Syntax, Next: ESA/390 Floating Point, Prev: ESA/390 Options, Up: ESA/390-Dependent
The opcode/operand syntax follows the ESA/390 Principles of Operation
manual; assembler directives and general syntax are loosely based on the
prevailing AT&T/SVR4/ELF/Solaris style notation. HLASM-style directives
are _not_ supported for the most part, with the exception of those
described herein.
A leading dot in front of directives is optional, and the case of
directives is ignored; thus for example, .using and USING have the same
A colon may immediately follow a label definition. This is simply
for compatibility with how most assembly language programmers write
`#' is the line comment character.
`;' can be used instead of a newline to separate statements.
Since `$' has no special meaning, you may use it in symbol names.
Registers can be given the symbolic names r0..r15, fp0, fp2, fp4,
fp6. By using thesse symbolic names, `as' can detect simple syntax
errors. The name rarg or r.arg is a synonym for r11, rtca or r.tca for
r12, sp, r.sp, dsa r.dsa for r13, lr or for r14, rbase or r.base
for r3 and rpgt or r.pgt for r4.
`*' is the current location counter. Unlike `.' it is always
relative to the last USING directive. Note that this means that
expressions cannot use multiplication, as any occurence of `*' will be
interpreted as a location counter.
All labels are relative to the last USING. Thus, branches to a label
always imply the use of base+displacement.
Many of the usual forms of address constants / address literals are
supported. Thus,
.using *,r3
L r15,=A(some_routine)
LM r6,r7,=V(some_longlong_extern)
A r1,=F'12'
AH r0,=H'42'
ME r6,=E'3.1416'
MD r6,=D'3.14159265358979'
O r6,=XL4'cacad0d0'
should all behave as expected: that is, an entry in the literal pool
will be created (or reused if it already exists), and the instruction
operands will be the displacement into the literal pool using the
current base register (as last declared with the `.using' directive).

File:, Node: ESA/390 Floating Point, Next: ESA/390 Directives, Prev: ESA/390 Syntax, Up: ESA/390-Dependent
Floating Point
The assembler generates only IEEE floating-point numbers. The older
floiating point formats are not supported.

File:, Node: ESA/390 Directives, Next: ESA/390 Opcodes, Prev: ESA/390 Floating Point, Up: ESA/390-Dependent
ESA/390 Assembler Directives
`as' for the ESA/390 supports all of the standard ELF/SVR4 assembler
directives that are documented in the main part of this documentation.
Several additional directives are supported in order to implement the
ESA/390 addressing model. The most important of these are `.using' and
These are the additional directives in `as' for the ESA/390:
A small subset of the usual DC directive is supported.
`.drop REGNO'
Stop using REGNO as the base register. The REGNO must have been
previously declared with a `.using' directive in the same section
as the current section.
`.ebcdic STRING'
Emit the EBCDIC equivalent of the indicated string. The emitted
string will be null terminated. Note that the directives
`.string' etc. emit ascii strings by default.
The standard HLASM-style EQU directive is not supported; however,
the standard `as' directive .equ can be used to the same effect.
Dump the literal pool accumulated so far; begin a new literal pool.
The literal pool will be written in the current section; in order
to generate correct assembly, a `.using' must have been previously
specified in the same section.
`.using EXPR,REGNO'
Use REGNO as the base register for all subsequent RX, RS, and SS
form instructions. The EXPR will be evaluated to obtain the base
address; usually, EXPR will merely be `*'.
This assembler allows two `.using' directives to be simultaneously
outstanding, one in the `.text' section, and one in another section
(typically, the `.data' section). This feature allows dynamically
loaded objects to be implemented in a relatively straightforward
way. A `.using' directive must always be specified in the `.text'
section; this will specify the base register that will be used for
branches in the `.text' section. A second `.using' may be
specified in another section; this will specify the base register
that is used for non-label address literals. When a second
`.using' is specified, then the subsequent `.ltorg' must be put in
the same section; otherwise an error will result.
Thus, for example, the following code uses `r3' to address branch
targets and `r4' to address the literal pool, which has been
written to the `.data' section. The is, the constants
`=A(some_routine)', `=H'42'' and `=E'3.1416'' will all appear in
the `.data' section.
.using LITPOOL,r4
BASR r3,0
.using *,r3
L r4,4(,r3)
L r15,=A(some_routine)
LTR r15,r15
AH r0,=H'42'
ME r6,=E'3.1416'
Note that this dual-`.using' directive semantics extends and is
not compatible with HLASM semantics. Note that this assembler
directive does not support the full range of HLASM semantics.

File:, Node: ESA/390 Opcodes, Prev: ESA/390 Directives, Up: ESA/390-Dependent
For detailed information on the ESA/390 machine instruction set, see
`ESA/390 Principles of Operation' (IBM Publication Number DZ9AR004).

File:, Node: i386-Dependent, Next: i860-Dependent, Prev: ESA/390-Dependent, Up: Machine Dependencies
80386 Dependent Features
The i386 version `as' supports both the original Intel 386
architecture in both 16 and 32-bit mode as well as AMD x86-64
architecture extending the Intel architecture to 64-bits.
* Menu:
* i386-Options:: Options
* i386-Syntax:: AT&T Syntax versus Intel Syntax
* i386-Mnemonics:: Instruction Naming
* i386-Regs:: Register Naming
* i386-Prefixes:: Instruction Prefixes
* i386-Memory:: Memory References
* i386-Jumps:: Handling of Jump Instructions
* i386-Float:: Floating Point
* i386-SIMD:: Intel's MMX and AMD's 3DNow! SIMD Operations
* i386-16bit:: Writing 16-bit Code
* i386-Arch:: Specifying an x86 CPU architecture
* i386-Bugs:: AT&T Syntax bugs
* i386-Notes:: Notes

File:, Node: i386-Options, Next: i386-Syntax, Up: i386-Dependent
The i386 version of `as' has a few machine dependent options:
`--32 | --64'
Select the word size, either 32 bits or 64 bits. Selecting 32-bit
implies Intel i386 architecture, while 64-bit implies AMD x86-64
These options are only available with the ELF object file format,
and require that the necessary BFD support has been included (on a
32-bit platform you have to add -enable-64-bit-bfd to configure
enable 64-bit usage and use x86-64 as target platform).

File:, Node: i386-Syntax, Next: i386-Mnemonics, Prev: i386-Options, Up: i386-Dependent
AT&T Syntax versus Intel Syntax
`as' now supports assembly using Intel assembler syntax.
`.intel_syntax' selects Intel mode, and `.att_syntax' switches back to
the usual AT&T mode for compatibility with the output of `gcc'. Either
of these directives may have an optional argument, `prefix', or
`noprefix' specifying whether registers require a `%' prefix. AT&T
System V/386 assembler syntax is quite different from Intel syntax. We
mention these differences because almost all 80386 documents use Intel
syntax. Notable differences between the two syntaxes are:
* AT&T immediate operands are preceded by `$'; Intel immediate
operands are undelimited (Intel `push 4' is AT&T `pushl $4').
AT&T register operands are preceded by `%'; Intel register operands
are undelimited. AT&T absolute (as opposed to PC relative)
jump/call operands are prefixed by `*'; they are undelimited in
Intel syntax.
* AT&T and Intel syntax use the opposite order for source and
destination operands. Intel `add eax, 4' is `addl $4, %eax'. The
`source, dest' convention is maintained for compatibility with
previous Unix assemblers. Note that instructions with more than
one source operand, such as the `enter' instruction, do _not_ have
reversed order. *Note i386-Bugs::.
* In AT&T syntax the size of memory operands is determined from the
last character of the instruction mnemonic. Mnemonic suffixes of
`b', `w', `l' and `q' specify byte (8-bit), word (16-bit), long
(32-bit) and quadruple word (64-bit) memory references. Intel
syntax accomplishes this by prefixing memory operands (_not_ the
instruction mnemonics) with `byte ptr', `word ptr', `dword ptr'
and `qword ptr'. Thus, Intel `mov al, byte ptr FOO' is `movb FOO,
%al' in AT&T syntax.
* 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
* The AT&T assembler does not provide support for multiple section
programs. Unix style systems expect all programs to be single

File:, Node: i386-Mnemonics, Next: i386-Regs, Prev: i386-Syntax, Up: i386-Dependent
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
* `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.

File:, Node: i386-Regs, Next: i386-Prefixes, Prev: i386-Mnemonics, Up: i386-Dependent
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')
* 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
* 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'.

File:, Node: i386-Prefixes, Next: i386-Memory, Prev: i386-Regs, Up: i386-Dependent
Instruction Prefixes
Instruction prefixes are used to modify the following instruction.
They are used to repeat string instructions, 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)
* 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

File:, Node: i386-Memory, Next: i386-Jumps, Prev: i386-Prefixes, Up: i386-Dependent
Memory References
An Intel syntax indirect memory reference of the form
is translated into the AT&T syntax
where BASE and INDEX are the optional 32-bit base and index registers,
DISP is the optional displacement, 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 addressing 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.
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
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.

File:, Node: i386-Jumps, Next: i386-Float, Prev: i386-Memory, Up: i386-Dependent
Handling of Jump Instructions
Jump instructions are always optimized to use the smallest possible
displacements. This is accomplished 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) displacement 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 *note i386-Arch::)
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

File:, Node: i386-Float, Next: i386-SIMD, Prev: i386-Jumps, Up: i386-Dependent
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.
* Integer constructors are `.word', `.long' or `.int', and `.quad'
for the 16-, 32-, and 64-bit integer formats. The corresponding
instruction mnemonic suffixes are `s' (single), `l' (long), and
`q' (quad). As with the 80-bit real format, the 64-bit `q' format
is only present in the `fildq' (load quad integer to stack top)
and `fistpq' (store quad integer and pop stack) instructions.
Register to register operations should not use instruction mnemonic
suffixes. `fstl %st, %st(1)' will give a warning, and be assembled as
if you wrote `fst %st, %st(1)', since all register to register
operations use 80-bit floating point operands. (Contrast this with
`fstl %st, mem', which converts `%st' from 80-bit to 64-bit floating
point format, then stores the result in the 4 byte location `mem')

File:, Node: i386-SIMD, Next: i386-16bit, Prev: i386-Float, Up: i386-Dependent
Intel's MMX and AMD's 3DNow! SIMD Operations
`as' supports Intel's MMX instruction set (SIMD instructions for
integer data), available on Intel's Pentium MMX processors and Pentium
II processors, AMD's K6 and K6-2 processors, Cyrix' M2 processor, and
probably others. It also supports AMD's 3DNow! instruction set (SIMD
instructions for 32-bit floating point data) available on AMD's K6-2
processor and possibly others in the future.
Currently, `as' does not support Intel's floating point SIMD, Katmai
The eight 64-bit MMX operands, also used by 3DNow!, are called
`%mm0', `%mm1', ... `%mm7'. They contain eight 8-bit integers, four
16-bit integers, two 32-bit integers, one 64-bit integer, or two 32-bit
floating point values. The MMX registers cannot be used at the same
time as the floating point stack.
See Intel and AMD documentation, keeping in mind that the operand
order in instructions is reversed from the Intel syntax.

File:, Node: i386-16bit, Next: i386-Arch, Prev: i386-SIMD, Up: i386-Dependent
Writing 16-bit Code
While `as' normally writes only "pure" 32-bit i386 code or 64-bit
x86-64 code depending on the default configuration, it also supports
writing code to run in real mode or in 16-bit protected mode code
segments. To do this, put a `.code16' or `.code16gcc' directive before
the assembly language instructions to be run in 16-bit mode. You can
switch `as' back to writing normal 32-bit code with the `.code32'
`.code16gcc' provides experimental support for generating 16-bit
code from gcc, and differs from `.code16' in that `call', `ret',
`enter', `leave', `push', `pop', `pusha', `popa', `pushf', and `popf'
instructions default to 32-bit size. This is so that the stack pointer
is manipulated in the same way over function calls, allowing access to
function parameters at the same stack offsets as in 32-bit mode.
`.code16gcc' also automatically adds address size prefixes where
necessary to use the 32-bit addressing modes that gcc generates.
The code which `as' generates in 16-bit mode will not necessarily
run on a 16-bit pre-80386 processor. To write code that runs on such a
processor, you must refrain from using _any_ 32-bit constructs which
require `as' to output address or operand size prefixes.
Note that writing 16-bit code instructions by explicitly specifying a
prefix or an instruction mnemonic suffix within a 32-bit code section
generates different machine instructions than those generated for a
16-bit code segment. In a 32-bit code section, the following code
generates the machine opcode bytes `66 6a 04', which pushes the value
`4' onto the stack, decrementing `%esp' by 2.
pushw $4
The same code in a 16-bit code section would generate the machine
opcode bytes `6a 04' (ie. without the operand size prefix), which is
correct since the processor default operand size is assumed to be 16
bits in a 16-bit code section.

File:, Node: i386-Bugs, Next: i386-Notes, Prev: i386-Arch, Up: i386-Dependent
AT&T Syntax bugs
The UnixWare assembler, and probably other AT&T derived ix86 Unix
assemblers, generate floating point instructions with reversed source
and destination registers in certain cases. Unfortunately, gcc and
possibly many other programs use this reversed syntax, so we're stuck
with it.
For example
fsub %st,%st(3)
results in `%st(3)' being updated to `%st - %st(3)' rather than the
expected `%st(3) - %st'. This happens with all the non-commutative
arithmetic floating point operations with two register operands where
the source register is `%st' and the destination register is `%st(i)'.

File:, Node: i386-Arch, Next: i386-Bugs, Prev: i386-16bit, Up: i386-Dependent
Specifying CPU Architecture
`as' may be told to assemble for a particular CPU architecture with
the `.arch CPU_TYPE' directive. This directive enables a warning when
gas detects an instruction that is not supported on the CPU specified.
The choices for CPU_TYPE are:
`i8086' `i186' `i286' `i386'
`i486' `i586' `i686' `pentium'
`pentiumpro' `pentium4' `k6' `athlon'
Apart from the warning, there are only two other effects on `as'
operation; Firstly, if you specify a CPU other than `i486', then shift
by one instructions such as `sarl $1, %eax' will automatically use a
two byte opcode sequence. The larger three byte opcode sequence is
used on the 486 (and when no architecture is specified) because it
executes faster on the 486. Note that you can explicitly request the
two byte opcode by writing `sarl %eax'. Secondly, if you specify
`i8086', `i186', or `i286', _and_ `.code16' or `.code16gcc' then byte
offset conditional jumps will be promoted when necessary to a two
instruction sequence consisting of a conditional jump of the opposite
sense around an unconditional jump to the target.
Following the CPU architecture, you may specify `jumps' or `nojumps'
to control automatic promotion of conditional jumps. `jumps' is the
default, and enables jump promotion; All external jumps will be of the
long variety, and file-local jumps will be promoted as necessary.
(*note i386-Jumps::) `nojumps' leaves external conditional jumps as
byte offset jumps, and warns about file-local conditional jumps that
`as' promotes. Unconditional jumps are treated as for `jumps'.
For example
.arch i8086,nojumps

File:, Node: i386-Notes, Prev: i386-Bugs, Up: i386-Dependent
There is some trickery concerning the `mul' and `imul' instructions
that deserves mention. The 16-, 32-, 64- and 128-bit expanding
multiplies (base opcode `0xf6'; extension 4 for `mul' and 5 for `imul')
can be output only in the one operand form. Thus, `imul %ebx, %eax'
does _not_ select the expanding multiply; the expanding multiply would
clobber the `%edx' register, and this would confuse `gcc' output. Use
`imul %ebx' to get the 64-bit product in `%edx:%eax'.
We have added a two operand form of `imul' when the first operand is
an immediate mode expression and the second operand is a register.
This is just a shorthand, so that, multiplying `%eax' by 69, for
example, can be done with `imul $69, %eax' rather than `imul $69, %eax,

File:, Node: i860-Dependent, Next: i960-Dependent, Prev: i386-Dependent, Up: Machine Dependencies
Intel i860 Dependent Features
* Menu:
* Notes-i860:: i860 Notes
* Options-i860:: i860 Command-line Options
* Directives-i860:: i860 Machine Directives
* Opcodes for i860:: i860 Opcodes

File:, Node: Notes-i860, Next: Options-i860, Up: i860-Dependent
i860 Notes
This is a fairly complete i860 assembler which is compatible with the
UNIX System V/860 Release 4 assembler. However, it does not currently
support SVR4 PIC (i.e., `@GOT, @GOTOFF, @PLT').
Like the SVR4/860 assembler, the output object format is ELF32.
Currently, this is the only supported object format. If there is
sufficient interest, other formats such as COFF may be implemented.

File:, Node: Options-i860, Next: Directives-i860, Prev: Notes-i860, Up: i860-Dependent
i860 Command-line Options
SVR4 compatibility options
Print assembler version.
Other options
Select little endian output (this is the default).
Select big endian output. Note that the i860 always reads
instructions as little endian data, so this option only effects
data and not instructions.
Emit a warning message if any pseudo-instruction expansions
occurred. For example, a `or' instruction with an immediate
larger than 16-bits will be expanded into two instructions. This
is a very undesirable feature to rely on, so this flag can help
detect any code where it happens. One use of it, for instance, has
been to find and eliminate any place where `gcc' may emit these

File:, Node: Directives-i860, Next: Opcodes for i860, Prev: Options-i860, Up: i860-Dependent
i860 Machine Directives
Enter dual instruction mode. While this directive is supported, the
preferred way to use dual instruction mode is to explicitly code
the dual bit with the `d.' prefix.
Exit dual instruction mode. While this directive is supported, the
preferred way to use dual instruction mode is to explicitly code
the dual bit with the `d.' prefix.
Change the temporary register used when expanding pseudo
operations. The default register is `r31'.

File:, Node: Opcodes for i860, Prev: Directives-i860, Up: i860-Dependent
i860 Opcodes
All of the Intel i860 machine instructions are supported. Please see
either _i860 Microprocessor Programmer's Reference Manual_ or _i860
Microprocessor Architecture_ for more information.
Other instruction support (pseudo-instructions)
For compatibility with some other i860 assemblers, a number of
pseudo-instructions are supported. While these are supported, they are
a very undesirable feature that should be avoided - in particular, when
they result in an expansion to multiple actual i860 instructions. Below
are the pseudo-instructions that result in expansions.
* Load large immediate into general register:
The pseudo-instruction `mov imm,%rn' (where the immediate does not
fit within a signed 16-bit field) will be expanded into:
orh large_imm@h,%r0,%rn
or large_imm@l,%rn,%rn
* Load/store with relocatable address expression:
For example, the pseudo-instruction `ld.b addr,%rn' will be
expanded into:
orh addr_exp@ha,%r0,%r31
ld.l addr_exp@l(%r31),%rn
The analogous expansions apply to `ld.x, st.x, fld.x, pfld.x,
fst.x', and `pst.x' as well.
* Signed large immediate with add/subtract:
If any of the arithmetic operations `adds, addu, subs, subu' are
used with an immediate larger than 16-bits (signed), then they
will be expanded. For instance, the pseudo-instruction `adds
large_imm,%rx,%rn' expands to:
orh large_imm@h,%r0,%r31
or large_imm@l,%r31,%r31
adds %r31,%rx,%rn
* Unsigned large immediate with logical operations:
Logical operations (`or, andnot, or, xor') also result in
expansions. The pseudo-instruction `or large_imm,%rx,%rn' results
orh large_imm@h,%rx,%r31
or large_imm@l,%r31,%rn
Similarly for the others, except for `and' which expands to:
andnot (-1 - large_imm)@h,%rx,%r31
andnot (-1 - large_imm)@l,%r31,%rn

File:, Node: i960-Dependent, Next: M32R-Dependent, Prev: i860-Dependent, Up: Machine Dependencies
Intel 80960 Dependent Features
* Menu:
* Options-i960:: i960 Command-line Options
* Floating Point-i960:: Floating Point
* Directives-i960:: i960 Machine Directives
* Opcodes for i960:: i960 Opcodes

File:, Node: Options-i960, Next: Floating Point-i960, Up: i960-Dependent
i960 Command-line Options
`-ACA | -ACA_A | -ACB | -ACC | -AKA | -AKB | -AKC | -AMC'
Select the 80960 architecture. Instructions or features not
supported by the selected architecture cause fatal errors.
`-ACA' is equivalent to `-ACA_A'; `-AKC' is equivalent to `-AMC'.
Synonyms are provided for compatibility with other tools.
If you do not specify any of these options, `as' generates code
for any instruction or feature that is supported by _some_ version
of the 960 (even if this means mixing architectures!). In
principle, `as' attempts to deduce the minimal sufficient
processor type if none is specified; depending on the object code
format, the processor type may be recorded in the object file. If
it is critical that the `as' output match a specific architecture,
specify that architecture explicitly.
Add code to collect information about conditional branches taken,
for later optimization using branch prediction bits. (The
conditional branch instructions have branch prediction bits in the
CA, CB, and CC architectures.) If BR represents a conditional
branch instruction, the following represents the code generated by
the assembler when `-b' is specified:
.word 0 # pre-counter
Label: BR
.word 0 # post-counter
The counter following a branch records the number of times that
branch was _not_ taken; the differenc between the two counters is
the number of times the branch _was_ taken.
A table of every such `Label' is also generated, so that the
external postprocessor `gbr960' (supplied by Intel) can locate all
the counters. This table is always labelled `__BRANCH_TABLE__';
this is a local symbol to permit collecting statistics for many
separate object files. The table is word aligned, and begins with
a two-word header. The first word, initialized to 0, is used in
maintaining linked lists of branch tables. The second word is a
count of the number of entries in the table, which follow
immediately: each is a word, pointing to one of the labels
illustrated above.
+------------+------------+------------+ ... +------------+
| | | | | |
| *NEXT | COUNT: N | *BRLAB 1 | | *BRLAB N |
| | | | | |
+------------+------------+------------+ ... +------------+
__BRANCH_TABLE__ layout
The first word of the header is used to locate multiple branch
tables, since each object file may contain one. Normally the links
are maintained with a call to an initialization routine, placed at
the beginning of each function in the file. The GNU C compiler
generates these calls automatically when you give it a `-b' option.
For further details, see the documentation of `gbr960'.
Normally, Compare-and-Branch instructions with targets that require
displacements greater than 13 bits (or that have external targets)
are replaced with the corresponding compare (or `chkbit') and
branch instructions. You can use the `-no-relax' option to
specify that `as' should generate errors instead, if the target
displacement is larger than 13 bits.
This option does not affect the Compare-and-Jump instructions; the
code emitted for them is _always_ adjusted when necessary
(depending on displacement size), regardless of whether you use

File:, Node: Floating Point-i960, Next: Directives-i960, Prev: Options-i960, Up: i960-Dependent
Floating Point
`as' generates IEEE floating-point numbers for the directives
`.float', `.double', `.extended', and `.single'.