gas/doc/c-i386.texi

@c Copyright (C) 1991, 92, 93, 94, 95, 1997 Free Software Foundation, Inc.
@c This is part of the GAS manual.
@c For copying conditions, see the file as.texinfo.
@ifset GENERIC
@page
@node i386-Dependent
@chapter 80386 Dependent Features
@end ifset
@ifclear GENERIC
@node Machine Dependencies
@chapter 80386 Dependent Features
@end ifclear

@cindex i386 support
@cindex i80306 support
@menu
* i386-Options::                Options
* i386-Syntax::                 AT&T Syntax versus Intel Syntax
* i386-Opcodes::                Opcode Naming
* i386-Regs::                   Register Naming
* i386-prefixes::               Opcode Prefixes
* i386-Memory::                 Memory References
* i386-jumps::                  Handling of Jump Instructions
* i386-Float::                  Floating Point
* i386-16bit::                  Writing 16-bit Code
* i386-Notes::                  Notes
@end menu

@node i386-Options
@section Options

@cindex options for i386 (none)
@cindex i386 options (none)
The 80386 has no machine dependent options.

@node i386-Syntax
@section AT&T Syntax versus Intel Syntax

@cindex i386 syntax compatibility
@cindex syntax compatibility, i386
In order to maintain compatibility with the output of @code{@value{GCC}},
@code{@value{AS}} supports AT&T System V/386 assembler syntax.  This is quite
different from Intel syntax.  We mention these differences because
almost all 80386 documents used only Intel syntax.  Notable differences
between the two syntaxes are:

@cindex immediate operands, i386
@cindex i386 immediate operands
@cindex register operands, i386
@cindex i386 register operands
@cindex jump/call operands, i386
@cindex i386 jump/call operands
@cindex operand delimiters, i386
@itemize @bullet
@item
AT&T immediate operands are preceded by @samp{$}; Intel immediate
operands are undelimited (Intel @samp{push 4} is AT&T @samp{pushl $4}).
AT&T register operands are preceded by @samp{%}; Intel register operands
are undelimited.  AT&T absolute (as opposed to PC relative) jump/call
operands are prefixed by @samp{*}; they are undelimited in Intel syntax.

@cindex i386 source, destination operands
@cindex source, destination operands; i386
@item
AT&T and Intel syntax use the opposite order for source and destination
operands.  Intel @samp{add eax, 4} is @samp{addl $4, %eax}.  The
@samp{source, dest} convention is maintained for compatibility with
previous Unix assemblers.

@cindex opcode suffixes, i386
@cindex sizes operands, i386
@cindex i386 size suffixes
@item
In AT&T syntax the size of memory operands is determined from the last
character of the opcode name.  Opcode suffixes of @samp{b}, @samp{w},
and @samp{l} specify byte (8-bit), word (16-bit), and long (32-bit)
memory references.  Intel syntax accomplishes this by prefixes memory
operands (@emph{not} the opcodes themselves) with @samp{byte ptr},
@samp{word ptr}, and @samp{dword ptr}.  Thus, Intel @samp{mov al, byte
ptr @var{foo}} is @samp{movb @var{foo}, %al} in AT&T syntax.

@cindex return instructions, i386
@cindex i386 jump, call, return
@item
Immediate form long jumps and calls are
@samp{lcall/ljmp $@var{section}, $@var{offset}} in AT&T syntax; the
Intel syntax is
@samp{call/jmp far @var{section}:@var{offset}}.  Also, the far return
instruction
is @samp{lret $@var{stack-adjust}} in AT&T syntax; Intel syntax is
@samp{ret far @var{stack-adjust}}.

@cindex sections, i386
@cindex i386 sections
@item
The AT&T assembler does not provide support for multiple section
programs.  Unix style systems expect all programs to be single sections.
@end itemize

@node i386-Opcodes
@section Opcode Naming

@cindex i386 opcode naming
@cindex opcode naming, i386
Opcode names are suffixed with one character modifiers which specify the
size of operands.  The letters @samp{b}, @samp{w}, and @samp{l} specify
byte, word, and long operands.  If no suffix is specified by an
instruction and it contains no memory operands then @code{@value{AS}} tries to
fill in the missing suffix based on the destination register operand
(the last one by convention).  Thus, @samp{mov %ax, %bx} is equivalent
to @samp{movw %ax, %bx}; also, @samp{mov $1, %bx} is equivalent to
@samp{movw $1, %bx}.  Note that this is incompatible with the AT&T Unix
assembler which assumes that a missing opcode suffix implies long
operand size.  (This incompatibility does not affect compiler output
since compilers always explicitly specify the opcode suffix.)

Almost all opcodes 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
@emph{from} and a size to zero extend @emph{to}.  This is accomplished
by using two opcode suffixes in AT&T syntax.  Base names for sign extend
and zero extend are @samp{movs@dots{}} and @samp{movz@dots{}} in AT&T
syntax (@samp{movsx} and @samp{movzx} in Intel syntax).  The opcode
suffixes are tacked on to this base name, the @emph{from} suffix before
the @emph{to} suffix.  Thus, @samp{movsbl %al, %edx} is AT&T syntax for
``move sign extend @emph{from} %al @emph{to} %edx.''  Possible suffixes,
thus, are @samp{bl} (from byte to long), @samp{bw} (from byte to word),
and @samp{wl} (from word to long).

@cindex conversion instructions, i386
@cindex i386 conversion instructions
The Intel-syntax conversion instructions

@itemize @bullet
@item
@samp{cbw} --- sign-extend byte in @samp{%al} to word in @samp{%ax},

@item
@samp{cwde} --- sign-extend word in @samp{%ax} to long in @samp{%eax},

@item
@samp{cwd} --- sign-extend word in @samp{%ax} to long in @samp{%dx:%ax},

@item
@samp{cdq} --- sign-extend dword in @samp{%eax} to quad in @samp{%edx:%eax},
@end itemize

@noindent
are called @samp{cbtw}, @samp{cwtl}, @samp{cwtd}, and @samp{cltd} in
AT&T naming.  @code{@value{AS}} accepts either naming for these instructions.

@cindex jump instructions, i386
@cindex call instructions, i386
Far call/jump instructions are @samp{lcall} and @samp{ljmp} in
AT&T syntax, but are @samp{call far} and @samp{jump far} in Intel
convention.

@node i386-Regs
@section Register Naming

@cindex i386 registers
@cindex registers, i386
Register operands are always prefixes with @samp{%}.  The 80386 registers
consist of

@itemize @bullet
@item
the 8 32-bit registers @samp{%eax} (the accumulator), @samp{%ebx},
@samp{%ecx}, @samp{%edx}, @samp{%edi}, @samp{%esi}, @samp{%ebp} (the
frame pointer), and @samp{%esp} (the stack pointer).

@item
the 8 16-bit low-ends of these: @samp{%ax}, @samp{%bx}, @samp{%cx},
@samp{%dx}, @samp{%di}, @samp{%si}, @samp{%bp}, and @samp{%sp}.

@item
the 8 8-bit registers: @samp{%ah}, @samp{%al}, @samp{%bh},
@samp{%bl}, @samp{%ch}, @samp{%cl}, @samp{%dh}, and @samp{%dl} (These
are the high-bytes and low-bytes of @samp{%ax}, @samp{%bx},
@samp{%cx}, and @samp{%dx})

@item
the 6 section registers @samp{%cs} (code section), @samp{%ds}
(data section), @samp{%ss} (stack section), @samp{%es}, @samp{%fs},
and @samp{%gs}.

@item
the 3 processor control registers @samp{%cr0}, @samp{%cr2}, and
@samp{%cr3}.

@item
the 6 debug registers @samp{%db0}, @samp{%db1}, @samp{%db2},
@samp{%db3}, @samp{%db6}, and @samp{%db7}.

@item
the 2 test registers @samp{%tr6} and @samp{%tr7}.

@item
the 8 floating point register stack @samp{%st} or equivalently
@samp{%st(0)}, @samp{%st(1)}, @samp{%st(2)}, @samp{%st(3)},
@samp{%st(4)}, @samp{%st(5)}, @samp{%st(6)}, and @samp{%st(7)}.
@end itemize

@node i386-prefixes
@section Opcode Prefixes

@cindex i386 opcode prefixes
@cindex opcode prefixes, i386
@cindex prefixes, i386
Opcode prefixes are used to modify the following opcode.  They are used
to repeat string instructions, to provide section overrides, to perform
bus lock operations, and to give operand and address size (16-bit
operands are specified in an instruction by prefixing what would
normally be 32-bit operands with a ``operand size'' opcode prefix).
Opcode prefixes are usually given as single-line instructions with no
operands, and must directly precede the instruction they act upon.  For
example, the @samp{scas} (scan string) instruction is repeated with:
@smallexample
        repne
        scas
@end smallexample

Here is a list of opcode prefixes:

@cindex section override prefixes, i386
@itemize @bullet
@item
Section override prefixes @samp{cs}, @samp{ds}, @samp{ss}, @samp{es},
@samp{fs}, @samp{gs}.  These are automatically added by specifying
using the @var{section}:@var{memory-operand} form for memory references.

@cindex size prefixes, i386
@item
Operand/Address size prefixes @samp{data16} and @samp{addr16}
change 32-bit operands/addresses into 16-bit operands/addresses.  Note
that 16-bit addressing modes (i.e. 8086 and 80286 addressing modes)
are not supported (yet).

@cindex bus lock prefixes, i386
@cindex inhibiting interrupts, i386
@item
The bus lock prefix @samp{lock} inhibits interrupts during
execution of the instruction it precedes.  (This is only valid with
certain instructions; see a 80386 manual for details).

@cindex coprocessor wait, i386
@item
The wait for coprocessor prefix @samp{wait} waits for the
coprocessor to complete the current instruction.  This should never be
needed for the 80386/80387 combination.

@cindex repeat prefixes, i386
@item
The @samp{rep}, @samp{repe}, and @samp{repne} prefixes are added
to string instructions to make them repeat @samp{%ecx} times.
@end itemize

@node i386-Memory
@section Memory References

@cindex i386 memory references
@cindex memory references, i386
An Intel syntax indirect memory reference of the form

@smallexample
@var{section}:[@var{base} + @var{index}*@var{scale} + @var{disp}]
@end smallexample

@noindent
is translated into the AT&T syntax

@smallexample
@var{section}:@var{disp}(@var{base}, @var{index}, @var{scale})
@end smallexample

@noindent
where @var{base} and @var{index} are the optional 32-bit base and
index registers, @var{disp} is the optional displacement, and
@var{scale}, taking the values 1, 2, 4, and 8, multiplies @var{index}
to calculate the address of the operand.  If no @var{scale} is
specified, @var{scale} is taken to be 1.  @var{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 @emph{must} have
be preceded by a @samp{%}.  If you specify a section override which
coincides with the default section register, @code{@value{AS}} does @emph{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:

@table @asis
@item AT&T: @samp{-4(%ebp)}, Intel:  @samp{[ebp - 4]}
@var{base} is @samp{%ebp}; @var{disp} is @samp{-4}. @var{section} is
missing, and the default section is used (@samp{%ss} for addressing with
@samp{%ebp} as the base register).  @var{index}, @var{scale} are both missing.

@item AT&T: @samp{foo(,%eax,4)}, Intel: @samp{[foo + eax*4]}
@var{index} is @samp{%eax} (scaled by a @var{scale} 4); @var{disp} is
@samp{foo}.  All other fields are missing.  The section register here
defaults to @samp{%ds}.

@item AT&T: @samp{foo(,1)}; Intel @samp{[foo]}
This uses the value pointed to by @samp{foo} as a memory operand.
Note that @var{base} and @var{index} are both missing, but there is only
@emph{one} @samp{,}.  This is a syntactic exception.

@item AT&T: @samp{%gs:foo}; Intel @samp{gs:foo}
This selects the contents of the variable @samp{foo} with section
register @var{section} being @samp{%gs}.
@end table

Absolute (as opposed to PC relative) call and jump operands must be
prefixed with @samp{*}.  If no @samp{*} is specified, @code{@value{AS}}
always chooses PC relative addressing for jump/call labels.

Any instruction that has a memory operand @emph{must} specify its size (byte,
word, or long) with an opcode suffix (@samp{b}, @samp{w}, or @samp{l},
respectively).

@node i386-jumps
@section Handling of Jump Instructions

@cindex jump optimization, i386
@cindex i386 jump optimization
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 (32-bit) displacement is used.  We do not support
word (16-bit) displacement jumps (i.e. prefixing the jump instruction
with the @samp{addr16} opcode prefix), since the 80386 insists upon masking
@samp{%eip} to 16 bits after the word displacement is added.

Note that the @samp{jcxz}, @samp{jecxz}, @samp{loop}, @samp{loopz},
@samp{loope}, @samp{loopnz} and @samp{loopne} instructions only come in byte
displacements, so that if you use these instructions (@code{@value{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 @samp{jcxz foo}
to

@smallexample
         jcxz cx_zero
         jmp cx_nonzero
cx_zero: jmp foo
cx_nonzero:
@end smallexample

@node i386-Float
@section Floating Point

@cindex i386 floating point
@cindex floating point, i386
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 opcode suffix and a constructor
associated with it.  Opcode suffixes specify operand's data
types.  Constructors build these data types into memory.

@cindex @code{float} directive, i386
@cindex @code{single} directive, i386
@cindex @code{double} directive, i386
@cindex @code{tfloat} directive, i386
@itemize @bullet
@item
Floating point constructors are @samp{.float} or @samp{.single},
@samp{.double}, and @samp{.tfloat} for 32-, 64-, and 80-bit formats.
These correspond to opcode suffixes @samp{s}, @samp{l}, and @samp{t}.
@samp{t} stands for temporary real, and that the 80387 only supports
this format via the @samp{fldt} (load temporary real to stack top) and
@samp{fstpt} (store temporary real and pop stack) instructions.

@cindex @code{word} directive, i386
@cindex @code{long} directive, i386
@cindex @code{int} directive, i386
@cindex @code{quad} directive, i386
@item
Integer constructors are @samp{.word}, @samp{.long} or @samp{.int}, and
@samp{.quad} for the 16-, 32-, and 64-bit integer formats.  The corresponding
opcode suffixes are @samp{s} (single), @samp{l} (long), and @samp{q}
(quad).  As with the temporary real format the 64-bit @samp{q} format is
only present in the @samp{fildq} (load quad integer to stack top) and
@samp{fistpq} (store quad integer and pop stack) instructions.
@end itemize

Register to register operations do not require opcode suffixes,
so that @samp{fst %st, %st(1)} is equivalent to @samp{fstl %st, %st(1)}.

@node i386-16bit
@section Writing 16-bit Code

@cindex i386 16-bit code
@cindex 16-bit code, i386
@cindex real-mode code, i386
@cindex @code{code16} directive, i386
@cindex @code{code32} directive, i386
While GAS normally writes only ``pure'' 32-bit i386 code, it has limited
support for writing code to run in real mode or in 16-bit protected mode
code segments.  To do this, insert a @samp{.code16} directive before the
assembly language instructions to be run in 16-bit mode.  You can switch
GAS back to writing normal 32-bit code with the @samp{.code32} directive.

GAS understands exactly the same assembly language syntax in 16-bit mode as
in 32-bit mode.  The function of any given instruction is exactly the same
regardless of mode, as long as the resulting object code is executed in the
mode for which GAS wrote it.  So, for example, the @samp{ret} mnemonic
produces a 32-bit return instruction regardless of whether it is to be run
in 16-bit or 32-bit mode.  (If GAS is in 16-bit mode, it will add an
operand size prefix to the instruction to force it to be a 32-bit return.)

This means, for one thing, that you can use @sc{gnu} @sc{cc} to write code to be run
in real mode or 16-bit protected mode.  Just insert the statement
@samp{asm(".code16");} at the beginning of your C source file, and while
@sc{gnu} @sc{cc} will still be generating 32-bit code, GAS will automatically add
all the necessary size prefixes to make that code run in 16-bit mode.  Of
course, since @sc{gnu} @sc{cc} only writes small-model code (it doesn't know how to
attach segment selectors to pointers like native x86 compilers do), any
16-bit code you write with @sc{gnu} @sc{cc} will essentially be limited to a 64K
address space.  Also, there will be a code size and performance penalty
due to all the extra address and operand size prefixes GAS has to add to
the instructions.

Note that placing GAS in 16-bit mode does not mean that the resulting
code will necessarily run on a 16-bit pre-80386 processor.  To write code
that runs on such a processor, you would have to refrain from using
@emph{any} 32-bit constructs which require GAS to output address or
operand size prefixes.  At the moment this would be rather difficult,
because GAS currently supports @emph{only} 32-bit addressing modes: when
writing 16-bit code, it @emph{always} outputs address size prefixes for any
instruction that uses a non-register addressing mode.  So you can write
code that runs on 16-bit processors, but only if that code never references
memory.

@node i386-Notes
@section Notes

@cindex i386 @code{mul}, @code{imul} instructions
@cindex @code{mul} instruction, i386
@cindex @code{imul} instruction, i386
There is some trickery concerning the @samp{mul} and @samp{imul}
instructions that deserves mention.  The 16-, 32-, and 64-bit expanding
multiplies (base opcode @samp{0xf6}; extension 4 for @samp{mul} and 5
for @samp{imul}) can be output only in the one operand form.  Thus,
@samp{imul %ebx, %eax} does @emph{not} select the expanding multiply;
the expanding multiply would clobber the @samp{%edx} register, and this
would confuse @code{@value{GCC}} output.  Use @samp{imul %ebx} to get the
64-bit product in @samp{%edx:%eax}.

We have added a two operand form of @samp{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 @samp{%eax} by 69, for
example, can be done with @samp{imul $69, %eax} rather than @samp{imul
$69, %eax, %eax}.