gas/doc/c-mips.texi

@c Copyright 1991, 1992, 1993, 1994, 1995, 1997, 1999, 2000, 2001,
@c 2002, 2003, 2004
@c 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 MIPS-Dependent
@chapter MIPS Dependent Features
@end ifset
@ifclear GENERIC
@node Machine Dependencies
@chapter MIPS Dependent Features
@end ifclear

@cindex MIPS processor
@sc{gnu} @code{@value{AS}} for @sc{mips} architectures supports several
different @sc{mips} processors, and MIPS ISA levels I through V, MIPS32,
and MIPS64.  For information about the @sc{mips} instruction set, see
@cite{MIPS RISC Architecture}, by Kane and Heindrich (Prentice-Hall).
For an overview of @sc{mips} assembly conventions, see ``Appendix D:
Assembly Language Programming'' in the same work.

@menu
* MIPS Opts::   	Assembler options
* MIPS Object:: 	ECOFF object code
* MIPS Stabs::  	Directives for debugging information
* MIPS ISA::    	Directives to override the ISA level
* MIPS symbol sizes::   Directives to override the size of symbols
* MIPS autoextend::	Directives for extending MIPS 16 bit instructions
* MIPS insn::		Directive to mark data as an instruction
* MIPS option stack::	Directives to save and restore options
* MIPS ASE instruction generation overrides:: Directives to control
  			generation of MIPS ASE instructions
@end menu

@node MIPS Opts
@section Assembler options

The @sc{mips} configurations of @sc{gnu} @code{@value{AS}} support these
special options:

@table @code
@cindex @code{-G} option (MIPS)
@item -G @var{num}
This option sets the largest size of an object that can be referenced
implicitly with the @code{gp} register.  It is only accepted for targets
that use @sc{ecoff} format.  The default value is 8.

@cindex @code{-EB} option (MIPS)
@cindex @code{-EL} option (MIPS)
@cindex MIPS big-endian output
@cindex MIPS little-endian output
@cindex big-endian output, MIPS
@cindex little-endian output, MIPS
@item -EB
@itemx -EL
Any @sc{mips} configuration of @code{@value{AS}} can select big-endian or
little-endian output at run time (unlike the other @sc{gnu} development
tools, which must be configured for one or the other).  Use @samp{-EB}
to select big-endian output, and @samp{-EL} for little-endian.

@cindex MIPS architecture options
@item -mips1
@itemx -mips2
@itemx -mips3
@itemx -mips4
@itemx -mips5
@itemx -mips32
@itemx -mips32r2
@itemx -mips64
@itemx -mips64r2
Generate code for a particular MIPS Instruction Set Architecture level.
@samp{-mips1} corresponds to the @sc{r2000} and @sc{r3000} processors,
@samp{-mips2} to the @sc{r6000} processor, @samp{-mips3} to the
@sc{r4000} processor, and @samp{-mips4} to the @sc{r8000} and
@sc{r10000} processors.  @samp{-mips5}, @samp{-mips32}, @samp{-mips32r2},
@samp{-mips64}, and @samp{-mips64r2}
correspond to generic
@sc{MIPS V}, @sc{MIPS32}, @sc{MIPS32 Release 2}, @sc{MIPS64},
and @sc{MIPS64 Release 2}
ISA processors, respectively.  You can also switch
instruction sets during the assembly; see @ref{MIPS ISA, Directives to
override the ISA level}.

@item -mgp32
@itemx -mfp32
Some macros have different expansions for 32-bit and 64-bit registers.
The register sizes are normally inferred from the ISA and ABI, but these
flags force a certain group of registers to be treated as 32 bits wide at
all times.  @samp{-mgp32} controls the size of general-purpose registers
and @samp{-mfp32} controls the size of floating-point registers.

On some MIPS variants there is a 32-bit mode flag; when this flag is
set, 64-bit instructions generate a trap.  Also, some 32-bit OSes only
save the 32-bit registers on a context switch, so it is essential never
to use the 64-bit registers.

@item -mgp64
Assume that 64-bit general purpose registers are available.  This is
provided in the interests of symmetry with -gp32.

@item -mips16
@itemx -no-mips16
Generate code for the MIPS 16 processor.  This is equivalent to putting
@samp{.set mips16} at the start of the assembly file.  @samp{-no-mips16}
turns off this option.

@item -mips3d
@itemx -no-mips3d
Generate code for the MIPS-3D Application Specific Extension.
This tells the assembler to accept MIPS-3D instructions.
@samp{-no-mips3d} turns off this option.

@item -mdmx
@itemx -no-mdmx
Generate code for the MDMX Application Specific Extension.
This tells the assembler to accept MDMX instructions.
@samp{-no-mdmx} turns off this option.

@item -mdsp
@itemx -mno-dsp
Generate code for the DSP Application Specific Extension.
This tells the assembler to accept DSP instructions.
@samp{-mno-dsp} turns off this option.

@item -mmt
@itemx -mno-mt
Generate code for the MT Application Specific Extension.
This tells the assembler to accept MT instructions.
@samp{-mno-mt} turns off this option.

@item -mfix7000
@itemx -mno-fix7000
Cause nops to be inserted if the read of the destination register
of an mfhi or mflo instruction occurs in the following two instructions.

@item -mfix-vr4120
@itemx -no-mfix-vr4120
Insert nops to work around certain VR4120 errata.  This option is
intended to be used on GCC-generated code: it is not designed to catch
all problems in hand-written assembler code.

@item -mfix-vr4130
@itemx -no-mfix-vr4130
Insert nops to work around the VR4130 @samp{mflo}/@samp{mfhi} errata.

@item -mfix-loongson2f-btb
@itemx -mno-fix-loongson2f-btb
Clear the Branch Target Buffer before any jump through a register.  This
option is intended to be used on kernel code for the Loongson 2F processor
only; userland code compiled with this option will fault, and kernel code
compiled with this option run on another processor than Loongson 2E and 2F
will yield unpredictable results.

@item -m4010
@itemx -no-m4010
Generate code for the LSI @sc{r4010} chip.  This tells the assembler to
accept the @sc{r4010} specific instructions (@samp{addciu}, @samp{ffc},
etc.), and to not schedule @samp{nop} instructions around accesses to
the @samp{HI} and @samp{LO} registers.  @samp{-no-m4010} turns off this
option.

@item -m4650
@itemx -no-m4650
Generate code for the MIPS @sc{r4650} chip.  This tells the assembler to accept
the @samp{mad} and @samp{madu} instruction, and to not schedule @samp{nop}
instructions around accesses to the @samp{HI} and @samp{LO} registers.
@samp{-no-m4650} turns off this option.

@itemx -m3900
@itemx -no-m3900
@itemx -m4100
@itemx -no-m4100
For each option @samp{-m@var{nnnn}}, generate code for the MIPS
@sc{r@var{nnnn}} chip.  This tells the assembler to accept instructions
specific to that chip, and to schedule for that chip's hazards.

@item -march=@var{cpu}
Generate code for a particular MIPS cpu.  It is exactly equivalent to
@samp{-m@var{cpu}}, except that there are more value of @var{cpu}
understood.  Valid @var{cpu} value are:

@quotation
2000,
3000,
3900,
4000,
4010,
4100,
4111,
vr4120,
vr4130,
vr4181,
4300,
4400,
4600,
4650,
5000,
rm5200,
rm5230,
rm5231,
rm5261,
rm5721,
vr5400,
vr5500,
6000,
rm7000,
8000,
rm9000,
10000,
12000,
mips32-4k,
sb1
@end quotation

@item -mtune=@var{cpu}
Schedule and tune for a particular MIPS cpu.  Valid @var{cpu} values are
identical to @samp{-march=@var{cpu}}.

@item -mabi=@var{abi}
Record which ABI the source code uses.  The recognized arguments
are: @samp{32}, @samp{n32}, @samp{o64}, @samp{64} and @samp{eabi}.

@item -msym32
@itemx -mno-sym32
@cindex -msym32
@cindex -mno-sym32
Equivalent to adding @code{.set sym32} or @code{.set nosym32} to
the beginning of the assembler input.  @xref{MIPS symbol sizes}.

@cindex @code{-nocpp} ignored (MIPS)
@item -nocpp
This option is ignored.  It is accepted for command-line compatibility with
other assemblers, which use it to turn off C style preprocessing.  With
@sc{gnu} @code{@value{AS}}, there is no need for @samp{-nocpp}, because the
@sc{gnu} assembler itself never runs the C preprocessor.

@item --construct-floats
@itemx --no-construct-floats
@cindex --construct-floats
@cindex --no-construct-floats
The @code{--no-construct-floats} option disables the construction of
double width floating point constants by loading the two halves of the
value into the two single width floating point registers that make up
the double width register.  This feature is useful if the processor
support the FR bit in its status  register, and this bit is known (by
the programmer) to be set.  This bit prevents the aliasing of the double
width register by the single width registers.

By default @code{--construct-floats} is selected, allowing construction
of these floating point constants.

@item --trap
@itemx --no-break
@c FIXME!  (1) reflect these options (next item too) in option summaries;
@c         (2) stop teasing, say _which_ instructions expanded _how_.
@code{@value{AS}} automatically macro expands certain division and
multiplication instructions to check for overflow and division by zero.  This
option causes @code{@value{AS}} to generate code to take a trap exception
rather than a break exception when an error is detected.  The trap instructions
are only supported at Instruction Set Architecture level 2 and higher.

@item --break
@itemx --no-trap
Generate code to take a break exception rather than a trap exception when an
error is detected.  This is the default.

@item -mpdr
@itemx -mno-pdr
Control generation of @code{.pdr} sections.  Off by default on IRIX, on
elsewhere.

@item -mshared
@itemx -mno-shared
When generating code using the Unix calling conventions (selected by
@samp{-KPIC} or @samp{-mcall_shared}), gas will normally generate code
which can go into a shared library.  The @samp{-mno-shared} option
tells gas to generate code which uses the calling convention, but can
not go into a shared library.  The resulting code is slightly more
efficient.  This option only affects the handling of the
@samp{.cpload} and @samp{.cpsetup} pseudo-ops.
@end table

@node MIPS Object
@section MIPS ECOFF object code

@cindex ECOFF sections
@cindex MIPS ECOFF sections
Assembling for a @sc{mips} @sc{ecoff} target supports some additional sections
besides the usual @code{.text}, @code{.data} and @code{.bss}.  The
additional sections are @code{.rdata}, used for read-only data,
@code{.sdata}, used for small data, and @code{.sbss}, used for small
common objects.

@cindex small objects, MIPS ECOFF
@cindex @code{gp} register, MIPS
When assembling for @sc{ecoff}, the assembler uses the @code{$gp} (@code{$28})
register to form the address of a ``small object''.  Any object in the
@code{.sdata} or @code{.sbss} sections is considered ``small'' in this sense.
For external objects, or for objects in the @code{.bss} section, you can use
the @code{@value{GCC}} @samp{-G} option to control the size of objects addressed via
@code{$gp}; the default value is 8, meaning that a reference to any object
eight bytes or smaller uses @code{$gp}.  Passing @samp{-G 0} to
@code{@value{AS}} prevents it from using the @code{$gp} register on the basis
of object size (but the assembler uses @code{$gp} for objects in @code{.sdata}
or @code{sbss} in any case).  The size of an object in the @code{.bss} section
is set by the @code{.comm} or @code{.lcomm} directive that defines it.  The
size of an external object may be set with the @code{.extern} directive.  For
example, @samp{.extern sym,4} declares that the object at @code{sym} is 4 bytes
in length, whie leaving @code{sym} otherwise undefined.

Using small @sc{ecoff} objects requires linker support, and assumes that the
@code{$gp} register is correctly initialized (normally done automatically by
the startup code).  @sc{mips} @sc{ecoff} assembly code must not modify the
@code{$gp} register.

@node MIPS Stabs
@section Directives for debugging information

@cindex MIPS debugging directives
@sc{mips} @sc{ecoff} @code{@value{AS}} supports several directives used for
generating debugging information which are not support by traditional @sc{mips}
assemblers.  These are @code{.def}, @code{.endef}, @code{.dim}, @code{.file},
@code{.scl}, @code{.size}, @code{.tag}, @code{.type}, @code{.val},
@code{.stabd}, @code{.stabn}, and @code{.stabs}.  The debugging information
generated by the three @code{.stab} directives can only be read by @sc{gdb},
not by traditional @sc{mips} debuggers (this enhancement is required to fully
support C++ debugging).  These directives are primarily used by compilers, not
assembly language programmers!

@node MIPS symbol sizes
@section Directives to override the size of symbols

@cindex @code{.set sym32}
@cindex @code{.set nosym32}
The n64 ABI allows symbols to have any 64-bit value.  Although this
provides a great deal of flexibility, it means that some macros have
much longer expansions than their 32-bit counterparts.  For example,
the non-PIC expansion of @samp{dla $4,sym} is usually:

@smallexample
lui     $4,%highest(sym)
lui     $1,%hi(sym)
daddiu  $4,$4,%higher(sym)
daddiu  $1,$1,%lo(sym)
dsll32  $4,$4,0
daddu   $4,$4,$1
@end smallexample

whereas the 32-bit expansion is simply:

@smallexample
lui     $4,%hi(sym)
daddiu  $4,$4,%lo(sym)
@end smallexample

n64 code is sometimes constructed in such a way that all symbolic
constants are known to have 32-bit values, and in such cases, it's
preferable to use the 32-bit expansion instead of the 64-bit
expansion.

You can use the @code{.set sym32} directive to tell the assembler
that, from this point on, all expressions of the form
@samp{@var{symbol}} or @samp{@var{symbol} + @var{offset}}
have 32-bit values.  For example:

@smallexample
.set sym32
dla     $4,sym
lw      $4,sym+16
sw      $4,sym+0x8000($4)
@end smallexample

will cause the assembler to treat @samp{sym}, @code{sym+16} and
@code{sym+0x8000} as 32-bit values.  The handling of non-symbolic
addresses is not affected.

The directive @code{.set nosym32} ends a @code{.set sym32} block and
reverts to the normal behavior.  It is also possible to change the
symbol size using the command-line options @option{-msym32} and
@option{-mno-sym32}.

These options and directives are always accepted, but at present,
they have no effect for anything other than n64.

@node MIPS ISA
@section Directives to override the ISA level

@cindex MIPS ISA override
@kindex @code{.set mips@var{n}}
@sc{gnu} @code{@value{AS}} supports an additional directive to change
the @sc{mips} Instruction Set Architecture level on the fly: @code{.set
mips@var{n}}.  @var{n} should be a number from 0 to 5, or 32, 32r2, 64
or 64r2.
The values other than 0 make the assembler accept instructions
for the corresponding @sc{isa} level, from that point on in the
assembly.  @code{.set mips@var{n}} affects not only which instructions
are permitted, but also how certain macros are expanded.  @code{.set
mips0} restores the @sc{isa} level to its original level: either the
level you selected with command line options, or the default for your
configuration.  You can use this feature to permit specific @sc{r4000}
instructions while assembling in 32 bit mode.  Use this directive with
care!

The directive @samp{.set mips16} puts the assembler into MIPS 16 mode,
in which it will assemble instructions for the MIPS 16 processor.  Use
@samp{.set nomips16} to return to normal 32 bit mode.

Traditional @sc{mips} assemblers do not support this directive.

@node MIPS autoextend
@section Directives for extending MIPS 16 bit instructions

@kindex @code{.set autoextend}
@kindex @code{.set noautoextend}
By default, MIPS 16 instructions are automatically extended to 32 bits
when necessary.  The directive @samp{.set noautoextend} will turn this
off.  When @samp{.set noautoextend} is in effect, any 32 bit instruction
must be explicitly extended with the @samp{.e} modifier (e.g.,
@samp{li.e $4,1000}).  The directive @samp{.set autoextend} may be used
to once again automatically extend instructions when necessary.

This directive is only meaningful when in MIPS 16 mode.  Traditional
@sc{mips} assemblers do not support this directive.

@node MIPS insn
@section Directive to mark data as an instruction

@kindex @code{.insn}
The @code{.insn} directive tells @code{@value{AS}} that the following
data is actually instructions.  This makes a difference in MIPS 16 mode:
when loading the address of a label which precedes instructions,
@code{@value{AS}} automatically adds 1 to the value, so that jumping to
the loaded address will do the right thing.

@node MIPS option stack
@section Directives to save and restore options

@cindex MIPS option stack
@kindex @code{.set push}
@kindex @code{.set pop}
The directives @code{.set push} and @code{.set pop} may be used to save
and restore the current settings for all the options which are
controlled by @code{.set}.  The @code{.set push} directive saves the
current settings on a stack.  The @code{.set pop} directive pops the
stack and restores the settings.

These directives can be useful inside an macro which must change an
option such as the ISA level or instruction reordering but does not want
to change the state of the code which invoked the macro.

Traditional @sc{mips} assemblers do not support these directives.

@node MIPS ASE instruction generation overrides
@section Directives to control generation of MIPS ASE instructions

@cindex MIPS MIPS-3D instruction generation override
@kindex @code{.set mips3d}
@kindex @code{.set nomips3d}
The directive @code{.set mips3d} makes the assembler accept instructions
from the MIPS-3D Application Specific Extension from that point on
in the assembly.  The @code{.set nomips3d} directive prevents MIPS-3D
instructions from being accepted.

@cindex MIPS MDMX instruction generation override
@kindex @code{.set mdmx}
@kindex @code{.set nomdmx}
The directive @code{.set mdmx} makes the assembler accept instructions
from the MDMX Application Specific Extension from that point on
in the assembly.  The @code{.set nomdmx} directive prevents MDMX
instructions from being accepted.

@cindex MIPS DSP instruction generation override
@kindex @code{.set dsp}
@kindex @code{.set nodsp}
The directive @code{.set dsp} makes the assembler accept instructions
from the DSP Application Specific Extension from that point on
in the assembly.  The @code{.set nodsp} directive prevents DSP
instructions from being accepted.

@cindex MIPS MT instruction generation override
@kindex @code{.set mt}
@kindex @code{.set nomt}
The directive @code{.set mt} makes the assembler accept instructions
from the MT Application Specific Extension from that point on
in the assembly.  The @code{.set nomt} directive prevents MT
instructions from being accepted.

Traditional @sc{mips} assemblers do not support these directives.