path: root/gas/doc/c-mips.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 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 IV.  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 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
@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 -mips32
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, @samp{-mips4} to the @sc{r8000} and
@sc{r10000} processors, and @samp{-mips32} to a generic @sc(MIPS32)
processor.  You can also switch instruction sets during the
assembly; see @ref{MIPS ISA, Directives to override the ISA level}.

@item -mgp32
Assume that 32-bit general purpose registers are available.  This
affects synthetic instructions such as @code{move}, which will assemble
to a 32-bit or a 64-bit instruction depending on this flag.  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 -mfix7000
@itemx -no-mfix7000
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 -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 -mcpu=@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,
4300,
4400,
4600,
4650,
5000,
rm5200,
rm5230,
rm5231,
rm5261,
rm5721,
6000,
rm7000,
8000,
10000,
mips32-4k
@end quotation


@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.
@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 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 4, or 32.  The values 1
to 4 and 32 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.