path: root/gas/doc/c-mips.texi

@c Copyright (C) 1991, 1992, 1993, 1994, 1995 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
@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
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.  You can also switch instruction sets during the
assembly; see @ref{MIPS ISA,, Directives to override the ISA level}.

@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 -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.

@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 -mcpu=@var{CPU}
Generate code for a particular MIPS cpu.  This has little effect on the
assembler, but it is passed by @code{@value{GCC}}.

@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 --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.  A value from 1
to 4 makes 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.