GNU as for MIPS architectures supports several
different MIPS processors, and MIPS ISA levels I through IV. For
information about the MIPS instruction set, see MIPS RISC
Architecture, by Kane and Heindrich (Prentice-Hall). For an overview
of MIPS assembly conventions, see "Appendix D: Assembly Language
Programming" in the same work.
The MIPS configurations of GNU as support these
special options:
-G num
gp register. It is only accepted for targets
that use ECOFF format. The default value is 8.
-EB
-EL
as can select big-endian or
little-endian output at run time (unlike the other GNU development
tools, which must be configured for one or the other). Use `-EB'
to select big-endian output, and `-EL' for little-endian.
-mips1
-mips2
-mips3
-mips4
-mips16
-no-mips16
-m4650
-no-m4650
-m4010
-no-m4010
-mcpu=CPU
gcc.
-nocpp
as, there is no need for `-nocpp', because the
GNU assembler itself never runs the C preprocessor.
--trap
--no-break
as automatically macro expands certain division and
multiplication instructions to check for overflow and division by zero. This
option causes 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.
--break
--no-trap
Assembling for a MIPS ECOFF target supports some additional sections
besides the usual .text, .data and .bss. The
additional sections are .rdata, used for read-only data,
.sdata, used for small data, and .sbss, used for small
common objects.
When assembling for ECOFF, the assembler uses the $gp ($28)
register to form the address of a "small object". Any object in the
.sdata or .sbss sections is considered "small" in this sense.
For external objects, or for objects in the .bss section, you can use
the gcc `-G' option to control the size of objects addressed via
$gp; the default value is 8, meaning that a reference to any object
eight bytes or smaller uses $gp. Passing `-G 0' to
as prevents it from using the $gp register on the basis
of object size (but the assembler uses $gp for objects in .sdata
or sbss in any case). The size of an object in the .bss section
is set by the .comm or .lcomm directive that defines it. The
size of an external object may be set with the .extern directive. For
example, `.extern sym,4' declares that the object at sym is 4 bytes
in length, whie leaving sym otherwise undefined.
Using small ECOFF objects requires linker support, and assumes that the
$gp register is correctly initialized (normally done automatically by
the startup code). MIPS ECOFF assembly code must not modify the
$gp register.
MIPS ECOFF as supports several directives used for
generating debugging information which are not support by traditional MIPS
assemblers. These are .def, .endef, .dim, .file,
.scl, .size, .tag, .type, .val,
.stabd, .stabn, and .stabs. The debugging information
generated by the three .stab directives can only be read by GDB,
not by traditional MIPS debuggers (this enhancement is required to fully
support C++ debugging). These directives are primarily used by compilers, not
assembly language programmers!
GNU as supports an additional directive to change
the MIPS Instruction Set Architecture level on the fly: .set
mipsn. n should be a number from 0 to 4. A value from 1
to 4 makes the assembler accept instructions for the corresponding
ISA level, from that point on in the assembly. .set
mipsn affects not only which instructions are permitted, but also
how certain macros are expanded. .set mips0 restores the
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 R4000 instructions while
assembling in 32 bit mode. Use this directive with care!
The directive `.set mips16' puts the assembler into MIPS 16 mode, in which it will assemble instructions for the MIPS 16 processor. Use `.set nomips16' to return to normal 32 bit mode.
Traditional MIPS assemblers do not support this directive.
By default, MIPS 16 instructions are automatically extended to 32 bits when necessary. The directive `.set noautoextend' will turn this off. When `.set noautoextend' is in effect, any 32 bit instruction must be explicitly extended with the `.e' modifier (e.g., `li.e $4,1000'). The directive `.set autoextend' may be used to once again automatically extend instructions when necessary.
This directive is only meaningful when in MIPS 16 mode. Traditional MIPS assemblers do not support this directive.
The .insn directive tells 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,
as automatically adds 1 to the value, so that jumping to
the loaded address will do the right thing.
The directives .set push and .set pop may be used to save
and restore the current settings for all the options which are
controlled by .set. The .set push directive saves the
current settings on a stack. The .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 MIPS assemblers do not support these directives.
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