The MIPS assembler was originally written by Christian Pirker.
Currently the assembler and disassembler covers most of the MIPS32 architecture and doesn’t support FP instructions.
The register names $a0–$a3 are not available to avoid
shadowing hex numbers. Use register numbers $4–$7
instead.
Nothing distinguishes registers from immediate values. Use explicit
opcode names with the i suffix for instructions with immediate
argument. E.g. addiu, in place of addu,.
Where the architecture manual specifies several formats for the
instruction (e.g., for jalr,),use the one with more arguments
(i.e. two for jalr,). When in doubt, see
arch/mips/testasm.fs for an example of correct use.
Branches and jumps in the MIPS architecture have a delay slot. You
have to fill it manually (the simplest way is to use nop,), the
assembler does not do it for you (unlike as). Even
if,, ahead,, until,, again,,
while,, else, and repeat, need a delay slot.
Since begin, and then, just specify branch targets, they
are not affected. For branches the argument specifying the target is
a relative address. Add the address of the delay slot to get the
absolute address.
Note that you must not put branches nor jumps (nor control-flow
instructions) into the delay slot. Also it is a bad idea to put
pseudo-ops such as li, into a delay slot, as these may expand
to several instructions. The MIPS I architecture also had load delay
slots, and newer MIPSes still have restrictions on using mfhi,
and mflo,. Be careful to satisfy these restrictions, the
assembler does not do it for you.
Some example of instructions are:
$ra 12 $sp sw, \ sw ra,12(sp) $4 8 $s0 lw, \ lw a0,8(s0) $v0 $0 lui, \ lui v0,0x0 $s0 $s4 $12 addiu, \ addiu s0,s4,0x12 $s0 $s4 $4 addu, \ addu s0,s4,$a0 $ra $t9 jalr, \ jalr t9
You can specify the conditions for if, etc. by taking a
conditional branch and leaving away the b at the start and the
, at the end. E.g.,
4 5 eq if, ... \ do something if $4 equals $5 then,
The calling conventions for 32-bit MIPS machines is to pass the first
4 arguments in registers $4..$7, and to use
$v0-$v1 for return values. In addition to these
registers, it is ok to clobber registers $t0-$t8 without
saving and restoring them.
If you use jalr, to call into dynamic library routines, you
must first load the called function’s address into $t9, which
is used by position-indirect code to do relative memory accesses.
Here is an example of a MIPS32 abi-code word:
abi-code my+ ( n1 n2 -- n3 ) \ SP passed in $4, returned in $v0 $t0 4 $4 lw, \ load n1, n2 from stack $t1 0 $4 lw, $t0 $t0 $t1 addu, \ add n1+n2, result in $t0 $t0 4 $4 sw, \ store result (overwriting n1) $ra jr, \ return to caller $v0 $4 4 addiu, \ (delay slot) return uptated SP in $v0 end-code