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MIPS Architecture

I wrote these notes on July 6th, 1998 and as of March 2007 no longer have any idea what I used them for. Ricardo Nabinger Sanchez has helpfully pointed out what is almost certainly an error, in that identical subu instructions are used to allocate and free the stack frame. I do not know which one should be corrected, and how. -- John Chew <poslfit@gmail.com>

Memory Model

start	end	description
0000 0000	003F FFFF	Reserved
0040 0000	____ ____	Code (.text)
____ ____	0FFF FFFF	Shared libraries
1000 0000	____ ____	Read-only data (.rdata)
____ ____	____ ____	Data (.data)
____ ____	$gp	Short-offset data (.sdata)
$gp	____ ____	Short-offset uninitialised data (.sbss)
____ ____	____ ____	Uninitialised data (.bss)
____ ____	____ ____	Heap (expanded by sbrk and break)
____ ____	$sp	Protected
$sp	7FFF EFFF	Stack
7FFF F000	7FFF FFFF	Not accessible
8000 0000	FFFF FFFF	Kernel

Registers

Integer Registers

Register Name	Software Name	Description
$0		Always set to zero
$at		Assembler temporary register
$2, $3	v0, v1	Expression evaluation, integer function results, static link for nested procedure calls
$4..$7	a0..a3	First four volatile int arguments
$8..$15	t0..t7	Not preserved across procedure calls
$16..$23	s0..s7	Preserved across procedure calls
$24, $25	t8, t9	Not preserved across procedure calls
$kt0, $kt1	k0, k1	Kernel use only
$28 or $gp	gp	Global pointer
$29 or $sp	sp	Stack pointer
$30 or $fp	fp	Frame pointer (if used)
$31	ra	Return address

Procedures

Calling a Procedure

The first four integer arguments must be placed in registers $4 through $7 and the first two floating point arguments must be placed in registers f12 and f14.

Return values go in $2 and $f0.

To call a procedure, use the jal instruction.

                jal     procedure_name

Writing a Procedure

Start all procedures with a .ent directive and entry label.

                .ent     procedure_name
procedure_name:

If your procedure requires stack storage, either because it calls other procedures, or because it needs to put local variables on the stack, allocate the required storage. You should not modify the stack pointer again, except to restore it on return.

                subu     $sp, framesize

Regardless of whether or not you use stack storage, include the following pseudo-op. It tells the debugger how big your stack frame is.

                .frame   framereg, framesize, returnreg

Framereg is usually $sp, returnreg is usually $31.

If you need to save integer registers, e.g. $16, $17 and $31, do so as follows.

                .mask    bitmask, frameoffset
                sw       $31, framesize+frameoffset-0($sp)
                sw       $17, framesize+frameoffset-4($sp)
                sw       $16, framesize+frameoffset-8($sp)

If you need to save floating point registers, use s.s or s.d instead of sw, and .fmask instead of .mask. Allow four bytes for singles, eight bytes for doubles.

Include your procedure body (if any) here.

To restore registers, use lw, l.s and l.d.

                lw       $16, framesize+frameoffset-8($sp)
                lw       $17, framesize+frameoffset-4($sp)
                lw       $31, framesize+frameoffset-0($sp)

Remove the stack frame.

                subu     $sp, framesize

Return.

                j        $31

End the procedure definition.

                .end     procedure_name