Register usage

Table 8 shows the registers available on MIPS.

Table 8: MIPS register summary

		CALLEE
REGISTER	USAGE	SAVE
$0	Always zero	n/a
$1/$at	assemb temp	NO
$2-$3	integer return	NO
$4-$11	1$^{\rm st}$ 8 iargs	NO
$12-$15	scratch	NO
$16-$23	saved regs	YES
$24-$25	scratch	NO
$26-$27	reserved for kernel	n/a
$28 ($gp)	global pointer	YES
$29 ($sp)	stack pointer	YES
$30	optional frame ptr	YES
$31	return address	NO
Floating Point Registers for -64
$f0,$f2	fp func return vals	NO
$f1,$f3	scratch	NO
$f4-$f11	scratch	NO
$f12-$f19	1$^{\rm st}$ 8 fp args	NO
$f20-$f23	scratch	NO
$f24-$f31	scratch	YES
Floating Point Registers for -n32
$f0,$f2	fp func return vals	NO
$f1,$f3	scratch	NO
$f4-$f11	scratch	NO
$f12-$f19	1$^{\rm st}$ 8 fp args	NO
$f21,23,25,27,29,31	scratch	NO
$f20,22,24,26,28,30	scratch	YES

Note that in parameter passing, you can only pass 8 args in registers, regardless of what type, and args always consume a register of the correct type, and cause a skip of the other type. So, here's the register passing of a simple routine with mixed arguments:

//          $f12    $5       $f14     $7
void bob(float s, int i, double d, int k);