> register        software        use
> name            name
> --------        --------        ---
> $0              v0              return value
> $1-$8           t0-t7           temps
> $9-$14          s0-s5           saved registers
> $15             fp or s6        contains frame pointer;
>                                 otherwise a saved register
> $16..$21        a0..a5          arguments
> $22..$25        t8-t11          temps
> $26             ra              return address
> $27             pv or t12       procedure value; otherwise a temp
> $28             at              assembler temp
> $29             gp              global pointer
> $30             sp              stack pointer
> $31             zero            always zero
>