register conventions

$s registers, including $sp - 
When calling some other function, you can expect that
when the function returns, values stored in $s registers will stay the same.
corollary: any function you write, have to save $s registers that you want to use
ON THE STACK.
$t, $a, $v (everything else) registers - subroutines will trash these, save them if
you want to preserve values.
either - save them on the stack
alternatively - move into $s register, and move back when your function call returned.

jal <label> - jumps to label, current PC (program counter) + 4 -> $ra
jr $ra - jumps to value of $ra