First save the "unsaved" registers if we will still care
about them after procedure returns
($t0, ..., $t9, $a0, ..., $a3
).
Where do they get saved?   the Stack (we'll talk about this in a minute)
$a0, ..., $a3
)
jal ProcedureAddress
) does 2 things:
$ra
. (Happens automatically.)