Answer from cs61c-ee (Michael Shen 15963205) for Question 1
By setting aside registers that are not preserved by a callee on a procedure call and those that are preserved on a register call, we always have a set of registers that we know we can use without erasing any important data, i.e. the 10 temporary registers, $t0-$t9.  Thus none of the data in $t0-$t9 needs to be transferred to memory.