Answer from cs61c-bc (Svetlana Simakova 16608048) for Question 1
MIPS Has 2 classes of registers: those that need to be saved on a procedure call by callee ($t0-$t9) & those, that must be preserved ($s0-$s1).
Thus, inside procedure can use temp registers without need to save them first. Spilling is required only if temporary registers are not enough.