Answer from cs61c-fy (Vinayak Aggarwal 17797902) for Question 1
To avoid saving and restoring a register whose value is never used, which might happen with a temporary register, MIPS software ofers two classes of registers.
- $t0-$t9:10 temporary registers not preserved by the callee on a procedure call.
- $s0-$s7:8 saved registers that must be preserved on a procedure call.

This simple convention reduces spilling.