Answer from cs61c-be (Tony Yiu 15099596) for Question 2
The values come from the ID which is the decoded value from register, from MUX which is the prior ALU result that has been written into memory or from data memory or previous ALU result.


add $4, $2, $3 (from register)

addi $4, $4, 1 (from mem stage)

sub $2, $1, $3
and $4, $2, $5
or  $4, $4, $2 (from prior ALU result)