Answer from cs61c-el (David Tung 16479725) for Question 3
The Add unit calculates which instruction we are going to jump to.  We need this separate Add unit because our other ALU is already computing the result of the branch condition.  If we wanted to use the same ALU, we should probably compute the branch condition first, and then the address, but we would need a separate register to store the result of computing the branch instruction.