A student writes:

        Dr. Patt,

        I would like to thank you for your lecture on Monday, for clearing up 
        my confusion about muxes, adders, and so on for the homework and the

Thank you.

        However, I have a question about problem number 12 on the
        homework set, the one that asks about the adder/subtractor.

        What type of binary digit system is supposed to be used?  
        Am I supposed to
        used the unsigned magnitude system, or the 2's complement system when
        implementing this adder/subtractor?

2's complement.

        Thank you.

        <<name withheld to protect the student who starts with a compliment>>

...and, perhaps another hint, to get you started:

Part a should get you started.  What is the output if X=0?  What is the
output if X=1?  Part b asks you to add whatever is necessary in order 
for the output to be A-B when X=1.  How can that be?  What additional 
stuff is needed to make that be the case?

Good luck.

Yale Patt