A student (representing her study group) writes:

     Dear Dr. Patt,
     How do I do number 9? My study group and I have no idea how
     to do it. Actually, we are having trouble doing it with only
     2 control signals. 

OK, let's see how we might do this one. Let's start with a very big box, so there is room inside to build a logic circuit. Four inputs from the top. One output out of the bottom. Two select lines in from the side. The two select lines can take on one of the four values 00, 01, 10, 11, depending on which of the four we want to output through the bottom.

The only thing we can put inside the big box are two-input muxes. We can use as many as we need. What does a 2 input mux do? Any thoughts?

     Also, we have a question on 10b. Is there a subtraction box
     or a way to change a positive number to a negative number with what
     we know? Or can we just write "negative B?" Help is appreciated!

This is the third question I have gotten on this.

No, you can't write "negative B." But you could "construct" negative B, using B as an input. How might you do that? What is involved? Does figure 3.39 provide anything useful that you can build on? Does this help?

Good luck.

Yale Patt

     << name withheld to protect the designated questioner >>