10/04/2006

'Tis the night before the midterm, 
And all through the dorm, 
Anxiety takes over,
Such is the norm.

I could go on, but I might make you vomit, and I do not want that, 
certainly not before the exam.

A student writes:

        dr. patt,
        this may be a stupid question, so get ready.

Not stupid.  Just not particulary productive.

        if during the test (hypothetically speaking), we have a problem that
        requires us to draw a gate level diagram for something that involves
        combinational logic circuits or storage elements, and we are sort of 
        in a time crunch, could we just draw a box and then label whatever it 
        is? like a box with register or decoder written in it, as long as we 
        make sure there are the correct number of input and output lines going 
        into and coming out of the box.
        Thanks,

        <<name withheld to protect one who can not sleep the night before>>

I really do understand your anxiety.  This is your first mid-term, and you
are worried.  But if you concentrate on the concepts and let me concentrate
on grading fairly and correctly, you will probably do better.  

I guess the real question is what is in the box?  With all that I have to
ask you, and all I have said about memorizing, do you really think I am going
to ask you to draw a gate-level representation of a decoder (which is Fig.3.11
in the book)?

On the other hand, it certainly could be the case that I might ask you 
to design something where one component of the logic circuit is a decoder.  
In that case, sure, it is not necessary to draw the individual gates of the 
decoder since that could be copied directly from your three sheets of paper.
Simply draw a box with the proper number of inputs and outputs, and write
decoder inside the box.

BUT: For example, if I ask you to draw a gate level logic circuit that takes 
five inputs and outputs the binary representation of the sum of the number 
of inputs that are 1, and you are crunched for time, and you write the answer 
as:

               ----------------------------
     a ------>|                            |
              |  Logic that counts the     |
     b ------>|  number of inputs that     |------> out(2)
              |  are 1 and outputs the     |
     c ------>|  binary value corresponding|------> out(1)
              |  to that count.            |
     d ------>|                            |------> out(0)
              |                            |
     e ------>|                            |
               ----------------------------


How much credit do you think you should get?  

Get some sleep and good luck tomorrow.

Yale Patt