Sat, 10 Sep 2011, 19:16

A student has been reading ahead, and writes:

> Dear Dr. Patt,
> 
> I had a question about the logic circuits drawn in chapter 3. I know it
> shows gates with more than 2 inputs, but do the circuits in computers (or
> other devices) actually use these types of gates, or is it just a shorthand
> notation?
> 
> Thank you,
> <<name withheld to protect the student who wants to know how many inputs>>

It is not a shorthand.  The designer is free to decide how many inputs a gate
has, and also how many circuits each output is applied to.  Incidentally, we
refer to the number of inputs to a gate as the gate's "fan-in."  We refer to
the number of subsequent inputs that the gate's output can be used for as the
gate's "fan-out."  


The choice of both fan-in and fan-out depend in large part on electrical 
considerations, which is beyond what we will talk about in EE 306.  I will 
say that I have seen gates with occasionally a fan-in of 20 or 30,
and I have seen a lot of gates with fan-in of only 2.

As far as outputs (which you did not ask about), most circuits use the output
of one gate as one of the inputs to another gate.  But I have also seen logic
circuits where the output of one gate is subsequently used for two inputs.
In fact, that was necessary in one of the homework problems in the first 
problem set.  

             ----------             -----------
        a ->|          |       --->|           |
            |          |______|    |           |___
            |          |      |    |           |
        b ->|          |       --->|           |
             ----------             -----------

It is usually the case that most gates are restricted to having a fan-out 
of at most 4.

OK?

Good luck with the rest of EE 306.

Yale Patt