Sat, 7th Nov 2015, 23:23 Re: Problem Set Question 19 SR2MUX

A student writes:

> I understood the last email about the SR1MUX picking which register that
> goes to the ALU, but on question 19  of the problem set, no control is
> identified for SR2MUX, yet the ALU is adding A and B. How do I know if it
> is adding the SEXT of [4:0] or SR2 if the selector is not specified for the
> <<name withheld to protect the student wondering what controls the SR2MUX>>

Indeed, the select line for the SR2MUX is not explicitly labeled.  Can you
figure it out?  First, what are the two sources of the SR2MUX?  Is there
any bit in the instruction that tells you how to obtain the source of the
second operand for the ALU?  Do you see that the select line of the SR2MUX
is coming from the finite machine.  Do you see all the inputs to the
finite state machine?  Is there anything associated with the finite state
machine that could be used to select the source of the SR2MUX?

Good luck putting this together.  Feel free to discuss my response with the
other members of your study group.  If you are all stuck, please see one of
my TAs.  If that doesn't work, then see me.  I will be glad to tell you.
But as you know I always like it better when my hints enable you to figure
it out for yourself.

Yale Patt