Dear Dr. Patt,

	Me and my study group had a question concerning how the state machine 
	changes state when performing a JSR. I understand how you can get to 
	state 4, but i do not understand how you are able to switch bit 4, which 
	will add 16 and goto state 20. I would think there 
	would have to be some 
	type of combinational logic above J[4] to do this (referring to the 
	microsequencer diagram).

Actually, I am concerned that you are missing something fundamental.  The address of the 
next state has NOTHING to do with the ADDRESS of the current state.  So, the 1 required by 
J[4] is not at all influenced by the value of J[4] in the address 000100 (that is, 4).

The address of the next state is computed using the J bits in the current microinstruction, 
along with all the other bits shown as input to the microsequencer.

So, if you want to next go to state 20 or 21 (that is 010100 or 010101), then the J field of your 
current microinstruction better be 010100, and your COND field better select the ability to 
change J[0].  This is all INDEPENDENT of what state you are coming from.  That is if you 
wanted to next go to state 20 or 21 from state 55, rather than from state 4, the microinstruction 
at state 55 would have to contain that information.

Finally (perhaps I am overdoing it, but I would rather err on the side of overdoing it!), a 
microinstruction is the CONTENTS of a control store location, not the ADDRESS of a control 
store location.  But you learned that in 306, right?

Good luck with this.


	Cheers,
	<<name withheld to protect the one who says "Cheers>>  

since Cheers is usually best used when holding up the glass before drinking, 
when the work is done and relaxing is about to begin.  ...and we are not 
quite at that point yet.

Yale Patt