ALU Implementation - the road traveled thus far

[WBahn]

Again, look carefully at the definition of the multi-way multiplexer. In particular, Fig. 1.10.

Let's say that you want to select the b input channel. What does sel[1] need to be? What does sel[0] need to be?

Your truth table says that you want to select channel b when zx is 0 and when nx is 1.

So, which signal needs to be connected to sel[1] and which signal needs to be connected to sel[0]?

Now look at your schematic. Which signal is connected to sel[1] and which signal is connected to sel[0]?

[ouverson]

I just saw this reply! I just saw the page "2" link at bottom-right of the page.

Yes, that was what I figured out.

I'm kinda glad I never saw this, as I really had to learn how to use the simulator; and it helped me slow down and really dig into the signals, etc.

Taking the incremental approach (thank you for exhortation) I was able to tackle the zr and ng bits separately; and did implement them successfully. It also helped to comment out the compare file in order to evaluate each output independently; I only used the compare file to confirm a successful implementation.

I can now move on to Sequential Logic; one step closer to "Building a Computer from First Principles" - G-d willing, and the creek don't rise.

Thanks again for all your help!

[WBahn]

Sure thing.

We (humans, that is) have a strong tendency to see what we expect to see -- which is something that usually serves us well. But when doing detailed work like most engineering disciplines it can really get in our way because we tend to see what we expect to see, not what is actually there. We know what we meant to do, so we have a natural confirmation bias that leads us to skim over the errors of what we actually did. We have to work at developing the ability to step back and look dispassionately at what we actually did; putting yourself in the mindset of an auditor reviewing someone else's work can help.

Incrementalism also helps tremendously in this regard. If you build a house and are told that one of the outlets has a loose connection, it could be a real nightmare to find. But if you wire each room and then test it and are told the same thing, it is a pretty straightforward process to fix. The fewer things we have to distract us, the less susceptible we are to seeing only what we expect to see.

Good luck moving forward. You're doing great.

[ouverson]

Thank you for the words of encouragement!