Δαφνη Ευτυχια Αρθουρος

Last night spent a while talking to my youngest brother. My mother had said she'd give me money to buy the prescription I was whining about a few days ago, so I went to Bowie to get that from her, and I wound up talking to my brother for a couple hours.

He had some computer questions, and one of them was what we meant by "gates". So I explained logic gates, and he asked the obvious next question -- what do we do with gates and how do we get from gates to computers? I picked an adder as an example of one of the parts of a CPU and explained that you'd build an adder out of gates. And that's when I realized just how long it had been since I had done anything with logic gates and how long since I'd last thought about full adders and half adders and look-ahead carry and stuff...

...and realized that since my old Computer Architectures textbook is buried in a box somewhere, I was going to have to re-invent some of those things or I'd never get to sleep. There are some things that it just bugs me to not remember. (The gas constant is another -- every so often I realize I've forgotten it again, and re-derive it by starting with 22.4 l/mole @ STP and working backwards. I almost never need the gas constant, but I can't stand not having it at my fingertips <reverb>Just In Case</reverb>)

Okay, I had some other things I had to do when I got home, so I didn't pull out a pen and paper until a little while ago. A basic adder was easy until I thought that I should re-do it with NAND and NOR gates instead of AND and OR, because (IIRC) the inverting ones are cheaper and smaller if you're using silicon TTL (... uh, right??). Out comes DeMorgan's Law, and I find out just how incredibly rusty I've gotten at this -- I had to run the truth tables to verify that I'd remembered DeMorgan's law correctly (maybe I shouldn't have admitted that aloud). And then I wanted to find a better way, since I figure ya' gotta minimize the number of gates the signal has to go through, for speed, right? Now I'm trying to remember how look-ahead worked.

I guess I ought to just STFW and stop beating myself up over the macho "derive it without looking at the answers" thing. I need to sleep.

Please tell me other people do things like this.