I almost did not send this out to all of you, since it is more of a distraction than critical knowledge. However, since it does relate to the notion of abstraction, and some of you may be interested in it, I decided I would. If you feel compelled to just "delete" and move on, that is okay, too.

A student writes:

     I'm a student in your 306 class. I didn't fully understand
     the CMOS you were talking about in class. What is it and what does
     it do?

CMOS is the name of a device technology that is used to make the transistor. It stands for complementary metal-oxide semiconductor. Kind of like (though not exactly) saying the filament in the light bulb is made out of tungsten.

Semiconductor describes a behavior characteristic of the material -- half way between a conductor (like copper) and an insulator (like rubber).

Metal-oxide is part of the material.

Complementary means there are both P-type and N-type devices on the same chip.

Not a big deal. You do not have to memorize that. For your purposes, CMOS is one technology which is used to make transistors that you can use as building blocks to form AND gates and OR gates, etc. They are not the only technology. You could use relays, or vacuum tubes (see the collection on the first floor of ENS) or beams of light, actually, to build gates out of. I chose CMOS because that is what the transistors in your laptop, cell phone, etc. use.

We never described how the electricity actually works in these CMOS transistors because it is below the lowest level or abstraction that we are interested in in 306. We also did not explain how quantum physics works, because it is even below the electricity.

The point is that at some point, we need to accept something if we are ever going to make progress in understanding how computers work. And I made the call to start at the level we did: how the transistor BEHAVES (that is, like a wall switch), and we have already started our move up from there. I told you in class that that is how a CMOS transistor works, and showed you how we can build on this. ...and asked you to trust me! CMOS transistors do behave that way.

If you want to go deeper, there are plenty of other courses for that and I invite you to take them. But, we will not be going there this semester in 306. OK?

     Also, does it relate at all to the CMOS battery that is on
     my motherboard in my PC?

Sort of. CMOS battery, near as I can tell, is "a battery to provide power for a circuit made up of CMOS transistors." So, the CMOS is about what the battery is used for. Indeed the CMOS circuit uses very little power, so the battery need not be very powerful. If you want more on CMOS batteries, you can Google CMOS battery and get more catalogues than you could ever get through this semester.

Good luck with 306.

Yale Patt

     << name withheld to protect the curious >>