In January 1970 I was on the downwind leg of my senior year in high school, and the Chicago Public Schools had installed a computer somewhere. A truck full of these fancy IBM typewriter gadgets was delivered to Lane Tech, and a bewildered math teacher was drafted into teaching computer science (as they had the nerve to call it) to a high school full of rowdy males.
I figured it out fairly quickly. You pounded out a deck of these goofy computer cards on the card-punch machine, dropped them into the card hopper of one of the typewriter gadgets, and watched in awe as the typewriter danced its little golf ball over the green bar paper, printing out your inevitable list of error messages. It was fun. I got straight A's. I even kept the first program I ever wrote that did something useful: a little deck of cards that generated a table of parabolic correction factors for hand-figuring telescope mirrors, astronomy being my passion at the time. (I still have the card deck, though the gummy mess left behind by disintegrating rubber bands would not be healthy for a card reader, assuming that one still exists.)
The question that kept gnawing at me was exactly what sort of beast RAX (the computer's wonderfully appropriate name) actually was. What we had were ram-charged typewriters that RAX controlled over phone lines—that much I understood—but what was RAX itself?
I asked the instructor. In brief, the conversation went something like this:
He: ''Eh? Um, a computer. An electronic computer.''
Me: ''That's what it says on the course notes; but I want to know what RAX is made of and how it works.''
Me: ''You mean, there's no levers and gears inside.''
He: ''Oh, there may be a few, but no vacuum tubes.''
Me: ''I wasn't worried about tubes. I suppose it has a calculator in it somewhere; but what makes it remember that A comes before B? How does it know what 'format' means? How does it tell time? What does it have to do to answer the phone?''
He: ''Now, come on, that's why computers are so great! They put it all together so that we don't have to worry about that sort of thing! Who cares what RAX is? RAX knows FORTRAN and will execute any correct FORTRAN program. That's what matters, isn't it?''
He was starting to sweat. So was I. End of conversation. That June I graduated with three inches of debugged and working FORTRAN punch cards in my book bag, and still had absolutely no clue as to what RAX was. It has bothered me to this day.
I was thinking about RAX six years later, while on the Devon Avenue bus heading for work, with the latest copy of Popular Electronics in my lap. The lead story described a do-it-yourself project called the COSMAC ELF, which consisted of a piece of perfboard full of integrated circuit chips, all wired together, plus some toggle switches and a pair of LED numeric displays.
It was a computer. (Said so right on the label, heh.) The article described how to put it together, and that was about all. What did those chips do? What did the whole thing do? There was no fancy robotic typewriter anywhere in sight. It was driving me nuts.
As usual, my friend Gus Flassig got on the bus at Ashland Avenue and sat down beside me. I asked him what the COSMAC ELF did. He was the first human being to make the concept of a physical computer hang together for me:
These are memory chips. You load numbers into the memory chips by flipping these toggle switches in different binary code patterns, where "up" means a 1-bit, and "down" means a 0-bit. Each number in memory means something to the CPU chip. One number makes it add; another number makes it subtract; another makes it write different numbers into memory, and lots of other things. A program consists of a bunch of these instruction-numbers in a row in memory. The computer reads the first number, does what the number tells it to do, and then reads the second one, does what that number says to do, and so on until it runs out of numbers.
If you don't find that utterly clear, don't worry. I had the advantage of being an electronics hobbyist (so I knew what some of the chips did) and had already written some programs in RAX's FORTRAN. For me, my God, everything suddenly hit critical mass and exploded in my head until the steam started pouring out of my ears. I got it!
No matter what RAX was, I knew that it had to be something like the COSMAC ELF, only on a larger scale. I built an ELF. It was quite an education, and enabled me to understand the nature of computers at a very deep level. I don't recommend that anybody but total crazies wire-wrap their own computers out of loose chips anymore, although it was a common enough thing to do in the mid to late 1970s.
As a side note, someone has written a Windows-based simulation of the COSMAC ELF that looks just like the one I built, and it will actually accept and execute COSMAC programs. It's a lot of fun and might give you some perspective on what passed for hobby computing in early 1976. The URL is as follows:
The site's author, Bill Richman, has also reprinted the Popular Electronics article from which I built the device. All fascinating reading—and a very good education in the deepest silicon concepts underlying computing as it was then and remains to this day.
In this chapter I will provide you with some of the insights that I obtained while assembling my own computer the hard way. (You wonder where the ''hard'' in ''hardware'' comes from? Not from the sound it makes when you bang it on the table, promise.)
Switches, Transistors, and Memory
Think about it: you flip the wall switch by the door, and the light in the middle of the ceiling comes on. It stays on. When you leave the room, you flip the switch down again, and the light goes out. It stays out. Poltergeists notwithstanding, the switch will remain in the position you last left it until you or someone else comes back and flips it to its other position. Even if the bulb burns out, you can look at the position of the switch handle and know whether the light is on or off.
In a sense, the switch remembers what its last command was until you change it, and ''overwrites'' that command with a new one. In this sense, a light switch represents a sort of rudimentary memory element.
Light switches are more mechanical than electrical. This does not prevent them from acting as memory; indeed, the very first computer (Babbage's nineteenth-century Difference Engine) was entirely mechanical. In fact, the far larger version he designed but never finished was to have been steam-powered. Babbage's machine had a lot of little cams that could be flipped by other cams from one position to another. Numbers were encoded and remembered as patterns of cam positions.
Whether a switch is mechanical, or electrical, or hydraulic, or something else is irrelevant. What counts is that a switch contains a pattern: on or off; up or down; flow or no flow. To that pattern can be assigned a meaning. Paul Revere told his buddy to set up a code in the Old North Church: ''One if by land, two if by sea.'' Once lit, the lamps in the steeple remained lit (and thus remembered that very important code) long enough for Paul to call out the militia and whup the British.
In general, then, what we call memory is an aggregate of switches that retain a pattern long enough for that pattern to be read and understood by a person or a mechanism. For our purposes, those switches will be electrical, but keep in mind that both mechanical and hydraulic computers have been proposed and built with varying degrees of success.
Memory consists of containers for alterable patterns that retain an entered pattern until someone or something alters the pattern.
One problem with building a computer memory system of light switches is that light switches are pretty specialized: they require fingers to set them, and their output is a current path for electricity. Ideally, a computer memory switch should be operated by the same force it controls. This enables the patterns stored in memory to be passed on to other memory storage locations. In the gross electromechanical world, such a switch is called a relay.
A relay is a mechanical switch that is operated by electricity, for the purpose of controlling electricity. You ''flip'' a relay by feeding it a pulse of electricity, which powers a little hammer that whaps a lever to one side or another. This lever then opens or closes a set of electrical contacts, just as your garden-variety light switch does. Computers have been made out of relays, although as you might imagine, it was a long time ago, and (with a typical relay being about the size of an ice cube) they weren't especially powerful computers.
Fully electronic computers are made out of transistor switches. (Very early computers were also made with vacuum tube switches.) Transistors are tiny crystals of silicon that use the peculiar electrical properties of silicon to act as switches. I won't try to explain what those peculiar properties are, as that would take an entire book unto itself. Let's consider a transistor switch a sort of electrical black box, and describe it in terms of inputs and outputs.
Figure 3-1 shows a transistor switch. (It is a field-effect transistor, which in truth is only one type of transistor, but it is the type that our current computers are made of.) When an electrical voltage is applied to pin 1, current flows between pins 2 and 3. When the voltage is removed from pin 1, current ceases to flow between pins 2 and 3.
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