But How Do It Know? - the Basic Principles of Computers for Everyone (2 page)

Computers are designed to do a small number of specific simple things, and to do these things quickly, one after the other. Which simple things are done, and in what order, determines what sort of task the computer accomplishes in any given time, but anything the computer does consists of nothing outside of its limited capabilities.

Once you see what a computer is made up of, you will come to realize how it is that they can do what they do, exactly what sorts of things they are capable of, and also, what they are not capable of.

So the secret of computers is not that they are complex, rather it is their speed. Let’s look at exactly how fast their speed is.

Since computers work on electricity, their speed is related to the speed of electricity. You may remember hearing that the speed of light is 186,000 miles per second. That’s pretty darned fast. Light can go around the entire earth seven times in one second, or from the Earth to the Moon in about a second and a half. Per the physicists, electricity has many properties in common with light, and its speed, when traveling in a wire, gets slowed down to about half the speed of light. But still, going all the way around the Earth three and a half times in one second is extremely fast.

As a point of comparison, imagine it is a hot day and you have an electric fan sitting on the table blowing cool air on you. The fan is spinning around so fast that the blades are a blur, but it is only spinning around about 40 times each second. A point on the edge of one of those blades will only travel about 150 feet in that second, it will take 35 seconds for that point to travel just one mile.

Since the fan blades are already a blur, it may be hard to imagine them going just ten times faster. If it did, that fan would be putting out quite a breeze. And if you could make it go a hundred times faster, it would almost certainly self-destruct, with fan blades breaking off and getting stuck in the ceiling. But electricity traveling in the same circle would go around about a hundred million times in one second, that’s two and a half million times faster than the fan blades. That’s fast.

A million is a very large number. If you took a big sheet of paper that was 40 inches square and took a ruler and placed it at the top edge, and drew 25 dots per inch along the top edge of the paper, you would have to draw one thousand dots to get across that sheet of paper. If you then moved the ruler down the page 1/25
^th
of an inch, and drew another thousand dots, and kept doing that, you would have to move the ruler down the page one thousand times, each time drawing one thousand dots. If you could complete such a boring task, you would end up with a piece of paper with a million dots on it. That’s a lot of dots or a lot of anything. And just to finish the thought, if you could find a thousand people who would each draw one of these million dot sheets, and stacked up those thousand sheets in a pile, you would then have a billion dots.

Now let’s say that the electricity moving around inside the computer can accomplish some simple task by traveling one foot. That means that the computer could do 500 million simple things in one second. Again for comparison, the fan on the table will spin for 7 hours to go around just one million times and it will take a full six months for it to spin around 500 million times.

When you talk about the speed that electricity can move between parts inside the computer, some of the parts you can see are a foot apart, some are closer, an inch, a tenth of an inch. And inside these parts are a multitude more parts that are very close to each other, some just thousandths of an inch apart. And the shorter the distance the electricity has to travel, the sooner it gets there.

There is no point in saying how many things today’s computers do in a single second, because that would date this book. Computer manufacturers continue to produce new computers that go twice as fast as the fastest computers of only two or three years past. There is a theoretical limit to how fast they can go, but engineers keep finding practical ways to get around the theories and make machines that go faster and faster.

During all of this time that computers have been getting faster, smaller and cheaper, the things that computers do, really have not changed since they were first invented in the 1940's. They still do the same few simple things, just faster, cheaper, more reliably and in a smaller package.

There are only a few sections to a computer, and they are all made out of the same kinds of parts. Each section has a specific mission, and the combination of these parts into a machine was a truly marvelous invention. But it is not difficult to understand.

Language

In this book, we are going to need to define some words that are used to describe the parts inside a computer.

In some professions, notably the Medical and Legal, there is a tendency to make up a lot of new words, and to take them from the ancient Greek and Latin languages, and to make them long and hard to pronounce.

In the world of computers, it seems that the pioneer inventors were a less formal sort of people. Most of the words they used are simple words from everyday language, words that already existed, but are used in a new way.

Some of the new words are words we already know, used as a different part of speech, like an existing noun now used as a verb. Some of the words are acronyms, the first letters from the words of a phrase.

Each word will be described thoroughly when it is first used. And although there are thousands of words and acronyms in use if you consider the entire computer industry, there are only about a dozen or two words needed to understand the computer itself. You have probably heard some of these words before, and figured out what they meant from how they were used, but now you will get the proper and full definitions. In many cases you may find that they are simpler than you thought.

Just a Little Bit

What is in a computer? It shows you still pictures, moving pictures, music, your checkbook, letters you have written, it plays video games, communicates all around the world, and much much more. But are there pictures inside the computer? If you got out a microscope and knew where to look, could you find little pictures somewhere inside the computer? Would you see “A”s and “B”s and “8”s and “12”s moving around in there somewhere?

The answer is no, there are no pictures, numbers or letters in a computer. There is only one kind of thing in a computer. There are a large number of this kind of thing, but there is only one kind of thing in there. It is called a bit.

When you flip a coin up in the air, and let it fall on the ground, it will end up on the floor in one of two possible states - with either the head showing, or the tail.

The light in your living room (assuming you have a switch and not a dimmer) can be either on or off.

The lock on your front door can be either locked or unlocked.

What do all of these things have in common? They are all places that contain a thing that can be in one of two possible states. This is the definition of a bit.

A bit is some kind of a physical object that has a size and a location in space, and it has some quality about itself, that at any given time can be in one of two possible states, and may be made to change back and forth between those two states.

A lump of clay is not a bit. It can be molded into a ball, a cube, a pancake, a ring, a log, a face or anything else you can think of. It has a size and a location in space, but there are too many states that it can be in for it to be called a bit. If you took that lump of clay, flattened it out, scratched “yes” on one side of it, and “no” on the other side, and then put it in a kiln and fired it until it was hard, then you might be able to call it a bit. It could sit on a table with either the “yes” or “no” showing. Then it would only have two states.

You have probably heard of bits before in relation to computers, and now you know what they are. In a computer, the bits are not like the coin or the lock, they are most like the light. That is, the bits in a computer are places that either have electricity or they do not. In a computer, the bits are very, very small and there are a very large number of bits, but that’s all that is in there.

Like the light in the living room, the bit is either on or off. In the living room, there is electricity in the wall coming into the switch. When you turn the switch on, the electricity goes from the switch, through the wires in the wall and ceiling, into the light socket and then into the light bulb. So this bit in the living room is several feet long, it includes the switch, the wires, the socket and the light bulb. In a computer, bits are mostly tiny, actually microscopic. Also, the computer bit doesn’t have a mechanical switch at one end or a light bulb at the other. If you removed the light bulb from the socket in the living room, the switch would still send electricity to the socket when it was on, and it would still be a bit – you just wouldn’t be able to see whether it was on or off by looking at a light bulb. Your computer has something resembling switches, like the keys on the keyboard, and something resembling light bulbs, like the tiny dots on the screen, but most of the bits are inside and unseen.

This is basically all there is in a computer – bits. There are lots and lots of them, and they are arranged and connected up in various ways, which we will examine in detail as the book progresses, but this is what is inside all computers – bits. A bit is always in one of its two possible states, either off or on, and they change between on and off when they are told to do so. Computer bits aren’t like the coin that has to physically flip over to change from one state to the other. Bits don’t change shape or location, they don’t look any different, they don’t move or rotate or get bigger or smaller. A computer bit is just a place, if there is no electricity in that place, then the bit is off. When electricity is present, then the bit is on.

If you want to change a coin from showing heads to showing tails, you have to physically move it to flip it over, which takes some amount of time. Because the only thing that has to move in a computer bit is the electricity, changing it’s state from off to on, or on to off can happen much more quickly than anything that has to be moved physically.

As another example, remember the wild American west from the movies? There were little towns separated by vast distances. The bigger towns would have a telegraph office. In this office was a guy wearing a funny hat who had a spring-loaded switch called a key, and he would send messages by pressing this key on and off in certain patterns that represented the letters of the alphabet. That key was connected to a battery (yes they had batteries back then) and a wire that was strung along poles until it got to another town. The key simply connected the battery to the wire when it was pressed, and disconnected the battery when the key was not pressed. In the other town there was another telegraph office, the wire came into that office, the end of it was wrapped around an iron rod (which turns into a magnet when there is electricity in the wire,) the magnetized rod attracted a little bar of iron held nearby with a spring, and made a clicking sound every time the electricity came on. The guy in the office listened to the pattern of the clicking and wrote down the letters of the message. They might have used a light bulb instead of the clicker, except that light bulbs had
not
yet been invented.

The point of bringing up this subject, is that this whole telegraph machine, from the key that gets pressed in one town, through the long wire that travels to another town many miles away, to the clicker, this whole apparatus comprises just one single bit. It is a place that can either have or not have electricity, and goes on and off as it is told. And this method of communication revolutionized the world in many ways. But this very important invention of the 1840s consisted of nothing more than one bit.

So I hope this begins to simplify the subject of computers for you. There is only one thing inside computers, bits. Lots of them to be sure, but when you understand bits, you understand what’s in there.

What the…?

Imagine it is a bright sunny day, and you walk into a room with lots of open windows. You notice that the ceiling light is on. You decide that this is a waste, and you are going to turn the light off. You look at the wall next to the door and see a switch plate with two switches. So you assume that the one closer to the door is for the ceiling light. But then you notice that the switch is already off. And the other switch is off too. So then you think “well, maybe someone installed the switch upside down,” so you decide to flip the switch anyway. You flip it on and off but nothing happens, the ceiling light stays lit. So then you decide that it must be the other switch, and you flip it on, off, on, off. Again nothing happens, that ceiling light continues to shine at you. You look around, there is no other door, there are no other switches, no apparent way to turn off this darned light. It just has to be one of these two switches, who built this crazy house anyway? So you grab one switch with each hand and start flipping them wildly. Then suddenly you notice the ceiling light flicker off briefly. So you slow down your switch flipping and stop when the ceiling light is off. Both switches say “on”, and the light is now off. You turn one switch off, then on, and the light goes on, then back off. This is backwards. One switch off equals light on? So then you turn the other switch off, then on, the same thing, the light goes on, then back off. What the heck? Anyway, you finally figure out how it works. If both switches are on, the light goes off. If one or the other or both switches are off, then the ceiling light is on. Kind of goofy, but you accomplish what you intended, you turn both switches on, the light goes off, and you get the heck out of this crazy room.

Now what is the purpose of this little story about the odd light switches? The answer is, that in this chapter we are going to present the most basic part that computers are made of. This part works exactly like the lighting system in that strange room.