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What is a computer

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These are the main parts of a computer, the motherboard (with a Central Processing Unit and memory), a disk drive, and a CD-ROM (or maybe DVD) drive.

Of course, there's also a power supply, a case, keyboard, mouse and monitor before you have something useful, but the motherboard and the disks are the important parts.

The Motherboard is the main part of a computer. It has the Central Processing Unit (CPU) on it and the computer's fast memory.

The CPU is the part of the computer that does the thinking. But, it doesn't think of very much. All it really does is look at about 40 wires and figure out if there's some voltage on them or not. It compares what it sees with about 30 sets of values and then puts some voltage on about 40 other wires.

That's pretty simple. But it does this several million times a second and that makes it interesting.

The other important thing on the motherboard is the memory. This is the memory the computer uses to remember what it's doing right now. It won't remember information in main memory for very long, but it knows what it's doing while it's doing it.

It's like how you can remember what you and your friends are talking about right now, but you don't remember what the teacher said about History yesterday.

When you write a computer game, all the information about how to play the game will be in this memory.

The computer's memory chips remember if they had a high voltage or low voltage set on them. They remember this until the computer tells them to change, or you turn the computer off. When the CPU asks a piece of memory whether or not there's voltage, it puts that voltage onto one of those 40 wires the CPU watches.

There are a few billion of these little chunks of memory in your computer. By the time you add up a billion little "am I on or not" pieces of information, you've got something.

If there's something the computer needs to remember for a long time, we tell the computer to write it onto the disk. This is like your mom giving your dad a shopping list so he can remember what to get at the store.

The disk drives are how the computer remembers things it's not working on right now, but it might need to know about later. Like how to play a game, what your high score was, and stuff like that.

The computer disk drive has a few billion magnetized spots on a spinning iron disk. Again, what it does is very simple and useless. The disk drive can look at one of those magnetized spots and tell you whether it's a north or south pole.

Finally, if we want to give the information to someone else, we put it onto a CD-ROM. The CD-ROM stores data by burning tiny holes with a tiny laser in a the CD disk. Later on, it tells the computer whether or not a given location on the CD had a hole burned in it or not.

You see the pattern - memory tells the CPU it's got voltage or not, disk drives tell the CPU whether a spot is a north pole or not, and the CD-Rom tells the CPU whether a spot has a hole burned in it or not.

We call this binary information, because it can only be one of two values. Everything the CPU sees is converted to voltage (on) or no-voltage (off), even if it started as hole or no-hole.

It's too much work for computer programmers to talk about voltage levels and magnets and holes, so we call everything a one or a zero and let the hardware worry about whether that means hole, magnet or voltage.

When we write a computer program, we are telling the computer which pieces of memory should be turned on or off, how to look at one place in memory then the next place, compare the on and off values and turn something else on or off.

That sounds like about as much fun as having your teeth pulled.

Early computer programmers actually had to sit down and know all of this in lots of detail and tell the computer what value to put on every single wire and when to change it.

This got old pretty fast.

The first programs they wrote were programs to take real words and make the computer convert the words to patterns of on and off values.

That got more interesting.

For the past 50 years, people have been working to make it easier and easier to tell the computer what we want it to do, and have the computer do the boring parts of converting this to on and off values.

The next lessons will start explaining how to tell the computer to set a bunch of on and off values (last time we'll discuss those!) so that the computer will know how to play a game.

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