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How do CD-ROMs work?
How do Computers work?

How are the disks made?

What is a hard disk

A CD-ROM, or Compact Disk - Read Only Memory, is similar to a floppy disk or hard disk in that it stores binary information that makes sense to a computer. The difference between a CD-ROM and a floppy disk or hard disk, is that the CD-ROM uses light reflections rather than magnetism to store information.

If you look at a CD-ROM, (one is included with your answer), the information is stored by making little tiny holes in a reflective material. Your eye sees these as a rainbow. A tiny laser shines on this material and determines of there is a hole or not in a certain place. This is translated into binary information that the computer reads as data.

To tell you about a hard disk, let's first mention floppy disks. A floppy disk is made of the same material as a cassette tape. It's brown and fairly flexible. I've included some old 8 inch diskette for you to look at. The more modern 3 1/2" floppy disks are almost the same except they are in a hard plastic shell. A hard disk uses an aluminum disk covered with the same brown material as the floppy disks. It's not flexible. Look at the one in the library. You can touch it because it's already broken, but never touch the brown surface on a floppy disk. Most hard disks these days are sealed so you can't touch it unless you break it open.

Now, to describe how a computer works.....


"Generally, any device that can perform numerical calculations-even an adding machine, an abacus, or a slide rule-may be called a computer. Currently, however, the term usually refers to an electronic device that can use a list of instructions, called a program, to perform calculations or to store, manipulate, and retrieve information."

In 1847, that's more that 140 years ago, a man named Charles Babbage designed something that he called "Difference Engine Number 2." His machine is considered to be the first computer. This machine could not be built, however, until 1991 because the gears and bearings were far too difficult to make in 1847. A working model was built by scientists at the Science Museum in London England and it worked perfectly. It also weighed as much as large car!

Before there were computers, scientists had to do calculations using printed tables of numbers. Often, these tables would have errors in them and so the results that the scientists got from their calculations also had errors. Charles Babbage once said to John Herschel, a famous British astronomer, after discovering an error in a printed table "I wish these calculations had been executed by steam!" Can you imagine a computer that ran on steam!

Today we're used to seeing a MAC or PC, sitting on a desk and giving precise answers to our questions. There are, of course, much bigger computers that are used by large companies, governments and universities. These are called mini-computers or main-frame computers and they could be big enough to fill your school library.

The invention of the transistor in 1947 and later the integrated circuit or chip, helped to make the small powerful computers that are so common today. The transistor is an electronic switch and an integrated circuit can contain millions of transistors in a tiny square smaller than your baby fingernail.

  1. computer is actually a pretty simple device. It very rarely needs repair except for the moving parts such as the disk drives. Most problems with computers are due to faulty software or computer programs rather than electrical bits.

    A computer doesn't really do much more than:

    % % SYMBOL 183 \f "Symbol" \s 10 \h

    Store a number into memory

    % % SYMBOL 183 \f "Symbol" \s 10 \h

    Get a number from memory

    % % SYMBOL 183 \f "Symbol" \s 10 \h

    Add two numbers and store the result in memory

    % % SYMBOL 183 \f "Symbol" \s 10 \h

    Do comparisons between two numbers such as: are they the same, is the first bigger than the second, is a number equal to zero

    % % SYMBOL 183 \f "Symbol" \s 10 \h

    A computer is made of both hardware, the things you can touch, and software, commands that the computer understands.


    CPU - Central Processing Unit

    This is the actual brain of the computer. It's the part that does the adding, storing, comparing and retrieving. These days, the CPU is just an Integrated circuit and doesn't look too exciting.


    Memory is really only a bunch of switches that can be on or off. The computer sort of looks at the switch to see if it's on or off when it reads memory or flips the switch when it writes memory. Each switch is called a BIT and the computer usually looks at 8 bits at a time. Eight bits is called a BYTE. If the switch is on then it represents a value of 1, if it is off then it has a value of 0. Computers can only add ones and zeroes. The way they seem to add bigger numbers is done by clever programs.

    Another type of memory is ROM or read only memory. This is also an integrated circuit but the computer can only read this information not change it. When you first turn on a computer it usually reads its first instructions from ROM. There is a special type of ROM called EPROM. The E stands for erasable. These memory chips can be erased if they are placed under a special light. The light shines through the tiny window in the chip and in a few hours will clear all of the memory. Information can then be stored in it again by using a special machine called an EPROM programmer.

    There is yet another type of memory and it has many types. This memory is stored on external devices such as floppy disks, hard disks and tape drives. These days a CD or optical disk is becoming quite common as well.

    Input/Output Devices - Keyboard, Mouse, Printer, Monitor

    A CPU with only memory wouldn't be very useful unless there was some way we humans could communicate with the computer.

    The early computers had a lot of lights on them. These were fun to look at but not very practical. You've probably seen old science fiction movies where the computer is a wall of lights and buzzers. In the 1960's computers actually did look like this.

    Later, electric typewriters were modified so that humans could type instructions using the keyboard and the computer would give the answers by typing on paper. These machines were called teletypes. The paper was replaced with a TV screen and these were called 'glass' teletypes. Today, almost all interaction with a computer is by using a keyboard and a TV screen or monitor. Output to paper is done using either printers that use ink and ribbon just like a typewriter or laser printers that work a lot like a photo-copier.

    Today, we use keyboards and mice to "talk" to computers and they respond by showing things on the computer screen or monitor or by printing on a printer.

    Thanks for your questions.

    Answer% % filename A253.DOC Page% % page 1 of% % numpages 3 % % date \@ "MMMM d, yyyy"May 31, 1998

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