Memory comes in three popular forms: RAM chips, ROM chips, and disks.
You already learned about RAM chips and ROM chips. Let's examine disks.
A computer disk is round, like a phonograph record.
Three kinds
You can buy three popular kinds of computer disks:
A floppy disk is made of flimsy material. It's permanently encased in a sturdy, square dust jacket.
A hard disk is made of firmer material. It typically hides in your computer permanently, unseen.
A CD-ROM is a compact disk. It's the same kind of CD compact disk that plays music.
Each kind has its own advantages and disadvantages.
Floppy disks are the cheapest (about 50œ per disk) and the easiest to mail to your friends: just stick the floppy disk in an envelope, perhaps with some padding. Unfortunately, floppy disks work the most slowly, and they hold the least data: the typical floppy disk holds about 1 megabyte, while the typical hard disk or CD-ROM can hold many hundreds of megabytes.
Hard disks work the fastest ___ over 20 times faster than the other kinds! But hard disks are also the most expensive. Moreover, they typically can't be removed from your computer and therefore can't be mailed to your friends.
CD-ROMs are the best value: CD-ROM disks cost less than 1œ per megabyte to manufacture. But they have a frustrating limitation: the information on CD-ROM disks cannot be edited.
Since each kind of disk has its own advantages and disadvantages, you'll want to buy all three kinds.
Spelling
Computer experts argue about spelling. Some experts write ``disk'', others write ``disc''.
Most manufacturers write ``disk'' when referring to floppy disks or hard disks, but write ``disc'' when referring to CD-ROMs. That inconsistency annoys me.
To be more consistent, I'll always write ``disk'', even when referring to CD-ROMs. Most computer magazines (such as PC Magazine and PC World) feel the same way I do: they always write ``disk''. The growing tendency is to always write ``disk''.
For hard disks, IBM used to write ``disc'' but now writes ``disk''. A floppy disk (or diskette) is round but comes permanently sealed in a square dust jacket. (Don't try to remove the floppy disk from its square jacket.)
The floppy disk is as thin and flimsy as a sheet of paper but is protected by the sturdy, square jacket that encases it.
Three sizes
Floppy disks come in three sizes.
The most popular size is called a 3«-inch floppy disk, because it comes in a square jacket that's about 3« inches on each side. (Actually, each side of the jacket is slightly more than 3« inches, and the disk's diameter is slightly less.)
An older size, used mainly on older computers, is called 5¬-inch. It comes in square jacket that's exactly 5¬ inches on each side.
An even older size, 8-inch, is used just on ancient computers that are no longer built.
Those three sizes have nicknames:
An 8-inch floppy disk is called a large floppy.
A 5¬-inch floppy disk is called a minifloppy.
A 3«-inch floppy disk is called a microfloppy.
Jacket colors
The jacket of a 5¬-inch or 8-inch floppy disk is usually black. The jacket of a 3«-inch floppy disk is usually black, blue, white, or beige (very light grayish brown).
If you pay a surcharge, you can get jackets that have wilder colors.
History
8-inch floppies were invented in the early 1970's by IBM. 5¬-inch floppies were invented in the late 1970's by Shugart Associates, which later became part of Xerox. 3«-inch floppies were invented in the 1980's by Sony. They've become the most popular size because they're the smallest, cutest, and sturdiest. They're small enough to fit in the pocket of your shirt, cute enough to impress your friends, and sturdy enough to survive when you fall on your face. They're also easy to mail, since they're small enough to fit in a standard white business envelope and sturdy enough to survive the U.S. Postal System. Yup, nice things come in small packages!
Magnetized iron
The round disk (which hides inside the square jacket) is coated with rust, so it looks brown. Since the rust is made of iron, which can be magnetized, the disk stores magnetic signals. The pattern of magnetic signals is a code representing your data.
Drives
To use a floppy disk, you must buy a floppy-disk drive, which is a computerized record player.
If the drive is external, it's a box sitting near the computer. If the drive is internal, it's built into the middle of the computer.
The drive has a slit in its front side. To use the drive, push the disk (including its jacket) into the slit.
When pushing the sheathed treasure into the box's slit, don't shove too hard. Oooh! Please be gentle!
When you push your disk into the slit, don't push the disk in backwards or upside-down! Here's how to push the disk in correctly. . . .
First, notice that the disk's jacket has a label on it and also has a big oval cutout. (If the disk is 3«-inch, the cutout is covered by a metal slider.) Insert the disk so that the oval cutout goes into the drive before the label does. If the drive's slit is horizontal, make sure the label is on the top side of the jacket; if the drive is vertical, make sure the label is on the left side of the jacket.
After putting the disk into the slit, close the latch to cover the slit. (If the disk is 3«-inch, there is no latch.) Since the slit and latch act as a door, closing the latch is called closing the door.
As soon as you close the door, the disk drive automatically positions the disk onto the turntable that's hidden inside the drive. The turntable's called the spindle. It can spin the disk quickly.
Like a record player, the disk drive contains an arm with a ``needle'' on it. The needle is called the read-write head, because it can read what's on the disk and also write new information onto the disk.
Here's how to write new information onto the disk. Put your fingers on the computer's keyboard. Type a command that tells the computer you want to use the disk. Then type the information you want to transfer to the disk.
To transfer the information to the disk, the computer lowers the read-write head onto the disk. An electrical charge passes through the head. The charge creates an electromagnetic field, which magnetizes the iron on the disk's surface. Each iron particle has its own north and south pole; the patterns formed by the north and south poles are a code that stands for the information you're storing.
Tracks As the disk spins, the head remains stationary, so that the head draws a circle on the spinning disk's surface. The circle's called a track. To draw the circle, the head doesn't use ink; instead, it uses a pattern of magnetic pulses. Since your eye can't see magnetism, your eye can't see the circle; but it's there!
When you start using a blank disk, the arm puts the head near the disk's outer rim, so that the head's track (circle) is almost as wide as the disk. That track's called track 0.
Then the arm lifts the head, moves the head slightly closer to the virgin disk's center, and puts the head back down onto the disk again. The head draw
