Storage is a term that describes how a computer retains data in a form for later use. For a personal computer, storage is generally in the form of either a hard or "floppy" disk. These work in a manner similar to your tape recorder. Your music is recorded in the form of magnetic areas on tape. Computer data appears as magnetic bits (1s or 0s) placed on a flat magnetic surface. While hard disks hold considerably more data, the floppy disk is very portable. You have to decide what's best for your application.

There are a number of terms associated with floppy disks and their use. Each has a section below.

Tracks

Music is recorded on up to four tracks on your cassette tape recorder (left and right channels, both sides).

Data is written in a similar manner on computer disks, except that there are many more tracks and the tracks are arranged differently.

The magnetic surface of a disk is circular. The drive mechanism spins this circular surface between two read/write heads that move along the radius of the circle (i.e., in toward the center of the disk and out toward the edge).

With this construction there are two ways that data can be recorded: in a single long spiral or in a series of concentric circles.

Since a series of concentric circles can be easily indexed for data recovery, that method of recording data on floppy disks was picked and is still being used today.

Different format disks have differing numbers of tracks. Tracks are numbered from 0 to the maximum with the lower numbers on the outer portion of the disk.

A 40-track disk would have tracks numbered from 0 to 39 starting about a quarter of an inch from the outer edge of the magnetic material. An 80-track disk would have 0 to 79. Tracks are placed onto the disk by a formatting program provided with the computer's operating system.

Some of the common formats include:

• DSDD 5.25-inch (360K) - 40 tracks
• DSHD 5.25-inch (1.2MB) - 80 tracks
• DSDD 3.5-inch (720K) - 80 tracks
• DSHD 3.5-inch (1.44MB) - 80 tracks

Whoa! Several different sizes, same number of tracks? How can that be?

The simple answer is that the different size and capacity disks divide the tracks up into different sizes and data is written to the disk in a more compact form.

Sectors

Indexing data to tracks results in that data being more easily found. However, on an 80-track floppy disk that would mean there would be only 80-different places to put data and be able to uniquely find it again.

To be able to better find data, each track on the floppy disk is further divided into an equal number of sectors.

You can more easily visualize sectors if you look down on the disk and imagine the tracks as concentric circles. Now divide the disk into an equal number of pie-shaped sections. Each arc-shaped section defines a sector.

Using this visualization, at first glance it might look like the sectors on the outer rim of the disk can hold more data than sectors toward the center of the disk. While technically true, in order to take advantage of this extra space the disk controller mechanism must be specially designed and for floppy disk drives the extra complexity makes this not worthwhile. Instead, the disk is allowed to spin at a constant speed and because the angular velocity is the same, data on the outer sectors just gets recorded a bit further apart than data on the inner sectors.

Each sector is labeled by the operating system's format utility. These electronic labels enable DOS to find information on the disk.

The number of sectors per track differs with the particular disk size and version of DOS used.

• PC-DOS version 1.1, the first version that handled floppy disks, used 8 sectors per track for 5.25-inch disks.
• With PC-DOS version 2.0 this changed to 9 sectors per track.
• When 1.2MB 5.25-inch floppy disks were introduced the number of sectors per track increased to 15 (it remained 9 for the 360K 5.25-inch floppy disks).
• The newer 3.5-inch floppy disks use either 9 or 18 sectors per track for 720K or 1.44MB capacity respectively.

In general, you should not use an old version of DOS to write to a disk formatted by a newer version of DOS. It may work, but in some circumstances will not. For example, the 1.44MB format was introduced with version 3.3 of MS-DOS. If you try to use a 1.44MB disk with DOS 3.2 you will not be able to read it and if you attempt to write to it you may render the entire contents unreadable.

It's probable that you will never have to interact directly with the disk at the track/sector level, but if you have to, be very careful... It's really easy to mess up a disk by changing so much as a single bit.

Single/Double Sided

Floppy disks come in two major grades: single and double sided.

This means exactly what is stated: the single sided disk is only certified to be good on one side. The double sided disk can place information on both sides.

This does not mean there is no magnetic surface on the other side of a single sided disk, just that the manufacturer does not guarantee the surface will reproduce magnetic pulses at their true intensity, resulting in possible data errors.

Turning a single sided disk over to use the other side is not recommended, although it is possible. While a concern in the early days of personal computers, this is rarely a concern now as almost all disk drives and disks are double sided.

Density

The final factor in the equation to determine amount of data on a disk is the density or number of bits per inch. Single density records at 2,768 bits per inch & double density is 5,876 bpi; newer disks even higher.

While the number of bits per inch is the technical definition of density, usually a more practical way to look at the problem is to look at the number of bytes stored in each sector. Early computers used 128 bytes per sector, some use 256 and the IBM-PC uses 512.

As an example: 512 bytes per sector (Assumes a high density x 18 sectors per track 3.5-inch floppy.) x 80 tracks per side x 2 sides per disk = 1,474,560 bytes (or 1,474,560/1024 = 1,440KB = 1.44MB)

Handling/Safeguards

Finally, there is the topic of disk safeguards. At the top of a 5.25-inch disk is a small notch. If this notch is covered then the drive mechanism will not write to the disk, thus protecting it and making it read only. If you want to write information to the disk, make sure the notch is uncovered. On the other hand, for 3.5-inch disks, write protection is just the opposite. The hole with the slider must be covered to be able to write to the disk. Uncovered is write protected.

Also, when the disk is spinning, the read/write head travels in contact with the media. Any obstruction will cause the head to jump parts of the data, or, if the obstruction gets caught by the head, it might even scratch it. What's big enough to do it? Here are some disk obstructions:

• Fingerprints (the oil forms ridges that collect dust)
• Smoke (another reason to quit)
• Hair (hair is like a large tree to a disk drive)

And, here are some things to keep magnetic materials away from:

• TVs (the power supply and picture tube have strong magnetic fields)
• Magnets (don't pin a disk to a refrigerator with a magnet)
• X-rays
• Telephones (ringer is magnetic)
• Cats & Dogs (static from fur)

Other Formats

The 360K 5.25-inch floppy disk was the staple for removable storage on IBM-architecture equipment for a long time. The AT-class (80286) computers introduced a new high density format: 15 sectors per track, yielding 1.2MB on a 5.25-inch disk. The magnetic properties of these 1.2MB disks make them unusable in 360K drives, but for a little more money they provide four times the storage.

Since its introduction, the Apple Macintosh computer used smaller 3.5-inch disks with hard plastic shells. With the introduction of the IBM PS/2 computer line, 3.5-inch disks became a standard for IBM-architecture as well. The first 3.5-inch drive yielded 720K per disk with a high density version now packing 1.44MB onto a 3.5-inch disk. New 3.5-inch formats double that to 2.88MB by increasing density.

Newer formats include: optical media or a combination of magnetic and optical to pack 20MB and much more on 3.5-inch or smaller disks.

Hard Disks