MAGNETIC DISK RECORDING
After completing this chapter, you'll be able to do the following:
Magnetic disk recording was invented by International Business Machines
(IBM) in 1956. It was developed to allow mainframe computers to store large amounts of computer programs and data. This new technology eventually led to what's now known as the computer revolution.
This chapter introduces you to the following aspects of magnetic disk recording:
MAGNETIC DISK RECORDING MEDIUMS
There are two types of disk recording mediums: flexible diskettes and fixed (hard) disks.
The following paragraphs describe (1) how flexible and fixed disks are made; (2) how data is organized on them; (3) how to handle, store, and ship them; (4) and how to erase them.
FLEXIBLE MAGNETIC RECORDING DISKETTES
Flexible diskettes, or floppy disks as they're more commonly called, are inexpensive, flexible, and portable magnetic storage mediums. They have the following characteristics.
Floppy Disk Construction
Floppy disks are made of round plastic disks coated with magnetic oxide particles. The disks are enclosed in a plastic jacket which protects the magnetic recording surface from damage.
Floppy disks come in three sizes: 8 inch, 5 1/4 inch, and 3 1/2 inch. Figure 8-1 shows each size. All disk sizes can either be single-sided or double-sided. Single-sided disks store data on only one side of the disk; double-sided disks store data on both sides.
Figure 8-1. - Floppy disk construction.
When floppy disks are manufactured, the magnetic oxide coating is applied to both sides. Each disk is then checked for errors. Disks certified as single-sided, are checked on only one side; disks certified as double-sided are checked on both sides.
Floppy disks are also classified by how much data they can store.
This is called a disk's density. There are three levels of floppy disk density: single-density, double-density , and high-density.
Some of the more common types of floppy disks and their storage capacity are listed below:
Floppy Disk Data Organization
Data is stored on a floppy disk in circular tracks. Figure 8-2 shows a circular track on a floppy disk. The total number of tracks on a floppy disk is permanently set by (1) the number of steps the disk drive's magnetic head stepper motor can make, and (2) whether the disk drive has a magnetic head for one or both surfaces of the floppy disk. These two things will also determine the type of floppy disk that's needed. Each type of disk is rated with a number that represents how many tracks per inch (TPI) it can hold. Some common track capacities are 40, 48, 80, and 96 TPI.
Figure 8-2. - Tracks and sectors of a magnetic disk.
Each track of a floppy disk is broken up into arcs called sectors. A disk is sectored just as you'd slice an apple pie. Figure 8-2 shows the sectors of a floppy disk.
How many slices are made? That depends on who made the disk and in what host computer the disk is used.
There are two methods for sectoring a floppy disk:
Hard Sectoring: This method sectors the disk physically. The disk itself will have marks or sensor holes on it that the floppy disk drive hardware can detect. This method is seldom used today.
Soft sectoring: This method sectors the disk logically. The computer software determines the sector size and placement, and then slices
the disk into sectors by writing codes on the disk. This is called formatting or initializing a floppy disk. During formatting, if the computer software locates a bad spot on the disk, it locks it out to prevent the bad spot from being used. Soft sectoring is by far the most popular method of sectoring a floppy disk.
Once a floppy disk is formatted, the computer uses the disk's side number, a track number, and a sector number (together) as an address. It's this address that locates where on the disk the computer will store the data.
Floppy Disk Handling, Storage, and Shipping
Floppy disks hold a lot of data. Even disks with only a 360,000-byte storage capacity can hold 180 pages of data! That's why it's important to handle, store, and ship floppy disks properly. One hundred and eighty pages of data is a lot of data to retype just because of carelessness.
Before we get into disk handling and storage procedures, let's first learn about head-to-disk contact. Do you remember reading in chapter 2 that the quality of magnetic tape recording is seriously degraded when dust, dirt, or other contaminates get between the magnetic head and the tape? Well, the same is true for magnetic disk recording. In fact, head-to-disk contact is extremely important with floppy disks. This is because floppy disk drives, unlike magnetic tape drives, spin at very high speeds - 300 to 600 revolutions-per-minute (RPM). If anything gets between the head and the recording surface, you can lose data, or even worse, you can damage the magnetic head and the disk's recording surface. Figure 8-3 shows the size relationship between a disk drive's magnetic head, the disk recording surface, and some common contaminants.
Figure 8-3. - Size relationship of distance between head and disk to contaminants.
You must handle, store, and ship floppy disks with great care if you want them to stay in good condition. Here's some specific precautions you should take:
Floppy Disk Erasing
There are two ways to erase a floppy disk: (1) degauss it and then reformat it, or (2) just reformat it. The process for degaussing floppy disks is the same as for degaussing magnetic tape. Refer back to chapter 2 for the details on this.
If the floppy disks were used to store classified, or unclassified but sensitive information, they can't be de-classified by erasing them. This is because, with the right equipment and software, the data that was on the disk can be reconstructed. Floppy disks are cheap and easy to replace. If you can't re-use the floppy disks to store other classified data, just destroy them, using the procedures in OPNAVINST 5510.1, DON Information and Personnel Security Program Regulation.
Q.1 Floppy disks are manufactured in what three sizes?
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