NUMERIC KEYPAD KEYS To activate the numeric keypad, press the NUM LOCK key. The NUM LOCK key may contain an indicator light within the key to indicate when it is ON. Use the numeric keypad just like a calculator keypad to enter numbers you will be using in mathematical functions.

CURSOR CONTROL KEYS The CURSOR CONTROL keys are most important. They allow you to move the cursor from one place to another on the display screen. Remember, the cursor is the indicator, the spot of light on the screen, which lets the user know where the next entry will be made. On the keyboard shown in figure 1-17, the cursor control keys are located in two locations, both on the numeric keypad and as a separate keypad. When the NUM LOCK key is OFF, you can use the arrows on keys 2,4,6, and 8 to control cursor movement one line up or down, or one position to the left or right, as shown by the direction of the arrow. The remaining cursor control keys on keys 1, 3, 7, and 9 are used to move the cursor to other parts of the screen or document/data (for example, the end of a line [END key] or the top of the next page [PgDn key]). Some software packages use the cursor control keys in combination with each other or with other keys to increase the number of ways and the speed with which you can move the cursor. Read the software documentation; the faster and more efficiently you are able to move the cursor through a document or database, the faster and more efficiently you will get the job done. Some keyboards have separate arrow keys for cursor control. This is particularly helpful when you are doing a lot of data entry of numbers.

SCROLL LOCK KEY Applications software uses the SCROLL LOCK mode of the key to control screen scrolling.

PAUSE KEY The PAUSE key is used to interrupt program execution.

PRT SCreen KEY The PRT SCREEN key is activated by depressing it in conjunction with the SHIFT key depressed. This sends whatever is displayed on the monitor's screen to the printer.

INSert and DELete KEYS These keys allow you to insert or delete a character at the position of the cursor when neither the SHIFT nor the NUM LOCK keys are depressed.

It is important to remember that any key or combination of keys can be assigned special meaning by a program. Therefore, the keys may have different meanings and functions, depending on the program you are using. Once again, we remind you, read all the documentation that comes with each program and with the computer system.

Disks and Disk Drives

Magnetic disks, regardless of their type or size, are, without a doubt, the most important secondary storage medium used with microcomputers. Disks provide fast retrieval of information. The disk's physical characteristics, flat and round, allow the disk drive direct access to data. Put simply, the processing unit goes directly to a designated disk drive, seeks out the specific location on the disk where the data is stored, and immediately retrieves it. The disk drive does NOT have to read through a series of records before reaching the one desired, as is the case with magnetic tape units. The two forms of magnetic disk typically used with microcomputers are the floppy disk (diskette) and the hard disk. Let's look at the sizes and construction of each and at the disk drive devices that read from and write to them.

DISKETTES AND THEIR DRIVES.\ A diskette is also referred to as a floppy disk, or just plain floppy, because it is a round, flexible platter.

Physical Characteristics.\ The platter is enclosed in a plastic jacket. At present, diskettes come in two sizes (diameters): 5 1/4 (5.25) inches and 3 1/2 (3.50) inches (see figure 1-18). The platter is made of a tough plastic that is coated with a magnetic oxide material, allowing it to be magnetized. The inside of the plastic jacket is lined with a soft wiping material that continuously cleans the floppy disk as it spins within the jacket. Look at figure 1-19; you will notice a hole in the middle of the diskette. It is referred to as the center hub. The hub makes it possible for the floppy disk drive's spindle to rotate the diskette inside the jacket. The recording window allows the read-write head mechanism of the floppy disk drive to come into contact with the diskette so it can read from or write data onto the diskette's surface. Located just to the right of the center hub is what we refer to as an index hole. This index hole (and corresponding hole [or holes] in the diskette), marks the first sector or starting point on the diskette. The sectors on a diskette are controlled by timing. On a soft-sectored diskette, the timing is controlled by the software; therefore, only one timing hole is needed on the diskette. On a hard-sectored diskette, the timing is controlled by the hardware, and the diskette has several timing holes.

Types of Floppies/Diskettes.\ The diskettes you will be using must be compatible with the floppy disk drives on your microcomputer system. Diskettes can be soft-sectored or hard- sectored. Depending upon the disk drive's characteristics, the disk drive can record data on one side of the diskette or both and can record data in one of several bit densities, depending upon how the diskette is formatted.

When you are working with a soft-sectored diskette, you must use your microcomputer and a utility routine or program (in this case, a formatter program) to sector or format each diskette for you. On most microcomputer systems, using the FORMAT command will automatically sector the diskette for you.

If you are working with hard-sectored diskettes, then you need not format them. They have already been presectored by the manufacturer for your specific

Figure 1-18.\Floppy disks/diskettes used on microcomputers.

Figure 1-19.\A typical 5.25-inch diskette.

microcomputer system. Floppy disk drives that use hard-sectored diskettes read and write data faster. However, the diskettes are more expensive and can only be presectored (reformatted) for a specific system, such as an IBM compatible or a Macintosh.

Most diskettes sold today are soil-sectored because the wide range of microcomputers and their operating systems vary considerably in respect to sectoring requirements. For now, the important thing to remember about sectoring is the fact that no matter what type of diskette you are working with (soft- or hard- sectored), it must be formatted before it is usable for storing data.

Storage Capacity.\ Although diskettes are relatively small in size, they can store a respectable amount of data. Some diskettes are single-sided, single-density, whereas others are double-sided, single-density; double-sided, double-density; or double-sided, quad-density.

When we refer to a diskette as being double-sided, double-density, what are we really saying? We are saying the floppy disk drive is able to read from and write to both sides of the diskette (hence, the term double-sided), and that each sector on the disk can store 512 bytes of information (instead of the 256 bytes that can be stored on a single-density diskette); hence, the term double-density. A diskette's capacity is also affected by the number of tracks per side. Therefore, if you wanted to know the total storage capacity of a double-sided, double-density diskette with 80 fifteen-sector tracks per side, then you would use the following formula to calculate the number of bytes:

To review, multiply bytes (5 12) times sectors (15) times tracks (80) times sides (2). This gives the total capacity of the diskette, 1.2M bytes.

Write Protect Feature.\ Like all other media, floppy disks/diskettes must also have a way of being protected from a user accidentally writing on them. This is accomplished with a write-protect notch (or cut-out) located in the upper right-hand corner of the plastic jacket (see figure 1-19). Whether or not the write-protect notch is covered to protect the data on the diskette depends solely on the disk drive manufacturer. For example, on most systems using 5 1/4-inch diameter diskettes, covering the write-protect notch makes it impossible to write on the disk's surface. However, the opposite is true on systems using 3 1/2-inch diameter diskettes. Uncovering the write-protect notch makes writing impossible. In short, find out which is which on your particular system before you accidentally wipe out an entire diskette's data.

Floppy Disk Drives.\ Floppy disk drives are manufactured to read and write data in one of several bit densities for both single- and double-sided diskettes. A single-sided disk drive can only read a single-sided diskette. However, a double-sided disk drive, like the one shown in figure 1-20, can read both a single-sided and a double-sided diskette.