The storage capacity of a disk depends on the bits per inch of track and the tracks per inch of surface. Using Winchester technology, the designers of disk drive units were able to increase the data density of a disk by increasing the number of tracks. Winchester was the code name used by IBM during the development of this technology. The designers originally planned to use dual disk drives to introduce the new concept. Each drive was to have a storage capacity of 30 million characters, and thus was expected to be a "30-30." Since that was the caliber of a famous rifle, the new product was nicknamed "Winchester." The designers found that data density could be improved and storage capacity increased by reducing the flying height, the distance of the read/write heads over the disk surfaces when reading and writing. By doing this, smaller magnetized spots could be precisely written and then read. The read/write heads were moved so close to the disk that a human hair looked like a mountain in the path of the flying head. Winchester technology reduces this potential problem by sealing the disks in a contamination-free container. This eliminates foreign objects from coming in contact with the read/write heads.
Data can be physically organized in one of two ways on a disk pack, depending on the manufacturer and the model of disk drive you are using. One way uses the cylinder method, and the other uses the sector method. On diskettes, data is organized using the sector method.
The cylinder method uses a cylinder as the basic reference point. When you look at figure 2-10, view A, you will see a disk pack containing six disk platters with 10 recording surfaces. Imagine you are looking down through the disk pack from the top. All the tracks with the same number line up vertically. Together they are called a cylinder. These 10 tracks, one on each recording surface, can be referenced by the 10 read/write heads on the five access arms at each discrete location where the access arms can be positioned.
Figure 2-10A. - Physical organization of data on a disk. CYLINDER METHOD.
Therefore, to physically reference a record stored using the cylinder method, a computer program must specify the cylinder number, the recording surface number, and the record number as shown in figure 2-10, view A. Here, the record is stored in cylinder 25 of recording surface 6 and is the first record on that track. Special data stored on each track specifies the beginning of the track so that the first record, second record, third record, and so on, can be identified.
Another way to physically organize data on the disk pack (and on diskettes) is to use the sector method. This requires that each of the tracks be divided into individual storage areas called sectors (shown in figure 2-10, view B). The number of sectors varies with the disk system used; however, there are usually eight or more. Each sector holds a specific number of characters. Before a record can be accessed, a computer program must again give the disk drive the record's address specifying the track number, the surface number, and the sector number of the record. One or more read/write heads are then moved to the proper track, the head over the specified surface is activated, and the data is read from or written to the designated sector as it spins under the head.
Figure 2-10B. - Physical organization of data on a disk. SECTOR METHOD.
Another type of storage device is magnetic tape which is similar to the tape used with commercial tape recorders. It is used mainly for secondary storage. It differs from commercial tape in that it is usually wider (ranging from one-half inch to an inch), and it is manufactured to more rigid quality specifications. It is made of a MYLAR base coated with a magnetic oxide that can be magnetized to store data. Magnetic tape comes in a variety of lengths (from 600 to 3,000 feet), and is packaged in one of three ways: open reel, cartridge, or cassette, as shown in figure 2-11. Large computers use standard open reels, 1/2-inch wide tape, 2,400 feet in length. Magnetic tape units are categorized by the type of packaging used for the tape. The tape unit (or drive) shown in figure 2-12 uses open reels, while cartridge tape units use tape cartridges and cassette units use tape cassettes. Cartridge tape units are often used on personal computers to provide backup for hard disk.
Figure 2-11. - Various types of magnetic tape storage.
Figure 2-12. - Mounting a magnetic tape
A standard 1/2-inch tape may have either seven (fig. 2-13, view A) or nine tracks (fig. 2-13 , view B) of data stored on it, depending upon the particular read/write heads installed in the tape unit. Read/write heads are usually designed to read (or write) data (in the form of bits) concurrently across the width of the tape.
Figure 2-13. - Multi-track magnetic tape.
The amount of data or the number of binary digits (0 and 1 bits) that can be written (stored) on a linear inch of tape is known as the tape's recording density. Common recording densities for multitrack tapes range from 200 to 6,250 bits/bytes per inch (BPI). Also note that sometimes the density of a tape is referred to as the number of frames per inch (FPI) or characters per inch (CPI) rather than BPI. Regardless of which term is used, a frame or byte is a group of related bits that make up a single character written across the width of the tape. Most magnetic tape units are capable of reading and writing in several different densities.
Magnetic tapes have many common features and data recording formats. Each tape is physically marked in some manner to indicate where reading and writing on tape is to begin (known as the beginning-of-tape [BOT]), and where it ends (known as the end-of-tape [EOT]). The length of tape between the BOT and EOT is referred to as the usable recording (reading/writing) surface or usable storage area. BOT/EOT markers are usually made of short silver strips of reflective tape (1/4-inch wide by 1/2-inch long) as shown in figure 2-14. The BOT marker is normally placed toward the front edge of the tape (the side nearest you when the tape is mounted on the tape unit). The EOT marker is placed toward the back edge (the side farthest from you when the tape is mounted on the tape unit). They are placed approximately 15 to 20 feet in from each end on the shiny side of the tape. Sometimes, holes or clear plastic inserts are used as markers in place of reflective strips. Regardless of the method used, the BOT/EOT markers are sensed by an arrangement of lamps and/or photodiode sensors to indicate where reading and writing is to begin and end.
Figure 2-14. - Beginning-of-tape (BOT) and end-of-tape (EOT) markers.
We can make records on magnetic tape any size we need to hold the data. We are restricted only by the length of the tape or the capacity of internal storage. For example, a record can be one character, several characters, or thousands of characters in length. The collection of records is called a file. A file containing payroll records is called a payroll file; a file containing supply inventory records is called a supply inventory file.
Records can be placed on tape either separately as single records (unblocked) as shown in figure 2-15, view A, or multiple records can be grouped together (blocked) as shown in figure 2-15, view B, to form a record block. The number of records stored in a record block is the blocking factor. In this example, the blocking factor is five.
Figure 2-15. - Record formats on magnetic tape.
All magnetic tape must be moving at a predetermined speed for data to be read from or written on the tape. Data cannot be read or written while the tape is coming up to speed, slowing down, or stopped. During this time delay, the tape moves a short distance creating a blank spot on the tape. This interrecord gap or interblock gap separates each single record or block of records on the tape. The length of the gap varies, depending upon the particular system and method of recording, but is approximately 2/5 to 3/4 inch in length. If single records are stored on the tape, the interrecord gap may be longer than the portion of tape used to store the record. Therefore, much of the tape's recording surface is wasted.
To overcome the inefficiency of storing single data records, we normally block records. In figure 2-15, view B, you will notice the tape is used more efficiently than the tape in figure 2-15, view A. Blocking allows more data to be stored on a reel of tape.
During reading, the record begins with the first character sensed following an interrecord or interblock gap and continues until the next gap is reached. All input records read are internally stored in accordance with the amount of storage area set aside by the applications program.
Magnetic tape, as a storage media, offers several useful features. We can store large amounts of data in a variety of convenient package sizes (open reels, cartridges, or cassettes). Magnetic tapes are easily interchangeable between similar tape units of different computer systems, and tapes are less prone to damage than other types of storage media.