Page 110 - Accounting Information Systems

CHAPT E R 2 Introduction to Transaction Processing 81

FI G U R E
2-34 RECORDS BLOCKED ON A MAGNETIC TAPE

Interblock Gaps (IBG)

Record 1 Record 2 Record 3 Record 100 IBG

Blocks of Records

the operating system for a piece of data to when it is read into the computer is called access time. The
access time of a particular hard disk is a function of several factors: (1) the seek time—how fast the read/
write head moves into position over a particular track, (2) the switching time—the time needed to activate
the read/write head, (3) the rotational delay time—the time it takes to rotate the disk area under the read/
write head, and (4) the data transfer time—the time it takes for the data to be transferred from the disk
track to primary storage. Most microcomputer hard disks have an access time of 5 to 60 milliseconds.
The file allocation table is an area on the disk that keeps track of the name of each file, the number
of bytes in the file, the date and time it was created, the type of file, and its location (address) on the disk.
A file may be stored in only one place on the disk, or it may be spread across several locations. In the lat-
ter case, the read/write head of the disk must skip among various addresses to read the entire file into the
primary memory.
We have used the term address several times to represent a disk storage location. Let’s now exam-
ine the elements of a disk address.

DISK ADDRESS
As we have seen, the surface of a disk is divided into magnetized tracks that form concentric circles of
data. The floppy disk for a microcomputer may have 40 or 80 tracks on a surface, while the surface of a
mainframe disk could contain several hundred tracks. These tracks are logically divided into smaller
blocks or record locations where data records reside. Each location is unique and has an address—a
numeric value. Depending on the disk’s size and density, hundreds or thousands of records may be stored
on a single track. Figure 2-35 shows data storage on a disk. For illustration purposes, the physical size of
the records is greatly exaggerated.
The concept of an address applies to all types of magnetic disks, including individual floppy disks
and hard disks used in microcomputers and the larger mainframe disk packs. A difference lies in the way
the disks are physically arranged. Mainframe disks are often stacked on top of one another in a disk-pack
arrangement that resembles a stack of phonograph records. Figure 2-36 illustrates this technique. The
disks are mounted to a central spindle that rotates at over 3,500 revolutions per minute. Each disk surface
is provided with a separate read/write head that is used for storing and retrieving data.

DATA STORAGE ON A DISK PACK
Every disk in a disk pack has two surfaces with the same number of tracks on each surface. Figure 2-36
shows that Track 100 exists on the top and bottom surfaces of each disk in the disk pack. Therefore, this