Page 111 - Accounting Information Systems

82 PART I Overview of Accounting Information Systems

FI GU RE
2-35 HOW DATA ARE STORED ON A DISK

Record (or block)
number

Record #1 Record #2

Record #3
Track
Record #200

Record
#100

disk pack containing 11 disks has 22 occurrences of Track 100. To protect the data from exposure to
damage, the very top and bottom surfaces of the disk pack are not used, yielding 20 data storage surfaces
for Track 100.
When viewed collectively, the same track on each surface in the disk pack is called a cylinder. There-
fore, in our example, Cylinder 100 contains 20 tracks of data. However, the cylinders on a microcomputer’s
floppy disk or hard disk contain only two tracks because these disks have only two surfaces.

LOCATING A RECORD BASED ON ITS ADDRESS
A disk address consists of three components: the cylinder number, the surface number, and the record (or
block) number. To find a record, the system must know the numeric value for each of these components.
For example, if a record’s address is Cylinder 105, Surface 15, and Record Block 157, the record in ques-
tion could be directly accessed as follows: First, the disk-pack control device moves the read/write heads
into position above Track 105 on each surface (Cylinder 105). Next, it activates the read/write head for Sur-
face 15. Finally, as Record Block 157 passes under the active read/write head, it is either read or written.
The key task in direct access storage and retrieval is ascertaining the record’s address. This may be
determined from tables or calculations based on its primary key. Several direct access techniques are
examined in Chapter 9.