Chapter 2 • Computer Systems 25records in one cell. Because of the existence of this
address, it is possible for the computer to store a record in
a particular file address and then to retrieve that record by
remembering the address. Thus, the computer can go
directly to the file address of the desired record, rather than
reading through sequentially stored records until it
encounters the desired one.
How does the computer know the correct file address
for a desired record? For instance, assume that an inventory
control application running on the computer needs to
update the record for item number 79032. That record,
which is stored somewhere in DASD, must be brought into
memory for processing. But where is it? At what file
address? This problem of translating from the identifica-
tion number of a desired record (79032) to the correspon-
ding file address is the biggest challenge in using direct
access files. Very sophisticated software, to be discussed
later in this chapter, is required to handle this translation.
Online processing (discussed in Chapter 5) requires
direct access files and so does Web browsing. Airline reser-
vation agents, salespeople in a department store, managers
in their offices, and Web surfers from their home or office
machines will not wait (and in many cases cannot afford to
wait) the several minutes that might be required to load and
read the appropriate magnetic tape. On the other hand, batch
processing can be done with either sequential access files or
direct access files. Sequential access files are not going to go
away, but all the trends are pushing organizations toward
increased use of direct access files. Advancements in mag-
netic technology and manufacturing processes keep pushing
down the costs per byte of direct access files. Most impor-
tant, today’s competitive environment is forcing organiza-
tions to focus on speed in information processing, and that
means an increasing emphasis on direct access files.
Control Unit
We have considered five of the six building blocks repre-
sented in Figure 2.1. If we stopped our discussion at this
point, we wouldn’t have much. Thus far we have no way of
controlling these various components and no way of taking
advantage of the tremendous speed and capacity we have
described. The control unitis the key. It provides the con-
trol that enables the computer to take advantage of the
speed and capacity of its other components. The thin
arrows in Figure 2.1 point out that the control unit controls
each of the other five components.
How does the control unit know what to do?
Someone must tell the control unit what to do by devising
a precise list of operations to be performed. This list of
operations, which is called a program, is stored in the
memory of the computer just like data. One item at a time
from this list is moved from memory to the control unit
(note the broad arrow in Figure 2.1), interpreted by the
control unit, and carried out. The control unit works
through the entire list of operations at electronic speed,
rather than waiting for the user to tell it what to do next.
What we have just described is the stored-program con-
cept,which is the most important idea in all of computing.The Stored-Program Concept
Some person must prepare a precise listing of exactly what
the computer is to do. This listing must be in a form that
the control unit of the computer has been built to under-
stand. The complete listing of what is to be done for an
application is called a program,and each individual step
or operation in the program is called an instruction.The
control unit carries out the program, one step or instruction
at a time, at electronic speed.
When a particular computer model is designed, the
engineers build into it (more precisely, build into its circuit-
ry) the capability to carry out a certain set of operations. For
example, a computer may be able to read an item of data
keyed from a keyboard, print a line of output, add two num-
bers, subtract one number from another, multiply two num-
bers, divide one number by another, compare two numbers
for equality, and perform several other operations. The com-
puter’s control unit is built to associate each of these opera-
tions with a particular instruction type. Then the control unit
is told which operations are to be done by means of a pro-
gram consisting of these instructions. The form of the
instructions is peculiar to a particular model of computer.
Thus, each instruction in a program must be expressed in the
precise form that the computer has been built to understand.
This form of the program that the computer understands is
called the machine languagefor the particular model of
computer.FIGURE 2.3 SanDisk Cruzer USB Flash Drive (Courtesy of
SanDisk Corporation)