Managing Information Technology

22 Part I • Information Technology

the upper-left corner of each box. The mail stored in each
box changes as mail is distributed or picked up. In comput-
er memory, each memory cell holds some amount of data
until it is changed. For example, memory cell 0 holds the
characters MAY, memory cell 1 holds the characters 2012,
memory cell 2 holds the characters 700.00, and so on. The
characters shown in Figure 2.2 represent the contents of
memory at a particular point in time; a fraction of a second
later the contents may be entirely different as the computer
goes about its work. The contents of the memory cells will
change as the computer works, while the addresses of the
cells are fixed.
Computer memory is different from the post office
boxes in several ways, of course. For one thing, computer
memory operates on the principle of “destructive read-in,
nondestructive read-out.” This means that as a particular
piece of data is placed into a particular memory cell, either
by being read from an input device or as the result of a
computation in the arithmetic/logical unit, the computer
destroys (or erases) whatever data item was previously in
the cell. By contrast, when a data item is retrieved from a
cell, either to print out the item or to use it in a computa-
tion, the contents of the cell are unchanged.
Another major difference between post office boxes
and memory cells is in their capacity. A post office box has a
variable capacity depending upon the size of the pieces of
mail and how much effort postal employees spend in stuff-
ing the mail in the box. A memory cell has a fixed capacity,
with the capacity varying from one computer model to
another. A memory cell that can store only one character of
data is called a byte,and a memory cell that can store two or
more characters of data is called a word.For comparability,

it has become customary to describe the size of memory

(and the size of direct access files) in terms of the equivalent

number of bytes, even if the cells are really words.

Leaving our post office analogy, we can note that

there are several important differences between the mem-

ory of one computer model and that of another. First, the

capacity of each cell can differ. In a microcomputer, each

cell might hold only 1 digit of a number, whereas a single

cell in a large (mainframe) computer might hold 14 digits.

Second, the number of cells making up memory can vary

from several million to several trillion. Third, the time

involved to transfer data from memory to another compo-

nent can differ by an order of magnitude from one

machine to another. The technologies employed in

constructing the memories can also differ, although all

memory today is based on incredibly small integrated

circuits on silicon chips.

BITS AND CODING SCHEMES Each memory cell con-

sists of a particular set of circuits (a small subset of the

integrated circuits on a memory chip), and each circuit can

be set to either “on” or “off.” Because each circuit has just

two states (on and off), they have been equated to 1 and 0,

the two possible values of a binary number. Thus, each

circuit corresponds to a binary digit, or a bit.In order to

represent the decimal digits (and the alphabetic letters and

special characters) for processing by the computer, several

of these bits (or circuits) must be combined to represent a

single character. In most computers, eight bits (or circuits)

represent a single character, and a memory cell containing

a single character, we know, is called a byte. Thus, eight

bits equal one byte in most machines.

MAY

0

2012

1

700.00

2

4

3

OSU

4

17

5

321.16

6

3

7

C

8

OMPU

9

TER

10

32

11

0

12

MARY

13

71.3

14

L

15

27

16

18

17

103.0

18

7

19

JOHN

20

41

21

100.00

22

0

23

0

24

0

25

0

26

37

27

B

28

0

29

62

30

1

31

FIGURE 2.2 Diagram of Computer Memory