Chapter 2 • Computer Systems 23Consider a particular example. Assume that we have
a computer where each memory cell is a byte. (A byte can
contain one character.) Then memory cell number 327, for
instance, will consist of eight circuits or bits. If these cir-
cuits are set to on-on-on-on-on-off-off-on (or, alternatively,
1111 1001), this combination may be defined by the
coding scheme to represent the decimal digit 9. If these
bits are set to 1111 0001, this may be defined as the deci-
mal digit 1. If these bits are set to 1100 0010, this may be
defined as the letter B. We can continue on like this, with
each character we wish to represent having a correspon-
ding pattern of eight bits.
Two common coding schemes are in use today. The
examples given earlier are taken from the Extended Binary
Coded Decimal Interchange Code (commonly known as
EBCDIC, pronounced eb’-si-dic). IBM originally devel-
oped EBCDIC in the 1950s, and IBM and other vendors still
use it. The other common code in use is the American
Standard Code for Information Interchange (ASCII), which
is employed in data transmission and in microcomputers.
The bottom line is that a coding scheme of some sort
is used to represent data in memory and in the other com-
ponents of the computer. In memory, circuits in a particular
cell are turned on and off, following the coding scheme, to
enable us to store the data until later. It turns out that cir-
cuits are also used to represent data in the control and
arithmetic/logical units. In the input, output, and files, the
coding scheme is often expressed through magnetized
spots (on and off) on some media, such as a disk. In data
transmission, the coding scheme is often expressed
through a series of electrical pulses or light pulses. In sum-
mary, the coding scheme is vital to permit the storage,
transmission, and manipulation of data.
Arithmetic/Logical Unit
Thearithmetic/logical unit,like memory, consists of
incredibly small integrated circuits on a silicon chip. In
many respects, the arithmetic/logical unit is very simple. It
has been built to carry out addition, subtraction, multipli-
cation, and division, as well as to perform certain logical
operations such as comparing two numbers for equality or
finding out which number is bigger.
The broad arrows in Figure 2.1 represent the way in
which the arithmetic/logical unit works. As indicated by
the broad arrow from memory to the arithmetic/logical
unit, the numbers to be manipulated (added, subtracted,
etc.) are brought from the appropriate memory cells to the
arithmetic/logical unit. Next, the operation is performed,
with the time required to carry out the operation varying,
depending on the computer model. The speeds involved
vary from millions of operations per second up to trillions
of operations per second. Then, as indicated by the broad
arrow from the arithmetic/logical unit to memory in
Figure 2.1, the result of the operation is stored in the
designated memory cell or cells.Computer Files
As applications are being processed on a computer, the
data required for the current computations must be stored
in the computer memory. The capacity of memory is limit-
ed (although it can go over a trillion bytes on some large
machines), and there is not enough space to keep all of the
data for all of the concurrently running programs
(e.g., Microsoft Excel, Microsoft Word, Adobe Photoshop)
in memory at the same time. In addition, memory is
volatile; if the computer’s power goes off, everything
stored in memory is lost. To keep vast quantities of data
accessible within the computer system in a nonvolatile
medium and at more reasonable costs than main memory,
file devices—sometimes called secondary memory or sec-
ondary storage devices—have been added to all but the
tiniest computer systems. File devices include magnetic
tape drives, hard (or fixed) disk drives, removable disk
drives, flash drives, and CD or DVD (optical) drives. All
but the optical drives record data by magnetizing spots on
the surface of the media, using a binary coding scheme.
The broad arrows in each direction in Figure 2.1
illustrate that data can be moved from particular cells in
memory to the file and that data can be retrieved from the
file to particular memory cells. The disadvantage of files is
that the process of storing data in the file from memory or
retrieving data from the file to memory is quite slow rela-
tive to the computer’s computation speed. Depending upon
the type of file, the store/retrieve time may vary from a
very small fraction of a second to over a minute.
Nevertheless, we are willing to live with this disadvantage
to be able to store enormous quantities of data at a reason-
able cost per byte.SEQUENTIAL ACCESS FILES There are two basic ways
to organize computer files: sequential access and direct
access. With sequential access files,all of the records that
make up the files are stored in sequence according to the
file’s control key. For instance, a payroll file will contain
one record for each employee. These individual employee
records are stored in sequence according to the employee
identification number. There are no addresses within the
file; to find a particular record, the file device must start at
the beginning of the sequential file and read each record
until it finds the desired one. It is apparent that this method
of finding a single record might take a long time, particu-
larly if the sequential file is long and the desired record is