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6.3 COMPUTER SYSTEMS 317

are beyond the reach of the mainframes, and cost more than ten million dollars. These are used
for weather prediction, image processing, and nuclear-energy studies that require high-precision
processing of ordered data achieved by a speed advantage due to parallel processors. Cyber 205,
Cray X-MP, and Cray 2 are some examples of supercomputers. In the 1980s, supercomputing
centers were developed at six American universities for high performance computing. By 1990 it
was possible to build chips with a million components; semiconductor memories became standard
in all computers; widespread use of computer networks and workstations had occurred. Explosive
growth of wide area networking took place with transmission rates of 45 million bits per second.

Organization

There are two principal components:hardwareandsoftware. The former refers to physical
components such as memory unit (MU), arithmetic and logic unit (ALU), control unit (CU),
input/output (I/O) devices, etc.; whereas the latter refers to theprograms(collections of ordered
instructions) that direct the hardware operations.
Figure 6.3.1 illustrates the basic organization of a digital computer. The MU stores both data
and programs that are currently processed and executed. The ALU processes the data obtained
from the MU and/or input devices, and puts the processed data back into the MU and/or output
devices. The CU coordinates the operations of the MU, ALU, and I/O units. While retrieving
instructions from programs resident in the MU, the CU decodes these instructions and directs the
ALU to perform the corresponding processing; it also oversees the I/O operations. Input devices
may consist of card readers, keyboards, magnetic tape readers, and A/D converters; output devices
may consist of line printers, plotters, and D/A converters. Devices such as terminals and magnetic
disk drives have both input and output capabilities. For communicating with the external world
and for storing large quantities of data, a variety of I/O devices are used.
Software may be classified assystem softwareanduser software.The former refers to the
collection of programs provided by the computer system for the creation and execution of the
user programs, whereas the latter refers to the programs generated by various computer users for
solving their specific problems.
Aprogramgenerally consists of a set of instructions and data specifying the solution of
a particular problem. Programs (and data) expressed in the binary system (using 0s and 1s)
are known asmachine-languageprograms. Writing programs in this form, which demands

Data

Control

Control

Address

ALU

(arithmetric

and

logic unit)

CU

(control unit)

CPU

(central processing unit)

Data

Data

Data

Disk

Printer

Input

Terminal

MU

(memory

unit)

I/O

(Input/

output unit)

Figure 6.3.1Typical organization of a computer.