Zimsec Computer Science Notes

1. COMPUTER SCIENCE
ORDINARY LEVEL
STUDY NOTES
COMPUTER SYSTEMS
D. Jafali
0773 493 306

2. What is a computer?
A computer is an automatic, programmable,
electronic data processing device.
A computer is an electronic device which
works under the control of stored programs,
it automatically accepts data as input, stores
the data, processes it and then gives out
information as output.

3. Key Terms
 Program: A program is a set of instructions which
directs the operations of a computer. A program tells
the computer what to do.
 Information: Information is data meaningfully put
together, or it can be referred to as data which has
been processed. In computing there is a difference
between data and information. The difference between
data and information is that information is processed
while data are the raw facts still to be processed.

4.  Input: The term input refers data sent into the
computer for processing. You should realise that in
most cases we enter data into the computer and not
information, this is so because the main purpose of a
computer is to process data.
 Store: To store is to keep, retain or save data for later
use, this is achieved through the use of storage devices
such as a magnetic disk.

5.  Process: To process is to make some calculations and
logical comparisons on data. Processing in a computer
is done in the CPU by the Arithmetic and Logic Unit
(ALU)
 Output: Output refers to the information obtained
from the computer after processing data.

6. HARDWARE AND SOFTWARE
Computer systems are a combination of both
hardware and software working together.
Hardware: the physical components of the
computer, such as the central processing unit
(CPU), hard disk, monitor, keyboard and mouse.
Software: a set of instructions
or programs instructing a computer to do specific
tasks. The software in a computer system enables
the hardware to function and the user to perform
tasks. Software within a computer system is divided
into two main types, system
software and application software.

7. System software
System software is any software that assists with the
running or management of the computer system. This can
include operating systems and utilities.
•An operating system is a suite of programs that manages
and controls the computer for example Windows, Linux
and Android.
•Utility software is computer software intended to analyse,
configure, monitor, or help maintain a computer for
example antivirus software, file compression tools and disk
cleanup tools.
Application software
Application software is any software added to the system
that enables the user to perform a task. For example, a word
processor allows the user to type a document and a browser
allows the user to view web pages.

8. Basic Components of a Computer System
What is a Computer System?
A complete collection of components making up a
single computer installation. A computer system
is a set of integrated devices that input, output,
process, and store data and information.

9. Block diagram of a computer system

10. 1. Input device: a peripheral unit that can accept
data, presented in the appropriate machine
readable form, decode it and transmit as
electrical pulses to the central processing unit.
The most common manual input devices are the
keyboard and mouse. Automatic input devices
include sensors and document readers.
2. Central Processing Unit (CPU): the main part
of the computer, consisting of registers, arithmetic
and logic unit and control unit. Usually the central
processing unit includes the main memory. It is
sometimes called the central processor, processor or
microprocessor.

11. -Microprocessor: it is an integrated circuit where
the components of the central processing unit
excluding the main memory are combined as a
single unit.
-Control Unit:-part of the central processor which
manages the execution of instructions. The control
unit fetches each instruction in sequence, decodes
and synchronises it before executing it by sending
control signals to other parts of the computer. This
is known as the fetch execute cycle. The fetch
execute cycle is the complete process of retrieving
an instruction from store, decoding it and carrying
it out.

12. -Register: a location normally used for a specific
purpose, where data or control information is
temporarily stored. Some registers are used in the
different parts of the fetch execute cycle whilst
others may be available for use by the program
being executed. Registers usually are much faster
to access than the immediate access store, since
they have to be accessed so often. The various
registers include:

13. •Program Counter: in the control unit contains
the address of the next machine code instruction to
be executed.
•Address Register: in the control unit holds the
address part of the instruction being executed.
•Memory Address Register (MAR): in the central
processor that stores the address of the memory
location currently in use.
•Memory Buffer Register (MBR): in the central
processor that stores the data being transferred to
and from the immediate access store. It acts as a
buffer allowing the central processor and memory
unit to act independently without being affected by
minor differences in operation.

14. -Arithmetic and Logic Unit: part of the central
processing unit where data is processed and
manipulated. The processing and manipulation of
data normally consists of arithmetic operations or
logical comparisons allowing a program to take
decisions.
3. Main Memory: part of the computer system
where data and instructions are held for use by the
central processor and where the central processor
puts results it generates. Those storage locations
that can be addressed directly by the central
processor are called primary storage or
immediate access storage e.g. Random Access
Memory (RAM).

15. 4. Backing/Auxiliary Storage: a means of storing
large amounts of data outside the immediate access
store. A computer system will have at least one
form of backing storage. Backing storage is
sometimes referred to as Secondary store or Mass
storage.
•Magnetic Disk storage: is backing store in which
flat rotatable circular plates, coated with magnetic
material are used for storing digital data.
•Magnetic Tape Storage: is backing store which
uses plastic tape coated on one side with magnetic
material.
•Optical Disk Storage: is backing store which
uses plastic disks on which data is stored as

16. 5. Output Device: a peripheral unit that translates
signals from the computer into a human readable
form or into a form suitable for re-processing by
the computer at a later stage.
Common output formats are printed paper, sound,
video and on-screen documents. Examples of
devices that take advantage of these formats are
monitor, printers, plotters etc.

17. 1. Draw and label a block diagram of a computer system, showing the five basic
hardware components of a computer. [10 marks]
2. (a) Identify four components of the C.P.U and explain the purpose of each.
Component 1
Purpose
Component 2
Purpose
Component 3
Purpose
Component 4
Purpose
[8]

18. (a) Hardware :
[2]
(b) Utility program:
[2]
(c) Input device:
[2]
(e) Document reader:
[2]

19. 4.