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The OS determines determine the type of processing that a computer system is able to perform, since it controls the allocation & use of the computer resources.

Operating systems can be classified according to:

  1. Number of tasks that the system can perform concurrently.
    1. Single-tasking (program) operating system.
    1. Multi-tasking operating system.
  • Number of users the system can support at the same time.
    • Single-user operating systems.
    • Multi-user operating systems.
  • Human Computer interface (i.e., how the user & the computer interact).
    • Command line.
    • Menu driven interface.
    • Graphical user interface (GUI).

Classification according to tasks handled concurrently.

Single-tasking OS.

Single-tasking OS allows only one user-program in the main memory to be processed at a particular time.

This means that, the user can only run one interactive program at a time.  The user must then exit from the program before loading & running another program.

Example of a single user OS;

  • MS-DOS.

Multi-tasking (Multiprogramming) OS.

A Multi-tasking OS allows a single CPU to execute/process more than one program, all of which are in memory, at the same time. 

Each program is allocated a time-slice.  In this case, the programs take turns at short intervals of processing time.  The CPU switches its attention between programs as it receives requests for processing, executing statements from one program, and then from another. 

The programs to be run are loaded into the memory and the CPU begins execution of the first one.  When the request is satisfied, the second program is brought into memory and execution starts on the second program, and so on.

Note. In multi-programming, the computer is able to work on several programs at the same time.  It works on the programs on sequence, one after the other, and that at any given instant it executes instructions from one program only.  However, the computer works so quickly that it appears to be executing the programs simultaneously.

Classification according to number of users.

Single-user OS.

A single-user OS is designed to be used by only one person.  It allow only one user/person to operate the machine at a time in an interactive, conversational mode, and runs only one user program at a time, e.g. MS-DOS.

Multi-user (or multi access) OS.

A multi-user OS allows more than one user ( many people) to interactively use/access the computer at the same time.


  • UNIX,
  • Novell Netware,
  • Ms-Windows 2000,
  • Ms-Windows NT,
  • Linux, etc

Classification according to Human Computer Interface (HCI).

The term Human Computer Interface (HCI) refers to the method of interaction between the computer & the user, and determines how easily the user can operate the computer.

The HCI enables communication to & from between the user and the computer.


HCI is expected to be “user-friendly”, i.e., it should be one that the end-user finds helpful, and easy to learn & use.

Features/characteristics of a user-friendly HCI.

  1. It should be relatively easy for the user to try to start using the system.
  1. The system should be self-contained, so that the user is not forced into accessing manuals.
  1. The amount of effort & the information required for the user to get the system complete required tasks should be minimal.
  1. The system should be robust & reliable, i.e., the user should be protected from unexpected system actions, including system failures.
  • The system should be able to adjust to different levels of expertise between users & also as users grow in competence.
  • The user should be made to feel in control of what is going on.
  • The system should behave in a logical & consistent manner, enabling the user to reason about what is going on and apply what has been learned.

Types of User interfaces.

There are different types of Human Computer Interfaces: –

  1. Command driven interface.
  2. Menu driven interface.
  3. Graphical User Interface (GUI).

Command driven interface.

This is an interaction between the user & the computer that requires the use of commands

The user types a command at the prompt found on a command line.  The computer then reads instructions from the command line and executes them.


To copy a file called Fruits.Dat from Hard disk C to Floppy disk A using MS-DOS; type

C:\ >COPY Fruits.Dat A:\ (press the Enter key for the command to be executed).

Commands enable the user to quickly instruct the computer what to do.

Command-driven software is more flexible, but it is more difficult to learn.  The user must know what commands are available, what they do & how they should be typed.  For this reason, commands are most popular with experienced technical persons, such as computer Operators, Programmers or in situations where the end-user continually works with the same program and has therefore mastered the commands.

To make commands more user-friendly, the following points need to be observed: –

  1. The command words used should be descriptive VERBS that clearly convey the intended action, e.g., PRINT, COPY, RENAME, DELETE, etc.
  • Unique abbreviations should be provided for more experienced users, e.g., PRI, COP, REN, DEL, CHKDSK, etc.
  • Multiple items on a single command line should ALWAYS be separated by blank spaces.

E.g., PRINT can be used in the following ways: –

  • PRINT Report1 – prints the named document on the default printer.
    • PRINT Report1 Report2 Report3 – prints the three documents on the default printer.

Examples of Command line interfaces:

  • MS-DOS,
  • Early versions of PC-DOS, OS/2, and UNIX.

Disadvantages of using command driven interfaces

  • They are more difficult to learn.
  • The user must know the command to type.
  • It is less user-friendly.
  • It is not easy to use, i.e., one is required to master the command format/syntax.

Menu driven interface.

This type of interface provides the user with a list of program commands displayed on the screen  to choose from & a simple means of selecting between them.

To activate a choice in the menu, one can use the Enter key, or move the cursor until it is positioned at the desired choice & then press the activation key so that the system can start acting upon the information given.

This interface is suitable for beginners and infrequent users who may have difficulties in remembering commands.

There are 2 types of menus: –

  • Pull-down menus – are special types of menu used mostly in Windows.
  • Pop-up menus & Pop-down menus.  These menus are made to appear above or below an item on the screen in order to elicit/obtain a choice from a user.

Later versions of DOS have a menu driven interface called the DOS Shell or DOS Editor.

Advantages of Menu driven interfaces

  • Menus provide many options to select from.
  • The user is presented with a choice and therefore, does not need to master any commands.
  • They are easier to use.

Graphical User Interface (GUI).

This is an interaction between the user & computer that involves issuing of commands to the computer by activating certain small graphic images displayed on the screen called Icons.

To issue a command, the icons can be selected using a pointing device like a Mouse.

GUI is mostly found on Workstations or PCs fitted with graphic adapters able to support high-resolution graphics.

Examples of GUI based OS;

  • Presentation manager of OS/2,
  • Ms-Windows,
  • Linux,
  • Apple Macintosh.

Features of a graphical user interface.                                            (2 marks)

  • Programs are represented graphically by use of Icons.
  • Commands are selected and issued using pointing devices, e.g., Mouse, trackball.
  • There is use of pull-down menus.
  • Programs open by displaying windows.

Advantages of using GUI based OS

  • They are user friendly.
  • Easy to learn & use.

Revision Questions

  1. What criteria are used to classify types of operating systems?
  2. (a)  Differentiate between multi-user and multitasking operating systems.

(b)  State any computer software that can be classified as a Multi-user operating system.

  • Name three types of user interfaces employed by different commercial Operating systems.
  • (a)  Differentiate between command-line interface and graphical user interface operating systems based on the way commands are entered.

(b)  State two main advantages of GUI interfaces.

Factors to consider when choosing an operating system.

The following factors should be considered when choosing an operating system for a computer;

  1. Hardware configuration of the computer, e.g., RAM memory size, hard disk capacity, type of processor, etc.
  • Basic design of the computer, – i.e., is it an IBM or IBM compatible, or an Apple computer?
  • Hardware compatibility.
  • User needs (requirements), i.e., the applications intended for the computer.
  • User friendliness or Human computer interface, i.e., is it Command line based, Menu-driven or a Graphical user interface?
  • Availability in the market, e.g. Microsoft Windows based OS are very common.
  • Portability.
  • Cost – how expensive the OS is.
  • Reliability, i.e., can it run without crashing or hanging (stop responding to commands).
  1. The method of communication with the computer, e.g. the number of peripherals.
  2. The method of operating the computer.

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