1. THE OPERATION SYSTEM The need for an operating system
Layered view of a computer
Functions of the operating system
Types of operating system
Microcomputer operating systems

2. Need for an OS
Computer hardware and machine code only is vary hard to use
Users have to communicate with the computer
Many users want to use resources of the computer simultaneously
Programs may interfere with others’ programs or the overall system
Many users may know little about computers

3. Layered View of a Computer
hardware
machine code
operating system
systems programming language
compiler / interpreter
applications programming language
applications package

4. Types of OS batch processing (jobs in a queue)
multiprogramming (or multitasking)
time sharing
real-time processing

5. Need for an OS Operating System
is a collection of system programs that supervise, control and coordinate the resources and overall operation of the computer system
Provides an interface between the user, the computer and the applications.

6. Functions of an OS Supervise programs while they are running
scheduling of jobs, overcome problems
Communicate with users (e.g. display messages)
Provide utilities to manage the computer system
File manager, utility tools
Optimize the use of resources of the computer system
hw/sw resources(CPU, memory, I/O devices), priorities

7. Functions of an OS Keep a log of actions taken on all programs
Records important events(e.g. illegal operation, errors)
Maintain security
User accounts and passwords, file security among users and groups
Handle errors

8. OS Communicate with users Supervise programs Provide untilities
Keep log of events
Optimize use of resources
Handle errors
Maintain security

9. Batch processing
jobs are put in a batch queue until it is their turn to be run
one job cannot start until the previous one is complete
most suitable for ‘long’ programs or processes involves large volume of data
waste of CPU time as it is waiting for the operation of peripheral devices
 spooling

10. Multiprogramming (multitasking)
a no. of different jobs are handled at the same time by the CPU
the OS makes use of the time during which a task is waiting for input/output operations to work on another program
the capacity of a given computer increases
support both batch & interactive jobs

11. Time sharing (multi-access)
due to the fact that the CPU is not in use for most of the time
hundreds of terminals are connected by telephone lines to the central computer
OS allocate time slice to each user to interact with the CPU (in the central computer) whenever needed
supports multiprogramming

12. Real-time processing computer responds in real time
data are processed immediately after input
the computer system is dedicated to 1 application only
for applications which a very fast response is needed (no delay for even 1 sec.)
e.g., anti-missile defence system, aeroplane landing control system

13. Microcomputer Operating Systems
OS programs are commonly stored on
a floppy disk or
a hard disk
Disk Operating System (DOS)
before using a microcomputer, one must
load the OS or
‘boot’ the system
OS then is transferred from disk to main memory, the system is activated - ‘alive’
Kernel – the core part that always reside in mem.

14. Examples of OS DOS 3.3 (Apple II, 1980’s) PC DOS, MS DOS (IBM XT/AT)
Window 3.1
Window 95, 98, Me, NT, 2000
Mac OS
Linux
Unix (minicomputer to mainframe)