1. The Operating System
Nick Sims

2. The beginnings of the OS
If an application required input, the program itself would have to conta9n the code to do this
This was the same for output
This was the same for everyone program and there was multiple occurrences within a program

3. The beginnings of the OS
A short piece of code (called subroutines was developed to handle these simple tasks
Reading in character form a keyboard
The joining together of the input and output routines led to the input-output control system (IOCS – circa 1950s)

4. The beginnings of the OS
Originally IOCS became more powerful as magnetic tape and disks were created
Then assembly and higher level languages were created – machines only used binary to understand instructions

5. The beginnings of the OS
Originally IOCS became more powerful as magnetic tape and disks were created
Then assembly and higher level languages were created – machines only used binary to understand instructions

6. The beginnings of the OS
So computers required translators programs (assembly and compiler)
To make use of the speed of new processors, more than one program could be stored in memory
Processor give time to each program
So if 2 programs are stored in memory and one is using an input and output device it made sense for the other program to use the processor

7. The beginnings of the OS
So computers required translators programs (assembly and compiler)
To make use of the speed of new processors, more than one program could be stored in memory
Processor give time to each program
So if 2 programs are stored in memory and one is using an input and output device it made sense for the other program to use the processor

8. The O. S. It is a program What is a program?
It is set of instructions
The most frequently used portion of instructions in the O.S. must be stored in main memory (RAM) and remain there whilst other programs such as application programs are executed

9. O.S. This portion of the O.S. has many different names such as:
control program
nucleus
monitor
supervisor or executive

10. Loading an O.S.
On large computers and micros, the O.S. is kept on disk and has to be loaded into the main memory once the computer is switched on, before any other program is run
The process of loading the O.S. is called booting up the system

11. Loading an O.S.
On a microcomputer system the O.S. is usually held on the hard disk
A small program held in ROM (the loader) will tell the computer where to look for the O.S. and give instructions for loading at least part of it into memory

12. Loading an O.S.
Once the O.S. is loaded then more instructions can be executed to load the rest of the nucleus
On small hand-held computers the control program is permanently held in ROM

13. Modes of Operation
O.S vary a lot in their capabilities, from relatively simple single-user microcomputers to sophisticated mainframe computers.
There are various modes of operation

14. Single-User Single-Process
The O.S. supervises the loading and running of one program at a time, and the input and output of data to and from peripheral devices

15. Multi-Programming
Defined as the apparent simultaneous execution of two or more programs
A multi-programming O.S. enables two or more programs to be held in memory at the same time, with each program being given a small amount of processor time before moving on to the next

16. Multi-Programming
This system makes good use of processor time because when one program is held up waiting for input or output, the processor can be allocated to another program
The job of the O.S. here is to maximise throughput: ensuring that all jobs are completed in a reasonable time

17. Multi-User
A multi-user system is defined as one that allows two or more users to communicate with the computer at any one time
Each user interacts with the computer via a terminal (has a keyboard and a monitor)

18. Multi-User
While some users are typing at the keyboard or using a disk or printer, the processor is working on the other user’s programs
As long as there are not too many users on the system, the user can think that they are the sole user