The Operating System Nick Sims

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

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)

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

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

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

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

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

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

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

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

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

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

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

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

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

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)

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