Introduction to information systems T. Cornford, M. Shaikh is1 060 2013

4.3.4 Processor management
The operating system also needs to manage the other main hardware 
resource − the processor itself. In the simple model of a computer that we 
are concerned with, we assume that there is just one processor, and that 
it can do just one thing at a time (real computers − even microcomputers 
have, in reality, a number of processors dedicated to various specific tasks 
such as controlling main memory, doing arithmetic, manipulating graphics 
images, etc.).
The operating system is also a program, so it needs to use the processor 
in order to achieve all the tasks described above. In the case of the 
spreadsheet, the operating system will undertake the task of loading the 
program into the main memory (when the processor is being used by the 
operating system), and then passing control to that program (when the 
processor is then being used by the spreadsheet). When the spreadsheet 
wishes to achieve an input or output task − such as printing some 
information − it passes a request to the operating system.
4.3.5 Program management
The description given above suggests that the operating system actually 
manages one other resource − programs. In the example, there is just one 
processor, but two programs − the operating system and the spreadsheet. 
In a modern microcomputer, there can be many programs, all wishing 
to share the processor. Indeed, hundreds of separate programs may be 
running simultaneously. In such a case, the operating system has to ensure 
that all the programs get an appropriate slice of processor time − using 
it in rotation or when they have specific needs. In general, the operating 
system should be able to pre-empt any other program (for example, push 
to the front of the queue) and use the processor immediately when it 
needs to. In any case it should prioritise other programs to ensure that the 
most important ones get more of the processor resource. This is known as 
pre-emptive multi-tasking.