Computer Science - Operating System All the jobs and aspects of the operating system are explained and defined. The 5 main jobs of the operating system are outlined, this includes scheduling, managing input and output, memory management, virtual memory and paging and file management.

1. Computer Science
(A Level)
Operating System

2. A suite of programs (collection of
software) designed to control the
operations of the computer
(Definition)

3. • The operating system acts as an interface between the
user and the computer and manages the overall
operation of the computer.
• It links together the hardware, the applications and the
user.
Operating System

4. • Controls the start-up configuration of the computer
• Recognise mouse clicks and responds with an action
• Send signals to the hard disk controller, telling it what program to transfer to
memory
• Decides which sections of the memory to allocate to the program you intend to use
and manages memory to ensure all the programs you want to run are allocated the
space they need
• Attempts to cope with errors as and when they occur
• Make sure the computer shuts down properly
• Manage print queues
• Manage users on the network
The Operating System carries out many tasks:

5. How an operating system manages
hardware and software to optimise the
performance of the computer
(Definition)

6. What an OS can do?
Scheduling
Manage Input
and Output
Memory
Management
Virtual Memory
and Paging
File
Management

7. A technique to ensure that different users or
programs are able to work on the same
computer system at the same time.
(Definitions)
A device that carries out computation on
data by following instructions, in order to
produce an output.

8. A computer system may seem like it is multi-tasking by
appearing to run multiple programs at once, but in fact, a
single processor system will swap programs in and out very
quickly so that they appear to run simultaneously.
Scheduling:
The operating system needs to schedule access to the
processor, this is done by allocating time slices to each
task or program. This means each task is given an equal
amount of processor time.

9. The operating system controls the way in which the various input and
output devices are allocated, controlled and used by the programs
that are using them.
Managing Input and Output:
Accessing some devices may be relatively slow compared to the speed
at which the processor can handle requests. Therefore, the OS can
effectively create a queue of commands that are waiting for the device
and then handle each sequence in sequence or based on priority.
Examples
Allocating print jobs to printers
Rendering
Controlling read/write access to hard drive

10. Every input and output device has a device driver, this is often built into
the OS or installed when new devices are attached. When the OS starts up
it loads the various drivers for all the input/output devices that it detects.
Managing Input and Output:
(Definition)
A piece of software that enables the device
to communicate with the OS

11. How the operating system uses RAM
to optimise the performance of the
computer.
(Definition)

12. • The operating system stores details of all the unallocated locations in a
section of memory known as the HEAP.
• When an application needs some memory, it is allocated from the heap,
once it is closed or finished with the memory location, the unneeded
memory locations return to the heap.
Memory Management
The OS controls the use of the main memory by creating
a memory map. This way an OS can control more than
one task in the RAM at any one time.
The amount of memory needed for each task is dependant on
the size of the program itself.

13. It shows which block of memory
have been allocated to each task.
(Definition)

14. • An application or file may be too big to fit into the available RAM, this is when
virtual memory is used.
• Secondary storage (e.g. hard disk) is used to store code or files that would
normally be held in the RAM. The OS will then treat that part of the secondary
storage as if it was part of the RAM.
Virtual Memory
• The Kernel or Central Block (main part) of the code is held in the RAM. The rest of
the code (also known as ‘Pages’) are loaded from the secondary storage when
needed.
• This method allows large applications to run in a small section of the RAM, this can
also then free up memory for other applications to use.
Paging

15. How an operating system stores
and retrieves files.
(Definition)

16. File Management
• The OS has to manage files, this includes controlling the structures that
are used to store the files.
• Usually, files may be saved in folders of folders, this arrangement is
known as hierarchical structure.
• As the hard disk gets larger and larger, it becomes increasingly common
to split up or partition a hard disk. Although you still have one hard
disk, the OS will split it up into a number of partitions or logical drives.
Therefore, the computer will seem to be fitted with multiple hard
drives. You may use this system to store applications on one logical
drive and data on another.