Outcome 1:

Describe the structure and function of an operating system.

Systems Software

Computer

System

Hardware Software

Systems

Software

Applications

Software

Examples of Software types

Applications Software:Word Processor, Spreadsheet,

Database, Desktop Publishing

Systems Software:Compilers, InterpretersSystem utilitiesOperating systems

Operating Systems

An operating system is a collection of system programs that together control, manage and monitor the operation of a computer system.

Examples:• Controls the execution of applications software• Controls the operation of the hardware• Monitors the use of the hardware• Provides a user-interface

Operating System

User

Applications

Software

Operating

System

Hardware

An operating system can be regarded as providing a layer between the user, applications and the underlying hardware.

Types of Operating Systems

o General Purpose• Used in computers. • Provide the ability to run a number of

different applications.• Examples: all versions of Windows

o Embedded Systems• Do not manage user programs at all• Perform a well-defined task, usually in a real-

time environment• Used to control hardware devices such as

cars, electronic appliances etc.• Examples: DVD players, car fuel injection etc.

Types of Operating Systems

Types of Operating Systems

Single-user Multi-user

Single-tasking Multi-tasking Multi-tasking

Real-time

Types of Operating Systems

Single-user, single-taskingExample: MS-DOS

Single-user, multi-taskingExample: Windows XP

Multi-user, multi-taskingExample: Linux

Real-timeExample: OSE, QNX

Functional Areas of an OS.

User Interface

File Management

Memory Management

Kernel

Input Output

The Kernel

• Part of the O/S that is responsible for process management, multi-tasking and interrupt handling.

• Loads and runs other parts of the o/s when required.

• Must reside permanently in memory during computer operation.

Processes

ProcessA program in execution.The OS maintains additional information about running

programs (location in memory, stack pointer, program counter).

When a program is run, the operating system converts it into a process. It is the process, rather than the simple program, that the processor executes.

Foreground ProcessA process that currently uses the screen and keyboard

Background ProcessA process that does not require access to the screen and keyboard

The Boot Process

The core of an OS (known as the kernel) must be present in main memory.

At power-on, • the basic hardware is checked (POST)• The video display is enabled• ROMs found on adapter cards are executed• A bootable drive is searched for and when

found its volume boot record loads the OS kernel.

• The kernel then loads and prepares the remainder of the O/S.

These power-on activities are carried out by the ‘boot’ program.

This program is held on ROM (in the BIOS chip) on the motherboard.

The Boot Process

Storage of the Operating System

Storage of an O/S in a computer system may be either on

• ROM chip

Or

• Hard Disk

ROM based O/S

The operating system is held on a ROM chip. The ROM is configured as part of the main memory.

Advantages:• The O/S is immediately in control at power up.• Cannot be corrupted

Disadvantage• Updates to O/S require replacement of ROM chip

Most usually found in embedded systems.

Disk based O/S

The O/S must be loaded from disk into main memory when the computer is powered up. This is called ‘booting’ the O/S.

Advantages:• The O/S is easily updated• No limit to the size of the O/S

Disadvantages:• The O/S is easily corrupted• Loading of the O/S may take a considerable time.

Single-user, single-tasking O/S

Definition: O/S that allows a single user to carry out a single task at any one time.

CPU: Only one program is allowed to run on the CPU at any one time. Programs must run to completion.

Memory: Only the currently running program can reside in memory. It has access to all of memory (apart from the O/S)

Single-user, single-tasking O/S

I/O: The currently running program is given full dedicated access to all I/O devices.

Security: Very few security measures are required.

Single-user, multi-tasking O/S

Definition: O/S that allows a single user to carry out a one or more tasks at any one time.

CPU: Programs compete to run on the CPU. The O/S multi-tasks the programs giving the appearance that programs are running simultaneously.

Single-user, multi-tasking O/S

Memory: Programs compete for access to memory. The currently running program must be in memory. Other programs may be swapped to disk to make room.

I/O: Some devices can be shared by multi-tasking programs (e.g. hard disks). Others must be dedicated to a program until task completion. (e.g. printer)

Security: Few security measures are required.

Multi-user, multi-tasking O/S

Definition: O/S that allows one or more users to carry out one or more tasks at any one time.

CPU, Memory: Management similar to a single-user, multi-tasking O/S.

I/O: Normally supports a large number of devices e.g. terminals that each user can use to communicate with the O/S.

Multi-user, multi-tasking O/S

Security: Uses a variety of techniques:Login proceduresUsernames / PasswordsFile Access RightsHome directories

Data sharing: Multi-user O/S uses:File ownershipFile access rightsEmailto allow users to share information

Real-time O/S

Definition: O/S where the predictability of the speed of response and reliability are essential.

Usually part of a feedback loop: Responds to events so that it can influence those events.

Real-time O/S

Often found in embedded systems within process control.

Can be single-user or multi-user.

Normally multi-tasking.

Real-time O/S

CPU, Memory, I/O, Security:Similar management of these resources depending on whether single-user or multi-user.

Reliability: Often incorporates support for redundant components.e.g. multiple hard disks and processors, for mission-critical applications.

Predictability: A predictable response time is essential in a real-time o/s.