# A-level Computing 2009/AQA/Problem Solving, Programming, Operating Systems, Databases and Networking/Operating Systems/Operating system classifications

There are several different operating system classifications you need to know. You need to be able to compare and contrast them, as well as to identify the correct system required for a given problem or situation.

## Interactive

In an interactive operating system, the user interacts directly with the operating system to supply commands and data as the application program executes and the user receives the results of processing immediately. The user is in direct two way communication with the computer.
Example: An ATM.

## Real time

A real time operating system will give guaranteed feedback within a predefined set period of time, therefore taking away the unpredictability of the operation being carried out. This allows for the system to be mathematically modelled, to allow the time needed for operations to be carried out, to be determined, even under the predicted maximum stress levels. It allows deadlines to be set on operations being carried so as to provide a "best of" result within the time period able to be given for an operation.
Example: If a real time operating system was used to control a plane's rudders (via a RT application) it would need to set deadlines on the calculations used to calculate rudder movements. Without these deadlines a calculations results may be returned too late and cause the plane to crash.
One very important factor to remember about real time operating systems and applications is that "the right answer late is wrong". After all, what is the point in having the correct answer to a plane's rudder calculation if it comes too late, and the plane has already crashed because the rudder angle was not changed earlier?

## Network

A layer of software is added to the operating system of a computer connected to the network. This layer intercepts commands that reference resources elsewhere on the network, e.g. a file server, then redirects the request to the remote resource in a manner completely transparent to the user.

## Device

Not all computers have operating systems. If a computer only needs to perform one task, then this can be done most efficiently with just one firmware program running all the time. It would be overkill to add an operating system, adding complexity where it isn't needed and increasing production costs.
Devices that carry out more than one task benefit from an operating system. Some of the benefits of an operating system are:
* the device is able to multi task.
* the device has the ability to operate in real-time when necessary.
* the hardware can be changed or upgraded without the application code needing to be changed.
* it is easier to add new applications.
* changes to the functionality of the system can be implemented by upgrading the operating system without the need to change the hardware.
* applications can be developed on the device or created on another device and then installed.
* the entire OS can be replaced by another e.g. replacing Windows with Linux.

### Mobile devices

Mobile devices present their own limitations for the types of operating systems they can run. They have to be built with a consideration for the limited memory or processing power and also for the mobility of the device. They also need to be able to manage a network connection, predictive text keyboard, smaller display, phone dialler and limited battery capacity. They may include embedded and real-time elements as well.
Your standard mobile phone (i.e. not a smartphone) will run apps in what is known as a sandbox. A sandbox provides strictly controlled resources for a guest program to operate in where network access and the ability to read from the file system are usually not allowed.

#### Smartphones

Smartphones are able to run a complete operating system and provide a standardised interface and platform for app developers. The operating system must be able to support things like email and internet. We tend to take these things for granted on a phone these days, but smartphones today have more processing power than desktop computers 10 years ago.

## Embedded

Many pieces of everyday machinery contain computer systems, for example cars, washing machines and telephones. These systems are called embedded systems. An embedded system has a limited or non-existent user interface and is designed to operate completely, or mostly, independently from within other machinery. Embedded systems also have limited memory capacity. Operating systems for embedded systems are designed to work within the constraints of limited memory size and limited processing power.

## Desktop

All desktop computers have operating systems, the most common of these are Windows, Mac OS, and GNU/Linux. Desktop operating systems have to be able to support a wide range of tasks and manage many types of hardware and software. In order to do this they require large amounts of memory, multiple processors and large amounts of disk storage capacity. They also have the ability to read and write to optical disks and flash memory drives. Desktop operating systems also require real-time components for multimedia applications and are able to support a broad range of network protocols.
Desktop operating systems are written in a layered or modular way so that they can be updated easily. They support complicated GUIs and because of this they can have slow load times due to a large memory footprint. Desktop operating systems provide a virtual machine so that users do not have to interact directly with the hardware. In a client-server system the client computers usually use a desktop operating system.

## Server

Server Operating System is an operating system that provides its networked clients some specific services.
Server Operating system doesn't have GUI because:
* it would then become more expensive as it would cost more to have a GUI.
* it takes up processing power.
Client-server System is a system where services are requested by clients and these services are provided by other computers.