CIE IGCSE COMPUTER SCIENCE
Theory of computer science
Chapter 4 – Operating systems and computer architecture
4.1 Introduction
The operating system makes it possible to communicate with the software and hardware that make up a typical
computer system.
There are many ways of representing computer architecture, but one of the most common ones is the von
Neumann model.
4.2 Operating systems
The OS is essentially software running in the background of a computer system. It manages many of the basic
functions shown below. Obviously not all operating systems carry out everything shown below but it shows the
importance and complexity of this software. Without it, most computers would be very user-unfriendly and the
majority of users would find it impossible to work with computers on a day-to-day basis.
One of the most common examples of an OS system is Windows and is used on many personal computers.
Other examples include Linux, Android, UNIX and DOS. Windows is an example of a single-user multitasking
operating system – this means only one user can use the computer at a time but can have many applications
open simultaneously.
When a computer is first powered up, the initiating programs are loaded into memory from the ROM (read only
memory) chip. These programs run a checking procedure to make sure the hardware, processor, internal
memory and bios (basic input-output system) are all functioning correctly. If no errors are detected, then the
operating system is loaded into memory.
Simple devices with embedded microprocessors do not always have an operating system. Household items,
such as cookers, microwave ovens and washing machines only carry out single tasks which do not vary. The
input is usually a button pressed or a touchscreen option selected which activates a simple hardware function
which does not need an operating system to control it.
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, Activity 4.1
Find out how appliances fitted with microprocessors can be controlled and activated by web-enabled devices
such as smart phones.
For a device to be controlled remotely it requires two things:
o A receiver – the radio receiver allows the device to receive radio signals. When the signals are
received it acts on them, usually doing what they are command to do.
o A transmitter – the device needs to be able to talk back to the controller telling it that the signal was
received, and also probably telling the controller that the task has been completed.
An example would be a light switch. Z-wave is a special communications protocol which requires a hub to
control it. A SmartThings Hub plugged into the Wi-Fi router allows the light to be switched on when
spoken into the SmartThings App, where the hub sends out the signal to the z-wave light switch and then
the light switches on.
Google Home can also be connected to SmartThings which when said “Okay Google, goodnight”, the
Google Home tells SmartThings that all the lights should be turned off. The SmartThings sends all the
signals through the z-wave network telling the lights to turn off.
4.3 Interrupts and buffers
An INTERRUPT is a signal sent from a device or from software to the processor. This will cause the processor
to temporarily stop what it is doing and service the interrupt. Interrupts can occur when, for example:
a disk drive is ready to receive more data
the error has occurred, such as a paper jam in a printer
the user has pressed a key to interrupt the current process – an example could be
<CTRL><ALT><BREAK> keys pressed simultaneously
a software error has occurred – an example of this would be is an .exe file could not be found to initiate
the execution of a program.
Once the interrupt signal is received, the processor either carries on with what it was doing or stops to service
the device/program that generated the interrupt.
Interrupts allow computers to carry out many tasks or to have several windows open at the same time. For
example, downloading a file from the internet while listening to some music from the computer library.
Whenever an interrupt is serviced, the status of the current task being run is saved. This is done using an
INTERNET HANDLER and once the interrupt has been fully serviced, the status of the interrupted task is
reinstated and it continues from the point prior to the interrupt being sent.
Buffers are used in computers as a temporary memory area. These are essential in modern computers since
hardware devices operate at much slower speeds than the processor. If buffers were not there, processors
would spend the majority of their time idle, waiting for the hardware device to complete its operation. Buffers
are essentially filled from the processor or memory unit and whilst these are emptied to the hardware device,
the processor carries on with other tasks. Buffers are used, for example, when streaming a video from the
internet. This ensures that the video playback does not keep on stopping to wait for data from the internet.
Buffers and interrupts are often used together to allow standard computer functions to be carried out. These
functions are often taken for granted. The image below shows how buffers and interrupts are used when a
document is sent to a printer.
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