1. Higher Computer Systems
Unit Test- Revision

2. Representing negative numbers
A system of representation called two’s complement allows us to efficiently represent negative numbers.
It is easy to obtain a two’s complement of a number by following these steps:
positive number in bits
changes all the ones to zeros and vice versa
then add 1 +1
negative number

3. Floating Point Representation
Real number are represented using floating point.
The number contains a mantissa and exponent.
The mantissa is the number. The exponent is where the point is placed.
The more bits set aside to represent the mantissa, the more precise the number will be.
If there are not enough bits set aside for the mantissa the system has to round it down, losing precision.
Remember this PM - Prime Minister - Precision Mantissa!
Increasing the number of bits used to represent the exponent increases the range of numbers that can be represented.
Remember RE - Religious Education - Range Exponent!

4. Description of the bit map method of graphic representation using examples of colour/greyscale bit maps
Bit mapped packages paint pictures by changing the colour of the pixels which make up the screen display
Each pixel has to be stored individually in memory by a series of bits.
Bit depth
The more bits assigned to represent each pixel the greater the range of colours or shades of grey that can be represented.
This is known as the colour bit depth

5. Description of the relationship of bit depth to the number of colours using up to and including 24 bit depth (true colour)
You should already know that in a black and white image a white pixel is represented by a 0 and a black pixel is represented by a 1.
To enable colour within a bitmap graphic more bits are assigned to each pixel.
If there are 8 bits per pixel then we have 256 different binary patterns therefore 256 colours
If we have 16 bits per pixel then we have different binary patterns therefore colours .
If we have 24 bits per pixel then we have
different binary patterns therefore
colours .

6. Description of the vector graphics method of graphic representation
In vector graphics, the system stores the mathematical definitions of:
The shape of graphic objects;
Their position on the screen;
Their attributes such as the fill colour, the line colour and thickness.
The definition of a circle might hold:
The position of the centre;
The length of the radius
The width and colour of the line circumference;
The colour/pattern of the infill.

7. Advantages of Bitmapped Graphics
Description of the advantages and disadvantages of bit mapped and vector graphics
Advantages of Bitmapped Graphics
They allow the user to edit at pixel level
Storing a bit-mapped graphic will take the same amount of storage space no matter how complex you make the graphic

8. Description of the advantages and disadvantages of bit mapped and vector graphics
Disadvantages of Bitmapped Graphics
They demand lots of storage
Particularly when lots of colours are used
They are resolution dependent.
This means the resolution of the graphic, the number of pixels per inch, is set when the bitmap is produced. If you reduce the resolution, the system reduces the size of the pixel grid and eliminates pixels. This reduces the quality of the image.
Resizing a bit-map causes problems.
If you resize a bitmap graphic upwards it has the same number of pixels, and so the image becomes pixilated, the edges jagged.
If you resize it downwards it becomes dense.
You cannot isolate an individual object in a graphic and edit it

9. Advantages of Vector Graphics
Description of the advantages and disadvantages of bit mapped and vector graphics
Advantages of Vector Graphics
You can edit individual objects in a graphic.
They are resolution independent.
If you display the object on a system with higher resolution it will still display perfectly.
You can build up graphics by layering objects
They are less demanding on storage space.
When you resize a vector, it changes in proportions and keeps smooth edges.

10. Disadvantages of Vector Graphics You cannot edit individual pixels.
Description of the advantages and disadvantages of bit mapped and vector graphics
Disadvantages of Vector Graphics
You cannot edit individual pixels.
A very complex graphic with lots of layering will demand a lot of storage space.
Vectors have a flat perspective which comes from the fact that they are made up of objects fill in with a block of colour. They are best suited to logos and simple illustrations.

11. The Fetch-Execute Cycle
The memory address of the next instruction is placed on the address bus;
A read signal is activated on the Read Lines;
The data stored at the addressed memory location is placed on the data bus and transferred to a register which holds it until it is executed
Execute
The processor interprets the instruction;
The processor carries out the instruction.

12. Description of the function of the data bus and the address bus
The data bus allows data to be transferred between the processor, memory and any other devices attached to the bus.
It is bi-directional as data must go both ways.
The width of the data bus is measured by the number of wires
on the bus. Each wire can carry one bit. A 32-bit data bus (word) can transfer 32 bits at one time.
The width of the data bus is important in determining system performance.

13. Description of the function of the data bus and the address bus
The address bus holds the address of the memory location being accessed.
The more wires on the address bus, the more locations the system can, in theory, address.
The maximum number of addresses =
2 to the power width of the address bus

14. Description of the read, write and timing functions of the control lines
The control bus is made up of the read, write, clock, interrupt, non-maskable interrupt and reset lines
A read signal on the control lines instructs data to place data from the specified memory address on the data bus.
A write signal on the control lines instructs memory to take data on the data bus and place it in the location specified by the address bus
The clock lines carries a series of clock pulses at a constant rate in order to keep the processor and its related components in step with one another.

15. An interrupt is a signal to the processor from a peripheral device
Identification of other control lines, including reset and interrupt lines.
The reset line on the processor is used to return the processor to its initial state.
An interrupt is a signal to the processor from a peripheral device
A maskable interrupt may be ignored by the processor

16. Description of the following elements of computer memory: registers, cache, main memory, backing storage
The elements of memory are registers, cache, main memory and backing storage
Cache holds frequently accessed instructions
Main memory consists of a number of storage locations, each with a unique address
Backing Storage functions as permanent storage for programs and data in a computer system.
Main
Memory
(DRAM)
Cache fetches the
next instruction from
main memory in
advance of the
processor
2. Processor checks to
see if the next
instruction is in cache
Processor
Cache
Memory
(SRAM)
If not, the processor
has to fetch instruction
from main memory -
a much slower process
3.If it is, then instructions
is fetched from cache –
a very fast process

17. The concept of addressability
Main memory consists of a number of storage locations, each of which is identified by a unique address.
The ability of the processor to identify each storage location is called its addressability.
Each storage location can hold a quantity of data called a word.

18. The measure of computer performance are:
Description and evaluation of the following measures of performance: clock speed, MIPS, FLOPS and application based tests
The measure of computer performance are:
Clock speed
The clock signal is carried by on of the lines of the control bus. The clock rate is the frequency the clock generates pulses.
MIPS
Millions of Instructions Per Second- this is a measure of performance based on the average number of machine code instructions executed.
FLOPS
Floating Point Operations Per Second- this is a measure of the arithmetical calculating speed of a computer system. One MegaFLOP is a million FLOPs.

19. Application based tests
Description and evaluation of the following measures of performance: clock speed, MIPS, FLOPS and application based tests
Application based tests
No matter which one of these measurements is used, it does not really mean much to the ordinary user.
It is of more importance to the user how fast applications run, and this depends on the performance of the whole computer system, not just one particular component
A benchmark is a standard set of computer tasks to allow a computers performance to be measured.
A typical task may involve timing how long it takes to reformat a 100 page word processed document, how many pages can be printed per minute or how long it takes to save a 1000 record database to disk.

20. Description of the effect the following factors have on system performance: data bus width, use of cache memory, rate of data transfer to and from peripherals
Data bus width
Determines the quantity of data which the bus can carry at any one time.
A computer with a 32-bit data bus can carry 32 bits of data at any one time. This means the word length is 32 bits.
Increasing the data bus will increase the quantity of data which the bus can carry at any one time. Doubling the width will in theory double the quantity of data which may be transferred between the processor and memory.

21. Use of cache memory May result in significant improvements.
Description of the effect the following factors have on system performance: data bus width, use of cache memory, rate of data transfer to and from peripherals
Use of cache memory
May result in significant improvements.
There are two reasons for this:
The internal (Level 1) cache is on the processor chip and can benefit from the increased width of the processor bus. The level 2 cache is connected to the processor by it own bus which is also wider than the normal data bus which connects the processor to the main memory.
The cache memory is made up of static RAM rather than slower dynamic RAM

22. The rate of data transfer to and from peripherals
Description of the effect the following factors have on system performance: data bus width, use of cache memory, rate of data transfer to and from peripherals
The rate of data transfer to and from peripherals
Is controlled by the type of interface used to connect the peripheral device to the processor.
Common interfaces are USB and firewire if you compare a device like a scanner, using a USB 1 interface, with a new model bearing a USB 2 interface, you will notice a significant increase in performance of the USB 2 scanner.

23. Description of the use and advantages of buffers and spooling
Buffering
Is an area of memory used for the transfer of data between a computer and a peripheral.
It provides temporary storage of data.
Using a buffer provides a link between a device and the processor and helps compensate for any differences in their working speeds or data organisation.

24. Description of the use and advantages of buffers and spooling
A spooler is a program which is used to address the same problem as a printer buffer.
A spooler uses fast backing storage such as a hard disk for the temporary storage of print jobs.
The data is held on disk and transferred to the printer at a speed which the printer can accept
Advantages of a spooler over a buffer
The buffer is limited to a fixed size because it is RAM (typically megabytes), but a spooler uses backing storage which has a much larger storage space (typically gigabytes)

25. Serial and Parallel Interfaces
Converting data to and from serial and parallel forms:
Serial interface - is where data is transmitted along a communication channel one bit after another in sequence.
Parallel interface - transmit several bits of data simultaneously across a series of parallel channels, often transmitting 16 at 32 bits at a time.
The buses internal to the processor are parallel channels. Any data coming from a serial device has to be sent to an interface which buffers the data then converts it to parallel form before it is passed to the processor.

26. Advantages of client-server networks:
Descriptive comparison of peer-to-peer networks and client server networks
Advantages of client-server networks:
Increased security ober peer-to-peer – each user must log in to server
Different users can be given different levels of access to data.
Resources are controlled through the server

27. Descriptive comparison of peer-to-peer networks and client server networks
Workgroup computing – many users can work on the same document simultaneously.
Flexible use of stations – any user ca log into the file server and can access their own data from any station.
Backup is done centrally – the system administrator is responsible for making backup copies of all the data on the file server.

28. Disadvantages of client-server networks:
Descriptive comparison of peer-to-peer networks and client server networks
Disadvantages of client-server networks:
If the file server is not working then users cannot access their data.
Client-server is more expensive than peer-to-peer – it is necessary to buy a server and server software.

29. Advantages of peer-to-peer networks:
Descriptive comparison of peer-to-peer networks and client server networks
Advantages of peer-to-peer networks:
Each station on a peer to peer network has a similar status, with its own local storage devices.
Cheaper than client/server – no need for file server software.
Easier to set up current operating systems for peer-to-peer operations, for example, Windows XP.

30. Disadvantages of peer-to-peer networks:
Descriptive comparison of peer-to-peer networks and client server networks
Disadvantages of peer-to-peer networks:
No central file storage – users must always use the same machine to access their own data.
Users must backup their own data.
If one station is switched off, it may not be possible to access some resources.
Peer-to-peer is less secure than client/server - users may not be required to login.

31. Description of bus, star, ring and mesh topologies using the term node and channel
The network topology are the way in which the nodes on a network are connected together, for example bus, star, ring and mesh
node
channel
node
channel
Ring Topology
Bus Topology
channel
node
Central
node
Channel
Ring topology
Mesh Topology

32. Description of the consequences for each of the above topologies of node and channel failure
Bus
Star
Ring
Mesh
Node
No effect on rest of network
Network down if central node down – otherwise no effect on rest of network
Failed node can be bypassed to maintain integrity of ring
Channel
Network down

33. Single User Operating Systems
A single user operating system is only capable of supporting one user at a time. A network operating system is designed to support multi-access operation.
The main functions of a single user operating system are:
Interpreting users commands
File management
Memory management
Input/output management
Managing processes
Resource allocation

34. Interpreting Users Commands- Command Language Interpreter (CLI)
The CLI’s function involves taking instructions from the user, checking them, then passing them to the correct part of the OS to be carried out.
The CLI may be command driven, which means the user must know and be able to enter typed commands into it. Alternatively, the CLI may be menu-driven, which means that the user can choose commands from a list.
Most computers in current use have a menu-driven CLI which is part of the graphical user interface or GUI, e.g. Microsoft Windows

35. File Management System (FMS)
This is the part of the OS that is concerned with the efficient use of the computer’s backing storage devices and media.
The functions are:
Maintaining a directory which keeps track of where files are stored on storage.
Providing a connection between the user’s logical view of the files and the actual physical location of the file.
Supporting the manipulation of the data in the file.
Requesting data transfer from the I/O devices.

36. Memory Management System (MMS)
This part of the OS organizes the storage of data in main memory. Its main functions are:
Allocates space in memory for programs and data.
To control where programs and data are placed in main memory
To keep track of the total amount of main memory available and which programs and data are currently loaded.
To ensure that any user programs do not interfere with the area of main memory which is used by the operating system.

37. I/O Management (I/O System)
This layer performs the actual transfer of data between peripherals and memory. Its main functions are:
to communicate directly with the peripherals
to handle the transfer of data between the peripherals and the processor.
to control the timing to coordinate the flow of data between the CPU and external devices.
data buffering to regulate the speed between processor and MM.
To detect errors such as mechanical or electrical failures or transmission errors.

38. Managing Processes- the kernel
Resource Allocation is the job of the kernel.
The kernel is part of the OS responsible for managing processes and handling any interrupts.

39. Description of utility programs (including virus checker, disk editor and defragmenter)
Virus checking software should be able to detect a virus infection and remove it from the computer system. When copying a file from a disk or downloading a file from the Internet, the virus checking software should also check that the file does not contain a virus.
A disk editor allows the user to edit data directly on the surface of a disk, bypassing the normal loading of and saving feature of the operating system. This is a very powerful feature, but must be used with caution, since you run the risk of completely destroying all your data if you do not know what you are doing.

40. Description of utility programs (including virus checker, disk editor and defragmenter)
A defragmenter program allows you to defragment your disk. This is a useful feature, because it gathers all the free space together and reunites scattered file fragments.
This process may result in an improvement in the performance of the disk, especially if it is nearly full.
Disk before defragmentation 1 2 4 7 5 6 3
Disk after defragmentation 1 2 3 4 5 6 7

41. Description of the following reasons for the increasingly widespread use of networks: - advances in computer hardware, including processors, main memory capacity, backing storage, data transfer rates - improved network related software , including browsers and network operating systems
Processors have become faster and one trend is to increase the number of processors on a single chip

42. Description of the following reasons for the increasingly widespread use of networks: - advances in computer hardware, including processors, main memory capacity, backing storage, data transfer rates - improved network related software , including browsers and network operating systems
Main memory capacity has tended to increase as the cost of RAM chips continues to fall. This has allowed the development and the implementation of larger, more complex network operating system software.

43. Description of the following reasons for the increasingly widespread use of networks: - advances in computer hardware, including processors, main memory capacity, backing storage, data transfer rates - improved network related software , including browsers and network operating systems
Backing Storage capacity has increased as the cost of hard disk storage falls an d new types of storage media, such as Blu-ray are introduced.
Large capacity hard disk drives make it possible to store the data from many network users on one file server.

44. Description of the following reasons for the increasingly widespread use of networks: - advances in computer hardware, including processors, main memory capacity, backing storage, data transfer rates - improved network related software , including browsers and network operating systems
Data transfer rates are also increasing, for instance, 10 Mbps Ethernet is being replaced by 100 Mbps. This improvement in bandwidth allows larger files to be transferred in less time. It is now possible to edit and transfer large files such as video files efficiently across a LAN

45. Description of the following reasons for the increasingly widespread use of networks: - advances in computer hardware, including processors, main memory capacity, backing storage, data transfer rates - improved network related software , including browsers and network operating systems
Browsers One notable difference in the development of browser software over the past few years has been the ability of modern browsers to have their functionality extended by the addition of various plug-ins.
Plug-ins allow browsers to display different types of data other than plain text HTML.

46. Description of the following reasons for the increasingly widespread use of networks: - advances in computer hardware, including processors, main memory capacity, backing storage, data transfer rates - improved network related software , including browsers and network operating systems
Network Operating Systems controls all the devices on a network so that resources can be shared efficiently and files can be transferred.. It handles the administration of all network functions.

47. The Computer Misuse Act covers hacking and planting viruses
Description of the application of the Computer Misuse Act, the Copyright Designs and Patents Act and the Data Protection Act to the misuse of networks
The Copyright Designs and Patents Act covers breaches of copyright such as illegal copying of software, music and movies
The Computer Misuse Act covers hacking and planting viruses
The Computer Misuse Act makes it a criminal offence to gain unauthorised access to a computer system, to hack or to write and distribute viruses which can damage data on a computer
The Data Protection Act cover how information may be held and for what purposes.
The Data Protection Act places limits on the storage and use of personal information.

48. Description of the function of a bootstrap loader
The bootstrap loader is a small part of the operating system.
The bootstrap loader’s function is to load the rest of the operating system from disk when the computer is switched on.

49. JPEG has defined standards for still image compression.
Description of the standard file formats for graphic files: jpeg, gif, TIFF
JPEG has defined standards for still image compression.
It is designed for either full colour or grey scale images of natural scenes.
It uses 24 bits per pixel to store RGB colour images
JPEG files are compressed to save backing storage space,
It uses lossy compression, which means that some of the detail is lost from the image.

50. Description of the standard file formats for graphic files: jpeg, gif, TIFF
TIFF is a format used for high resolution bitmapped images, although the files may be any resolution.
TIFF files are large, they are an uncompressed file format.
TIFF is widely used in applications such as Photoshop, Desktop Publishing and scanning.
It is used when a high quality image is required, e.g. printing a photo or publication in a book or magazine.

51. GIF is a format for storing bit-map colour graphics images as files.
Description of the standard file formats for graphic files: jpeg, gif, TIFF
GIF is a format for storing bit-map colour graphics images as files.
It is a compressed file format making them faster to load and transfer via the internet.
GIF images are limited to 256 colours
It is designed to handle images such as logos or line drawings which contain large blocks of the same colour.
GIF uses lossless compression which means that no detail in the original image is lost when it is compressed.

52. Description of Virus Detection Techniques: use of checksum, searching for virus signature, heuristic detection and memory resident monitoring
Use of checksum - this technique scans an uninfected program file and calculates a checksum using the binary values of the data in the file. It then scans the file whenever the program is run and repeats the calculation. If the answers are different there could be a virus.

53. Description of Virus Detection Techniques: use of checksum, searching for virus signature, heuristic detection and memory resident monitoring
Searching for virus signatures - the anti-virus uses a table, which has to be regularly updated, containing virus signatures. It scans and tries to find a match for these bits of code (signatures) in its table.
Memory Resident Monitoring - this is when the software resides in RAM which monitors all a computer’s actions for suspicious activity, e.g. copying files. If it finds anything suspicious it throws up an error.

54. Description of Virus Detection Techniques: use of checksum, searching for virus signature, heuristic detection and memory resident monitoring
Heuristic detectors - this is software that looks for code that is triggered by time or date events, for code that searches for .com or .exe files, and for instructions that try to write to disk without going through the normal OS procedures.

55. Simple description of the functions and uses of a hub, switch and router
A hub attaches several network cables putting several computers on the same network
A switch operates like a ‘smart’ hub, dividing the network into separate segments one for each machine.
Workstations connected via a switch benefit because there are no collisions between signals to reduce the speed of the network.
A router has a processor that allows it to decide where to send traffic and possibly to modify the details