Higher Computing Notes
JBK
HIGHER COMPUTING
COMPUTER SYSTEMS -1-
Higher Computing Notes
JBK
This unit is designed to develop knowledge and understanding of the principles of computer systems and practical skills related to computer systems through the use of contemporary hardware and software. The Computer Systems unit involves the study of the following topics:-
Data Representation
(6 hours)
Computer Structure
(7 hours)
Peripherals
(5 hours)
Networking
(9 hours)
Computer Software
(9 hours)
The unit is assessed using objective multiple choice questions in a closed book test, lasting less than 45 minutes. Pupils must also be able to demonstrate practical skills in the standard use of operating system functions, use of utility software, use of standard file formats for graphics files, accessing a LAN using a network client and accessing the Internet for WWW, e-mail and file transfer.
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Higher Computing Notes
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Topic 1: Data Representation DETAILED CONTENT LIST The pupil must demonstrate knowledge and understanding, practical skills and problem solving based on the following content statements:
representation of positive numbers in binary – range up to 32 bits conversion from binary to decimal and vice versa description of the representation of negative numbers using two’s complement, using examples up to 8 bits description of the relationship between the number of bits assigned to the mantissa/exponent and the range and precision of floating point numbers conversion to and from bit, byte, Kilobyte, Megabyte, Gigabyte and Terabyte description of Unicode and its advantages over ASCII description of the bit map method of graphic representation using examples of colour or greyscale bitmaps description of the relationships of bit depth to the number of colours using examples up to and including 24 bit depth (true colour) description of vector graphics method of graphic representation description of the relative advantages and disadvantages of bit mapped and vector graphics description of the relationship between the bit depth and file size explanation of the need for data compression using the storage of bit-map graphic files, as examples
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Higher Computing Notes
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Essential Knowledge - revision The computer uses binary to represent all information that has to be stored. The main reasons for this are as follows:• • • •
only 2 different states are required – on/off a slight change in voltage will not affect the clarity of digit being sent easy to represent in storage devices – pits on a CD surface easy arithmetic rules to implement
The binary number system has the huge advantage that only two symbols are required, 0 and 1. These can easily be represented in a computer system by a switch or transistor being on or off, or by a high or low voltage level. Imagine how difficult it would be to represent 10 discrete values, as required in the base 10 number system. However, large strings of 0’s and 1’s are then required to represent some numbers, and a human finds binary very difficult to understand and work with.
Counting in Binary The computer system works in groups of 8 bits (1 byte). Therefore, you need to be able to display binary numbers in 1 or 2 byte format. For example:Decimal Number 0 1 2 3 4 5 6 7 8 9 10
Binary Equivalent 00000000 00000001 00000010 00000011 00000100 00000101 00000110 00000111 00001000 00001001 00001010
The easiest way to convert to and from binary is to use the place headings. The heading values relate to the number system being used. Our base 10 number system would use the headings 103, 102, 101, 100. A binary (base 2) number system will use the place headings 23, 22, 21, 20. 28 (256)
27 (128) 0 0 1
26 (64) 0 1 0
25 (32) 0 0 0
24 (16) 0 0 1
-4-
23 (8) 1 1 0
22 (4) 0 0 1
21 (2) 0 1 1
20 (1) 1 0 1
=9 = 74 = 151
Higher Computing Notes
JBK
A set of terms that are commonly used to refer to the amount of data that can be stored in the computer is: BIT
binary digit
0 or 1
BYTE
group of 8 bits
01101101
KILOBYTE (Kb)
1024 bytes (210)
MEGABYTE(Mb)
1024 kilobytes (220)
GIGABYTE (Gb)
1024 megabytes (230)
TERABYTE (Tb)
1024 gigabytes (240)
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Higher Computing Notes
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Data Representation – Text The computer has to be able to represent each character on the keyboard with a unique code, whether it is a number, letter or symbol. When the computer user presses a key, the unique code is translated into binary for the processor to understand. A list of all the characters that a computer can process is known as its character set, and a different code has to be used for each character. ASCII (American Standard Code for Information Interchange) is a common form of this code. ASCII is a 7-bit code, with the 8th bit being used as a parity bit to check the code has been transmitted correctly. Using a 7-bit code allows 27 or 128 different characters to be used. Extended ASCII uses all 8 bits allowing 28, which is 256 different characters. Many different computer systems use ASCII to represent text. This made it much easier to transfer text between the different types of computer system. However, with the increase in international communications came the problem of how to deal with languages such as Japanese and Arabic which did not use the Latin based symbols of the main languages English, French and German. Unicode was developed. This code uses 16-bits for each character. Therefore, 216 allows 655,536 possible symbols to be used. Unicode is capable of including the characters from all known languages and alphabets in the world.
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Higher Computing Notes
JBK
Data Representation – Positive Integers If a computer system only had to deal with positive integers, then these could easily be converted to binary using the ‘place headings’ method. The maximum size of the number that can be handled depends on the number of bytes allocated in memory for storage. If 2 bytes are allocated, then the numbers will range from 0 to 216-1 = 0 to 1111111111111111= 65535 (65536 numbers) A 32-bit computer can store numbers in the range 0 to 232-1 = 0 to 4294967296
Data Representation – Negative Numbers The method used to store negative numbers is called ‘Two’s Complement’. To convert a negative number into binary you carry out the following steps:• • •
convert the positive number to binary using 8 bits change all the 0’s to 1’s and vice versa add 1 to the result
If this method works correctly, then normal arithmetic rules will apply. For example, -5 + 5 = 0
Step 1:
change 5 into binary using 8 bits
00000101
Step 2:
work out –5 in binary
11111010 +1 11111011
Step 3:
check that –5+5=0
11111011 00000101 00000000
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Higher Computing Notes
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Data Representation – Floating Point numbers Real numbers are not always stored accurately in computers as it is not always possible to convert precisely from decimal to binary and vice versa. It is like changing fractions ½ and 1/3. second is 0.33333333333333333 etc.
The first converts easily to 0.5, while the
Real numbers are stored as floating point numbers. Floating point is like standard form in scientific notation e.g. 34006.8 = 3.40068 x 10-4. In binary, we need to move the decimal point right to the start as the computer system can only store 0’s and 1’s. In floating point notation the computer stores the number in two parts. The mantissa is the term used for the digits being stored, and the exponent is the term used to store the numbers of places that the point has been moved (or floated). Use the place heading method again: 28 Step 1:
27 26
24
23
22
21
20
.
2-1
2-2
2-3
2-4
converting 7.75 into binary 7.75
Step 2:
25
=
7 and (0.75 = 1/2 + ¼)
111.11
move the point 3 spaces to the front . 11111
Step 3:
Write down the mantissa and exponent values Mantissa = 00011111
Exponent = 00000011 (3 spaces )
The normal way is to assign four bytes to the mantissa and one byte for the exponent. •
If you increase the size of the mantissa you increase the accuracy with which the real number can be stored eg chop off less from 1/3 = .3333333333333333
•
If you increase the size of the exponent you increase the range of numbers that can be stored.
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Higher Computing Notes
JBK
Data Representation – Bit-mapped graphics A bit mapped image is stored as a file of pixel data and is produced on screen at the same resolution at which it was created. Resolution refers to the total number of pixels in the width and height of the image. A bit map is a term that is applied to a 1bit per pixel graphic system, i.e. where images can have only two possible colour values, 0 and 1, usually black and white. The diagram below shows a simple graphic produced in a painting package and its corresponding 9 x 9 x 1-bit representation.
With multiple bits per pixel, colour and brightness is represented in a pixel map (pixmap). As the number of bits per pixel increases, the number of colours that can be represented increases. For instance, using 2 bits per pixel allows 4 colours to be represented, while 8 bits per pixel can represent 256 possible colours. The number of bits used to represent each pixel is called the bit-depth. Screen resolutions of 1280 x 1024 with 24-bits per pixel (true colour) are available on today's PC’s. This requires 1280 x 1024 x 24 = 3.75 Mbytes of VRAM. Video RAM represents the entire screen area and the term bit map is used to describe the one-to-one mapping of pixels in VRAM to pixels on the screen.
Advantages of bit mapped graphic representation • •
a bit mapped image can be manipulated at the pixel level. eg remove red eye a wider range of irregular shapes and patterns can be created by deleting pixels anywhere on the image.
Disadvantages of bit mapped representation • • •
requires large amounts of storage space; image becomes course (jagged) when scaled; resolution dependant – redrawn at same resolution it was created at
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Higher Computing Notes
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Calculating Storage Requirements An image using 24-bit colour graphics will be of an extremely high standard. Each pixel can represent over 16 million colours. On a high resolution display, a single image may require several megabytes of memory storage.
Resolution = number of pixels in width
x
number of pixels in height
Bit Depth = number of bits required, per pixel, to store the colours Memory Requirements = resolution x bit depth
e.g. Calculate the storage requirements of an image 800 x 600 using 256 colours. 8 bits are required to store 256 colours - 28 Memory
= = = =
800 x 600 x 8 3840000 bits 480000 bytes 468.75 Kb
( divide by 8) ( divide by 1024)
Compression of Graphics On a 1024 x 768 display, a 24 bit colour image will require 2.25Mbyte of storage. Even if you have enough disk storage to have such images, transmitting them over a communications network will take a long time. At 56kbps - the theoretical maximum achievable over a telephone line with a modem - it would take over 5 ½ minutes to transmit such an image. File compression techniques can be used to reduce storage requirements. Specialised techniques for image compression result in images that are indistinguishable from the original as far as the human eye is concerned, but much smaller file sizes.
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Higher Computing Notes
JBK
Data Representation – Vector graphics Vector graphic representation stores a description of the objects that make up the image. For example, if an image contains a coloured circle and a pattern filled square then these object descriptions could take the form:
SQUARE ( lower left(x,y), upper right(x,y), fill colour, line colour, line width) CIRCLE ( centre(x,y), radius, fill colour, lone colour, line width) A vector representation is resolution independent. Rasterisation is needed to convert the object descriptions to a bit map for subsequent display or printing. Some printers are able to rasterise vector images of a certain format, such as EPS (encapsulated postscript format) while others need assistance from the computer's CPU to carry this out. This can introduce additional processing requirements and many systems provide graphic co-processors to carry out these transformations, offloading the task from the CPU.
Advantages of vector graphic representation • • • • •
they do not lose their image quality on scaling. values are not held for every pixel, less storage space than a bit mapped image they can be edited at the "object" level - reposition, scale and delete objects objects can be grouped and then manipulated as a single image images are resolution independent meaning that the same quality of image will be rendered regardless of the resolution of the display or print device.
Disadvantages of vector graphic representation •
Look back to the advantages of bit mapped graphics.
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Higher Computing Notes
JBK
Topic 2: Computer Structure DETAILED CONTENT LIST The pupil must demonstrate knowledge and understanding, practical skills and problem solving based on the following content statements:
Detailed description of the purpose of the ALU and control unit Description of the purposes of registers: to hold data being processed, instructions being executed, and addresses to be accessed Description of the function of the data and the address bus Description of the read, write and timing functions of the control lines Identification of other control lines, including reset and interrupt lines Simple description, referring to the appropriate buses and control lines, of the steps in the fetch-decode cycle Description of the following elements of computer memory: registers, cache, main memory, backing storage Distinction between the above elements of memory according to function and speed of access The concept of addressability Description and evaluation of the following measures of performance: clock speed, MIPS, FLOPS, and application based tests Description of the effect of the following factors on system performance: data bus width, use of cache memory, rate of data transfer to and from peripherals Description of current trends in computer hardware, including increasing clock speeds, increasing memory and backing storage capacity
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Higher Computing Notes
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Computer Organisation You should already be familiar with the following terms and concepts from the Intermediate 2 course: CPU
the central processing unit or processor, is the main part of the computer system, consisting of the control unit, the ALU and registers.
ALU
the arithmetic and logic unit is responsible for carrying out all calculations and performs the logical operations.
Control unit controls all other parts of the CPU and makes sure that the program instructions of the computer are carried out in the correct order at the correct time. Registers
are a group of temporary storage locations inside the CPU and they are used to store data being processed, instructions being executed or addresses to be accessed. The more data kept on the processor, the faster processing will be. Time would be wasted accessing main memory.
Word
is the number of bits that can be processed by the CPU in one operation.
RAM
random access memory holds data as long as the computer is switched on. RAM is volatile memory.
ROM
read only memory holds its data permanently and is fixed when the computer is manufactured.
A two-state machine The following diagram is a simple representation of the main components of a computer. All components and all data storage devices have only 2 states: off or on. Binary numbers are therefore used to represent these states: 0 or 1.
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Higher Computing Notes
JBK
Linking it all together Information has to be carried backwards and forwards between memory and the processor. This is done using three sets of wires called buses which connect the CPU to the memory and input/output devices of the computer. Memory
Processor Control bus Control unit Internal buses
Data bus ALU Address bus Registers
Address Bus
a set of parallel lines used to carry the memory storage location address required by the processor. It is a one-way bus. The width of the address bus determines the number of memory locations that can be accessed. Addressability – each memory location is given a unique address.
Data Bus
a set of parallel lines used to transfer data to/from the processor and memory. It is therefore a bi-directional bus. The size of the data bus is often referred to as the word size of the computer. The larger the data bus, the more data that can be transferred in one operation, and the faster system performance.
Control Bus
a set of separate lines, each with a different purpose. They are used to initiate and control what the processor does. Functions include: • Read - initiates a memory read operation (load) • Write - initiates a memory write operation (save) • Clock - generates pulses to synchronise components • Interrupt - informs processor of an external activity • Reset - signals processor to abort its process and restart
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Higher Computing Notes
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The Fetch – Execute Cycle When a processor needs an instruction from memory or requires some data to perform a calculation, the steps and buses needed to fetch that information from memory are as follows:
Reading from Memory • • •
Set up the address bus with the required memory address Control bus activates the read control line Set up data bus to transfer data from memory to the processor
Writing to Memory • • • •
Set up the address bus with the required memory address Set up the data bus with the data to be saved Control bus activates the write control line Transfer data from the processor to memory location
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Higher Computing Notes
JBK
Computer Memory Main memory is the general term used to describe any type of computer memory other than backing storage. Main memory consists of a number of storage locations, each identified by a unique address. Each storage location can hold a quantity of data called a word – the number of bits that can be processed in one operation. Main memory consists of two types: RAM
holds data as long as power is supplied volatile – when power switched off the contents of RAM are lost holds the computers programs and data being processed Static RAM holds contents as long as power is on Dynamic RAM needs signal to be constantly refreshed
ROM
permanent store of data non-volatile – when power is off the contents are kept contents of ROM fixed at manufacture may be used to hold bootstrap loader program – starts up the computer access time of between 10 – 50 nanoseconds
Other memory includes:
CACHE
this memory is a form of RAM, and is checked first for required data Level 1 cache is normally held within the processor Level 2 cache is close to the processor used to temporarily store commonly used instructions faster for the processor to access cache rather than RAM access time – approx 10 nanoseconds
REGISTERS temporary storage locations within the processing unit Used to store data or instructions until needed by the processor
BACKING STORAGE used as a permanent store for programs and data stores contents of RAM if power to be switched off access time varies depending on the device slower to transfer data to/from backing store than main memory magnetic storage – tape, floppy, hard disk, zip disc optical storage – CD-RW, DVD, USB key
Speed of Access (fastest first) – registers, cache, main memory, backing store
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Higher Computing Notes
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Computer Performance When comparing one computer’s performance against another, you can consider the following:Bus Width
the wider the data bus – the bigger the word size - meaning the more data can be transferred in one operation, therefore less time is required to complete a process.
Clock Speed
clock speeds are now measure in GHz, thousands of millions of cycles per second. The time between each pulse is called the cycle time. Clock pulses regulate and co-ordinate processor activities. However, two different processors working at the same clock speed will take different lengths of time to complete the same operation.
MIPS
millions of instructions per second. (number of machine code instructions that can be processed) An IBM mainframe can process billions. Manufacturers can easily time the simplest and fastest instructions only, therefore giving a false impression of overall system performance. You cannot compare instructions processed by one chip against different instructions processed by another type of chip.
FLOPS
floating point operations per second. The procedures involved in doing a floating point multiplication are basically the same for every processor and, since these kinds of operations are used in most software, they provide a better comparison of system performance than MIPS.
Application Tests
Here, you would carry out a particular task using the actual software package you are planning to use. Eg test 3D graphics – Unreal Tournament. This same task would be carried out on various processors in a normal working situation to see which is best suited for the type of work to be done. Referred to as Benchmark tests.
There are various aspects of memory that affect system performance, but you cannot just take the fastest memory type and add it to your computer. The motherboard and processor will dictate what memory type must be used. Memory can be added to improve performance if software is struggling to run on what is available. However, adding memory far beyond what is needed will have little effect, except maybe ‘future proofing’. In many cases, there is not just one single factor that affects system performance e.g. clock speed and data bus width.
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Higher Computing Notes
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Topic 3: Peripherals DETAILED CONTENT LIST The pupil must demonstrate knowledge and understanding, practical skills and problem solving based on the following content statements:
Description of the use and advantages of buffers and spooling Description of suitable hardware, including peripherals, to support typical tasks such as producing a multimedia catalogue, setting up a LAN in a school and developing a school website. Justification of the hardware selected in terms of resolution, capacity, speed, cost and compatibility Description of the features, uses and advantages of solid state storage devices including flash cards Description of the development trends in backing storage devices Description of interface functions: buffering, data format conversion (serial to parallel, analogue to digital), voltage conversion, protocol conversion, handling of status signals Distinction between parallel and serial interfaces Description and explanation of the trends towards higher bandwidth and wireless communications
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Higher Computing Notes
JBK
Introduction These notes introduce the input, output (I/O) and storage devices that are in common use today and discuss some of the principles involved in handling data. You have to be able to describe a range of peripherals in terms of the following characteristics: accuracy, capacity, speed and cost. (Not every characteristic may be appropriate to every device.) •
Accuracy
refers to how closely the data captured or displayed is to the original. Accuracy relates to resolution, bit depth, sampling and compression.
•
Capacity
refers to the amount of data that can be stored e.g. number of images held in a digital camera.
•
Speed
refers to the rate of transfer of data between the computer and a peripheral device.
•
Cost
refers to the money to be spent
INPUT DEVICES 1.
Modified Keyboards
When a key is pressed, a switch under the key is closed and a signal is sent to the processor. The keyboard is wired in such a way that each key switch sends a different code – ASCII code. Modified keyboards have many different uses:• • • •
Improve ergonomic design to reduce RSI Reduce the number of keys (chording keyboard) to reduce size Disabled users could have enlarged keys Specialist applications for business (eg call centres) and military
Proper keyboard position should allow 90o degree bend between the elbow and forearm. The mouse should be located as close as possible to the keyboard.
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Higher Computing Notes
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JBK
Digitiser
A digitiser is an input device that changes analogue data into digital so that it can be recognised and processed by the computer. The resultant digital representation of a scanned image is a bit map. Scanner:
There are four types of scanner in current use. Flatbed has a glass plate upon which scanned material is placed and a light beam, the width of the glass, moves backward and forward to scan the material. Hand scanner is pulled over the material to be scanned by the user. Miniature hand scanners, similar to a light pen, can be used to scan single lines of text. Drum scanner consists of a rotating cylinder on which the material to be scanned is placed. They produce very high quality output and are used by professional scanning companies. They are very expensive and require expertise to operate. Film scanner operates the same way as a flatbed. They are capable of a higher resolution
Two main application packages are used in association with a scanner. Bit mapped graphics and OCR. TWAIN is the standard that allows devices to be operated from within different software packages. If the scanner and software are both TWAIN compliant then they should work together successfully. TWAIN will add a new menu option in to the software to allow a scanned image to be imported directly. This saves scanning separately, saving in the correct format and then importing. Note: There is no point in scanning an image at greater than 75 dpi for a screen based display. Similarly, there is also no point in scanning at 600 dpi if a printer can only print at 300 dpi. You must scan at a resolution to match the purpose intended.
Digital Camera
Both digital and normal cameras use a lens which focuses reflected light from the subject on to the sensor. In a normal camera the film is the sensor and storage. On a digital camera the sensor is an electronic chip packed with light-sensitive elements.
Most digital cameras use removable memory cards, but these have limited capacity. There are three common memory card standards, all three use Flash Memory which can store data when the power is off. A typical uncompressed image would take up 5 – 10 Mb of storage. Most cameras use jpeg compression which reduces the image to about 1Mb. Therefore a 32Mb memory card will hold about 30 pictures.
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Higher Computing Notes
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JBK
Video Capture Devices
Video signals have to be converted to digital using an ADC. This conversion is known as video capture. A video capture card changes each frame into a series of bitmapped images that can be displayed on a computer monitor. Uncompressed video samples take up a huge amount of memory (each frame >1Mb). Compression can either be Lossy or Lossless. Lossy Compression sacrifices some of the data – therefore reduces quality. The technique loses some types of visual information that our eyes and brain usually ignore. (Quantization) Different areas of the frame could be compressed to different degrees depending on the amount of detail. The higher the compression ratio, the worse the results. Lossless compression uses maths techniques to reduce the quantity of information. Standards include MPEG, QT, AVI.. There is no loss in quality. Digital video cameras use DV format compression. Their video input to the computer is already compressed and digitised, which eliminates the need for a specialised video capture card to perform these functions. DV format runs at a constant rate of 3.6Mb/second.
4.
Sound cards
Most PC’s (apart from MAC) have only enough sound to produce a beep. They therefore require a sound card to produce decent quality sound. Due to competition, sound cards are now cheaper and more sophisticated. Sound is analogue and must be converted to digital. The sound card carries out the ADC in a process called sampling. The sound is ‘chopped’ into a number of slices per second. At each slice, the amplitude (height) of the signal is measured and rounded to the nearest digital value. The sampling rate is the frequency at which sounds are taken. The sampling resolution is the number of bits used for data storage. (44 KHz sampling rate at 16 bit sampling resolution for CD quality). A high capacity media, such as a hard disk, will quickly run out of space if any reasonable length of sound at top quality is to be stored. There are several approaches to reducing file sizes. These include: • • •
downsampling - reducing the sample rate; reduce the sample size (e.g. 16-bit to 8-bit); use a compression technique to reduce the file size.
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Higher Computing Notes
JBK
OUTPUT DEVICES 1.
Ink Jet printers
Ink jets are quiet in operation and can produce high quality printouts of both graphics and text. The most common inkjet technology, drop on demand (dod), works by squeezing small droplets of ink onto paper. Two main methods of producing ink droplets are thermal technology and piezo-electric technology. Thermal technology uses a heating element in the print-head to heat up and then form a bubble of vapour in the ink. This bubble forces the ink from the print-head. Canon and HP use this technology. Cyan, Magenta and Yellow inks are output via a combined CMY print-head. The smallest dot size visible is 30 microns. The number of holes in the print-head determines the printers resolution. The speed that the nozzle can fire out ink drops determines the printers speed. Average is between 4 and 8 pages per minute for black and white text and 2 to ppm for colour text and graphics. Piezo-electric technology is Epson’s inkjet technology. Piezo-electric crystals move when an electrical current passes through them. This crystal is used instead of a heating element to force out the ink droplet. This method allows closer control of droplet size and shape, smaller drops being possible. Also, the ink does not have to be heated and cooled between each cycle, so time is saved.
2.
Laser Printer
Laser printers use a special powder, called toner, produce the image on paper. A laser beam is used to project the image on to the printer drum, with the toner sticking to the parts of the drum with the image on them. It does NOT BURN the image the paper. The paper, with the toner, is passed between heated rollers called the fuser, to melt and seal the toner on the paper. Laser printers can have a number of features:•
An on-board processor to process pages using a page description language. This would produce a better quality image faster than a printer without a processor.
•
The printer buffer can temporarily hold a print job while it is being printed. Typical buffer sizes vary from 1Mb to 32Mb.
•
A network interface to allow connection to a LAN
Speed commonly measured in pages per minute (ppm).
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Higher Computing Notes
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Speakers
Multimedia computers usually have so-called active speakers. These are mains powered and have a built in amplifier. They may have a large woofer for low notes and a tweeter for high notes. More advanced speakers will have five or more separate units that can be positioned in order to give surround sound. There are two main methods of sound production using a sound card:-
4.
•
FM synthesis, on budget sound cards, creates the entire sound from scratch
•
Wave Table synthesis gives a more realistic sound output because it uses samples of real sounds held in a lookup or wave table. The card generates the sound by using a chip called a Digital Signal Processor (DSP) which replays the required notes by reading from different areas of the wavetable memory at different speeds to give the required pitches. A typical wavetable card may contain 700 instrument samples within 4Mb of ROM.
Multiscan Monitor
The most common type of monitor is a cathode ray tube monitor (CRT). A CRT looks like a large light bulb with a flat end where the image is displayed. Any colour can be made by mixing different amounts of red, green and blue (RB). The inside of a colour CRT is coated with phospors., which glow when they are hit by electrons. The phosphor coating is arranged in horizontal lines or rasters of triads. A triad being a set of three dots, one red, one blue, one green. Three separate beams of electrons are focused on the triad making each dot glow to greater or lesser extent in order to produce the required colour and brightness. The image on the screen needs to be able to change quickly, so the screen is refreshed many times per second. This is known as the refresh rate or scan rate. Alternate lines of triads are refreshed on each pass, the odd numbered lines, then the even. This method is known as interlacing and is used on most TV’s. A computer image is drawn on a single pass and is non-interlaced. A good quality monitor has a refresh rate of at least 72 Hertz in order to avoid flicker. Monitors that can operate at different rates are called multiscan.
5.
Video Capture Card
A video capture card is required to moving images from analogue video to the computer. Each frame is converted into a bit mapped image. The digitised video must be decompressed before it is displayed on the monitor.
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Higher Computing Notes
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Peripheral Devices – Terms Interface
the hardware and associated software needed to allow communication between the CPU and peripherals. Compensates for differences in speed, codes and voltage levels. The functions of an interface are: •
data storage – use buffers to temporarily store data in transit
•
data conversion – changes data received by peripheral into a form the processor can understand, and vice versa (Analogue to Digital Conversion)
•
voltage conversion – ensures peripherals sending data at different voltage can work together
•
protocol conversion – usb, midi, IEEE
•
handle status signals – status information shows whether a peripheral device is ready to send or receive data. Also, paper jams, out of ink or paper
Buffering and Spooling are required to help compensate for any differences in the working speeds of computer devices. Buffer
an area of memory used for the transfer of data between the computer and a peripheral device. An essential component of an interface. Fixed in size.
Spooler
a printer spooler is a program which uses fast backing storage (ie hard disk) for the temporary storage of print jobs. The user data is then spooled to the printer when it is ready, leaving the processor free. Not fixed, depends on free space on hard disk. Often referred to as background printing.
Parallel
parallel interfaces allow data to be transmitted simultaneously along a set of separate lines. Fast, but only useful over short distances.
Serial
serial interfaces send bits one after the other along the same data line. Slow, but more efficient over long distances like a network.
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Higher Computing Notes
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Backing Storage Backing storage is used as a permanent storage for programs and data in a computer system. It is required by the computer system because data held in RAM will be lost when the computer is switched off. Media
the term used to describe the actual hardware that is used in the backing storage device e.g. disk
You should be able to describe the main features, uses and advantages of: Magnetic Disk disk surface coated in a special magnetic material. Data is written to and read from a set of concentric circular tracks. Disks are formatted (prepared) before they can be used. Magnetic disks include floppy disks, hard disks or hard disk packs and zip disks. The rotational speed of hard disks has improved, from 3000 revolutions per minute (rpm) of very early disks, to current rotational speeds of 5,400 and even 7,200 rpm. Performance is also measured in terms of the rate of data transfer from the disk. This will depend on the type of interface being used. e.g. SCSI (Small Computer Systems Interface) can transfer at a rate of 5Mbytes per second, while Ultra Fast SCSI III can transfer at rates of 40 Mbytes per second. Hard disks have improved tremendously in their capacity to store data in the last 10 years. From the modest 10Mb disks of the early 80s to current 80Gb disks on many of today's PCs. The hard disk is a direct access device, meaning that data can be directly read or written to any portion of the disk
Magnetic Tape still commonly used in commercial situations. A cheap storage medium with a huge capacity. Since their introduction, tape drives have passed through many stages of improvement with extremely reliable Digital Audio Tape (44.1 kHz, 16-bit record and playback DAT) drives representing the current state of the art. A 4mm DAT tape can now store up to 24 Gb of data! Tapes are sequential access devices, meaning you must wind through all the data on the tape to get to a point near the end.
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Tapes are not suitable as storage media for applications where data needs be used regularly. They are generally used only for long-term storage and backup.
Optical Disk
This includes CD-ROM, CD-R, CD-RW, DVD. The data is moulded into tiny holes on the plastic surface called ‘pits’. Data is read from the disks using laser light. Typically 650 Mb in capacity. However, the future use of blue lasers, instead of red laser light, will deliver increased CD-ROM storage capacity to around 3 Gb. The tracks on a DVD are closer together than a CD, thus allowing more data to be stored. A DVD can also have 2 layers for storing data on each side of the disk.
Magneto-optical storage devices combine magnetic and optical technologies to read and record data. With a magneto-optical disk, a laser beam and a magnetic field is used to write the data. Only the laser is used to read the data. Solid-state storage devices are made up entirely from electronic components i.e. they have no moving parts. They are also called RAM disks, as they take the place of a magnetic disk as a mass storage device. They can be in the form of a plug-in card or cartridge containing memory chips. The chips of a SSSD are typically static RAM or Electrically Erasable Programmable ROM (EEPROM or Flash EPROM). SSSD are used with devices where space is at a premium e.g. in a camera, or when portability is desirable e.g a USB flash drive
Flash Cards
compact memory used mostly with digital cameras. Data is stored while the power is off. Flash ROM can be updated. A 32Mb card will hold approx 30 compressed (jpeg) images from a camera.
So, where do we go from here? •
The trends are towards higher bandwidth and wireless communication.
Wireless communications can be achieved using WAP (Wireless Application Protocol). This, like any protocol, is a specification for a set of communication rules to standardise the way that wireless devices, such as cellular telephones and radio transceivers, can be used for Internet access, including e-mail, the World Wide web, newsgroups, and Internet Relay Chat (IRC). Interconnecting devices centred around an individual person is called a wireless personal area network (WPAN). Typically, a WPAN uses technology that permits communication between devices in a short radius of about 10 metres. One such technology is Bluetooth.
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Topic 4: Networking DETAILED CONTENT LIST The pupil must demonstrate knowledge and understanding, practical skills and problem solving based on the following content statements:
Comparison of LAN’s, WAN’s, Intranet and Internetwork in terms of transmission media, bandwidth, geographical spread and functions Distinction between a mainframe with terminals and a network of computers. Descriptive comparison of peer-to-peer networks and client server networks. Description of the functions of file, print and web servers. Descriptions of bus, star, ring and mesh topologies using the terms node and channel, and the consequences for each of node or channel failure. Simple description of the functions and uses of a hub, switch and router. Identification of the need for a network interface cars (NIC) Descriptions of the following technical 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.
Description of the misuse of networks for the illegal purposes: breaching copyright, hacking and planting viruses. Description of the application of the Computer Misuse Act, the Copyright Designs and Patents Act to the misuse of networks.
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Computer Networks A network is defined as an interconnected set of independent computers connected by a communications channel. Networks are used to transmit and share data as well as enable communication. • Sharing system resources e.g. file space, programs, printers • Sharing information e.g. common files, multi-user files, e-mail Sharing of resources can save money on hardware, as well as increasing the work performance, as very little work has to be duplicated.
Mainframe A mainframe is a large, very powerful computer system that can occupy a whole room. It can be connected to thousands of Terminals, operating 24 hours a day. High processing speed, centralised storage and processing. Thorough backup, recovery and security system.
Terminals A dumb terminal has no CPU or local storage devices, as all processing and storage is done within the mainframe.
Server Far less expensive than a mainframe system. Stores the data files for all users on the network. Computers would be attached to a server, with part of the applications to be used installed on each computer hard drive.
Network Computer Each networked computer has its own CPU and backing storage.
Clients Thin clients are workstations that can be relatively low specification since they run virtually all their applications on the server. Thin client systems are very low maintenance because all upgrades are done on the server, not the stations. They are best for situations where users are restricted to a small number of applications, or even just an Internet browser. - 28 -
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Comparing Networks Local Area Networks (LAN’s) Functions
Used for sharing data files, peripherals and enabling communications via e-mail.
Geographical Spread
Limited to one building or office, usually up to 2km
Transmission Media & Bandwidth
Twisted-pair copper cable( 10-100Mbps) , fibre-optic, coaxial cables Wide Area Networks (WAN’s)
Functions
Supports transfer of files, communications via e-mail, shared use of multi-user databases, video conferencing
Geographical Spread
No physical limitation – world wide.
Transmission Media & Bandwidth
Depends on nature of telecommunications link e,g, dial up modem (56 kbps), ISDN (128 kbps), leased line (44.7 Mbps), broadband(up to 8 Mbps) Intranet
Functions
Internal communication within an organisation. Supports internal e-mail and file sharing only.
Geographical Spread
No physical limitation, could be world wide – but limited to the one company – no outside access.
Transmission Media & Bandwidth
Same as WAN’s for large distances Internetwork
Functions
Consists of several networks joined by devices such as routers or switches. Same functions as a LAN.
Geographical Spread
Can vary from linking several LAN’s in the same building to across the city or country.
Transmission Media & Bandwidth
Depends on network. If linking several LAN’s in same building then high-speed fibre optic cable (up to 100 Gbps). A wider spread would use the telecommunications system.
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Network Types There are two main types of network and you should be able to compare each one.
Client/Server Network Client work stations log-on to the File Server to gain access to the network resources. The file server may be a desktop computer but with more RAM and faster or multiple processors. Services:
password security – users log-on, access rights set network storage facilities for user files internal and external e-mail communications applications managed centrally centralised storage of user data files support services – virus checking, backup, etc
Advantages:
centralised software control sharing of resources – printers, modems, disk space easy to extend and upgrade security and backup facilities available centrally
Disadvantages:
server or cable failure trained personnel to manage large LAN’s
Peer – to – Peer Network All machines are connected and equally resourced. One station may act as a file server, while another acts as a print server – as well as ordinary stations to be used, no one station has control. Information passes from one to the other. This kind of system works well in a home or small office. Advantages:
cheap and easy to operate.
Disadvantages:
security - packets of info pass through each station on route to the destination; bottlenecks if several users want to access the same resource at the same time; difficult to track where the file is actually stored; must be careful to ensure compatibility of software versions across each machine.
Once the number of computers connected together goes above 5 or so, it becomes more practical to use a dedicated machine to control and provide access to the network resources.
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Server Types A network may consist of the following servers: • File Server Provides central disk storage for user programs and data on the network. Has centralised backup facilities. Should be designed so as to serve as many users in as fast a time as possible. Runs 24 hours a day, 7 days a week. • Print Server Receives the print jobs from the client stations, forming a queue. The jobs are spooled to an area on the hard disk and send to the printer one at a time, when the printer is ready to receive them. • Web Server Software running on a network station that sends out web pages to requests received from users. This server is connected to an outside communications channel by means of a router (similar to a modem).
Network Operating System The main function of a network operating system is to allow users to treat resources on other computers as if they were local resources. Resources could be physical devices, software applications or files. Functions provided include: • • • • •
mapping directories/drives of remote machines; copying remote files to local drives; managing different operating systems on the same network; ensuring data integrity; providing privacy and security of network resources;
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Additional Network Hardware NIC
a network interface card is fitted inside each network station to provide the physical connection and the electronics needed to connect it to a LAN. A NIC can uniquely identify itself and converts the signals from the station to a form that can be transmitted over the connection.
HUB A hub connects all nodes on a network. When a signal is received from one node a modern hub amplifies and then broadcasts the signal to all connected nodes. Hubs divide the network into a series of different segments, thus reducing the risk of a cable fault crashing the whole network. Stackable hubs can have 6, 16 or 24 ports and can be used to connect small LAN segments together to form larger LAN’s.
Advantage: hubs have an automatic shutdown capability which can be used to block the connection of any port with problems, thus isolating it. Disadvantages: the bandwidth is divided between nodes attached to the hub. Use: mostly used in home networks
ROUTER
A computer with two network cards which is responsible for routing data packets from one network to another. It keeps track of the IP addresses of the computers on the networks connected to its network interface cards and directs IP packets to the correct destination over the most efficient route available. Use: routers are used to connect smaller networks into larger internetworks. They can be used to divide larger networks into smaller, more manageable units, thus improving performance.
SWITCH
(a switching hub) Switches are used to connect nodes to a network. Each port on the switch gives the connected nodes the full bandwidth available on the network. A switch makes point-to-point connections between nodes, so messages being sent across a network are not broadcast to all stations. This cuts down network traffic and improves performance.
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Network Topologies A topology refers to the structure of a network – the way the nodes and server are connected. A Node is any device attached to a network e.g. work station, server, printer. A Channel is a path over which data is transmitted between one computer and another e.g. cables or wireless link Bus Topology
All nodes connected to a single channel. Used in LAN’s. The channel is often twisted pair or copper wires. Each node recognises and accepts only its own message. Advantage: a node failure will not disrupt the network, easy to expand and cheap to set up. Disadvantage: a channel break crashes all communication
Ring Topology
Each node is connected to a single channel whose ends join to form a circle. Data packets travel round the channel in one direction only, with each node passing on the packet until the destination is reached. Advantage: support high data transfer rates, very stable. Disadvantage expensive to install because of electronics. A bypass switching mechanism required to avoid node failure. Channel failure crashes the network.
Star Topology
All nodes are connected via their own individual channel to the central hub. Used to connect LAN’s together. Advantage: node or channel failure does not disrupt network. Disadvantage: cabling costs are high, central node failure crashes network. Congestion at central node can slow down network.
Mesh Topology
Multiple channels between the nodes on the network. Data can be sent through one of several routes through the network. Advantage: channel or node failure does not disrupt network. Transmission bottlenecks are avoided with alternative routes. Disadvantage: additional cost of cabling, connections and network maintenance.
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Widespread Use of Networks As the benefits of connecting computers together in a network have become more obvious, there has been an increased demand for the hardware and software to make it possible. The demand for this hardware has increased competition and manufacturers have been forced to make the technology easier to use and cheaper to purchase. Wide Area Networks (WANs), and in particular the Internet, have become increasingly popular due to the decline in the cost of communication and telecommunication technologies. This has meant that as more and more people are connected, new markets have opened up for the sale of goods and services.
Technical Improvements Computer hardware continues to evolve at a very fast rate and the cost of even more powerful computing systems constantly decreases. They can support high-speed, high quality data transmission between networked computers. • Processors Concentrates on key features such as clock speed, bus widths and use of cache. • Memory Capacity and type of memory used; Cache that can be included on or off the chip • Backing Storage Large capacity devices can be connected directly to the network as a Server. •
Data Transfer Rates The demand for bandwidth, particularly where WANs are concerned has always outstripped the ability of networks to provide it. The bandwidth available to any individual depends on the number of people they are sharing that part of the network with, as well as the rate at which data can be transmitted. Improvements in compression and caching have helped. •
Improved Network Related Software Key features in this software evolution include the fact that nearly all network software now has a graphical interface as well as increased functionality. A workstation on a network can treat all devices on that network such as disk drives or printers in exactly the same way as if they were directly connected to that machine. Browser Software – easy to use with wide range of features Communications Software – works in the background to transmit data Networking OS – GUI allows easy file sharing, security, setting access etc
As the economic demand for networking technology has grown, the trend has been for equipment prices to fall and performance to increase.
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Social Implications Many people feel that the Internet has accentuated the difference between the rich and the poor. People in western countries are described as "Information rich" whereas most people in the Third World are "Information poor", since connecting to the Internet requires expensive equipment and access to a networking infrastructure that does not exist in many poor countries. There is a fear that the increased use of electronic communication will mean that many people will become physically more isolated from each other. The Internet encourages the creation of global communities, but may result in neighbours not seeing each other for weeks on end. Problems with chat rooms – who are you really talking to?
Netiquette Some basic network rules of etiquette are: • do not use other peoples identities and passwords; • typing mail messages all in upper case is considered SHOUTING! and rude; • do not send people chain letters or hoax virus warnings; • avoid sending anyone spam; • do not attach large files to an email message.
Ethical Implications With modern communications networks it is theoretically possible to trace the movements of any individual around the world - they use cash machines, pay with credit cards, use the Internet, use mobile phones, appear on surveillance cameras, and may use electronic road tolling systems. Organisations who manage LANs are becoming concerned about the security and employment issues surrounding email, and are starting to monitor all email activity on their networks to make sure that their employees are not divulging company secrets or using the network facilities for their own personal use. Encrypting data is a method of coding it, in order to make it difficult or even impossible for someone to read it unless they have authorisation from you. Some governments make encrypting data illegal, others regard it as an individual right. As with all security systems, there is a trade-off between security and convenience. The more secure you make a communication system, the more inconvenient it is to use. What is legal in one country may not be legal in another. Not all governments encourage the free debate of political subjects and many governments try to block access to opinions or political debate that is critical of them. Many people believe that children should be protected from violent, pornographic or extreme political material, though of course what is considered to be extreme in one society is not always considered to be extreme in every society. - 35 -
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Censorship From the point of view of those wishing to impose censorship, the world-wide nature of the Internet makes it very difficult if not impossible to control. An international standard of "rating" web pages for violent or sexual content has been developed which makes controlling access easier. Many educational institutions and some Internet Service Providers install filtering software on their Proxy Server which can be set to deny access to web pages or news groups which contain offensive, violent or pornographic material. Commercial organisations may deny their users access to entertainment or other services because they believe that their employees should only access material which is relevant to their work. Inevitably what should or should not be banned is a matter of considerable debate, and is difficult to get right. Too strict a policy on banning access to parts of the Internet can deny access to legitimate material. Too lax a policy can allow access to unsuitable material. Hackers can illegally gather information about a network and valid accounts on that network. They can then use this information to break the security system and gain unauthorised entry. Networks are very efficient at transferring data across the world. Unfortunately people use this capability to break copyright. The Internet has made the distribution of software cheap and easy, but of course this also means that the distribution of illegal copies of software is just as easy. Software piracy is a major problem in many parts of the world. Many people argue that software piracy increases the cost of software because distributors need to charge more to recover the cost of research and development if only a percentage of those using the software are paying for it. • The Computer Misuse Act This Act is designed to make all unauthorised entry into a computer system illegal and so is specifically aimed at the hacker. It is designed to stop people planting viruses, gaining unauthorised access, corrupting or modifying material without permission. • Copyright, Designs and Patents Act This Act protects software copyright. Authors have the same rights as authors of books or music. It is an offence to copy or adapt software without authority. It is illegal to translate one program in one computer language into an equivalent in another language.
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Topic 5: Computer Software DETAILED CONTENT LIST The pupil must demonstrate knowledge and understanding, practical skills and problem solving based on the following content statements:
Describe the function of a bootstrap loader Describe and give examples of the main functions of a single user operating system: interpreting keyboard commands, file management, memory management, input/output management, resource allocation, managing processes Definition of a Utility program Description of 4 utility programs (virus checker, disk editor and defragmenting) Description of the standard file formats for graphics: jpeg, gif, bit-map, TIFF Description of suitable selection of software to support typical tasks including the production of a multimedia catalogue, setting up a school LAN and development of a school website. Description and examples of software compatibility issues – including memory and storage requirements, and OS compatibility Classification of viruses by type of file infected: file virus, boot sector virus, macro virus. Description of the following virus code actions: replication, camouflage, watching, delivery. Distinction between a virus, worm and Trojan horse. Description of anti-virus software detection techniques: use of checksum, searching for virus signature, heuristic detection and memory resident monitoring.
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Computer Software A processor can do nothing without instructions. These instructions with their associated data make up the programs or software that are needed to do the various tasks in a computer. Software can be divided into the following categories: • •
system software application software.
System Software This is the set of programs used to operate and maintain a computer system. It includes the operating system and utilities such as a disk formatter. The system software provides a layer of software that lets users operate the computer without having to know about the underlying processes that are going on all the time.
The operating system is responsible for managing the hardware and communicating with the user. It can be broken down into various layers which then allows manufacturers to update only part of the operating system if necessary.
Older computers often had their operating system stored in ROM. This meant that it was ready as soon as the computer was switched on. It was impossible to corrupt the system but very difficult to upgrade it as it involved changing the ROM chips on the motherboard. Most modern computers keep their operating system on disk and load it into RAM on startup. The main advantage of this is that it is very simple to upgrade the operating system. It also means that only those parts of the operating system that are really needed have to be loaded: other parts can be left on disk until needed. The main disadvantage is that other programs can corrupt the operating system. However, even modern disk-based operating systems need a small part in ROM, called the bootstrap loader. The processor runs this program on startup, and its task is to load in the rest of the operating system from the hard disk.
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Single-user operating systems The task of a single-user operating system is to manage all the resources of a computer system, hiding their complexity from the user and the rest of the system. The main functions are as follows:File Management The operating system supervises the creation, deletion and updating of files. A directory has to be kept which keeps track of where files are stored, as well as filename, id, type, date created, last modified, filesize and access rights. This is usually a hierarchical directory structure in which files are placed in directories and subdirectories. Memory Management The operating system has to decide where programs and data are to be placed in memory and, in a multi-tasking environment, keep track of what stage each program is at and ensure that no program tries to access the memory space of a different program. This applies especially to the operating system’s space. If it is corrupted in some way, it could cause the whole computer system to crash. Interpreting User Commands The operating system has to take user commands from the keyboard, mouse etc., interpret the commands and pass them to the appropriate part of the operating system which deals with that command. If the command cannot be understood, a suitable error message should be given. Input/Output Management All peripherals work at different speeds and have different characteristics. The I/O system hides these differences and makes them all appear to operate in a similar manner. It is the task of the I/O system to do all the actual data transfers and issue the appropriate control signals to the peripherals. The computer manufacturer normally supplies the basic I/O operating system as only they know the details of the initial hardware in their computer. If extra hardware is bought for the computer, say a printer, then the printer manufacturer will supply the appropriate software (often called a driver) to handle that peripheral. It will then be up to the user to install that driver into the operating system on the hard disc. Resource Allocation This refers to the process of allocating memory and CPU time to programs. In a single-use system the only processes are those being generated by the user or processor. Process Management The operating system has to decide which of the tasks in a multi-tasking environment gets to use the processor. In a single-program operating system it only has to choose between itself and the package being run, but on systems which can run several programs simultaneously this becomes much more complex. Process management is usually called ‘the kernel’ as it is the core function of the operating system.
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Network operating systems All stations on a network should have the same operating system to make it easy to communicate with each other. One feature of a network operating system that is additional to those described in the single-user system, is the ability to handle the messages from other stations on the network. This includes tasks such as addressing (putting the receiver’s and sender’s address on the message being sent), looking at incoming messages and checking if they are for that particular station.
Data integrity The network operating system also has to ensure data integrity and provide error correction features. When data has been sent and a transmission error is detected, the network operating system has to inform the sender that an error has occurred and the data has to be retransmitted.
Multi-user access A user has to log on to the network by giving an identity and a password before being allowed to work on the network. The identity controls what access that user has on the network. This is usually done by placing them at a specific place in a hierarchical structure and letting them have access to what is below them in the hierarchy. In a typical situation, a user will have read and write access to his/her own files and read access only for shared files.
Encryption of data On a ring or bus network all communications pass through every station. Thus the sending of passwords (or any other sensitive data) across the network has to be protected from the view of other stations through which the message passes.
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Utility Programs Utility programs are programs that aid the maintenance of the computer system or make the user’s life easier. They are not normally essential for the running of the computer and are normally kept on disk until needed. They are sometimes supplied with the operating system and sometimes bought as extras. They include such programs as a disk editor, disk recovery (for repairs, corrupt files etc.), programs for defragmenting a disk, virus checking etc.
Fragmentation Most modern filing systems save files wherever they can on a disc. They will attempt to save a file in consecutive sectors but, as time goes on and files are saved and/or deleted, this is not always possible. So the file may be saved with part of it in one sector, another part in a sector in a different part of the disc etc. This will obviously slow down the loading of the file as the drive head will keep having to move to different parts of the disc to load the file. A defragger is a utility program that moves files (and parts of files) around the disc until each file is stored in consecutive sectors with no fragmentation. •
Virus Checker
This utility checks your system for virus software. It is essential that this utility is run on a regular basis. •
Disk Editor (Clean Up)
A disk editor enables you to get rid of all unwanted files that can clutter up your system, some of which you might not know exist – temporary internet files, temporary files used by applications, offline web pages. •
Recovery
This utility is used to restore files that have been corrupted. The utility normally has a wizard which will help you locate an intact copy of your files from a backup and use it to replace the corrupted files.
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Application Software Although you do not need to know about every software package, you need to be able to distinguish between them in order to correctly match a computer specification to task requirements and to minimise expenditure. Selection of appropriate software can be complicated by general purpose packages which may include separate modules for processing text, spreadsheet data, databases, graphics and communications. • • • • • • •
text number data communications graphics multimedia integrated packages.
(word processor, text editor, web browsers); (spreadsheet, accounts packages); (database, world wide web); (www, e-mail, file transfer); (painting, drawing, CAD); (including sound and video);
File Formats Each package type is capable of saving data in a range of formats. It makes sense to save in a standard file format if data files are to be used in other packages. Standard file formats for graphics include:•
.gif
• • •
.jpeg joint photographic expert group, still picture compression
•
.bmp bit mapped, image file format used on windows paint programs
graphics interchange format lossless compressed images(no detail lost), allows inter-laced images a lower quality version of the image is displayed as the rest is loading
.tiff
tagged image file format. Used for large high-res bit mapped images
Compatibility When buying software applications it is important to ensure that they will not conflict with your operating system or your hardware. The main factors to consider are:
Memory requirements; Storage requirements; Operating System.
A program written for an OS from 1995 may not work properly on a computer that is using a more up-to-date version of the OS. Device drivers are another area requiring checks. Most drivers come with the operating system but some peripherals may need a new driver installed when you connect the device to a new computer.
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Computer Viruses A virus is a piece of programming code that causes some unexpected and usually undesirable event in a computer system. They are often designed so that they automatically spread to other computer users on a network. Viruses are classed by three main types: • • •
File virus; Boot sector virus; Macro virus.
File viruses attach themselves to program files such as .exe or .com files. When the program is loaded, the virus is also loaded. A file virus can also take the form of a complete program, or script, attached to something else, e.g. an e-mail. They then take up residence in the computer ready to cause havoc. Boot Sector viruses infect executable code found in certain system areas on a disk. They attach to the boot sector on disks or the master boot record on hard disks. To infiltrate the boot sector, the virus is read while the system is running and then activated the next time the operating system is loaded. Macro viruses are fairly common viruses, but tend to do the least damage. They infect applications and typically cause a sequence of actions within the application e.g. inserting unwanted words or phrases in a document.
Virus Actions Viruses don't all follow the same course of action. They can, and do, use a combination of the following actions: • • • •
Replication; Camouflage; Watching; Delivery.
Replication. A computer virus has the unique ability to replicate. They can attach themselves to almost any type of file and spread as files are copied and sent between computer users. A virus can take a long time to replicate itself before activation. This gives it time to be spread over many computers before being discovered. Camouflage. It is possible for a virus to avoid detection by taking on the characteristics that detection software is programmed to look for and ignore. Today's anti-virus software does much more than simply check particular characteristics (or signature) of a virus. They also check the virus code and even checksum the virus code to identify it. With these cross-checks it would be extremely difficult for a virus to camouflage itself and get past detection.
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Watching. A virus can lie in wait and ambush a computer when something routine is carried out e.g. opening a particular application. The damage routines may activate on a certain date, or when the infected user performs a particular action may trigger the virus. Delivery. Infected disks brought in from the outside used to be the main source of viruses until e-mail provided the ideal delivery vehicle.
Virus Forms A worm is a self-replicating virus that does not alter any files but takes up residence in the computer's active memory and duplicates itself. They only become noticeable once their replication consumes the memory to the extent that the system slows down or is unable to carry out particular tasks. Worms tend to use the parts of the computer's operating system that is not seen by the user until it is too late. •
A virus tends to spread from file to file, whereas a worm spreads from one computer to another, usually through security holes in a network.
•
Worms spread themselves by attaching copies of themselves to e-mail documents and TCP/IP packets and using them to move to other e-mail servers and from there to user systems.
•
Worms are used to flood a network with useless traffic which overwhelms the networks processing capability and halts communication.
A Trojan horse is a program where harmful code is contained inside another code which can appear to be harmless. Once the apparently harmless code is in the computer, it releases the malicious code to do its damage. Trojan horses may even claim to be anti-virus in order to get the user to install it. A classic Trojan horse activity is to pretend to be a network login screen so that it can steal a user ID and password which it then e-mails to a hacker or stores in a file that hackers can access.
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Protection against Viruses The best protection against a virus is to know that each file you open from an e-mail, disk or from the web is free from any virus. This requires anti-virus software that can screen e-mail attachments, web downloads, and checks all of your files from time-totime removing any viruses that are found. Techniques used by ant-virus software to detect a virus include: • • • •
Checksum; Signature; Heuristic; Memory monitoring.
Checksum. In checksum detection, a checksum of key files (boot an executable) is recorded at source and stored in the system. When these files are called to execute the checksum is calculated and compared with what it should be. If there are any anomalies, then the file about to be run could have be infected and a warning given. There is also the facility to recalculate the checksum when boot or executable file are legitimately altered by the user Signature. A virus signature is a unique pattern of bits within a virus. It can be used to detect and identify specific viruses. Once known, the anti-virus software uses the virus signature to scan for the presence of malicious code and removes it. Heuristic. Heuristic detection describes the technique of approaching a problem through past knowledge. The technique is used to find unknown viruses that have not yet been identified by their signatures by looking for characteristics in files that have previously been associated with a known virus. Heuristics can also detect a virus that has disguised its signature, by recognising a particular characteristic of the virus behaviour. Heuristic techniques work on the probabilities of a file being infected i.e. it will give a percentage likelihood of a file being infected by weighing up the file behaviour. For example, if a file attempts to access your address book then that might be suspicious. If the same file includes code that checks a date, then the suspicion rises. There will come a point when a warning is issued on the possibility of a virus. Memory Monitoring. Some anti-virus software is memory resident and is loaded on start up. It actively monitors the system for viruses whilst the computer is switched on and checks programs for infection every time they run. This will include the boot files on start up, checking any disk as it is accessed, checking any files accessed during operation and checking any files being loaded on the hard drive. The price to be paid with memory resident programs is that they can cause delays in program loading and execution whilst the checks are being carried out. Literally thousands of viruses have been detected and catalogued using a Virus Information Library (VIL). One such VIL can be found at: http://vil.nai.com/vil/ - 45 -
