Computer systems.

computer systems

1- Data Representation 2 – Computer Structure 3 - Peripherals 4 - Networking 5 – Computer Software

1- Data Representation

Storing Data – Bits, Bytes & Binary
All information used by a computer system must be stored as a pattern of ones and zeros. Each of these 1’s or 0’s used by the computer is called a Binary Digit (or Bit for short). 8 of these bits make up what is called a Byte. (E.g ) The table below shows the units of storage and how to convert between them: Name Value Bit 1 or 0 Byte 8 bits Kilobyte (KB) 1024 bytes Megabyte (MB) 1024 KB (roughly 1 million bytes) Gigabyte (GB) 1024 MB (roughly 1 billion bytes) Terabyte (TB) 1024GB (roughly 1000 billion bytes) Bytes to Bits Multiply by 8 Bits to Bytes Divide by 8 KB to Bytes Mulitply by 1024 Bytes to KB Divide by 1024 MB to KB Muliply by 1024 KB to MB Divide by 1024 GB to MB Multiply by 1024 MB to GB Divide by 1024 TB to GB Multiply by 1024 GB to TB Divide by 1024

Representing Data – Numbers, Text & Graphics
We know that the computer can only store data in binary (1’s and 0’s). This means that all the numbers, text and graphics that we use must be stored as 1’s and 0’s inside the computer. Storing Numbers You & Me This is called the decimal system. In our daily lives, we use a combination of the same 10 digits to represent every number. 1 2 3 4 5 6 7 8 9 The Computer This is called the binary system. The computer is different. It uses a combination of only 2 digits to represent every number. 1

How a computer counts How we count 1 2 10 3 11 4 100 5 101 6 110 7 111 8 1000 9 1001 10 1010 11 1011 12 1100 etc.

Decimal Numbers The table below shows how the decimal system (base 10) is used to represent numbers: Thousands Hundreds Tens Units 103 (10 x 10 x 10) 102 (10 x 10) 101 100 1000 10 1 Number 9 99 3 4 7 1347 The number ninety-nine in decimal is written as 99 (9 tens and 9 units).

26 25 23 21 20 Binary Numbers 27 24 22 128 64 32 16 8 4 2 1 Decimal 3
This table below shows how the binary system (base 2) is used to represent numbers: 27 (2x2x2x2x2x2x2) 26 (2x2x2x2x2x2) 25 (2x2x2x2x2) 24 (2x2x2x2) 23 (2x2x2) 22 (2x2) 21 20 128 64 32 16 8 4 2 1 Decimal 3 5 99 100 200 The number ninety-nine in binary is written as (1 x 64) + (1 x 32) + (1 x 2) + (1 x 1)

Why Use Binary? For Base Against
There are only 10 types of people in this world – those who understand binary, and those who don’t. The binary number system (base 2) is simpler than the decimal number system (base 10). Computers are able to represent all data (including numbers, text, graphics, videos, sounds) as combinations of binary digits (0s and 1s). Computers therefore represent 0 and 1 by means of electronic components (e.g. magnetic particles on hard disks, pits and lands on CD/DVD). This is why computers are called two state machines. BINARY It’s as easy as 01, 10, 11 Comparing Number Systems For Base Against Familiar to humans as it is in common use. Base 10 Decimal Too many symbols – 0,1,2,3,4,5,6,7,8 & 9. Too many rules for +, -, × and . Difficult to represent – requires 10 voltage levels (signal degradation). Circuits for every combination of two digits e.g. 100 rules for adding. Only two symbols – 0 & 1 0 & 1are easy to represent by a voltage and no voltage. Only a few circuits required as the only combinations for e.g. adding are 0+0, 0+1, 1+0 and (4 rules) Base 2 Binary Numbers are long. Difficult for humans to read, recognise, remember and copy. Value is “hidden” from humans.

Range of Values that can be Represented in Binary
As this table shows, the greater the number of bits used to represent a number, the greater the range of numbers that can be represented. Number of bits (n) Range (0 – 2n-1) Number of values (2n) 1 (i.e. 0 and 1) 0 – 1 2 (i.e. 00, 01, 10 and 11) 0 – 3 4 (i.e to 1111) 0 – 15 16 8 (i.e. the number in 1 byte) 0 – 255 256 (i.e. the number in 2 bytes) 0 – 65,535 65,536 24 (i.e. the number in 3 bytes) 0 – 16,777,215 16,777,216 32 (i.e. the number in 4 bytes) 0 - 4,294,967,295 4,294,967,296

Storing Integers The integers (from the Latin integer, which means with untouched integrity, whole, entire) are the set of numbers consisting of the natural numbers including 0 (0, 1, 2, 3, ...) and their negatives (0, −1, −2, −3, ...). They are numbers that can be written without a fractional or decimal component, and fall within the set {... −2, −1, 0, 1, 2, ...}. Decimal to Binary One technique of converting decimal numbers to binary, involves repeatedly dividing by two until we reach the answer zero and then read the remainders upwards. For example, let’s convert the integer 47: 2 47 We start with 47 units. 23 r 1 We now have 23 groups of two and one unit over. 11 We now have 11 groups of four and one two over. 5 We now have 5 groups of eight and one four over. We now have 2 groups of sixteen and one eight over. 1 r 0 We now have 1 group of thirty-two and no sixteens over. We now have no groups of sixty-four and one thirty-two over. Therefore 47 in decimal is in binary. (Note: we would normally add leading zeroes to show the value in one byte i.e ).

Storing Integers Binary to Decimal
To convert binary to decimal we simply use the table of place values that we looked at earlier. For example let’s convert to decimal: 128 64 32 16 8 4 2 1 = 154 Therefore in binary is 154 in decimal.

Representing Negative Numbers
Of course, computers must also be able to store negative numbers. One possible solution is to indicate the sign (+ or -) by using the most significant bit (i.e. the leftmost bit). Thus a 0 indicates positive while a 1 indicates negative. For example: 27 would be while -27 would be Sign Bit 0 or 1 + or - 64 32 16 8 4 2 1 = 27 0 or 1 + or - 64 32 16 8 4 2 1 = - 27 Though this seems to be a reasonable system, it has problems. (See next slide)

Representing Negative Numbers Continued…
Problems with Sign Bit Binary Integer 0000 + 0 0001 + 1 0010 + 2 0011 + 3 0100 + 4 0101 + 5 0110 + 6 0111 + 7 1000 - 0 1001 - 1 1010 - 2 1011 - 3 1100 - 4 1101 - 5 1110 - 6 1111 - 7 Though this seems to be a reasonable system, it has problems. Let us consider values using 3 bits, with the 4th bit used as the sign bit. Here is a table of the values starting at 0 and adding 1 each time: There are two problems here: There are two ways to represent zero! (0000 = +0 and 1000 = -0) Arithmetic doesn’t work! (Adding 1 onto 7 we get -0!) The solution? Use Two’s Complement! 0111 0001 1000 ??

Twos Complement When numbers are stored using two’s complement, the leftmost bit has a negative place value. So with 8 bits, we have the following: -128 64 32 16 8 4 2 1 Decimal Value = 14 = -125 = -64 = -108 To represent negative values we: - work out the binary number for the positive value - reverse the 1s and 0s (1s become 0s and 0s become 1s) - and then add 1. Method 1 -5 -12 -25 1. Work out the binary value for the positive number (using 8 bits) 2. Reverse the 1s and 0s 3. Add 1 +1

Twos Complement Continued
An easier method to get the two’s complement for a number is as follows: - Starting from the right, find the first ‘1’ - Invert all of the bits to left of that ‘1’ Method 2 -5 -12 -25 1. Starting from the right, find the first ‘1’ 2. Invert all of the bits to left of that ‘1’ Examples of Arithmetic 5 + (-3) = = 2 Ignore extra bit 3 + (-7) = = -4

Range of Values that can be Represented Using Twos Complement
The number of bits allocated to store each integer value determines the range of values which can be stored: Number of bits (n) Range (-2n-1 to 2n-1) Number of values (2n) 2 (i.e. 10, 11, 00 and 01) -21 to +21-1 -2 to 1 4 (i.e to 0111) -23 to +23-1 -8 to +7 16 8 (i.e to ) -27 to +27-1 -128 to 127 256 (i.e to ) -215 to to 32767 65536

Storing Real Numbers Decimal Sign Mantissa Exponent Equivalence Binary
We know that positive and negative integers can be stored using a combination of 0’s and 1’s. Since storing very large numbers would require too many digits, these numbers can be stored in a computer using a technique called floating point representation. This notation represents numbers as a base number and an exponent. For example, in decimal could be represented as x 102, with base = 10 and exponent = 2. Using floating point the number is stored in 2 parts: - The mantissa is the actual digits that make up the number - The exponent gives the position of the decimal point or binary point Mantissa Exponent Examples of floating point representation in decimal: Decimal Sign Mantissa Exponent Equivalence + +3 x 103 - .2639 -2 x 10-2 Examples of floating point representation in binary: Binary Sign Mantissa Exponent Equivalence + +011 x 23 - .1011 -110 x 2-2 The range of the number is represented by the exponent, while the precision is represented by the mantissa.

Why do Higher Computing Students mix up Halloween and Christmas Day?
Hilarious Computing Jokes to Tell Your Mates Why do Higher Computing Students mix up Halloween and Christmas Day? A. Because OCT 31 = DEC 25 There are only 10 types of people in the world. Those who understand ternary, those who don’t and those who confuse it with binary!

Storing Text – ASCII vs. Unicode
The set of characters that can be represented by the computer is known as the character set. Many computers have the flexibility of using several character sets, but we will restrict our discussions to ASCII and Unicode. ASCII ASCII is a 7-bit code which provides 128 code values. This gives us 96 characters and 32 control codes. Many systems use extended ASCII code which is an 8-bit code providing a range of 256 characters. The table opposite shows the ASCII codes for a range of characters. Unicode What you should know about Unicode Unicode’s advantages over ASCII Unicode is a 16-bit code which supports characters This is many more than ASCII code, enabling Unicode to define a code for characters: of every character-based alphabet in the world - of the large ideographic languages such as Chinese, Japanese and Korean - for all punctuation symbols and control characters The first 256 characters in Unicode are used to represent ASCII code. This makes conversion between the two codes easy Of the characters, codes are predefined and 6400 are reserved for private use This means they can be defined by the user of the software This still leaves around characters in the code not yet made use of These can be used in future developments Note: Unicode file sizes are large because it takes 2 bytes to store each character. This places greater demands on storage capacity and increases time taken to transmit files across a network.

Representing Graphics
Bitmap Graphics Bitmap representation of graphics means that each pixel in a graphic is represented by a series of bits. Bitmaps are typically used for creating realistic images e.g. photographs. Black & White Bitmaps Here is an example of a graphic that has been enlarged so that you can see each pixel. When storing black and white graphics the computer uses a very simple system. A graphic is made up of a grid of pixels and stored in memory as a bit pattern of 1’s and 0’s. Each black pixel is represented by a 1, while each white pixel is represented by a 0. 1 100 Pixels = 100 Bits (or 12.5 Bytes)

Representing Graphics
Colour Bitmaps 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 colour bit depth. Advantages of Bit-mapped Graphics They allow the user to edit at pixel Disadvantages of Bit-mapped Graphics They demand lots of storage space 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. I you reduce the resolution, the system reduces the size of the pixel grid and eliminates pixels, which in turn reduces the quality of the image. Number of bits per pixel Colours or shades of grey represented 1 2 4 8 256 16 65,536 24 16,777,216 (true colour – the max number human eye can distinguish) Increasing the number of bits per pixel increases the file size Resizing bit-mapped graphics causes problems. If you resize a bitmap graphic upwards it has the same number of pixels, and so the image becomes pixellated, the edges jagged. If you resize downwards it becomes dense. You cannot isolate and edit an individual object in a graphic and edit it

Relationship between Bit Depth and File Size
Let’s look at the file size of a tiny 1 inch square graphic. Resolution (pixels per square inch) Pixels per 1 inch sq graphic Number of bits representing each pixel Number of colours available File size in bytes File size in megabytes 600 x 600 360,000 8 bits (1 byte) 256 0.343 16 bits (2 bytes) 65,536 720,000 0.687 24 bits (3 bytes) 16,777,216 1,080,000 1.030 The more bits that are used to represent a pixel, the more colours you get, but the greater the file size. If the graphic was larger, say 6 inches square, then the table looks like this: Resolution (pixels per square inch) Pixels per 1 inch sq graphic Number of bits representing each pixel Number of colours available File size in bytes File size in megabytes 600 x 600 12,960,000 8 bits (1 byte) 256 12.36 16 bits (2 bytes) 65,536 25,920,000 24.72 24 bits (3 bytes) 16,777,216 38,800,000 37.8 Why is Compression needed? You can see from the tables that sizes for bit-mapped graphics can be very large. This means that: - they demand lots of storage space - they can take quite a time to transmit across a network Compressing the files means that less space is required for storage and transmission times can be reduced.

An Illustration of Bit Depth
1 Bit – Monochrome – 2 Colours 4 Bit – 16 Colours 8 Bit – 256 Colours 24 Bit – True Colour – 16 Million Colours

Vector Graphics Advantages of Vector Graphics
In vector graphics, the system stores 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 Where there are several objects in an image, the vector graphic file will store information about the layering of the objects. The definition of a circle might hold: - the position of the centre - the length of the radius - the with and colour of the line marking the circumference - the colour/pattern of the infill Advantages of Vector Graphics You can edit individual objects in a graphic They are resolution independent. If an object is displayed on a system with high resolution output it will display perfectly. You can build up an image by layering objects Vector Graphics take up less storage space than a bitmap graphic equivalent When resizing a vector graphic, it will change in proportion and maintain smooth edges. (x,y) - 100,200 Radius – 2.5 Width – 1.5pt Line – Pink Fill – Pale Blue

2 – Computer Structure

Back to Basics The animation below illustrates how RAM is used to store programs while they are running

Back to Basics You should already be familiar with the following diagram BACKING STORAGE INPUT OUTPUT RAM ROM MEMORY Cache ALU Control Unit Registers PROCESSOR CPU

Back to Basics SRAM Vs DRAM RAM BACKING STORAGE INPUT OUTPUT RAM ROM
To execute a program you must first load the program and any relevant data from backing storage into the computer’s memory (RAM). BACKING STORAGE INPUT OUTPUT RAM ROM MEMORY Cache Registers Control Unit ALU PROCESSOR CPU SRAM Vs DRAM SRAM – stands for static random access memory, a type of memory that is faster and more reliable than the more common DRAM (dynamic RAM). The term static is derived from the fact that it doesn't need to be refreshed like dynamic RAM. DRAM stands for dynamic random access memory, a type of memory used in most personal computers. In DRAM to store 1 bit of information 1 transistor and 1 capacitor is used. The information is stored in the capacitor in form of charge, so it required refreshing to retain the charge or data in the capacitor.

Back to Basics BACKING STORAGE INPUT OUTPUT RAM ROM MEMORY Registers
Cache Registers Control Unit ALU PROCESSOR CPU RAM The processor has to be able to pinpoint any memory location needed, so each memory location is assigned an address. This is a unique binary number from zero up to the number of locations -1. The amount of data each storage location can store is known as the word size for that computer.

Back to Basics BACKING STORAGE INPUT OUTPUT RAM ROM MEMORY ALU
Cache ALU Control Unit Registers PROCESSOR CPU ALU The Arithmetic & Logic Unit (ALU) is the part of the Central Processing Unit (CPU) where calculations, and Boolean logic operations (AND, OR, NOT) and comparisons take place.

Back to Basics Control Unit BACKING STORAGE INPUT OUTPUT RAM ROM
MEMORY Cache ALU Control Unit Registers PROCESSOR CPU Control Unit The Control Unit (CU) sends out control signals: - within the processor to move data from one register to another and to activate specific ALU function - to the control bus to read or write from memory - to I/O modules

Distinguishing Between Computer “Memory” - Registers
BACKING STORAGE INPUT OUTPUT RAM ROM MEMORY Cache ALU Control Unit Registers PROCESSOR CPU The Registers Registers are storage locations that are internal to the processor. They are used to: - hold data that is being transferred to or from memory - hold the address of the location in memory which the processor is accessing to read or write data - hold the instructions that are being carried out

Distinguishing Between Computer “Memory” - Cache
BACKING STORAGE INPUT OUTPUT RAM ROM MEMORY Cache Registers Control Unit ALU PROCESSOR CPU Cache Memory This is a small fast memory unit which the processor checks for data and instructions before accessing main memory. When the processor attempts a read from memory, the cache is checked first. If the data is already stored there it is transferred directly to the processor. This saves a read from memory operation which is much slower than cache memory. The overall effect is to speed up system performance.

Distinguishing Between Computer “Memory” – Main Memory
BACKING STORAGE INPUT OUTPUT RAM ROM MEMORY Cache Registers Control Unit ALU PROCESSOR CPU Main Memory This is the main internal storage area for the computer where instructions and data are stored. It is divided into RAM and ROM sections. Reading from main memory is slower than accessing either registers or cache memory. Use of cache memory avoids slower accessed to main memory.

Distinguishing Between Computer “Memory” – Backing Storage
INPUT OUTPUT RAM ROM MEMORY Cache Registers Control Unit ALU PROCESSOR CPU Backing Storage This is the slowest form of memory and is used to store data and software. It retains the data when the power is switched off, unlike the RAM area in main memory.

Distinguishing Between The Different Parts of Memory
You should be able to distinguish between registers, cache, main memory and storage memory according to function and speed of access. This table should help you: Type of Memory Function Speed of Access Registers Internal to the processor. Holds data while being processed, e.g. Instruction Register Fast access time internal to the processor Cache Stands between the processor and main memory. Processor checks the cache memory for data/instructions before accessing main memory Slower access than a register but faster than accessing main memory Main memory Stores user data and software in RAM and some system software in ROM Next in terms of speedAccessing data in main memory is slower than accessing either cache memory or registers Backing storage Stores data, software retains the data when power is off Slowest of all the types of memory

FETCH-EXECUTE CYCLE – READ FROM MEMORY
0000 0101 0110 0001 1101 0010 0011 0100 1111 1100 0111 1000 1001 1010 1011 1110 ALU COMPUTER TASK: READ the data currently held in memory location 1101 1 1 1 13 Address Bus REGISTERS 1 1 1 1 1 1 Data Bus CONTROL UNIT READ WRITE Control Bus A Processor may need an instruction from memory or require data to perform a calculation. Here are the steps: Processor sets up address lines with the required address (1101) PROCESSOR Processor activates READ line on control bus MEMORY Memory releases data ( ) from memory location (1101) onto data bus Data is received by the processor

FETCH-EXECUTE CYCLE – WRITE TO MEMORY
0000 0101 0110 0001 1101 0010 0011 0100 1111 1100 0111 1000 1001 1010 1011 1110 ALU COMPUTER TASK: WRITE the instruction to memory location 0011 1 1 3 1110 0101 Address Bus REGISTERS 1 1 1 1 1 1 1 1 1 1 1 Data Bus CONTROL UNIT READ WRITE Control Bus A Processor may need to store an instruction or piece of data in memory. Here are the steps: Processor sets up address lines with the required address (0011) PROCESSOR Processor sets up data bus with data to be stored ( ) MEMORY Processor activates WRITE line on control bus Data is copied from data bus into the memory location

Distinguishing Between The Different Parts of Memory
Computer memory is divided up into memory locations. Each location has its own unique address. The processor uses the address to find the data and instructions it needs. The number of memory locations that a processor can address is, in theory, limited by the number of lines on the address bus. Look at this table: Address Memory location contents 16 bit addresses = 16 line address bus Each location stores a 32-bit number Address Bus 3 The maximum capacity of memory is calculated as follows: Maximum capacity = no. of addresses x capacity of each memory location e.g. assuming i) that the width of the data bus matches the capacity of each memory location; ii) a 16-bit data bus; iii) a 24-bit address bus then maximum capacity = 2width of the address bus x 16-bits = 32 Megabytes.

READ WRITE CLOCK INTERRUPT NMI RESET
Control Bus Lines Some common lines on the Control bus are: READ Signal sent from processor to start a memory read operation after the address bus has been set up with address to be read WRITE Signal sent from the processor to start a memory write operation after the address bus has been set up with the address to be written to. READ Clock Signal that synchronises the processors operations by providing pulses along the control line. In a processor with a speed of 2GHz, the clock “ticks” 2000 million times every second and an operation takes place on each tick. WRITE Interrupt An interrupt is a signal to the processor that a peripheral requires to be serviced (e.g. pressing a keyboard key or moving the mouse will generate an interrupt) CLOCK NMI A Non-Maskable Interrupt signal means the processor is required to service the device immediately. E.g. power failure, disk read/write error. INTERRUPT Reset A signal on the reset line returns the system to its original state, stops all processes, clears the registers and RAM and returns system to state when it was first switched on. NMI RESET Control Bus

Measuring System Performance Good Indicator of Performance?
You should be able to describe and evaluate the following measures of performance: Measure Description Good Indicator of Performance? Clock Speed Clock pulses regulate and co-ordinate processor activities Pulses are measured in Megahertz (MHz) and Gigahertz (GHz) 1 MHz = 1 Million pulses per second, 1 GHz = 1000 MHz There is more to system performance than the power of the processor! While salespeople and adverts highlight clock speed, be careful not to overemphasise its importance. Other factors (e.g. Data bus width) must also be considered! MIPS Short for Millions of Instructions Per Second Based on number of machine code instructions that can be processed in 1 second. A rough indication of performance, it does NOT take into account the size or complexity of the instructions being carried out. FLOPS Short for Floating Point Operations Per Second Measures how many floating point operations can be carried out in 1 second. FLOPS can be seen as being a more reliable indicator of performance than MIPS. It is an objective approach measuring the number of clearly definable, arithmetical tasks that can be carried out per second. Application Based Tests Most computer mags use application-based tests (bench mark tests) to compare system performance. Set out series of practical tasks using standard applications packages and award scores to make overall comparisons. (E.g. Spreadsheet - Use a multi-document spreadsheet to carry out statistical changes to 200 rows of data per sheet with related graphs. Database – Three database table opened and filled with 120,000 records, then queried and reports produced Media – Convert 25 minute WAV file into mp3 and then to Windows Media format 3D Graphics – 3D Graphics run at resoluation of 1024 x 768 with 32-bit colour) While the other measures give (to varying degrees) reasonable indicators of system performance, they do not provide evidence of how well a system will perform I a given practical task. Application based tests provide actual, reproducible evidence of system performance. This is why they are seen as reliable measures of performance. Explaining System Performance – Key Points By now, you should appreciate the effect on system performance of: The width of the data bus The use of cache memory The data transfer rate of peripherals (All peripherals operate a slower speeds than the processor, which will slow the system down – for example if the CPU needs to read data from a CD.)

Current Trends in Computer Hardware
Complete the tables below with information on the latest trends: Current Trends in Storage Storage Medium Capacity Data Transfer Rate Hard Drive CD-R, CD-RW DVD-R, DVD-RW Solid State Drive (SSD) Other Portable Storage Current Trends in Processors Manufacturer Clock Speed Data Bus Width Address Bus Width Current Trends in Main Memory Capacity Other trends in Hardware Input Devices Output Devices Other Developments

3 – Peripherals

Peripherals – Compensating for Speed Differences
“A peripheral is a device attached to a host computer...whose primary functionality is dependent upon the host, and can therefore be considered as expanding the hosts capabilities, while not forming part of the system's core architecture.” Wikipedia Buffer A buffer is an area of memory which is used to temporarily store data while it is waiting to be transferred from an input device or to an output device. A printer for example has on-board memory called a printer buffer which lets the computer dispose of its printer output at full speed without waiting for each page to print. If the printer buffer is full, the printer will use the system RAM. Some printers come with upgradeable memory options. Why use buffers? The use of buffers is a technique for improving system performance. Peripherals operate at much slower speeds than the CPU. Using buffers helps the computer system compensate for the differences in operating speeds between the CPU and its peripherals. When transferring data out to a peripheral such as a printer, the faster CPU can transfer data into the buffer then return to other processing tasks. The use of buffers reduces the frequency with which the CPU is interrupted to deal with input. When data is being transferred to the CPU from a relatively slow input device, like a keyboard, a buffer is used to store the data until a significant block of data is assembled for the CPU to deal with. Spooling Spooling is a similar technique used in the transfer of data to a slow peripheral. When spooling, the data intended for the peripheral (i.e. the printer) is transferred to storage, usually the hard disk. This frees up the much faster CPU to process other tasks. Like the use of buffers, spooling is another possible method of improving system performance.

The Need for Interfaces between CPU and Peripherals
Computer peripherals, such as CD-ROM drives, scanners, keyboards and microphones all work differently. They ALL operate at much slower speeds than the CPU They each have particular data transfer rates They may transmit data in either serial or parallel form They use a wide variety of codes and control signals They may even work at higher voltages than the CPU Interfaces An interface is a combination of hardware and software needed to link the CPU to the peripherals and enable them to communicate with the CPU despite all their differing characteristics. The main functions of an interface are: Buffering Converting data between serial to parallel forms Converting data between analogue and digital forms Voltage conversion Protocol Conversion Handling of status signals

The Main Functions of Interfaces
1 - Buffering We have already seen how the interface uses the buffer to compensate for the differences in speed between the peripherals and the CPU by temporarily storing incoming data so that the faster CPU can process it in manageable blocks rather than waiting for the slower peripheral. 2 - Converting data between serial and parallel forms The buses internal to the processor are parallel communication channels. Any data coming from a serial device has to be sent to an interface which buffers the data then converts the data then converts it to parallel form before it is passed to the processor. To understand this clearly you need to know the difference between serial and parallel transmission of data. Parallel Serial 1 Serial data transmission is where data is transmitted along a communication channel one bit after another in sequence. Parallel interfaces transmit several bits of data simultaneously across a series of parallel channels, often transmitting 16 or 32 bits at a time. 3 - Converting data to and from analogue and digital forms A key job of interfaces is to convert the analogue signals that are sent from peripherals to the digital form that the CPU can handle. A mouse click generates an electrical or a wireless analogue signal that is sent to the computer. The interface buffers the signal, changes it to digital form and then sends it onto the CPU. Similarly a digital audio file will be converted to analogue sound waves via an interface between CPU and speakers!

The Main Functions of Interfaces
4 - Voltage conversions Peripherals mainly work at higher voltage levels than CPU. These signals need to be reduced to the CPU’s level and this is one of the jobs of the interface. For example, a signal coming from a keyboard at 9 V needs to be reduced to a level which can be handled by the CPU, a maximum of 5 V. 5 - Protocol conversion Peripherals send data in units of varying sizes and at speeds that are different from those that operate in the CPU. The interface has to deal with the differences between them. 6 – Handling of Status Signals Peripherals and the CPU exchange a series of signals before and during the exchange of data, for example a peripheral will signal that it is ready to accept data. These signals are passed through the interface

Why are wireless connections between peripherals becoming popular?
The Latest Interfaces One way in which computer system performance can be speeded up is by increasing the data transfer rate between peripherals and the CPU. Since the interface plays a key part in getting data to and from the peripherals and the CPU there is a trend towards greater and greater interface speeds. Interface Description Transfer speeds USB Universal serial bus A means of connecting external devices such as scanners, keyboards, mouse, audio equipment to a PC port. Fast transfer rate: 12 Mbps for fast devices and 1.5 Mbps for keyboards and mice. USB2 An improved version of the USB. Three operating speeds of 1.5, 12 and 480 Mbps. Firewire (IEEE 1394) A high speed serial interface used for connecting audio/visual and multimedia applications like digital camcorders, digital cameras and digital TV equipment, music systems. In its latest version, IEEE 1394b, up to a maximum of 800 Mbps. 1.6 Gbps and 3.2 Gbps versions are under development. Why are wireless connections between peripherals becoming popular? Wireless connections have the following obvious advantages: you do not need to have wires trailing all around your desktop; it gives you a lot more mobility and freedom to position you peripherals exactly where you want them without having to worry about cable lengths and extending cables. The most popular way of setting up wireless connections uses Bluetooth. This is a short range wireless transmission system used for the wireless connection of peripherals such as keyboards/mouse, printers and modems to computer systems. It can also be used to connect mobile phones and PDA’s to computer systems. It has a range of 10 metres or 100 metres within a signal booster. Its speed is at present a practical maximum of 720 kbps although by 2005, a 10 Mbps version is planned.

Peripherals – Solid State Storage Devices
Unlike hard drives and CD drives, solid state devices have no mechanical or moving parts. They store data using memory chips which can be written and rewritten to. They are often packaged and sold as ‘flash cards’, compact removable units which fit into your pocket. There are two types: flash ROM and flash RAM. Flash RAM needs power to retain its data and flash ROM does not. Flash ROM uses a type of EEPROM chip, an Electronically Erasable and Programmable ROM chip. It uses an electrical charge to change the value of blocks of data stored in memory. It stores data when the power is off. Flash memory has to be erased and written in fixed blocks generally from 512 bytes to 256 Kilobytes. There are commonly connected to a system via the USB interface and appear on the desktop as removable disks. Recent advances have made Solid State Storage a viable alternative to a traditional Hard Disc Drive. Advantages of Solid State Solid state devices are much faster than mechanical disks: the access time, the time taken to read in the data from a solid state machine is “instant”. They are compact and lightweight and so are highly portable They have no moving parts and so do not make a sound Open casing of 2.5” traditional hard disk drive (left) and solid-state drive (centre). Disadvantages of Solid State They are more expensive per megabyte of storage than a hard disk Flash chips generally have a limit to the number of times they can be written to, normally between The disassembled components of a hard disk drive (left) and components of a solid-state drive (right).

4 – Networking

Network Strategy Network Strategy
A network strategy allows an organisation to manage how data is distributed across the enterprise. It details the types of networking currently used by an organisation and the requirements for future developments. This strategy needs to address the following: - Data transfer - Distribution/coverage - Access and security - Facilities - Storage capacity Data Transfer (Traffic) How much data will be moving across the network and what are the projections for the future? Is the network carrying voice, data and video? Distribution / Coverage Where is the data to be located? Which areas of the organisation will be granted access and to what extent? Is structured cabling being used? Access and Security What precautions prevent unauthorised access to the computer systems and data? Facilities What applications and tools are available to process data on the network? What future applications are required? Storage Capacity What is the current capacity for data storage on the network and what are the projected requirements for storage?

Local Area Network - LAN
Network Architecture A network exists where any group of two or more computer systems are linked together. These tables will help you compare LANs, WANs, Intranets and Internetworks Local Area Network - LAN Functions Sharing data files, sharing peripherals, enabling communication (e.g. via s) Geographical Spread Usually limited to building/campus such as school, office block, factory. Usually up to 2km. Transmission Media Twisted-pair copper cable, fibre-optic cable, co-axial cable. Bandwidth Bandwidth depends on transmission media: - twisted-pair copper cable: Mbps - fibre-optic cable - co-axial cable Wide Area Network - WAN Functions Supports file transfer, communication ( ), multi-user databases etc Geographical Spread There is no physical limitation. It could cover a city, country or be worldwide. Transmission Media Telecommunication systems covering large distances. Bandwidth Bandwidth depends on nature of telecommunications link.

Network Architecture Intranet Internetworking Functions
Used for internal communication within an organisation. Supports internal , sharing of data files, web pages. Examples include using web pages to advertise internal jobs or training courses or sharing sensitive data within an organisation. Geographical Spread There is no physical limitation. It could cover a city, country or be worldwide. Transmission Media Intranets use the same telecommunications systems technologies as WANs to cover large distances. Bandwidth Bandwidth depends on nature of telecommunications link, from 56kbps for dial-up, 128kbps for ISDN, 10Mbps+ for broadband, 50mbps T3 telecoms leased line. Internetworking Functions An internetwork consists of several networks joined by devices such as routers or switches. The functions available to the users include those outlined for other networks. Geographical Spread An internetwork can vary in its geographical spread. It could be used to link several LANs in the same complex or, using the telecommunications systems it could link networks spread across a city or country. Transmission Media Depends on the configuration of the internetwork. If it is linking several LANs, it will probably use a high speed backbone medium such as fibre-optic cable. Bandwidth Using a fibre-optic cable to link LANs would support a bandwidth of up to 100Gbps. Linking netowrks across a wide area would involve using telecommunications.

Networks, Terminals, Mainframes, Servers
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 communications. Terminals Terminals are generally composed of a monitor and keyboard with little or no local storage or processing power which serve as access points to the storage/processing capabilities of mainframe. Mainframe Mainframes are powerful computers systems which are designed to provide: - High capacity input/output capability. This enables them to service thousands of users simultaneously by accessing and storing data at high speeds. - High speed processing. This enables them to process data for thousands of users simultaneously. - Centralised storage, processing and management for large amounts of data. - Reliability, security and centralised control. - High availability: this means they normally operate 24 hours a day. - Thorough backup, recovery and security systems. Server File Server – Stores data files for all network users. Holds information about users access rights (read/write etc) to files, folders etc Print Server – Receives print jobs from clients Queues the jobs, spools the jobs to disk while they are in the queue Sends the jobs in turn, to the printer Web Server – Enables HTML pages and other HTTP documents to be shared and accessed by systems using a browser Server – Manages, stores and controls access to,

Network Strategy Network Topology Bus Network Advantages Disadvantages
The way that workstations are connected to the network through the actual cables that transmit data, the physical structure of the network, is called the topology. Common network topologies include: - Bus - Star - Ring - Mesh - Tree Bus Network Consists of a main cable with a terminator at each end. All networked devices (nodes) are connected to the cable. Advantages - Easy to implement and extend Cheapest topology to implement - Can survive node failure Disadvantages - Main cable fault means entire network fails Requires terminators at both ends of backbone cable to absorb signals Can be difficult to locate faults Bus topology not intended to be used in large businesses because of these problems

Network Strategy Star Network Advantages Disadvantages
A star topology is designed with each node connected directly to a central node. The central node can be a network hub or switch. Each network device has its own direct cable connection to central node. This means that in many cases more cable is required than for a bus network. However, adding or moving computers is a relatively easy task. Data on a star network passes through the central node before continuing to its destination. The central node controls all functions of the network. It also acts as a repeater for the data flow by boosting the signal before sending it onto its destination. Advantages - Easy to install and extend Easy to detect and correct faults - Faults to individual nodes do not affect others Disadvantages Significantly more network cabling required If central node fails, then whole network fails More expensive because of hub or switch that is needed to function as central node

Network Strategy Ring Network Advantages Disadvantages
A ring topology consists of a set of nodes connected serially by cable. In other words, it is a circle or ring of computers through which data travels in a clockwise direction. In a ring network, a signal is transferred sequentially via a ‘token’ from one station to the next. When a station wants to transmit, it ‘grabs’ the token, attaches data and an address to it and then passes it round the ring. The token travels around the network until it reaches its destination address. The receiving computer acknowledges receipt with a return message to the sender. Each station on the ring has equal access but only one station can talk at a time. The ring employs an active topology. Each station ‘boosts’ the signal before passing it to the next station. Advantages - Network can grow without significantly impacting on performance of the system All stations have equal access to communication because of the use of the token Disadvantages Ring networks require specialist hardware to function which makes them very expensive Failure of one computer may impact on others on the network

Network Strategy Mesh Network Advantages Disadvantages
In a Mesh topology, devices are connected with many redundant interconnections between network nodes. In a fully connected mesh network, every node has a connection to all other nodes. In a partially connected mesh, there are still multiple connections but all nodes are not connected. Full mesh is very expensive to implement and yields the greatest amount of redundancy. In the event that one node fails, network traffic can still be directed to any of the other nodes. Partially connected mesh is less expensive to implement and yields less redundancy. Fully connected mesh Partially connected mesh Advantages Provide multiple paths along which data can travel If one of the network connections is cut, there will normally be another route available Disadvantages Expensive to implement because of cable links Difficult to install because of number of physical connections Complexity makes it difficult to manage Errors can be difficult to detect

Tree Network Network Strategy Advantages Disadvantages
A tree topology is like a series of interconnected star topology networks. Tree topologies allow for expansion of an existing network, and enable organisations to configure new networking to meet their needs. One segment Advantages - Easy to install Easy to add additional nodes to a segment or additional segments to the main central node Easy to detect faults / remove parts Errors/faults should only affect one node / segment Disadvantages Length of each segment is limited by the cable type Whole network fails if central connecting node fails Whole segment fails if segment central node fails

Client-Server vs. Peer-to-Peer
Peer-to-peer (or P2P) networking is a type of network in which each node has equivalent capabilities and responsibilities in terms of communication on the network. No single node is responsible for storing all the files or controlling data traffic. Peer-to-peer networks are generally simpler than client/server networks, but they usually do not offer the same performance as client/server networks under heavy loads. Peer-to-peer networks are commonly found in homes or small business situations where only two or three machines are networks. Client/Server In client/server architectures some computers are dedicated to serving the others. These computers, called servers, can be file servers, web servers and so on. The nodes on the network, the clients, access the server for data. The server then sends the data to the client in response to the request. Peer-to-peer Client-server Centralised Storage No centralised storage. Each workstation stores data independently. Data stored centrally at server. Backup Regime Implementing back-up strategy is very difficult. Each workstation backs-up its data independently. Centralised storage means centralised back-up strategy, regularly e.g. daily. Security Security is difficult to implement because there is no mechanism centrally managing access. Individual workstations can set up IDs and passwords. Server holds database of user information that contains IDs and Passwords and user access privileges. Type of Environment Is best suited to ‘trusting’ environment, for example in the family home. Commonly used in businesses and organisations.

Network Hardware Network Strategy
A wide variety of hardware devices are required for the correct operation of a computer network. Server A server is a computer or device on a network that manages network resources. For example, a file server is a computer and storage device dedicated to storing files. Any user on the network can store file in the file server. A print server is a computer that manages one or printers, and a network server is computer that manages network traffic. A database server is a computer system that processes database queries. Servers are often dedicated, meaning that they perform no other tasks other than their server tasks. Multiprocessing operating systems allow multiple programs to run at the same time. A server, in this case, could refer to the program that is managing resources rather than the entire computer. File Server Print Server Hub Hubs are commonly used to connect segments of a network. A hub contains multiple ports. When a block of data (a packet) arrives at a port, it is copied to other ports so that all segments of the LAN can see all packets. Hubs repeat everything they receive and can be used to extend the network. Hubs pass on traffic to the network regardless of their intended registration; the PCs to which the packets are sent use the address information in each packet to work out which packets are meant for them. Hub

Network Hardware cont. Switch Repeater Network Strategy
A switch acts very much as a hub, however a switch can interpret the destination of data packets and will route packets only to their destination. This increases the capacity of the network because the incoming data is not repeated to every node. Switches control the flow of network traffic based on the address information in each packet. A switch learns which devices are connected to its ports, and then forwards on packets to the appropriate port only. This allows simultaneous communication across the switch, improving bandwidth. Switches reduce the amount of unnecessary network traffic. Switches and hubs can be used on the same network. Hubs extend the network by providing more ports, and switches divide the network into smaller, less congested sections. Switch Repeater A repeater is a network device used to regenerate or replicate a signal on a network. Repeaters boost signals that are distorted by transmission loss. Transmission loss can occur when a workstation is a considerable distance from the nearest hub or switch. In such a case, a repeater would be required to boost the signal due to the length of the cable. A repeater can also relay messages between segments of networks that use different protocols or cable types. Hubs can operate as repeaters by relaying messages to all connected computers. A repeater cannot do the intelligent routing performed by bridges and routers. Repeater

Network Hardware cont. Router Bridge Network Adapter (or NIC)
Network Strategy Network Hardware cont. Router A router is a device which forwards data packets along networks. It is normally connected to at least two networks, often two LANs or WANs or a LAN and an internet service provider’s network. Routers are located gateways, the place where two or more networks connect. Routers use headers and forwarding tables to determine the best path for forwarding the packets, they use protocols to communicate with each other and configure the best route between any two points on the networks. A router typically links networks which use different transmission protocols. Router Bridge A bridge is a device that connects two local area networks (LANs), or two segments of the same LAN that use the same protocol. A bridge essentially joins together the two networks, allowing data transmission between them. Bridge Network Adapter (or NIC) A network adapter, also known as a Network Interface Card (NIC), is a computer circuit board or card that is installed in a computer so that it can be connected to a network. The network adapter encodes and decodes networks transmissions. Every computer attached to a network needs a network adapter card. The card is often built into the computer at the time of manufacture. The network cable plugs into the card and the other end into a socket and it therefore allows the computer to send and receive data across the network.

Perth High School Network
Internet Library To Primaries 3-13 H.T.O. Server Room MS2

Network Software Network Operating Systems (NOS)
Network Strategy All computer networks require software in order to function. Computers and other devices connected to the network must be network-enabled. This means that these devices must run a network operating system. In addition, networks often require management tools which assist in the monitoring and administration of the network. Network Operating Systems (NOS) A network operating system (NOS) is any operating system product which has device drivers and communication software which allow connection to network services. There are 2 parts to the network operating system; the version that runs on the server and the version that runs on the personal computers to turn them into network stations. The server software is needed to control which users and workstations can access the server, keep each user’s data secure, and control the flow of information around the network. It is also responsible for file and data sharing, communication between users and hardware peripheral sharing. Each network workstation needs the network operating system installed before it can connect successfully to the network facilities. In its early form, Windows did not support networking, so Novell NetWare became the first popular network operating system for the PC (Windows 95 was Microsoft’s first networking operating system product). Today, nearly any consumer operating system qualifies as a NOS due to the popularity of the Internet and the need to support basic Internet Protocol (IP) networking. The user is asked to log in before they can access either the computer or the network. Network Management System (NMS) Network management software is a collection of tools which allow the performance, setup, security and functionality of the network to be controlled and managed. It can be used to monitor both users’ activity and workstation activity. Small programs called agents run on and monitor each of the computers attached to the network. User/workstation activity is logged by the agent and then sent to the central Network Management System. Information gathered by agents is used to monitor and control network performance, network configuration, network faults and network security. Performance Management – Measures and controls network performance so that it can be maintained at an acceptable level Configuration Management – Monitors the configuration (the arrangement) of the network and devices attached to it. Fault Management – detects, logs, notifies users of, and sometimes automatically fixes network problems Security Management - provides control over access to network resources to prevent unauthorised access and sabotage

Networks & The Law Networks and the Law The Data Protection Act (1998)
This very important act is designed to protect people from having false information about them stored on a computer database. It concerns 2 main groups of people: Data Subjects (People whose personal details are stored – you and me!) Data Users (Organisations who store and use personal data – E.g. Mobile Phone Co.) Data Subjects have the right to: - See what data is being held about them (they may have to pay for the privilege!) - have the data corrected if it is wrong. - (You may be entitled to claim compensation for damage caused (“damages”) if personal data held about you is inaccurate, lost, or disclosed. Data Users holding the data must adhere to the following: - Personal data shall be processed fairly and lawfully. - Personal data shall be obtained only for lawful purposes, and shall not be further processed in any manner incompatible with those purposes. - Personal data shall be adequate, relevant and not excessive in relation to the purposes for which it is processed. - Personal data shall be accurate and, where necessary, kept up to date. - Personal data processed for any purpose shall not be kept for longer than is necessary. - Personal data shall be processed in accordance with the rights of data subjects. - Appropriate measures shall be taken against unauthorised or unlawful processing of data and against accidental loss or destruction of, or damage to, personal data. - Personal data shall not be transferred to a country outside Europe, unless that country ensures protection for the rights of data subjects in relation to the Act. Changes from 1984 Data Protection Act – The 1984 DPA had certain shortcomings that unscrupulous companies exploited. For example it only covered data in electronic form and companies used printed mailing lists and photocopied names and addresses onto labels to circumvent the DPA. It also had no European or worldwide dimension and there was no obligation on any data user to tell the data subject that they held any data about them. The 1998 Act covers the transmission of data in electronic form, which was not really an issue in 1984, and harmonised the European Union data protection legislation. It also made it a requirement of the Act to ask for the prior consent of data subjects to have data held about them, and that included paper-based records.

Legal Implications Networks and the Law The Computer Misuse Act (1990)
The Computer Misuse Act (1990) was introduced to deal with the issue of computer hacking and other unauthorised access to computer systems and unauthorised modification of computer data. The act defines 3 offences in each of its 3 sections: Section 1: unauthorised access to computer material Section 2: unauthorised access with intent to commit or facilitate commission of further offences Section 3: unauthorised modification of computer material 1: Unauthorised Access to Computer Material It is an offence to gain unauthorised access to a computer system. This includes causing a computer to perform a function with intent to gain access to any program or data, knowing that access is unauthorised. This is an offence regardless of whether the motives for access are well meaning or malicious. Examples of such offences would include unauthorised use of another person’s username and password, persistently trying to guess a username and password, and laying a trap to obtain a password (possibly using a Trojan or virus). 2: Unauthorised Access with Intent to Commit or Facilitate Commission of Further Offences It is an offence to gain access with the intent to commit another crime, for example, gaining access to a person’s online bank account with the intention of stealing their money. 3: Unauthorised Modification of Computer Material This part of the act outlaws the unauthorised modification of computer material. A person found guilty of this act could be convicted with a maximum prison sentence of 5 years or an unlimited fine (or both!) Examples of offences under this section would include deleting another user’s file, modifying system files, the introduction of computer viruses, or deliberately generating information to cause a complete system malfunction. The act also states that on a charge of attempting to commit an offence, as outlined above, it is immaterial where the attack was made from, so a hacker from outwith the UK could be prosecuted. There have been several successful prosecutions of hackers and computer virus developers using this law.

Legal Implications Networks and the Law
The Copyright, Designs and Patents Act (1988) Legal Implications The Copyright, Designs and Patents Act is a large and complex act. We only really need to concern ourselves here with the Copyright section of the Act. It covers published materials, software, music, film, pictures, books etc. We will consider how it applies to software (including software piracy), computer databases, web content and digital media. Copyright of Software There are three methods of licensing software and slightly different copyright laws apply to each kind: 1. Commercial Software can be legally bought and installed on as many computers as the licence permits. Home and small business users usually buy a single user licence, whereas schools, colleges, local authorities and large businesses generally buy multiple licences. 2. Shareware is generally downloaded from the Internet and can be legally installed, usually for 30 days, after which time a payment should be made to the author or the software uninstalled from the computer. 3. Freeware is similar to shareware except that there is no need to pay and therefore no trial period. It can be downloaded and installed free of charge. Software Piracy One area of great concern is that of software piracy, where illegal copies of software are sold or where software is installed on machines when no licence has been purchased. So concerned are the software houses that an organisation called FAST – Federation Against Software Theft – has been set up to try and prevent the large scale of software theft which is thought to exist within organisations in the UK. Copyright of Computer Databases As well as the applications themselves, the content created using a computer application is also protected by copyright. One area of great concern commercially is that of computer databases. They can store vast amounts of very useful information, which may be commercially valuable and may have cost the originators a lot of money to set up. The copyright laws cover the database, or an extract from it. Copyright of Web Content It is illegal for you to copy anybody else’s work without permission. This applies directly to material found on the Internet. It considered plagiarism to copy material directly from Internet sites and try to pass it off as your own original work. If you must use the material it must be acknowledged in some way.

Networks & The Law The Regulation Of Investigatory Powers Act (2000)
This Act provides for, and regulates the use of, a range of investigative powers by a variety of public authorities, such as the Police, Special Branch, GCHQ and MI5. It updates the law on the interception of communications to take account of technological change such as the growth of the Internet. It allows organisations to monitor employees’ and web usage. It also provides powers to help combat the threat posed by rising criminal use of strong encryption to try to break into electronic transactions. The Act contains five parts providing for powers in relation to specific investigative techniques or establishing systems of scrutiny, oversight and redress: 1. Relates to the interception of communications and the acquisition and disclosure of communications data. 2. Relates to the use of covert surveillance, agents, informants and undercover officers. 3. Covers the investigation of electronic data protected by encryption. 4. Provides for independent judicial oversight of the powers in the Act. 5. Covers miscellaneous and supplemental matters such as consequential amendments, repeals and interpretation. In practice the act allows the authorities to monitor our personal and Internet usage in terms of the sites we visit. Furthermore businesses, local authorities and government departments can and do monitor s, even internally, and Internet usage of staff, students and pupils. This sounds very like ‘Big Brother’, and it may enrage and disturb many people to realise that this monitoring occurs. Others believe that, at a time when terrorists can be anywhere in our society it may be a relief to know that the authorities are taking active steps to catch them.

5 – Computer Software

Computer Software – Systems Software
Intro to Software Computer hardware can do nothing without sets of instructions and their associated data – called software! Software can be divided into the following categories: * Systems Software * Applications Software * Programming Languages This is the set of programs used to operate and maintain a computer system. It includes: * Single User Operating System * Network Operating System * Utilities (e.g. Disk Defragmenter or File Backup) The systems software provides an user interface or layer of software that lets users operate the computer without having to know about the underlying processes that are going on all the time! For example, - Users can save a file to disk without worrying where the file is stored and whether all the details about the file are saved to allow the file to retrieved at a later date. - Users can print a document currently stored in memory, without worrying which memory locations are used, how the output is routed to the printer and whether the data is to be transferred via a direct link or over a network connection. Systems Software

Disk Based Vs ROM Based Operating Systems
The operating system is responsible for managing the hardware and communicating with the user. Most modern computers store their operating system on disk, and load it into RAM at 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 loaded. Other parts can be left on disk until they are needed. The main disadvantage of storing the OS on disk, is that other programs can corrupt it. Older computers often had their operating system in ROM. This meant that it was ready as soon as the computer was switched on! It was impossible to corrupt it the system (since it was READ ONLY) but it was expensive and clumsy to upgrade. Disk Based Vs ROM Based Operating Systems Bootstrap Loader Even the modern disk-based operating systems need a small part in ROM – the bootstrap loader. The bootstrap loader is the program that the processor runs on startup and whose task it is to check the system memory and peripherals and then load in the rest of the operating system from the hard disk into the main memory.

The Main Functions of a Single User Operating System
The task of a single user operating system is to handle efficiently all the resources of a computer system, hiding their complexity from the user and the rest of the system. The main resources are the processor, memory, backing storage and peripherals. * Interpreting User Commands (CLI) * File Management * Memory Management * Input/Output Management * Processor Management * Resource Allocation Main functions of Single User OS Interpreting User Commands This operating system function involves the computer system taking instructions or commands from the user, checking them and passing them onto the correct part of the operating system which deals with that command. In early operating systems, these were typed text commands. In modern systems the operating system will have to check menu selections and mouse click commands.

File Management The operating system supervises the creation, deletion and updating of files. A directory is used to keep track of where files are stored on the various backing storage media. It has to maintain a hierarchical directory structure, where files are placed in directories/folders and sub-directories/sub-folders. Input/Output Management The input/output layer of the operating system performs the actual transfer of data between peripherals and memory. Its main functions are: - control and timing to co-ordinate the flow of data between the CPU and external devices - communicating with the processor and peripherals by accepting CPU commands, handling information about whether peripherals are ready to send/receive data and managing the actual exchange of data to and from memory and peripherals - data buffering to regulate the speed between processor and main memory - detecting errors during transmission

Memory Management This part of the operating system organises the storage of data in main memory. Its main functions are: to load programs into memory so that each has the memory required monitoring the use of memory freeing memory locations when data is no longer needed RAM ROM Process Management A process is defined as a program that is being executed plus all the resources which are associated with that that program (buffers, memory locations, input/output devices, data files etc) The operating system: - allocates resources to the process, such as memory, files, buffers - schedules CPU time - maintains the integrity of the process - terminates the process and restores all system facilities so that they can be accessed by other processes Resource Allocation This refers to the process of allocating memory and CPU time to programs. Allocating memory is usually a matter of admitting processes to the memory if there is enough space or delaying until there is space available.

Disk Clean-Up/Disk Editor
Utility Programs Utility Programs These are programs designed to carry out specific tasks related to the management or maintenance of a computer system. They are not part of the core operating system as they are not needed on a continuous basis. Virus Checker This utility checks your system for virus software using a range of techniques outlined on slide **. It is essential for system security and maintenance to run this utility on a regular basis. If connected to a network, it may be a good idea to have this utility running in the background whenever you are online. Disk Clean-Up/Disk Editor A disk editor or disk clean-up utility enables the user to remove all sorts of unnecessary files that clutter up your system, some of which they may not even know exist, such as temporary internet files, temporary files used by applications and offline web-pages that are no longer needed. Using this utility will free up disk space. 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 on your computer system. Disk Defragmenter When the operating system saves a file to disk, it uses the first available empty sectors it finds. This means that the disk will eventually have parts of files scattered across the disk surface which can decrease system performance. To fix this, you can use a defragmenter. This utility rearranges the contents of the hard disk so that the data blocks that make up a file are contiguous (right beside each other). Defragmenting a disk frees up space and speeds up access times.

Viruses, Trojans and Worms
What is a virus? A virus is a destructive piece of software that attaches itself to a file, reproduces itself, and spreads to other files. A virus will often lurk in a system before disrupting it, for example by corrupting data. To understand the many types of virus is it helpful to classify them into types: Classification of Viruses by Type of File Infected File Virus This type of virus attaches itself to an application program such as a game or any executable (.exe) file. When you run the program the virus instructions are also carried out. Boot Sector Virus A boot sector virus infects the system that your computer uses every time you power up. Macro Virus A macro is a set of legitimate instructions to automate operations, for example producing a worksheet. Hackers create a destructive macro, attach it to document and then often distribute it over the internet. When a user opens the document the macro duplicates itself into the macro library from where it attaches itself to other doucuments, spreading even further.

Techniques Viruses Use
Techniques Viruses Use to Disrupt a System Replication The virus inserts copies of itself into other program files. Each time the infected program is run by the computer, it reproduces itself, copying itself into another program, often into that part of the code containing information about the program running: the program header. Camouflage A virus can disguise itself to avoid detection by anti-virus software by adding fake instructions to its code so that the anti-virus software is unable to spot the pattern of instructions which identify it. Watching Some viruses copy themselves to memory and wait there checking for a condition before carrying out their destructive action, for example on specific date or a certain combination of key-presses. Meantime it replicates. Delivery Some viruses copy themselves to memory and wait there checking for a condition before carrying out their destructive action, for example on specific date or a certain combination of key-presses. Meantime it replicates.

Trojans and Worms Worms Trojans
What is the Difference between a Worm and a Virus? Whereas a virus spreads from file to file, a worm spreads from one computer to another, usually via security holes in a network. Like a virus it then reproduces itself. Unlike a virus it does not need to be attached to a document or an executable program. How do Worms Spread Themselves? The most common method used by worms to spread themselves is by attaching copies of themselves onto documents and TCP/IP packets and using them to move to other servers and from there to user systems. What do Worms do? Worms are often used to activate a ‘Denial of Service' attack. This means they can flood a network with useless traffic which overwhelms a network’s processing capability and halts communications. Trojans What is a Trojan? A Trojan is software which appears to be doing one thing but actually secretly does another. A classic Trojan activity is to pretend to be a network login screen so it can steal an ID and password which it either s to a hacker or stores in a file tha the hacker can easily get to later. Others pass themselves off as graphics files or adverts.

Virus Detection Techniques Used by Anti-Virus Software
Searching for Virus Signatures Whereas a virus spreads from file to file, a worm spreads from one computer to another, usually via security holes in a network. Like a virus it then reproduces itself. Unlike a virus it does not need to be attached to a document or an executable program. 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 calculation produces a different checksum, it knows that the code has been altered, possibly by a virus. Memory Resident Monitoring This is anti-virus software residing in RAM which monitors all a computers actions for suspicious activity, for example copy or decompressing files, attempts to modify programs, instructions that remain in memory after they’ve been executed. If it finds anything suspicious it throws up an error message and halts all operations. Heuristic Detectors .exe This software 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 normal operating system procedures.

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