Computer Systems 1. Data Representation i.
The Binary System Reasons for using binary
Easy to represent ‘ON’ and ‘OFF’ as 0 Volts and a higher voltage (often above 1V – nominally 5V). Fewer rules needed with binary (0, 1) arithmetic than with decimal (0..9) arithmetic. Any loss of voltage due to resistance etc does not change value. Easy to represent 0, 1 with other devices e.g. pits and lands on a CDROM or directions of magnetic field on a hard disc.
Binary Arithmetic
Bits – a binary digit. Byte – 8 bits Kilobyte – 1024 bytes (210 bytes) Megabyte – 1024 x 1024 bytes Gigabyte – 1024 x 1024 x 1024 bytes Terabyte – 1024 x 1024 x 1024 x 1024 bytes
Place Values the values are doubled as you add more digits starting from the least significant bit 128 64 32 16 8 4 2 1 Most significant bit
Least significant bit
Converting between binary and decimal Add the place values e.g. 1011 1001 = 1 +8 + 16 + 32 + 128 = 185 in decimal Converting between decimal and binary Divide repeatedly by 2.
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Storing Numbers Positive integers are stored as described above Negative integers are stored either using signed bit or 2’s complement. Signed Bit The most significant bit is 0 if the number is +ve, 1 if –ve. However there are two zeroes (+0 and –0) and the ordinary rules of arithmetic don’t work. 2’s complement To find the 2’s complement of a number change all 0’s to 1’s and 1’s to 0’s. Then add 1. Only one zero and all the ordinary rules of arithmetic work. The most significant bit will be a 1 if the number is –ve.
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Currie Community High School
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Computer Systems Floating Point Numbers
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are stored using mantissa and exponent. E.g. 1011.11 is equal to .101111 X 2 100 (since 100 in binary is 4 in decimal and the point has been moved 4 places) This is stored as 101111 in the mantissa, 100 as the exponent. Because of the number of bits being used to store the number is limited. The mantissa cannot be exactly accurate. Increasing the number of bits used to store the mantissa can increase precision (accuracy); increasing the number of bits used to store the exponent increases the range of numbers that can be stored.
Storing Text character – a symbol or letter on the keyboard. A list of all the characters a computer can process is called the character set. ASCII – American Standard Code for Information Interchange is an agreed 7bit code, which gives 128 characters. Text data is stored using an 8-bit code; the 8th bit either being used for parity checking or to extend the number of characters. UNICODE – a 16 bit agreed code covering characters from more languages. The lowest numbers are the same codes as ASCII.
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Storing Graphics Graphics are made up of dots and on screen displayed using pixels (picture elements). For black and white, each pixel can be set ON with the value 1 or OFF with the value 0. For colour, more bits are needed for each pixel. E.g. a colour bit depth of 8 bits allows 28 (256) different colours. Bitmapped images store the screen display as a set of values of each pixel colour for the whole document. This can use a large amount of memory. File size = height x breadth x colour depth. Compression is used to reduce the file size of bitmapped images. Photographs are typical bit mapped images. Vector graphics just store the attributes of each object e.g. for a circle, object number, type of object, X and Y coordinates of centre, radius, line thickness, line colour, fill colour, line pattern, fill pattern. Vector graphics use less memory and are resolution independent, as the resolution of the screen does not affect the stored details.
2. Computer Structure i.
The Two state machine The processing and storage devices have only two states ‘ON’ and ‘OFF’, represented using the binary system - ‘1’ and ‘0’.
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Currie Community High School
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Computer Systems ii.
The Structure of the Central Processing Unit (CPU) In a microcomputer, this consists of a microprocessor, containing a control unit, arithmetic and logic unit (ALU) and registers. The control unit controls the fetching, decoding and executing of instructions. The ALU carries out the calculations and the logic operations. The registers are a group of storage locations within the microprocessor that are used temporarily to hold:
data being processed, instructions being executed and addresses to be accessed.
More Registers
Arithmetic and Logic Unit (ALU)
Memory Address Register
Address Bus
Memory Data Register
Data Bus
Control Unit
Main Memory
Control Bus Read & Write
Buses are the sets of wires connecting the CPU to the main memory and peripherals Address Bus carries address information from the CPU to main memory or peripherals. It is unidirectional. The width of the address bus determines the n
maximum amount of memory that the computer can have. This is 2 where n is the width of the address bus. Data Bus carries data between the CPU and the memory or peripherals. During a write operation data is carried from the CPU and in a read operation, to the CPU. It is bi-directional. The width of the data bus determines how much data can be transferred in a single operation. Control Bus is made up of a number of separate wires, connecting the control unit to memory and peripherals. Read and Write wires Clock wire carries clock pulses, which keep the CPU and other units in step. The clock rate is measured in hertz (MHz or GHz). Interrupt wires carry signals to the CPU, which can cause the CPU to break in the carrying out of its program. The CPU may be able to ignore the interrupt (maskable) or not (non-maskable interrupt - NMI). Reset line is used to return the CPU to its initial state. E.g. used to recover from ‘crashes’.
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Currie Community High School
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Computer Systems iii.
The Fetch Execute Cycle Program instructions are got (fetched) one at a time from main memory, decoded and then carried out (executed). The CPU fetches the instructions and data in the program from memory. This is a read operation. The program may require data to be put into memory – a write operation. Memory Read Operation 1. The required memory address is put in the Memory Address Register. 2. The control unit activates the Read Line. 3. The contents of the memory at the address are put on the data bus and into the Memory Data Register
Memory Write Operation 1. The required memory address is put in the Memory Address Register. 2. The value to be written is put into the Memory Data Register 3. The control unit activates the Write Line. 4
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The contents of the Memory Data Register are transferred by the data bus to the required memory address.
Memory – Registers, Cache and Main Memory Main memory consists of memory locations each with a unique address. Each location can hold a word. A word is the number of bits of data that can be processed in a single operation by the CPU. The word length is the same as the width of the data bus. There are two types of main memory – RAM and ROM.
Random Access Memory RAM is volatile memory - data held in RAM is lost when the power is switched off. RAM holds the computer’s programs and data while they are being processed. DRAM (Dynamic RAM) is normal RAM and needs to have its contents refreshed otherwise the contents are lost. SRAM (Static RAM) doesn’t need refreshed as long as power is applied. It is used for holding date etc when the computer is switched off; power being supplied by a rechargeable battery. SRAM is also fast and used for cache memory. Cache memory is memory close to (or in) the processor, which is used to hold often-used instructions to speed up processing. VRAM (Video RAM) is used to hold the data, which makes up the screen display. It is related to the number of colours and screen resolution. It may be part of the main memory or, more usually, on a separate graphics card.
Read Only Memory ROM is permanent memory – data is retained when the power is switched off. ROM is used to hold programs and data permanently, the data being fixed during manufacture. In computers with a disc-based operating system, ROM
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Currie Community High School
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Computer Systems is used to hold the bootstrap loader program, which loads the operating system from disc. In some games machines, ROM cartridges are used to hold software and ROM is used in many digital automated systems.
Virtual Memory With limited memory, there may be insufficient room for the required program(s) and data. Virtual memory uses the hard disc as a temporary storage area and swaps programs and data between memory and hard disc as required. This is much slower than just using RAM.
Speed of Memory Access Processor registers are accessed fastest, cache memory in the processor next, then the cache outside the processor, the main memory and slowest is virtual memory on disc.
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Factors that Affect System Performance System Performance can be measured using MIPS (millions of instruction per second), FLOPS (floating point operations per second) or Benchmarks (timing the computer carrying out tasks such as recalculating in a spreadsheet or redrawing complex graphics) Bus Width Increasing the data bus width will increase the amount of data fetched in each cycle and hence increase system performance. Increasing the width of the address bus increases the maximum amount of memory that the computer could have. More memory allows more programs and data to be loaded at the same time, which can improve productivity. Also with a larger memory, slower virtual memory does not need to be used as often. Clock Speed The clock generates a series of pulses at a constant rate, which keeps everything that the CPU does in time. The faster the clock speed, the faster the operations will be carried out. Remember that the overall speed is determined by the slowest component. Memory Adding more memory can speed up the computer (see i above). Also some memory has faster access time than other types. Use of cache memory made of fast static RAM can also improve performance.
Speed of data transfer to peripheral devices Speeds of Interfaces increasing e.g. USB 1 → USB 2, Firewire 400 → Firewire 800. Cables are being replaced by Wireless communication between CPU and peripheral e.g. WIFI and Bluetooth
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Currie Community High School
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Computer Systems 3. Peripherals i.
Buffers and Spoolers A buffer is an area of memory used for the temporary storage of data being transferred between computer and peripheral. It compensates for any difference in their speeds, e.g. a printer buffer. A printer spooler is a program that uses fast backing storage such as hard disk for the temporary storage of print data. This is printed as a background task while the user continues his/her work – the foreground task.
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Hardware Hardware should be selected in terms of appropriate characteristics including resolution, capacity, speed, cost and compatibility
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Solid State Storage Devices Examples include digital camera flash card storage or USB memory sticks. Advantages are small size, speed of data transfer and the fact that there are no moving parts which could wear out and break down.
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Interfaces An interface is the hardware (and associated software) needed to allow communication between the CPU and peripheral devices and to compensate for any differences in their characteristics.
Functions of an Interface will include some of the following Buffering – holding data temporarily while in transit between the CPU and peripheral. Data Conversion – changing data into a form the CPU can understand and vice-versa - protocol conversion e.g. serial to parallel, voltage conversion Handling Status Information – to show whether the device is ready to receive or send data e.g. is the printer ready or out of paper.
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Serial and Parallel Transmission of data between devices Serial Data Transmission – sends the bits of the data one after each other along the same data line. Relatively slow but more efficient over long distances. Parallel Data Transmission – sends the bits of data along separate lines at the same time. This is fast but only suitable for short distances.
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Currie Community High School
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Computer Systems 3. Networking i
Nodes and Channels Networked computers can exchange data and messages. Each computer on the network is called a station, terminal or node. They can be connected by different means e.g. by cable, which could be wire (most common is UTP – Unshielded Twisted Pair) or optical fibre or radio (wireless) links. The means of connection is the communications channel.
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Mainframe This is very large computer, which may be connected to hundreds of terminals, many of which may be dumb terminals with no processing power. Most of the processing will be done by the mainframe processor, which shares its time between the terminals – a multi-access system.
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Types of Network LAN – Local Area Network commonly using UTP wire cable (perhaps with optical fibre) or wireless. WAN – Wide Area Network – may use public communication channels e.g. telephone lines – wire, optical fibre, microwave. Intranet – private network which may be local or covering a wider area, using same protocols (TCP/IP) as the Internet. Secure intranets are much less expensive to build and manage than private networks based on other protocols. Client Server Network has dedicated servers, which provide facilities such as file serving, print serving, web serving or mail serving for the client machines – the other stations on the network. Peer to Peer Network is similar to the client server network but each station usually deals with its own storage.
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File, Print and Web Servers In the client/server model, a File Server is a computer responsible for the central storage and management of data files so that other computers on the same network can access the files. A Web Server is a program often running on a dedicated computer, which uses the web protocol, HTTP, to serve out files that form Web pages to Web users (whose computers contain HTTP clients – web browser programs). Every computer on the Internet that hosts a Web site must have a Web server program. A Print Server is a program, a network device, or computer that manages print requests and makes printer queue status information available to users.
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Network topologies can be Bus, Star, Tree, Mesh, and Ring etc. Consequences of channel and node failure need to be known.
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Currie Community High School
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Computer Systems vi
Network hardware A network interface card (NIC) is an electronic card fitted to a computer. It provides a dedicated full-time connection to a network and has a unique hardware address. A hub is common connection point for segments of a LAN. A hub contains multiple ports. When a packet arrives at one port, it is copied to the other ports so that all segments of the LAN can see all packets. A switch actually reads the destination address (the hardware address of the NIC) on each packet and then forwards the packet to the correct port on the switch. A router is connected between networks, commonly two LANs or WANs. A router keeps tables of the states of the networks it is connected to and uses these to decide which way to send each information packet.
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Trends in Networks Bandwidth In computer networks, means the same as data transfer rate - the amount of data that can be carried from one point to another in a given time usually expressed in bits (of data) per second (bps) or bytes per second (Bps). Network bandwidth is steadily increasing. Wireless networks are becoming more popular.
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Misuse of Networks Misuse covered by the following Acts in British Law. Misuse of Computers Act Makes hacking (attempting to access to computer data without authorisation) a criminal offence. Writing or wilful distribution of viruses made illegal. Data Protection Act Protects personal data Principles include o Data controllers who use personal data must register, o Data must be processed lawfully. o Data must be accurate o Subjects have right to find out what data is held about them only and have it corrected if wrong. o If subject can show that they have suffered because data was wrong they can take legal action for compensation. Exemptions include National Security Copyright, Designs and Patents Act Gives the creators rights to control ways their material can be copied or distributed. . Computer programs and pages on websites (text, pictures etc.) are protected.
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Currie Community High School
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Computer Systems 1. Systems Software i
Operating System Single User OS
CLI MMS
This is a program that controls the entire operation of the computer and any devices that are attached to it. The OS is often thought of as a series of modules or layers. One design is shown.
FMS I/O
Kernel
Command Language Interpreter is the layer that the user interacts with to give instructions to the computer. It can be command-driven, menu-driven or a GUI which may use a WIMP system. Memory Management System controls where programs and data are placed in main memory. Many OS’s allow more than one program to be in memory at the same time. MMS tries to make sure that any program does not overwrite any other program (including the OS itself!). File Management System deals with the user of the computers backing storage devices. It uses part of the disc as a file catalog, which holds details of where data is stored on disc. Files are organised into a hierarchical filing system storing files in separate directories and subdirectories. Input Output System communicates directly with peripherals and handles the transfer of data between peripherals and the CPU. Kernel manages processes (when a program runs it starts a process and this may start further processes) and handles interrupts. The layers work together to carry out operations, data being passed between them.
Resource allocation The OS allocates resources to other programs such as applications. For example: Scheduler to share processor - batch, foreground/background, timesharing Memory management – e.g. Virtual memory paging, swapping memory contents to hard disc ensures drivers are given access to hardware devices Mechanisms for accounting; limit enforcement - e.g. no. of pages printed, file space per user etc.
Operating system can be ROM based or disc based If disc based, then there must be a small part of the OS in ROM – the bootstrap loader – to load the main OS from disc to RAM.
Network Operating Systems A Network OS provides all the features of a single user OS with the addition of the following. Multi-user access and security. If more than one person can access the system, then they need to ‘log on’ by giving an ID and password.
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Currie Community High School
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Computer Systems File Server. Different users have different levels of access to the files e.g. file read and write privileges for a certain file. Print Server. The OS may provide a print server program to queue print jobs for all users, using a central printer spooler.
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Utility Software Utility programs are programs that aid the maintenance of the computer. They are not normally essential for the running of the computer and are usually kept on disk until needed. They include: Virus Checkers Scans discs/memory for viruses – see below. Disk Editors Allows the user to view the bytes of data held on disc and make changes to them. This is useful if disc errors occur to allow for data to be repaired or parts of files to be extracted. Disk Defragmenter When disks have been used many times (i.e. files saved and deleted from them), as the disc fills up, free space appears between sectors with data, files have to be split up into these gaps as there is no room to save them in one continuous place. This makes loading programs slow as lots of fragments of file have to be found and linked. The defragmenter program move files around on disk until they are all stored in consecutive sectors and the free space is maximised.
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Standard Graphics File Formats Supported across a wide range of operating systems and application software.
Jpeg – Joint Photographic Expert Group
compression
Colour depth
Yes, lossy Compression can be varied. More compression, less resolution
Up to 24 bit
Supports transparency
Supported on Web Pages
Used for
No
yes
Photographs, pictures on Web
Gif – Graphics Interchange Format
Yes, lossless
Up to 8 bit
Yes
Yes
Web Graphics with blocks of colour
TIFF – Tagged Image File Format
24 bit Not in standard colour or 8 TIFF bit greyscale
No
No
Saving high resolution graphics.
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Currie Community High School
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Computer Systems iv. Malicious Programs i.
Definition of a Virus A computer virus is a small program written to alter the way a computer operates, without the permission or knowledge of the user. A virus executes itself and will replicate. For example, it may replace other executable files with a copy of the virus infected file. Viruses can infect desktop computers and network servers alike.
ii. Types of Virus File viruses infect program files. These viruses normally infect executable code, such as .com and .exe files. They can infect other files when an infected program is run from floppy, hard drive, or from the network
Boot sector viruses infect the system area of a disk-that is, the boot record on floppy disks and hard disks. activate when the user attempts to start up from the infected disk.
Macro viruses The virus code written as a macro mainly infects data files (especially Microsoft Office files.) Because of the ease with which these viruses can be created, there are now thousands of them in circulation.
iii Virus Actions Replication They spread as files are copied and sent between computer users.
Camouflage It is possible for a virus to avoid detection by changing its characteristics from that which detection software is programmed to look for.
Watching The virus damage routines may activate when certain conditions are met e.g. opening a particular application or on a particular date.
Delivery This is how the virus gets from one computer to another. o Infected disks brought in from the outside o E-mail now provides the ideal delivery vehicle.
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Currie Community High School
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Computer Systems iv. Trojan Horses The difference between Trojan horse programs and true viruses is that Trojans do not replicate themselves. Trojans contain malicious code that when triggered cause loss, or even theft, of data. For a Trojan horse to spread, you must, invite these programs onto your computers--for example, by opening an email attachment or downloading and running a file from the Internet.
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Worms Worms spread from computer to computer, but unlike a virus they do not infect other files. A worm takes advantage of data transport features on your system e.g. using email, which allows it to travel unaided. It could send out hundreds or thousands of copies of itself. The end result in most cases is that the worm consumes too much system memory (or network bandwidth), causing Web servers, network servers, and individual computers to stop responding.
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Antivirus techniques Virus signature This is a unique pattern of bits within a virus code. The anti-virus software searches for the presence of the virus signature and may remove it. Useful only for known viruses.
Checksum This is created by an algorithm from the bytes that make up a file creating a unique numeric code that represents the file. Any subsequent change to the file by a virus will produce a change in the checksum calculation.
Heuristic detection describes the technique of finding unknown viruses by looking for characteristics in files that from experience been associated with known viruses. Heuristics can detect a virus that has disguised its signature, by recognising a particular characteristic of the virus attributes or behaviour. Occasionally can stop legitimate programs running.
Memory Resident Monitoring Memory Resident programs operate in the background, while other programs are running, unlike virus scanner programs which operate in batch mode, scanning files as requested by the user They take up memory and processor time tending to cause a slowing down of the system and can cause false alarms.
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Currie Community High School
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Computer Systems 3. Application Software Types of Application Software (and Contemporary Software) i. ii. iii. iv. v. vi. vii.
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Ron Loan
Text e.g. Text Editor (Wordpad), Word Processor (Word) Number e.g. Spreadsheet (Excel) or Accounting (Money) Data e.g. Database (Access, FileMaker) Communications e.g. WWW browsers (Internet Explorer, Firefox), Email (Outlook) Graphics e.g. painting (Photoshop), drawing (Illustrator, Freehand), CAD (AutoCAD) Multimedia e.g. presentation (PowerPoint), multimedia authoring (Macromedia Director) Integrated – 2 or more applications in a single program (AppleWorks). It is easy to link data between them and they have a common HCI. Page Layout or Desktop Publishing (PageMaker, InDesign, Quark Express)
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