Higher Comp. Comp Syst

8/8/2019 Higher Comp. Comp Syst

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Computer Systems

Ron Loan Currie Community High School 1

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 CD-

ROM 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 bit128 64 32 16 8 4 2 1

Converting between binary and decimal

Add the place values e.g. 1011 1001 = 1 +8 + 16 + 32 + 128 = 185 indecimal

Converting between decimal and binaryDivide repeatedly by 2.

ii. Storing NumbersPositive integers are stored as described aboveNegative integers are stored either using signed bit or 2s 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 dont work.

2s complement To find the 2s complement of a number change all 0s to1s and 1s to 0s. 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.

Least significant bitMost significant bit


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Floating Point Numbers are stored using mantissa and exponent. E.g. 1011.11 is

equal to .101111 X 2 100 (since 100 in binary is 4 indecimal 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 numbersthat can be stored.

iii. Storing Text

character a symbol or letter on the keyboard. A list of all the characters acomputer can process is called the character set.

ASCII American Standard Code for Information Interchange is an agreed 7-

bit 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.

iv. 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 orOFF 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 graphicsjust 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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ii. The Structure of the Central Processing Unit (CPU)

In a microcomputer, this consists of a microprocessor, containinga 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.

Buses are the sets of wires connecting the CPU to the mainmemory 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

maximum amount of memory that the computer can have. This is 2n

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 controlunit to memory and peripherals.

Read and Write wires

Clock wire carries clock pulses, which keep the CPU and other units instep. The clock rate is measured in hertz (MHz or GHz).

Interrupt wires carry signals to the CPU, which can cause the CPU tobreak 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.

Memory AddressRegister

Memory DataRegister

Control Unit

MoreRegisters

Arithmeticand LogicUnit (ALU)

MainMemory

Address Bus

Data Bus

Control Bus

Read & Write


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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 Memory Write Operation

1. The required memory address is put in

the Memory Address Register.

1. The required memory address is put

in the Memory Address Register.

2. The control unit activates the Read

Line.

2. The value to be written is put into the

Memory Data Register

3. The contents of the memory at the

address are put on the data bus and

into the Memory Data Register

3. The control unit activates the Write

Line.

4 The contents of the Memory Data

Register are transferred by the data

bus to the required memory address.

iv. 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 computers programs and data while they arebeing processed.

DRAM (Dynamic RAM) is normal RAM and needs to have its contents

refreshed otherwise the contents are lost.

SRAM (Static RAM) doesnt need refreshed as long as power is applied. It is

used for holding date etc when the computer is switched off; power beingsupplied 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 screendisplay. 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 fixedduring manufacture. In computers with a disc-based operating system, ROM


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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.

v. Factors that Affect System Performance

System Performance can be measured using MIPS (millions ofinstruction per second), FLOPS (floating point operations per second) orBenchmarks (timing the computer carrying out tasks such as recalculating in aspreadsheet or redrawing complex graphics)

Bus Width

Increasing the data bus width will increase the amount of data fetched in eachcycle 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 isdetermined 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 ofcache 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 andperipheral e.g. WIFI and Bluetooth


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3. Peripherals

i. Buffers and Spoolers

A buffer is an area of memory used for the temporary storage of data beingtransferred 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.

ii. Hardware

Hardware should be selected in terms of appropriate characteristics including

resolution, capacity, speed, cost and compatibility

iii. Solid State Storage DevicesExamples 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.

iv. Interfaces

An interface is the hardware (and associated software) needed to allow

communication between the CPU and peripheral devices and to compensate forany differences in their characteristics.

Functions of an Interface will include some of the followingBuffering 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 conversionHandling Status Information to show whether the device is ready to

receive or send data e.g. is the printer ready or out of paper.

v. 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 longdistances.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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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.

ii 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 sharesits time between the terminals a multi-access system.

iii Types of Network LAN Local Area Network commonly using UTP wire cable (perhaps withoptical 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 stationusually deals with its own storage.

iv File, Print and Web ServersIn 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 Webusers (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.

v 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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vi Network hardwareAnetwork 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.Ahub 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.Aswitch actually reads the destination address (the hardware address of the

NIC) on each packet and then forwards the packet to the correct port on theswitch.

Arouter 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.

vii Trends in NetworksBandwidth In computer networks, means the same as data transfer rate - theamount of data that can be carried from one point to another in a given timeusually expressed in bits (of data) per second (bps) or bytes per second (Bps).

Network bandwidth is steadily increasing.Wireless networks are becoming more popular.

viii Misuse of NetworksMisuse 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 datawas 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.) areprotected.


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1. Systems Software

i Operating System

Single User OS

This is a program that controls the entire operation of thecomputer and any devices that are attached to it.

The OS is often thought of as a series of modules orlayers. One design is shown.

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 OSs allow more thanone 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 SystemsA 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 thesystem, then they need to log on by giving an ID and password.

Kernel

I/O

FMS

MMS

CLI


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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.

ii. 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 tothem. 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 filesaround on disk until they are all stored in consecutive sectors and the free

space is maximised.

iii. Standard Graphics File Formats

Supported across a wide range of operating systems and application software.

compressionColourdepth

Supportstransparency

Supportedon Web

Pages

Used for

Jpeg JointPhotographic

Expert Group

Yes, lossy

Compression

can be varied.

More

compression,

less resolution

Up to 24bit

No yesPhotographs,pictures on Web

Gif Graphics

Interchange

Format

Yes, lossless Up to 8 bit Yes YesWeb Graphics

with blocks ofcolour

TIFF Tagged

Image File

Format

Not in standardTIFF

24 bitcolour or 8

bitgreyscale

No NoSaving high

resolution

graphics.


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iv. Malicious Programs

i. Definition of a Virus

A computer virus is a small program written to alter the waya computer operates, without the permission or knowledgeof the user. A virus executes itself and will replicate.

For example, it may replace other executable files with a copy of the virusinfected 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 (especiallyMicrosoft 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 aremet 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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iv. Trojan Horses

The difference between Trojan horse programs and true viruses is thatTrojans 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.

v. 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.

vi. 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 forcharacteristics 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 inbatch 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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3. Application Software

Types of Application Software (and Contemporary Software)

i. Text e.g. Text Editor (Wordpad), Word Processor (Word)ii. Number e.g. Spreadsheet (Excel) or Accounting (Money)

iii. Data e.g. Database (Access, FileMaker)

iv. Communications e.g. WWW browsers (Internet Explorer,Firefox), Email (Outlook)

v. Graphics e.g. painting (Photoshop), drawing (Illustrator,

Freehand), CAD (AutoCAD)

vi. Multimedia e.g. presentation (PowerPoint), multimedia authoring(Macromedia Director)

vii. Integrated 2 or more applications in a single program

(AppleWorks). It is easy to link data between them and they have

a common HCI.viii. Page Layout or Desktop Publishing (PageMaker, InDesign,

Quark Express)

Documents

Higher Comp. Comp Syst