Computer Systems

Outcomes• 1 Computer Organisation

• 2 Computer Software

• 3 Input, Output & Storage Devices – Managing the Computer

• 4 System Specification

1 Data Representation1.1 Text

– Each character was originally stored in 7 bits called ASCII. Now we use an 8 bit binary code called extended ASCII or ANSI code.

E.g. A is stored as 65 (01000001 in Binary).– 1 byte can store 256 different characters –

enough for all the keys on the keyboard and several foreign symbols (for currency etc.).

– Languages with many characters such as Chinese or Japanese are represented by Unicode which uses 16 bits.

1 Data Representation1.2 Numbers

– Numbers converted to binary – number base 2– Example of 154 represented as a binary

number

128 64 32 16 8 4 2 1

1 0 0 1 1 0 1 0

128 +0 +0 +16 +8 +0 +2 +0 =154

1 Data Representation1.2 Numbers

2 69

2 34 R1

2 17 R0

2 8 R1

2 4 R0

2 2 R0

2 1 R0

2 0 R1

How to convert decimal to binary e.g. 69

Giving 1000101

•Using 1 byte for storage gives 256 possible numbers. (0-255)

•2 bytes gives 65536, 0-65535

•Increasing the size of the storage for numeric data increases the range of numbers which can be stored.

Data Representation1.3 Integers

• Positive numbers– Converted directly to binary– 2 bytes = 16 bits gives 0 to 216-1 or 0 to 65535

Data Representation1.5 Bit-mapped Graphics

• For a graphic drawn in a painting package the computer stores the data as a 2 dimensional array of pixels.

• Each pixel in a black and white image is 1 bit in memory.• 2 bits can store 4 colours or shades of grey.• 8 bits can store 256 colours.• 2 bytes can store 65,536 colours.• Bit mapped images are often generated from scanned pictures

or digital cameras and can create huge files.• Editing can be performed at pixel level using a painting /

photo-editing package.• When images are enlarged they can become “lumpy” and lose

definition or when image reduced, fine detail is lost.

2 Categories of Computers

• Mainframe – multiple users sharing memory and processors.

• Desktop Computer – single user with one processor.• Laptop Computer – single user, all in one unit, LCD

screen, trackball or touchpad mouse. Some now have screens which can be written on with a stylus.

• Palmtop Computer – May write on with a stylus, handwriting recognition and probably interfaces with a laptop or desktop. OS and Apps usually on ROM.

• Networked Computer – Usually a desktop with a network card connected to a local or remote server.

3 Computer Organisation3.1 A two State Machine

• Typical 4 box diagram

• Only 2 states used in all components and data storage, on or off, 1 or 0.

• Advantages of 2 state.– Simplicity – 2

voltage levels.– Good tolerance– Simple calculations.

InputProcessor

Memory

RAM & ROM

Output

Backing

Storage

CPU

3 Computer Organisation3.1.1 Memory – RAM & ROM

• RAM• Has same access time for all locations.• Volatile – loses contents on power off.• Static – holds contents as long as there is power.• Dynamic – has to be refreshed (every 2 ms).

• ROM• Contents permanent or non-volatile.• Software & data fixed into ROM at manufacture.• Operating systems and specialised ROMs (e.g.

cameras and CD players etc.).

3 Computer Organisation3.1.2 Links between processor & memory

• To execute a program.• Program and data loaded from disk to memory.

• Stored in RAM

• Stored Program Concept

• Fetch – Execute Cycle• 1st instruction fetched from memory and decoded

then executed.

• Then next instruction and so on until program finished.

• Each memory location has a unique address.

3 Computer Organisationword length

• The Word Length is the size of data, in bits, which can be manipulated as a single unit by the processor.

3 Computer Organisation3.2 The structure of the CPU (a)

• ALU (Arithmetic & Logic Unit)– Data is processed and manipulated.– Involves arithmetic operations and logical

comparisons.

• Control Unit– Manages execution of instructions.– Sends control signals around the computer.

• Registers– Storage location with the CPU– Hold calculations, store addresses etc.

3 Computer Organisation3.2 The structure of the CPU (b)

Processor

Control unit

ALU

Registers, A, MAR, MDR, PC, SP

4 Factors Affecting Performance4.2 Measures of Processor Speed

• Clock Speed– Generally the faster the clock speed the faster

the processor – 1.4 GHz is faster than 933 MHz

4.6 Networked Computers

• Main purpose of networks– To share system resources

• In a LAN, file space on server, printers, multi-user software

– To share information• Common files

• Multi-user database files

• E-mail & internet services

2 – System Software 2.1 -The Operating System

• Manages the hardware & communicates with the user.

• Some parts of O/S held in ROM chips, but most parts held on disk – can be corrupted.

• Bootstrap program held in ROM runs on start-up and loads in the rest of the O/S from disk.

2 – System Software 2.1.1 –Single-user Operating System

• The tasks of a single-user operating system is to manage all the resources of a computer system.

• The main resources are.– File management.

Memory management.Command language interpreter.Input - Output SystemProcess Management

2 – System Software Single-user Operating System -File management

• The operating system supervises the creation, deletion and updating of files.

• A directory keeps track of where files are stored.This is a hierarchical directory structure in which files are

placed in folders (directories) and sub-directories

2 – System Software Single-user Operating System -Memory

management

• The operating system decides what programs and data ar to be placed in memory

• It also keeps track of what stage a program is at and this applies equally to the operating system's own memory space.

• If corrupted it could cause the computer to crash

2 – System Software Single-user Operating System -Command

Language Interpreter

• The operating system takes commands from the keyboard and mouse.

• Passes them to the operating system which;– Carries out the command.

– If command cannot be understood an error is reported and a suitable message given.

2 – System Software Single-user Operating System - Input-Output

System

• Peripherals all work at different speeds• The I/O system hides the differences and it makes them

all appear to operate in a similar manner.• The I/O system does all the actual data transfers and

issues the appropriate control signals to the peripherals.• Manufacturer usually supplies software (called a

driver) for the user to install on their hard disk.

2 – System Software 2.2 Utility Programs

• Utility programs aid the maintenance of the computer or make the user’s life easier.

• Includes disk formatter, disk tools and defragmentation tools.– Files are saved wherever the system can find space in

consecutive sectors. As files are added and deleted spaces become available and are all over the filing system. A de-fragger moves files around to create a contiguous system with no empty sectors.

2 – System Software 2.3 Translators

• A Translator turns a program written in a high-level language into machine code.

• The high-level language is the source code and the machine code is the object code.

• Compilers – translate the source code in one go and produces executable machine code.

• Interpreters – Translate and execute a program line by line. Interpreted programs run much slower than compiled. Interpreter is simpler to learn and use.

4 – Application Software4.1 Types of Applications

• You need to be aware of applications which handle:-– Text (word processor, web browser)

– Number (spreadsheet, accounts package)

– Data (database, world wide web)

– Communications (e-mail, file transfer)

– Graphics (painting, drawing, CAD)

– Multimedia (sound, video)

– Integrated Packages (Works v Office)

4 – Application Software4.2 Features and Requirements

• All application packages have features which distinguish them e.g.– Database – search and sort data, print reports.– Multimedia – allows user to create presentations using sound and

video.

• Application packages have minimum hardware requirements.– Memory – Minimum RAM used is specified.– Hard Disk – Minimum disk space for installation and running

usually specified.– Processor and operating system.– Any specific peripherals.

4 – Application Software4.3 Standards

• Each application package saves files in its own way. These are called native file formats.

• Saving as standard file formats allows other packages to read the data more easily but may affect the usefulness or appearance of the file.

– Text or ASCII, simple text only with no formatting

– RTF (Rich Text Format), standard for text and WP

– JPEG, GIF and PNG for graphics

Outcome 3Input, Output and Storage Devices

• We will examine devices in terms of:-– Accuracy– Capacity– Speed– Cost

1 – Input Devices1.1 Modified Keyboards

• Different layouts for ergonomic reasons and also to allow access for the disabled.

• Accuracy– Keyboard must be 100% accurate

• Capacity– No internal buffering. Any buffering will be at the interface.

• Speed– All keyboards the same.

• Cost– Generally more expensive than conventional keyboards which

can be bought for around £5

1 – Input Devices1.2 Digitisers

• A digitiser provides text, sound, graphics or video in a digital form (e.g. a scanner)

• Scanners– Modern scanners use high bit depths to allow high

resolutions.

– Images must be matched to their purpose• No point in scanning at a resolution of more than 75 dpi for

a screen based display.

• No point in scanning at 600 dpi for a printer rated at 300 dpi.

1 – Input Devices1.2 Digitisers - Scanners

• Accuracy– Accuracy usually main reason for choosing a particular model.– Accuracy evident in the bit depth and resolution available.

• Capacity– Little internal buffering– Rely on techniques to transfer the data.

• Speed– Depends on the computer rather than the scanner

• Cost– Dropped dramatically in recent years– Bundled software often the major selling point.

1 – Input Devices1.3 Digital Camera

• Film replaced by an array of photosensitive cells.

• Images stored electronically.

• Compression usually takes place.

• Image transferred to computer– Serial Cable– “Floppy Disk” adapter

1 – Input Devices1.3 Digital Camera

• Accuracy– Accuracy depends on the array of photosensitive cells..– The more sensors and the smaller they are the higher the

resolution.

• Capacity– Based on resolution and memory in the device.– Compression v altering resolution

• Speed– Download time depends on the computer and the interface

• Cost– Dropping as they become more common.– Resolution main factor and also facilities (zoom, flash etc.).

1 – Input Devices1.4 Video Capture Devices

• Video capture card.– Converts analogue video to digital– Compresses video between 4:1 and 200:1– MPEG a common standard– Software used by the board is called CODEC

(Compressor/Decompression)– Example is QuickTime by Apple.

1 – Input Devices1.4 Video Capture Devices

• Accuracy– Based on the quality of the source– Resolution settings on the board (often via software)

• Capacity– Depends on the amount of VRAM on the board.– Computer’s ability to transfer data quickly to disk

• Speed– Need at least 15 frames/second for adequate results.

• Cost– Lower compression ratio then higher cost.– Video print facilities on same card push up price.

1 – Input Devices1.5 Sound Input

• Naturally Occurring Sound– Natural sound is analogue in form

– To input sound to a computer• Software samples the incoming signal

• Coverts the signal into digital form

• Usually compresses the file

– This is called ADC – Analogue to Digital Conversion

– Simplest input device is a microphone with sound card

– Sound card performs the ADC and compression

1 – Input Devices1.Sound Input

• Sampling

Sampler listens to sound repeatedly and stores a number representing the amplitude each time

Sampling Rate

No of times per second sampler listens to the sound e.g. 22 kHz is 22,000 times a second

Sample Size

No of bits stored per sample e.g. 8 or 16 bit samples

Compression

Reduce storage space and reduce quality

Speed

Irrelevant in conversion

1 – Input Devices1.Sound Input

• Digitised Sound– Sound can be created digitally.– Keyboards have a MIDI interface to allow direct connection to

computer for sequencing.– ADC not required

• Accuracy– No of bits used to store sound.

• Capacity– No built-in cache. Depends on fast access via the sound card to hard

disk storage

• Cost– 8-bit sound card quite cheap– 64-bit CD quality card fairly expensive.

2 – Output Devices2.1 The Ink Jet Printer

• Colour– Colour printers now the norm and cost has

reduced dramatically. Good quality at 720dpi.

• Accuracy– Resolution based on dots per inch (dpi)

• Capacity– Generally depends on background printing

from computer.

• Speed– Ink jets becoming faster. Faster drying inks

help.

• Cost– Prices have tumbled and are very low

nowadays.

2 – Output Devices2.1 The Laser Printer

• High quality results but quite expensive• Usually a page is composed in the printer (often

PostScript).• Capacity

– On board RAM & processor needed to compose pages. The more RAM the higher quality graphics can be printed.

• Accuracy– 600 dpi quite common (300 cheaper, 1200 expensive)

• Speed– The faster in pages per minute (ppm) the dearer.

• Cost– Can be cheap for personal lasers but high quality,

fast (20ppm) networked printers still expensive.

2 – Output Devices2.3 Speakers for Sound Output

• Accuracy– CD audio requires 16 bits– Stereo needs 32 bits to store data

• Cost– Cost of sound card based on number of bits used in

the ADC/DAC conversion.– Quality of audio amplifier and speakers will affect

cost

• Quality– Modern PC based speakers and sound systems as

good as music centre systems.

2 – Output Devices2.4 Multiscan Monitor

The CRT is the basis of most visual display technology.

The screen is arranged as a series of lines of dots and each dot is made up of three small areas of red, green and blue called a triad. The intensity of light shone on each triad determines the actual colour of the pixel.

The picture is redrawn between 25 and 75 times a second. This is the refresh rate.

A monitor which operate at different refresh rates is known as a multiscan or multisync monitor. The refresh rate is controlled by the video adapter.

Screen resolution is quantified by the dot pitch, the distance between the dots on the screen. Typically between 0.28 and 0.38mm, corresponding to 100 to 70 dpi.

2 – Output Devices2.5 Video Output

To playback video on a standard computer it will need to be decompressed by hardware or software, usually on the card.StandardsAVI – (Audio Video Interleave) or Video for Windows. Being replaced by Active Movie which will playback AVI, QuickTime and MPEG.QuickTime – CODEC s/w developed by Apple but used by both Mac and PC.MPEG – Video board uses hardware to make compression much faster.

Accuracy – Depends on Compression Technique, frame rate and resolution.Speed – Hardware must be fast enough to cope with stream of data to memory and to the hard disk.Cost – Not only card but good Multiscan Monitor required (17” and 19” nowadays)

3 – Storage Devices3.1 Magnetic Disk

Widely used for a long time as a fast backing storage media. Floppy disks cheap and rugged. Hard disks fairly fast, capacity growing fast and cost per Mb (or even Gb) is dropping rapidly.Capacity – increased dramatically for hard disks (40Gb typical – 1Tb soon?). Floppy disks stuck at 1.4Mb.Speed – Access time is seek time + search time (latency). Seek time is average time for head to get to right track and latency is time for disk to rotate to correct part (average is ½ a rev)Cost – Based on cost per Mb or even Gb. Dropping rapidly even now.Access – Direct or Random access

3 – Storage Devices3.2 Magnetic Tape

Magnetic tape used in commercial situations for many years. Advantages, cheap, huge capacity & secure. Disadvantages, slow due to serial access.Tape now used in cartridges for backup of hard disks and servers.

Capacity - Typical max capacity 50% more than largest common disk.Speed – Relatively slow. Used in batch processes.Cost – Very cheap which is why it used for backups.Access - sequential

3 – Storage Devices3.3 Optical Storage

CD-ROM

CD – R

CD - RW

A plastic disk is scanned using a laser. It reflects off pits on the surface differently from lands (bumps)

Capacity – About 650Mb

Speed – from single to 32x (or even 40x). The x refers to the times faster than CD Audio.

Cost – CD-ROM Drives fairly cheap.

Access – Always random

3 – Storage Devices3.4 Magneto - Optical Storage

Based on a combination of magnetic and optical technologies.Active layer is magnetic material.Recording – magnetic material heated beyond a particular temperature by laser, allows magnetisation to be reversed.Reading – laser operates at much lower temp and reflected beam rotated by magnetic field and detected by read head.Capacity – 3.5” disks of 128, 230 and 384 MbSpeed – Varies as multiple of standard single speedCost – decreasing with time with different formats and capacities becoming available.

3 – Storage Devices3.5 DAT

Digital Audio Tape (DAT) widely used in music industry for professional quality tapes. Now used in micro-computers as backup medium.Capacity – 90m tape holds 2Gb of dataSpeed – Typically data read at 2 megabits/secAccess – sequential as with all tape.