# Which Computer Should I Buy

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```					 Outcome 1 - Contents

• 1 Data Representation
• 2 Computer Structure
• 3 Computer Performance
• 4 Peripherals
• 5 Networking
• 6 Using Networks
• 7 Computer Software
• 8 Supporting Software
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1 Data Representation
1.1 Introduction
• Everything the computer stores uses
binary
• Binary = base 2 so only 2 digits (1and 0)
• We use base 10(decimal), ten digits
0,1,2,3,4,5,6,7,8,9
• +’s for binary: 2 states so some voltage
can represent 1 and no voltage for the
0, less likely that voltage drop will corrupt
data
• Fewer rules for arithmetic than decimal
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1 Data Representation
1.2.1 Decimal Numbers

We use base 10
When expressing large numbers in terms of powers of 10 the
following abbreviations are used:
•101 = 10
•102 = 100
•103 = 1000 = 1 kilo
•106 = 1,000,000 = 1 Mega
•109 = 1,000,000,000 = 1 Giga
•1012 = 1,000,000,000,000 = 1 Tera

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1 Data Representation
1.2.1 BinaryNumbers
Computers work in number base 2 which uses 2 symbols, 0 and 1 to
represent a value.
In computing systems, large numbers are expressed in terms of powers of
2 and use the following abbreviations:
20 has a decimal equivalent of 1
21 has a decimal equivalent of 2
22 has a decimal equivalent of 4
23 has a decimal equivalent of 8
24 has a decimal equivalent of 16
25 has a decimal equivalent of 32
26 has a decimal equivalent of 64
27 has a decimal equivalent of 128
28 has a decimal equivalent of 256
29 has a decimal equivalent of 512
210 has a decimal equivalent of 1024 and is abbreviated to 1 kilo
220 has a decimal equivalent of 1,048,576 and is abbreviated to 1 Mega
230 has a decimal equivalent of 1,073,741,824 and is abbreviated to 1 Giga
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240 has a decimal equivalent of 1,099,511,627,776 and is abbreviated to 1 Tera
1 Data Representation
1.2.2 Decimal to Binary
How to convert decimal to binary e.g. 29
Use the headings going from the right
26 25 24 23 22 21 20
128 64 32 16 8 4 2 1
0 0 0 1 1 1 0 1

•So in 8 bit binary 29=00011101
•The more bits we use the larger the
range of numbers we can store

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1 Data Representation
1.2.3 Binary to Decimal

- Another example
- 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
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1 Data Representation

Long binary numbers can be difficult to read correctly.
Computers have memory addresses of 2 or 4 bytes long
which give addresses of 16 or 32 bits.

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1 Data Representation
1.2.5 Why Binary
• Logic circuits based on two state
logic – use only 0 and 1.
• We only need two voltages: no
voltage = 0 and a voltage of any
value = 1
• There are only 4 rules of arithmetic
with binary (100 in base 10).
• Robust – can cope with

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Data Representation
1.2.6 & 1.2.7 Integers
• Positive numbers
– Converted directly to binary
– 2 bytes = 16 bits gives 0 to 216-1 or 0 to 65535
– n bytes gives n*8 bits - 2n*8 - 1
• Negative Numbers
– 216 : Same range but –32768 to 32767
– Using sign bit 0 +ive and 1 –ive e.g. 011 = 3 and 111 = -
3
• Using a sign bit has a few flaws:
• Addition does not work properly (-5+-10 gives 15)
• Two 0’s (00000000 and 10000000)

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Data Representation
1.2.8 Twos Complement
– Two’s Complement is another way of
representing negative numbers.
– Addition works and there is only one zero
– All 0’s are converted to 1’s and 1’s to 0’s
– To convert 5 to –5
0      0     0    0      0    1   0    1
OR 5

1      1     1    1      1    0   1    0
+1
1      1     1    1      1    0   1    1
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OR -5
Data Representation
1.2.9 Real Numbers
• Real numbers are stored as floating point
notation. In binary a mantissa and an
exponent are stored
• In binary 1101.1001 is .11011001*2100 (the 4 is 100 in
binary)
• The mantissa is 11011001 and the exponent is
100
• Usually 4 bytes used for mantissa and 2 for
exponent

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Data Representation
1.2.9 Real Numbers
• Increasing the size of the mantissa
increases precision
– The more digits we have in the
mantissa the more accurately we can
store the number
• Increasing the size of the exponent
increases the range of numbers
which can be stored
– Allows more flexibility in the movement
of the decimal point
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1 Data Representation
1.3 Text
ASCII
– Each character is stored in an 8 bit binary
code called the ASCII system.
– 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.).
• Unicode
– Need to represent non-Latin chars e.g.
Japanese and Chinese
– Characters encoded using 16 bits – 65,536
symbols.
– MS Office stores documents in Unicode
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Text Representation
• ASCII vs Unicode

• ASCII files take up less space as they use ½ the
amount of bits to represent the characters
• UNICODE can represent more characters as it
has more bits so can support non Latin
characters e.g. Asian languages
– ASCII – 8 bits – 256 characters max
– UNICODE -16 bits – 65536 characters max

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Blog Entry
• Title – Number and Text
Representation
– Include
• Why binary is used
• Regular binary with an example
• Compare 2’s complement and signed bit
for negative number representation
• Give an example of an 8 bit 2’s
complement number
• Describe the 2 methods for text
representation
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Bit Mapped Graphics
• Graphics which appear on a
computer screen are made up of a
series of dots.
• These dots are called pixels (picture
elements).
• On a monochrome (black/white)
system each pixel can either be on
or off. This means that graphic
images can be represented by a
series of bits.
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Example bitmap
If you look closely at the bits on the right you
can see the shape of the letter.

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Bitmap Graphics
• The pixels used to generate the
computer screen is normally called
the resolution of the monitor.
• This is normally worked out by
multiplying the number of pixels
horizontally by the number of pixels
vertically, for example, the
resolution of the screen used to
create this is 1024 x 768 (786432
pixels in total)
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Resolution
• The resolution may also be measured in
Dots Per Inch (DPI) instead of pixels
horizontally by pixels vertically.
• The characters and graphics which
appear on the Apple Macintosh screen
are of a resolution of 72 DPI. This means
that 1 square inch of the Macintosh
screen has a resolution of 72 x 72 = 5 184
DPI.
• This is because in every inch there are 72
pixels vertically and 72 pixels horizontally
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Resolution
• This image is stored at 300 x 400

If we stretch it we will see
that the resolution of a
bitmap is static. The pixels
are stretched to fill the
space and this is what
causes the blocky look in
pictures

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Using Colour
• We have seen that each pixel of a
monochrome image can be
represented by 1 bit. If we wish to
display colour then each pixel
requires more than 1 bit.

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This image uses 16 colours

How many bits required for each pixel?

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This image uses 256 colours

How many bits required for each pixel?

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This image uses 16,777,216
colours

How many bits required for each pixel?
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No. of bits used       No. of possible colours
1                      21 = 2 colours
2                      22 = 4 colours
3                      23 = 8 colours
4                      24 = 16 colours
5                      25 = 32 colours
6                      26 = 64 colours
7                      27 = 128 colours
8                      28 = 256 colours
• The more colours an image has, the
greater the size of the image on disc (or
in memory).
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Data Representation
1.4.2 Calculating Memory Requirements
• We need to know the size of the
image, resolution and bit depth.
– Size – usually inches e.g. 6” x 4”
– Resolution – say 500 dpi (pixels per
inch)
– Bit Depth – e.g. 1 byte for 256 colours
No of bytes is
Pixels Across   Pixels down       Total Pixels   * Bit depth
6 x 500         4 x 500           24 x 250,000   6,000,000
= 6,000,000    bytes / 1024 =
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5859K = 5.7
MB
Data Representation
1.4.6 Compression
• A colour bit – mapped image with
a high resolution and 24 bit colour
needs a lot of storage (50MB for a
smallish photo).
• File compression is used to reduce
storage requirements.
• Different techniques – colours
removed that are indistinguishable
to the human eye.
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Data Representation
1.4.5 Colour

One colour can be represented by one byte
giving 256 colours (GIF format).
Monitors etc. have 3 primary (additive) colours,
Red, Blue and Green. Other colours obtained
We use 8 bits for Red, 8 for Blue and 8 for Green
which give us 256 x 256 x256 colours – over 16
million.
We need 3 bytes to describe RGB coded
colours.
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Data Representation
1.4.7 Vector Graphics
• Objects not described pixel by pixel but by its
attributes (start & end positions, thickness &
colour of lines etc.)
• Sometimes called object-orientated graphics.
• Vector graphics are resolution independent.
Rasterisation is the process of converting them
for display or printing purposes
• Editing at pixel level not possible.
• Can be scaled without losing original image
quality
• Less storage required
• Can be grouped and edited at the object
level.
over another graphic without
• Can be placed DMcAlpine
rubbing it out as happens with bit-mapped.
2 Computer Structure
2.1 An Introduction

This unit on Computer Structure describes in detail
the function of the component parts of a processor
in the manipulation of data.
This is extended to the methods of transferring data
within a processor and between a processor and
memory.
The concept of a stored program is considered and
the steps in the fetch-execute cycle to access and
run programs. Memory types are considered, from
registers to backing storage and how memory is
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2 Computer Structure
2.2.1 A Two State Machine
• Typical 4 box
diagram
• Only 2 states                   CPU
used in all                   Processor
components and        Input               Output
Memory
data storage, on
or off, 1 or 0.               RAM & ROM

state.                        Backing
– Simplicity – 2
voltage levels.             Storage
– Good tolerance
– Simple
calculations.
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2 Computer Structure
2.2.2.1 The structure of the CPU (a)

Memory                         Processor

Control Bus           Control
unit

ALU

Data bus – 2 way      Registers,
A, MAR,
MDR, PC,
Address bus – 1 way   SP

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2 Computer Structure
2.2.2.1 The structure of the CPU (b)
• 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.
• Main Memory
• External Memory
• Peripheral Devices

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The processors three buses
A one-way bus which comes from the processor to
memory. The lines of the address bus work in parallel and
hold the address of the memory location being
accessed. The bigger the address bus the more memory
locations. (8 bits gives 256 memory locations)

2 Data bus
The data bus can operate in two directions - from
processor to memory for WRITE operations and memory
to processor for READ. This bus is used to carry the data.
The width of the data bus determines the WORD size. The
bigger the word size the more data that can be
processed per clock cycle.

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The processors three buses(
cont.)
3 Control bus
This bus is unlike the other two in that the wires
work independently from each other. The wires
on the control bus send signal to control the
different operations e.g.

operation
WRITE line - when activated invokes a write
operation
RESET line - when activated clears all registers and
fetches a startup up program
CLOCK- synchronises the clock pulse
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The components of the CPU
ALU
The Arithmetic and logic unit carries out all arithmetical
and logical operations

Control Unit
sequences, decodes and synchronises the execution of
program instructions.

Registers
These are temporary storage units inside the processor.
Some like the MAR (memory address register) are
at a time) while others like the MDR (memory data
register) are connected to the data bus

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Main memory
Main memory is either RAM or ROM. Memory consists of
many individual locations each with it’s own unique
determines the number of these locations. Multiply this by
the size of these locations (word size) gives the total
memory
RAM
Random access memory - is cleared when the computer
is switched off. Applications and files are stored here
when you are working on them.
ROM
not prone to viruses, Some systems like early Archimedes
and BBC’s had the operating system stored in ROM.

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2 Computer Structure
2.2.3 The stored program concept

All computers based on same basic
design, known as the Von Neumann
Architecture.
executing machine instructions. A
series of these instructions is called a
machine code program held in main
memory as a stored program, a
concept first proposed by John Von
Neumann in 1945.
Central Processing Unit (CPU) fetches, decodes and executes
the machine instructions.
By altering the stored program it is possible to have the
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computer carry out a different task.
2 Computer Structure
2.2.4 The fetch execute cycle

To execute a machine code program it
must first be loaded, together with any
data that it needs, into main memory
(RAM). Once loaded, it is accessible to
the CPU which fetches one instruction
at a time, decodes and executes it at
electronic speed.
Fetch, decode and execute are
repeated until a program instruction to
HALT is encountered. This is known as
the fetch-execute cycle.

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• The CPU sets up the address bus
Does this by placing it in the MAR
• The CU activates the Read line on
the Control Bus
• The contents of the specified main
memory location are released onto
the data bus and into the MDR

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Memory Write Operation
• The CPU sets up the MAR with the
address that is to be written to
• The CPU sets up the MDR with the
data to be written
• The CU activates the write line
• The contents of the MDR are
released onto the data bus and

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2 Computer Structure
2.3.1 RAM
• RAM
• Has same access time for all locations.
• Volatile – loses contents on power off.
– 2 Types of RAM Static and Dynamic
• Static – holds contents as long as there is
power.
• Dynamic – has to be refreshed (every 2
ms).
• Each memory location has a unique

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2 Computer Structure
2.3.2 ROM

• 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.).
– Some ROMs can be reprogrammed
• EPROM – electrically programmable read only
memory (data erased by ultraviolet light and new
program burned onto ROM
• EEPROM – electrically erasable programmable
read only memory – selective parts can be
reprogrammed & used in developmental work

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Blog Entry
• Blog Entry titled Processor Structure
– Cover the following
• Components of the Processor
• Describe the role of each
• Internal buses, address/data bus, control
bus
• Main memory
• Stored program concept
• Fetch Execute Cycle

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2 Computer Structure
2.3.4 Cache Memory
• Faster processors mean data is being processed before the
next instructions can be read from memory (system busses
slow).
• Most systems have 2nd smaller area of fast SRAM called Cache
memory – much quicker to access than main memory
• Whilst the processor is executing one instruction the next one is
brought into Cache memory
• Frequently used instructions are also stored there for quick
access

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2 Computer Structure
2.3.6 External Memory

External memory, such as the hard disk,
holds quantities of data too large to store in
main memory.
It is also used to keep a permanent copy of
programs and data.
Examples of external memory devices are:
• hard disk;
• floppy disk;
• zip disk;
• CD-R;
• magnetic tape;
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• flash drive.
2 Computer Structure
2.4 Central Processing Unit

Central Processing Unit. The CPU
coordinates and controls the activities of all
other units in the computer system. It executes
program instructions and manipulates data in
accordance with the instructions.
It uses a standard architecture composed of
the following three components:
Arithmetic and logic unit (ALU);
Control unit;
Registers.
All three components work together to form the
processor.            DMcAlpine
2 Computer Structure
2.4.1 Architecture of the microprocessor

We will now study the
internal architecture of the
microprocessor (CPU)
itself. Because of the stored
program concept, we must
consider the relationship
between the CPU and             This is a diagram of a
fairly typical
memory.                         microprocessor design,
showing the internal
structure of the CPU
and its relationship to
the memory of the
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2 Computer Structure
2.4.2 Accessing Memory

The CPU has to access memory both for instructions
and to receive and transmit data from or to memory.
Memory Address Register (MAR) - specifies the
from or to memory;
The Memory Data Register (MDR) or Memory Buffer
Register (MBR) - contains the data to be written to
•MAR register connected to the address bus
•MDR register connected to the data bus.
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2 Computer Structure
2.4.2 Accessing Memory (2)

The MAR and MDR registers have a large part to
play in the fetch-execute cycle.

of the memory location into the MAR and activates the
memory-read control line of the system bus. This will
cause the required data to be transmitted from
memory via the data bus to the MDR;
To write from the CPU to memory, the CPU places
the data to be written in theMDR; the address of the
memory location where they are to be written is placed
in the MAR; and the memory-write control line is
activated.           DMcAlpine
2 Computer Structure
2.4.3 Functions of Control Bus
• Control bus has several lines, used singly to
initiate a process.
• Write Line - initiates memory write operation.
• Clock – Generates pulse to synchronise
components.
• Interrupt – Signal to processor of an interrupt like a
key press or mouse click. Processor saves stack
and deals with the interrupt.
• NMI – Non-Maskable Interrupt. Interrupt which
cannot be ignored.
• Reset – Clears all registers, aborts program and
gives control back to the operating system.

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2 Computer Structure
2.4.3.1 Getting the processors attention
• Polling
– The processor checks each part of the
system in turn and if any part wants the
processors attention it signals to the
processor.
• Interrupts
– This is how a PC works. When a key is pressed
or mouse clicked an interrupt is generated
and the processor carries out that task
(sometimes it is “doing nothing” and is
interrupted.
– Some interrupts need not be actioned
actioned (NMI) e.g Ctrl+Alt+Del
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2 Computer Structure
2.4.4 The Arithmetic Logic Unit
• ALU
– Where data is processed and
manipulated.
– ALU involves arithmetic operations
and logical operations .
– ALU uses temporary registers to hold
data.
– Accumulator is main register.

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2 Computer Structure
2.4.5 Registers

location in main memory.
• memory buffer register (MBR) holds data that has just
been read from main memory or is to be written to main
memory.
• instruction register (IR) holds the current instruction that is
being executed.
• program counter (PC) holds the address of the next
instruction to be fetched from memory.
• Processor also has a set of general purpose registers.
They are called general purpose because their role is not
defined at manufacture and can be used by programmers as
appropriate.

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2 Computer Structure
2.5 Buses
• Data is transferred               Inside the processor.
between memory and
processor by buses.
– Pinpoint memory
Control Bus Control Unit
location.
– One-way Bus
• Data Bus
– Transfers the data                     Data Bus
– Same size as Word size
Processor
• Control Bus
– Initiates and controls
operations.

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2 Computer Structure

• The Word Length is the size of data, in bits, which can be
manipulated as a single unit by the processor.
• In an ideal computer the size of the data pathways and
the size of memory locations will match.
– 8 bit address bus can access 28 = 256 locations
– 16 bit address bus can access 216 = 65,536 locations (64K)
– 24 bit address bus can access 224= 1,677,216 locations
(16MB)
– 32 bit address bus can access 232 = 4294967296 locations
(4GB)
– If we have 2 bytes or 4bytes for each memory location we
get for a 24 bit bus 16MB of addressable locations, but 32MB
or 64MB of actual storage.
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If I have a 16bit address bus and a 1byte
word length/location size, how much
main memory can the CPU address?

16 bit address bus = 216 locations = 65536

Each location is 1 byte so total memory is
65536bytes

65536/1024 = 64KB of memory

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3 Computer Performance
3.2 Measuring Performance
• When we measure performance
we usually mean how fast the
computer carries out instructions.
The measure we use is MIPS, millions
of instructions per second.
• MIPS affected by
– The clock speed of the processor
– The speed of the buses
– The speed of memory access.

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3 Computer Performance
3.2.1 The Clock
• Every processor has a clock which ticks
continuously at a regular rate.
• Synchronises all the components.
• Cycle time measured in MHz or GHz
• 200 MHz (megahertz) means the clock
ticks 200,000,000 times a second (P1 -
1995)
• 1.4 GHz (gigahertz) is 1,400,000,000 times
a second (P4 – 2001)
• 2.3 – 4+ GHz on P5 in 2004

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3 Computer Performance
3. 2 Measures of Processor Speed
• Clock Speed
– Generally the faster the clock speed the faster the
processor – 3.2 GHz is faster than 933 MHz
– But this is not the only indicator of performance
• Mips – Millions of Instructions per Second
– Better comparison but beware of false claims e.g. only
using the simplest & fastest instructions and different
processor families.
• Flops – Floating Point Operations per sec.
– Best measure as FP operations are in every processor
and provide best basis.
• Benchmark Tests
– Well defined standardised routine to test the
performance of a computer.

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• Create a spreadsheet using the
details of the 5 desktops from the
• Create a chart(s) to show visually
how they compare on clock speed,
at least 2 benchmark tests and
system memory
• Use your comparison to decide
which of the desktops a user should
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3 Computer Performance
3.3.1 Data Bus Width
• A WORD is the basic number of bits a
processor can handle in one operation.
– If word size and data bus same size then data
transfers carried out in single operation.
• Width of data bus defines how much data can
be carried in one fetch.
– 32 bit data bus (word length) carries twice as much
data as a 16 bit bus and a 32 bit system should be
faster.
• Width of Address bus affects the amount of
memory which can be accessed as we’ve
seen with the concept of addressability

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3 Computer Performance
3.3.2 Peripherals & System Performance
• Peripherals work at much slower
speeds than the CPU.
– Buffers and spooling can help.
– Sound cards can have their own
processor, RAM and ROM.
– Video cards their own RAM (up to
256Mb)

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3 Computer Performance
3.3.2 Interfaces & Input/output Devices
• An interface makes the link between the processor and a
peripheral (disk drive, printer etc.).
• Peripherals work at different speeds, use different formats.
• Parallel to serial conversion is often needed
– Some devices are serial – 1 bit at a time is transferred. Serial
used for long (over 2m) distances.
– Some are parallel (printers) – 8 bits at a time. Used for short
distances problems with skewing – loss of data integrity.
• Interface transfers data so the processor is delayed as little
as possible. It has buffers to store blocks of data in transit.
• Memory mapped I/O uses memory linked to peripheral.
• Spool files are used when large quantities of data are sent
to a slow peripheral, like a printer. Enables background
printing.

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3 Computer Performance
3.3 Memory & System Performance
• Speed of access & Word size
– 15-120 nanosecond but memory speed and word size
dictated by motherboard and processor
• Amount of memory
performance esp. graphics & multimedia.
• Cache (pronounced cash) memory
– Cache exists between memory and processor
– Very fast memory speeding data transfer in shorter
fetch cycle.

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Blog Entry
• Create a new blog entry titled
Measures of Computer
Performance
–   MIPS
–   Clock speed
–   FLOPs
–   Benchmarks
• Explain each and explain how they
can be fudged. Explain the any
drawbacks
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4 Peripherals
4.1 Introduction
• Examine a range of hardware devices
• We will examine devices in terms of:-
–   Speed
–   Cost
–   Resolution
–   Compatibility
• Developments and trends in storage
devices
• Serial & parallel interfaces and wireless
communications
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4.2 – Input & Output Devices
4.2.1 Keyboards
• QWERTY keyboard
– has its roots in mechanical typewriters – to slow down
operators to avoid jamming the keys.
– Key press causes a scan code to be sent to the
computer.
• Sent via serial coble to keyboard controller.
• Sent as ASCII Codes
• Modified Keyboards
– Used to alleviate Repetitive Strain Injury (RSI)
– Customised keypads can have more (or fewer) keys
all programmable to suit particular situations.
– Adjustable split keyboard in 3 parts to allow flexibility.

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Alternate Layouts
• Find 2 alternate key layouts e.g. not
QWERTY, find the reasons for their
alternate layout
• Find 2 alternate designs e.g. the
shape of the keyboard and the
placement of the keys is totally
different to a regular keyboard, find
the reasons for this alternate
designs
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4.2 – Input & Output Devices
4.2.2 Scanners
• Scanners
– Flat bed scanner allows for up to A4 size
documents
• Document placed face downwards on glass
panel and scanned.
• Light beam reflects light from the document and
photocells measure the light reflected.
• Analogue data needs converted to digital (A DC)
– 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. DMcAlpine
4.2 – Input & Output Devices
4.2.2 Scanners

• Accuracy – measured by how close the image
is to the original.
– Resolution is the dots per inch (dpi) that can be
detected by the scanner hardware. A 600 dpi
scanner has 600 photocells per linear inch.
– Bit depth usually 24 bits (8 red, 8 green, 8 blue).
• Capacity
– Little internal buffering, rely on techniques to transfer
the data.
– Storage can be high e.g. A4 page at 600 dpi requires
33.28MB for 8 bit and around 100MB for full colour.
• Cost
– Dropped dramatically in recent years
– Bundled software often the major selling point.
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4.2 – Input & Output Devices
4.2.3 Sound
• 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 but sound files can be taken from a CD and
– Sound card performs the ADC and compression

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4.2 – Input & Output Devices
4.2.3 Sound sampling
• 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

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4.2 – Input & Output Devices
4.2.3 Sound
• Accuracy
– Resolution – Three sampling resolutions in common use.
• 11.025 KHz (8-bit) – voice quality
• 22.05 KHz (8-bit) – Quality of AM radio
• 44.1 KHz (16-bit) – CD quality stereo [data sampled
44,100 times per second]
– Bit-Depth
• 8-bit sample size can hold 256 amplitudes per sample
• 16-bit sample size can hold 65,536 amplitudes per
sample
• Capacity
– No built-in cache. Depends on fast access via the sound
card to hard disk storage. 10.09MB to store 2 mins stereo
audio.
– Compression required e.g. reduce sample rate / size or use
a compression technique to reduce file size.

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Storage requirements for sound
• Capacity = sample rate * sampling depth * length of
sound in secs

How much storage is required for a 3minute song stored at
CD quality: 44.1KHz sampling rate and 24bit sample
depth

3 mins = 180 secs
44.1KhZ = 44100 samples per second
24 bits = 3 bytes

44100 * 3 bytes * 180 = 23814000bytes
23814000/1024 = 23255.9 KB
23255.9 / 1024 = 22.8MB

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4.2 – Input & Output Devices
4.2.4 Video
• Video Digitising
– is performed by special video digitising
circuitry installed on the motherboard of
the computer.
– File Formats
• Quick Time
• MPEG
• AVi

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4.2 – Input & Output Devices
4.2.4 Video
To playback video on a standard computer it will need to be
decompressed by hardware or software, usually on the card.
Standards
AVI – (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
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4.2 – Input & Output Devices
4.2.5 Digital Camera
• Film replaced by an array of photosensitive
cells.
– Images stored electronically using photosensitive
diodes called charge coupled devices (CCDs)
– Intensity of light recorded in an image.
– Analogue values converted to digital using ADC
• Compression usually takes place.
– Bit map files turned into JPEG
• Image transferred to computer
– Serial Cable
– Can then be printed, e:mailed etc.

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4.2 – Input & Output Devices
4.2.5 Digital Camera
• Accuracy
• Resolution
– Measured in megapixels
– Accuracy depends on the array of photosensitive cells.. The
more sensors and the smaller they are the higher the
resolution.
• Bit Depth
– Number of bits is proportional to the number of colours that
can be represented like in bitmap images
• Capacity
– Based on resolution and memory in the device.
– Compression v altering resolution
• Cost
– Dropping as they become more common.
– Resolution main factor and also facilities (zoom, flash etc.).

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4.2 – Input & Output Devices
4.2.6 Printers – Ink Jet Printer
• Ink-jet Printers are based on one of three different types of
technology: continuous flow ink-jet, liquid ink-jet or phase-
change ink-jet. We will look at how a liquid ink-jet printer
works.
• Liquid ink-jet or bubble-jet, operates by squirting tiny
droplets of ink onto the page. The ink is first heated by
passing an electric current through a coil. In milliseconds a
bubble of vapour appears, forcing a tiny drop of ink from the
nozzle onto the paper.
• Resolution is typically 300 to 600 dots per inch, support the
printing of text and graphics, colour and a range of shades.
• Speed is pretty slow with a range of 4 pages per minute to
8 pages per minute, depending upon the model.

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4.2 – Input & Output Devices
4.2.6 Printers – Laser Printer
• This type of printer uses lasers to "write" a page image onto
a special drum as an electrostatic charge. The charged
drum attracts toner particles which are transferred to the
page and heated to set the image.
• Usually a page is composed in the printer (often PostScript).
• Capacity
– On board RAM (buffer) & processor needed to compose
pages. The more RAM the higher quality graphics can
be printed.
• Resolution
– 600 dpi quite common (300 cheaper, 1200 expensive)
• Speed
– ranges from 4 ppm to 20 ppm with 12 ppm being about
average.

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4.2 – Input & Output Devices
4.2.7 Multiscan Monitor
The CRT is the oldest of 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
determines the actual colour of the pixel.
The picture is redrawn between 50 and 100 times a
second. This is the refresh rate.
A monitor which operate at different refresh rates is known
as a multiscan monitor. The refresh rate is controlled by the
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.
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4.3 Selecting hardware to match
operational requirements
• When given a scenario like setting up a library system you will
have to consider:-
– RAM requirements
• Memory must be enough to run the software and
support all the data in the system.
– Backing Storage
• Big enough to hold the O/S, Applications and data.
– Processor Performance
• Usually as fast as you can afford but must be fast
enough to support all the applications recommended.
– Peripherals
• Specify type of printer, monitors etc.
– Communications
• Attached to a network or set up a new network.

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4.4 Buffers and Spoolers
4.4.1 Buffering
• Buffer
• area of memory used to transfer data between
the computer and a peripheral.
• Used when a fast acting part of the system
exchanges data with a slow peripheral
• Buffer stores the data until peripheral can act on it.
– Peripheral Buffer
• E.g. printer – very slow. Has on board RAM to store
the incoming data (laser may have 8MB)
• E.g. Mass storage (disks). Data transferred in blocks
so whole block transferred, managed by buffering
– Interface Buffer
(UART) handles transfer of serial to parallel and
vice versa.
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4.4 Buffers and Spoolers
4.4.2 Spooling
When large amounts of data are to be sent to a peripheral
device, or when the peripheral is shared across a network
then spooling is a preferred method of compensating for
the difference in speeds of the processor and the
peripheral.
Spooling involves the input or output of data to a tape or a
disk.
This, for example, allows output to be queued from many
different programs and sent to a printer by a print spooler
(special operating system software).
The print spooler stores the data in files and sends it to the
printer when it is ready, using a print queue.

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4.5 – Storage Devices
4.5.1 Magnetic
Magnetic storage devices include hard disks, floppy disks,
Zip disks and magnetic tape.
They are called magnetic storage devices because their
recording surfaces are coated with a material that
responds to magnetic fields to enable data to be stored.
Storage devices can be fixed or
removable. Removable storage
devices allow the user to
disconnect the device and
physically transport data from one
computer to another. E.g. external
hard drive

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4.5 – Storage Devices
4.5.1.1 Magnetic Disk
All the sectors around the disk, equidistant from the centre,
form a track. With multiple platters, the collection of tracks
on each platter, equidistant from the spindle is called a
cylinder. When data is to be read or written, the read and
write heads are moved to the appropriate track, where they
wait until the relevant sector spins past.
Speed
Rotational speed of hard disks has improved, from 3000 (rpm)
of early disks, to current rotational speeds of 5,400 and even
7,200 rpm.
Performance is also measured in terms of the rate of data
transfer from the disk.
SCSI - transfer rate 5Mb/sec
Ultra Fast SCSIIII transfer rates - 40 Mb/sec.
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4.5 – Storage Devices
4.5.1.1 Magnetic Disk
Capacity
Hard disks have improved tremendously in their capacity to
store data in the last 10 years. From the modest 10Mb disks
of the early 80s to current 80 Gbyte disks on many of today’s
PCs.
Access
The hard disk is a direct access device, meaning that data
can be directly read or written to any portion of the disk.

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4.5 – Storage Devices
4.5.1.2 Magnetic Tape
Storing data on tapes used to be the only solution to backing up
hard disks of large capacity. Now, with large, removable magnetic
disks and optical CD-RW technology, this is no longer the case.
However, removable storage media is comparatively expensive,
costs 10 times tape. Tape, therefore, still has the edge in this
market.
Tape is read and written on a tape drive. A single operation writes
each block
Data is stored on magnetic tape as magnetised regions on the
surface of the tape induced by the magnetic recording head. To
stored magnetised regions produce very small voltages in the
analysed to give a representation of the stored binary data.
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4.5 – Storage Devices
4.5.1.2 Magnetic Tape

Capacity
Magnetic tapes have large capacities, reaching up to several gigabytes
and come in a variety of sizes and formats.
Since their introduction, tape drives have passed through many stages of
improvement with extremely reliable Digital Audio Tape (44.1 kHz, 16-
bit record and playback DAT) drives representing the current state of the
art. A 4mm DAT tape can now store up to 24 Gbytes of data!
Access
Tapes are sequential access devices. Accessing data on tapes is therefore
much slower than accessing data on disks.
They are not suitable as storage media for applications where data needs
be used regularly - where a disk is a more appropriate medium. Because
tapes are so slow, they are generally used only for long-term storage and
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backup.
4.5 – Storage Devices
4.5.2 Optical Storage
CD-ROM

A plastic disk is
scanned using a laser. It
reflects off pits on the
surface differently from
lands (bumps)
Re-writeable CD-ROM            Capacity – About 700Mb
becoming more
Speed – from single (150KB/sec) to
common.
32x (or even 40x). The x refers to the
times faster than CD Audio.
Cost – CD-ROM Drives fairly cheap.
Access
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4.5 – Storage Devices
4.5.2 Optical Storage
• DVDs fall into the same category as CDs
• They use light to read data from the disk
• Like a CD there are pits and lands on the DVD surface.
However a DVD can store 4.7GB compared to 700MB on
a CD. How is this possible on a disk of the same physical
size?
• Well the track on a DVD is much closer together meaning
that the track is longer and therefore more pits lands on
the disk
• There is also more than one layer on a DVD which
requires the lens that reads the data to be able to focus
on both layers, if you’ve ever noticed a slight pause whilst
watching a DVD movie then this is the reason for it, the
DVD has come to the end of one layer and is moving
onto another

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• This diagram shows a cross section
of a DVD

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4.5 – Storage Devices
4.5.3Magneto 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
Capacity – 3.5” disks of 128, 230 and 384 Mb
Speed – Varies as multiple of standard single speed
Cost – decreasing with time with different formats
and capacities becoming available.
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4.5 – Storage Devices
4.5.4 Solid State Storage Devices (SSSD)
Solid-state storage devices are made up entirely from
electronic components i.e. they have no moving parts.

They are also called RAM disks, as they take the place of a
magnetic disk as a mass storage device.
They can be in the form of a plug-in card or cartridge
containing memory chips.

The robustness of this technology has led to it now being
implemented as the main form of backing storage on new
computers, however a yet they are proving to be very
expensive

SSSD are used with devices where space is at a premium
e.g. in a camera, or when portability is desirable e.g a USB
flash drive.
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4.6 Interfacing (1)
Interfacing means making the hardware connections so
that two devices can communicate effectively.
Data Format – data has to be consistent e.g. serial output to
a serial device. Interface makes data consistent (also ADC)
Parallel/Serial – time & space division. Time separates
transmission of actual bits and space can be used for
multiple bits in parallel. Serial can be slow but use of fibre-
optic cable very fast.
Voltage – different voltage levels between peripheral &
computer need to be ironed out.
Protocols – rules that govern transmission of data. E.g. no of
bits per packet, voltage levels etc.

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4.6 Interfacing (2)
Status signals from a device indicate what the device is doing at
any given moment. E.g.if a device is unable to receive data, then
a transmitting device can delay transmission and retry later.
Speed - Different devices send and receive data at different rates.
The devices agree a rate prior to transmission by utilising a
protocol.
Wireless communications can be achieved using WAP (Wireless
Application Protocol) - a specification for a set of communication
rules to standardise the way that wireless devices, such as cellular
telephones and radio transceivers, can be used for Internet
access, including e-mail, the World Wide web, newsgroups, and
Internet Relay Chat (IRC). Interconnecting devices centred around
an individual person is called a wireless personal area network
(WPAN). Typically, a WPAN uses technology that permits
communication between devices in a

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The Operating System

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Structure of Operating System
CLI
MMS
FMS
I/O

Kernel

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CLI
• Outer layer or shell of the OS
driven/or GUI
• Makes sense of commands from
user and does it’s best to carry
them out
• If user friendly, responses will be
given when there are errors and
prompts to assist the user

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Memory Management
System
• Controls where programs and data are
stored in main memory
• Keeps track of main memory available
– Where programs and data are stored
• Will always try to put programs or data in
memory
– Sometimes not possible

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Memory Management
System
• If space left in memory prevents
something being stored the MMS
passes an error message to the CLI
– The CLI then presents the error to the
user
• What could the user do?

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MMS
• Many computers have an MMS that
– What does multi tasking mean?
– In actual fact only one process is
running at any one time
• MMS manages the switching between
processes

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MMS
• Makes sure programs don’t
interfere with the section of memory
reserved for Operating System
• Contrast this with a ROM based OS
compared to a disk based one

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File Management System
• Efficient use of backing storage
– Also information about where files are
stored
• Usually the outer track of the disk
• Specifies precise address on disk
• This info required by I/O when requested
to find and load files by FMS

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FMS
• Hierarchical System
– Directories and sub-directories
– Keep related files together

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I/O system or BIOS
• Basic Input Output System
– Tailored to suit the hardware
• Produced by manufacturer
• Communicates directly with
peripherals
– Transfer of data between CPU and
peripherals

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The Kernel (Kernel Sanders)
• The kernel is the centre of our onion
– Manages the processes
– Handles interrupts
– Interrupts used by peripherals to
communicate with CPU
• The temporary suspension of the CLI
is how the Kernel deals with
interrupts

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Working in concert
involves all of the layers of the OS
– This must be carefully choreographed
– This is quite complex, what follows is a
simplified version of events

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Working in concert
• You issue command to CLI to load myfile by
selecting from a menu or typing in a command
• Kernel suspends current CLI process, passes
request to FMS
• FMS requests the I/O to read the disk’s catalog
track, loads a list of file names
• If file is present, the FMS passes physical location
to the I/O System
• I/O system loads file and passes to MMS
• MMS places the file in correct place in Main
Memory
• Kernel allows CLI process to resume

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Things going wrong
• Consider what should happen in
the following scenarios
– The requested file wasn’t present on
the disk
– There was insufficient memory to hold
the file in the computer’s main
memory
– The application that created the file
memory
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Single and Multi User OS
• Single User – used by one person at
a time
• Multi User – run on a computer that
forms part of a network
– Means that there are several special
facilities that a Multi User OS must
provide
– Windows NT

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Multi User OS
• Access and Security
– Log-on
• Precautions?
– Log-off
• Why?
– Time-out
• Automatic log-off after certain amount of
inactivity

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Multi User OS
• File and Print Services
– Depends on type of network e.g. Peer-to-
peer or Client/Server
• What type do we have in school?
• Creates users and allocates portion of backing
storage on file server
• Workstation set as print server
– Lots of backing storage as temporary storage for
print documents
– Spooler
– Why not a buffer?

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Multi User OS
• Data Sharing
– Make sure only you can access your
data
• Achieved by setting aside space on the
server
– If 2 people access the same file then
only one can alter it
• Or all shared files set to read only

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Utility Software
• Type of software designed to
– Disk editing
– Back up copies
• Could be described as a single
purpose package
– Tasks it performs are confined to a
single purpose

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Virus checking
• What is a virus?
– Malicious program
– Cause damage
– Can copy itself to other computers
• What should virus checking software do?
– Detect virus programs on the computer and
delete
copied from a disk

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Disk repair
• Scan damaged disks and repair any
mistakes
– These mistakes are usually in the disk catalog
• Where is the disk catalog stored?
• Should install before any disk errors take
place
– Much better chance of recovery
– Disk repair utility makes it’s own copy of the
disk catalog

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De-fragmentation
• As we know hard disks are formatted to
contain blocks of a set size
• Filing systems try to ensure that files and
programs are stored on adjacent blocks
• But as more files are saved and deleted
this becomes increasingly difficult
• Take the following diagram as an
example

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Imagine the table below is a section of the hard
drive. It has become fragmented through use,
saving and deleting files. We can de-frag it to
improve performance. Note: De-fragmentation
does not create space, it organises space and
existing files better!

1/1          1/2      2/1       2/2

2/3   3/1             3/2

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Benefits of De-Fragmentation
programs/files?
– Should load quicker as it will take less time to
find them
– Computer start up should also be quicker
• What effect will this have on saving new
files?
– Should be quicker as the free space is better
organised, more contiguous blocks free

• De-fragmentation can take a long time
depending on disk size, disk speed and
overall performance of the system
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Back-up
• Helps automate the process of backing
up
– Select the type of back-up media
– What are posssible media types?
• Tape, CD,DVD, USB, External Hard-disk
– Select the frequency of the back-ups made
– Synchronise files on two separate devices

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Emulators
– Use the internet to find out what an
emulator is. Write the description in your
jotter
– What are the benefits of an emulator?
– What are the drawbacks?
– Is any additional hardware required?
– Find three examples of different
emulators

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Emulators
• Allows your computer to behave
like it’s another type of machine
– E.g. Mac behaving like a PC, and
crashing all the time!
• Also allows your computer to be
used like a peripheral, enables
microcomputers to act as terminals
for mainframes

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Emulators
• Drawbacks
– Much slower than normal because of
extra processing required.
– Fast CPU required and lots of RAM for
best performance
• Emulation can also be achieved in
hardware

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Converters
• If I create a document in MS Word, can I
open it with another application?
formatting is lost
• All applications save files in a unique
format
• Data converters can help overcome this
problem
– Change the data format from one
application type to another

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Compressors/Expanders
• Some packages create files that are very big
– Video, graphics
• Compressors can reduce the file size and save
backing storage space
• We might also compress a file before e-mailing
it
• When received the expander program carries
out the reverse process and restores the file to
it’s original form and size

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Installers
• Installer used to place an application or
an OS on a computer system
• Installers allow users to “custom” install
programs
– E.g. only install the parts they want or need
to save space
• Usually have a Wizard
– What is a wizard?
• Guides you through installation process step by
step

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Uninstallers
• Reverse the process of installing
– Much more than removing the
program or deleting a file
– Application may have installed new
fonts for example
– A good uninstaller will remove all
traces of a piece of software from a
computer system

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Printer Drivers
• Printers can print many different
fonts and styles
– Before it can do this it must be sent the
correct code from the computer
system
– These are different on different printers
• E.g. the code for bold on an HP laser
printer might produce italic on an Epson
printer

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Printer Drivers
• Printer drivers overcome this problem
• They translate the codes sent from the
computer to the correct code for the
specific printer
• Drivers also allow us to connect more
than one printer to a computer
• Driver must be installed before a printer
can be used

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• Use a piece of utility software (probably
best to use ZIP)
• Create a report, including screenshots,
telling someone how to use the software
and why they might use it
• Include in the report reasons why the
utility is useful and what benefits it
provides the computer system

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Some starter questions
• What piece of utility software would
you use if you wanted to create
some space in backing storage?
• What does a de-fragmenter do?
• What part of a disk does a disk
repair program repair?
• Why are printer drivers required?

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