Higher Computing
Systems Unit
Higher Systems
Data Representation
Topic 1
Data Representation

Representation of positive numbers in binary including place values and range
up to and including 32 bits

Conversion from binary to decimal and vice versa

Description of the representation of negative numbers using two’s complement
using examples of up to 8 bit numbers

Description of the relationship between the number of bits assigned to the
mantissa/exponent and the range and precision of floating point numbers

Conversion to and from bit, byte, Kilobyte, Megabyte, Gigabyte, Terabyte. (Kb,
Mb, Gb, Tb)

Description of Unicode and its advantages over ASCII

Description of the bit map method of graphic representation using examples of
colour/greyscale bit maps

Description of the relationship of bit depth to the number of colours using
examples up to and including 24 bit depth (true colour)

Description of the vector graphics method of graphic representation

Description of the relative advantages and disadvantages of bit mapped and
vector graphics

Description of the relationship between the bit depth and file size

Explanation of the need for data compression using the storage of bit-map
graphic files, as examples
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Higher Systems
Data Representation
Introduction
Computers are called two-state devices because all data is stored using two values.
All the logic circuits used in digital computers are based upon two-state logic. That
is, quantities can only take one of two values, typically 0 or 1. These quantities will
be represented internally by voltages on lines, zero voltage representing 0 and
the operating voltage of the device representing 1. The reason two-state logic is used
is because it is easy and economic to produce such devices.
Measures:
o A bit is a Binary digit a 0 or a 1.
o 8 bits make a byte
o 1024 bytes in 1 Kilobyte (1024 = 210)
o 1024 Kbytes in 1 Megabyte (220)
o 1024 Mbytes in 1 Gigabyte (230)
o 1024 Gbytes in 1 Terabyte.
To go from bits to bytes, divide by 8.
To go from Kbytes to Mbytes, divide by 1024, etc.
Numbers
We use the base 10 number system to represent whole numbers, integers and
fractional numbers. This number system uses the 10 digits 0  9 to represent
numbers. The value of a decimal digit is given by its position within the base 10
number system.
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Higher Systems
Data Representation
Example:
34 043 is
10 000 1 000 100
3
4
0
10
4
1
3
3 x 10 000 + 4 x 1 000 + 4 x 10 + 3 x 1
The binary number system
When numbers are represented electronically, the most convenient base is 2, where
each column, reading from the right is a power of two. The base 2 number system
uses 2 symbols, 0 and 1 to represent a value.
Example 1:
10110 is
16
1
8
0
4 2
1 1
1
0
1 x 16 + 1 x 4 + 1 x 2 = 22
Example 2:
1101100110011010 is
32768 16384 8192 4096 2048 1024 512 256 128 64 32 16 8 4 2 1
1
1
0
1
1
0
0
1
1
0 0 1 1 0 1 0
= 32768 + 16384+4096+2048+256+128+16+8+2 = 55 726
So, to convert a binary number to our numbers (denary, or base 10), you put the
headings starting from the right : …. 32 16 8 4 2 1 above the number and add
up the headings where there is a 1.
The binary number system has the huge advantage that only two symbols are
required, 0 and 1. These can easily be represented in a computer system by a switch or
transistor being on or off, or by a high or low voltage level. Imagine how difficult it
would be to represent 10 discrete logic values for the base 10 number system.
You can easily store data in binary e.g. magnetic discs using N/S magnetism or CDs
using pits and lands to reflect light.
Binary representation also simplifies the number of arithmetic rules that need to be
applied in calculations. Binary arithmetic has fewer rules. You need 100 rules for
adding our numbers, you just need to know 0+0, 0+1, 1+0 and 1+1 (=10) to add
binary numbers.
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Higher Systems
Data Representation
So the advantages of binary are:
 Simple arithmetic
 Simple electronic circuits
 Wide range of storage devices can use 2 values.
Another advantage is called ‘signal degradation’. If you used 0V for 0, 1V for 1 up to
9V for 9, voltages are never stable and if that 9V drops to 8.5 is it an 8 or a 9? With
binary you can have a large difference between the values (e.g. 0V for 0 and 8V for 1)
Converting our numbers to binary:
There are two ways to convert decimal numbers into binary.
Method 1.
To convert 29 into binary: write down the binary headings (don’t go past 29), then
work out which headings add up to 29:
16 8 4 2 1
To get 29 we need a 16, an 8 a 4 and a 1, so 29 =
11101
Method 2.
This is guaranteed to work on any number and is useful for very large numbers. Here
you continuously divide by 2, writing down the remainder each time until there is
nothing left. The binary number is formed by reading the remainders up the way.
2 29
2 14 R 1
2 7 R 0
2 3 R 1
2 1 R 1
= 1 1 1 0 1
0 R 1
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Higher Systems
Data Representation
Something to know about storing numbers on a computer is that a fixed number of
bits is always used. Let’s say a computer uses 32 bits to store numbers, then
3 would be stored as:
324 564 046 stored as:
00000000000000000000000000000011
00010011010110000111010001001110
The programming would be too difficult if variable length numbers were used, the
computer wouldn’t know when a number ended! This means that there is a fixed
range of values that can be used determined by how many bits you use.
The range of positive integers.
With 1 bit you can get 2 possible values: 0 or 1.
With 2 bits you can get 4 values: 00, 01, 10 and 11
With 3 bits you get 8 values: 000, 001, 010, 011, 100, 101, 110 and 111
1 bit
2 bits
3 bits
…
8 bits
…
n bits
21
22
23
…
28
…
2n
values
values
values
…
values
…
values
=
=
=
2
4
8
range 0  1
range 0  3
range 0  7
=
256
range 0  255
=
2n
range 0  2n - 1
The range of positive numbers you can code is always one less because we start at 0.
For n bits the range is 0 to 2n - 1
If we lived in a binary world we
would probably have a 16 hour
clock (and 32 hour day), but the
clock numbers would range
from 0000 to 1111 (0 – 15) !
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Higher Systems
Data Representation
You can add, subtract, multiply & divide in binary exactly the same as our numbers
although you would find division difficult to get the hang of.
The only thing you need to remember is 1+ 1 = 10 (and 1+1+1 = 11). Also 10 – 1 = 1
Simple really!
So 1010 + 1011 =
10 10
+ 1 01 1 1
10101
EXERCISE 1:
1. Convert these binary numbers to decimal:
a) 10101 b) 11001 c) 11100010 d) 10101010 e) 11110000
2. Convert these decimal numbers to binary:
a) 27 b) 37 c) 56 (use 8 bits) d) 97 (use 8 bits) e) 765 (use ÷ by 2 method)
3. Why are computers called two state devices?
4. Give two reasons why computers use binary.
5. What range of positive numbers can be stored using:
a) 1 byte b) 16 bits
use powers of 2 for these answers:
c) 20 bits
d) 32 bits
6. Add these binary numbers:
a) 1 0 1 0
+ 11
b) 1 1 0 1 1 1
+1 0 1 1 0 1
c) 1 0 1 0 1 1 0 0
+11010101
7. If you use a fixed number of bits to store numbers, what will happen if there is
a carry at the end?
8. What number has been tattooed on this leg:
a) If you go ankle to knee?
b) If you go knee to ankle?
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Higher Systems
Data Representation
Negative numbers
An obvious way of getting computers to store –ves would be make the first bit a 1 for
negative, 0 for positive. This is called sign and magnitude but unfortunately doesn’t
work because adding gives the wrong answer and there is a +ve 0 and a –ve 0.
So two’s complement is used because there is only one 0 and arithmetic works
correctly.
In actual fact the ALU in a processor can only carry out two operations:


Binary addition
Inverting (or flipping) bits  changing 1 to 0 and vice versa.

Two’s complement allows the ALU to store negatives and to subtract using these two
operations. Subtraction is just adding the negative ( i.e. 7 – 2 = 7 + (-2) )
To store a –ve in two’s complement:
Step 1: Write down the positive value in binary
Step 2: Flip the bits ( 1 becomes 0 and 0 becomes 1)
Step 3: Add 1
For two’s complement to work correctly you must use a fixed number of bits for each
number, so add 0s to the front to get the required amount.
Example 1: What is -17 in two’s complement using 8 bits
Step 1:
Step 2:
Step 3:
17 = 00010001
flip: 11101110
add 1: 11101111
So 11101111 is -17 in 8-bit two’s complement.
Example 2: What is -88 in 8-bit two’s complement?
Step 1:
Step 2:
Step 3:
88 = 01011000
flip: 10100111
add 1: 10101000
So 10101000 is -88 in two’s complement.
You must remember that positive numbers are still stored in ordinary binary. So you
only need to do step 1 for +ve numbers.
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Higher Systems
Data Representation
Using two’s complement, -ve numbers will always start with a 1, +ves with a 0. Also
two’s complement is its own inverse. So to convert back you flip and add 1.
So going from binary to our numbers:
Example 3: What is this two’s complement number? 10110001
We can see it is a –ve because it starts with 1.
So Flip : 01001110
Add 1: 01001111
Put the usual binary headings above this number and you get: 64 + 8 + 4 + 2 + 1 = 79
So the original number was –79
Example 4: What is the value of the two’s complement number11001111?
Flip: 00110000
Add 1: 00110001 work it out as 49, so answer -49.
N.B. If the number starts with a 0 it is positive, just put the headings above it to find
out what it is.
EXERCISE 2:
1. Write these numbers in 8-bit two’s complement:
a) -34
b) -19
c) -97
d) -64
e) 28
2. These binary numbers are stored in 8-bit two’s complement, work out their
value in denary.
a) 11000000
b) 10111111
c) 10011000
d) 01010001
3. Why do computers use two’s complement?
4. Work out these subtractions by i) subtracting them (remember 10 – 1 = 1 ) and
ii) by adding the two’s complement.
a) 01001010 – 00000110
b) 01100111 – 00111111
(throw away the carry at the end when you add the two’s complement)
5. What range of values can be stored using 8 bits in two’s complement?
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Higher Systems
Data Representation
That covers how to store Integers. What about numbers outside of the range of
integers? Also what about decimals?
For these numbers, computers use Floating Point. This is the same as Standard Form
except the computer does not store the point or the base and counts the places from
the BEGINNING OF THE NUMBER.
93 000 000 in Standard Form :
9.3 x 107
93 000 000 in Floating Point :
93
8
The 93 is called the MANTISSA
The 8 is called the EXPONENT.
Again, computers use a fixed number of bits to store floating point numbers. For
instance with 32 bits they might use 24 bits for the mantissa and 8 bits for the
exponent. Or they could use 20 bits mantissa, 12 bits exponent. This has an effect of
the ACCURACY that numbers are stored in and the RANGE of values that can be
stored.
With our numbers imagine we have a calculator that can only store 3 digits for the
mantissa and 1 digit for the exponent:
E
93 000 000 would be stored:
12 875 000 would be stored:
1
9
3
8
2
9
8
Note the loss of accuracy.
The length of the mantissa determines how accurately (or precisely) floating
point numbers can be stored.
2 340 000 000 cannot be stored at all. The point moves 10 places and this is out of
our range. So the exponent determines the range of numbers that can be stored
in floating point.
The more bits for the mantissa, the higher the accuracy
The more bits for the exponent, the bigger the range.
And vice versa.
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Higher Systems
Data Representation
TEXT
ASCII
American Standard Code for Information Interchange (ASCII) was first
developed for teletypewriters and is now an internationally agreed
standard for storing information.
ASCII uses 7 bits per character, giving a possible 128 different
characters. It has 96 displayable characters, enough to represent every
letter, number and punctuation mark of the English alphabet which forms
its character set. Each character has its own unique code.
There are 32 special character codes known as control characters. They
make something happen like new line, clear screen etc.
Now computers use bytes (groups of 8 bits), so the extra bit can be used either for
error checking or extending the character set to include French, German etc characters
like : ê, å, ñ, etc.
Now the problem with ASCII is it was designed for our Latin alphabet and as we have
seen can be extended to cover Western European character sets, but what about Urdu,
Arabic, Chinese and so on.
So ASCII is being superseded by Unicode. Unicode is a 16 bit code giving 65 536
characters which is enough to include all the world’s alphabets. ASCII forms the first
128 characters and extended ASCII forms the next 128 codes.
So Unicode has the advantage of including existing ASCII but extends to all character
sets, so you can code every alphabet in the world (including ancient unused ones).
The disadvantage is text takes up twice as much storage or twice as much bandwidth.
EXERCISE 3
1. Describe how very large or small numbers are stored.
2. What is the effect of increasing the number of bits allocated to the mantissa?
3. When running a programming you can get an ‘overflow’ error. What do you
think this error means in relation to floating point storage?
4. Give one advantage and one disadvantage of using Unicode rather than ASCII.
5. How many different characters (or ‘glyphs’) can be stored in Unicode?
6. How many bytes of storage would be needed for the sentence inside the
rectangle below if it was stored in Unicode?
This is quite a short sentence, only 9 words.!!
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Higher Systems
Data Representation
GRAPHICS
There are two methods of storing graphics:- bitmap and vector.
BITMAP
In Maths a mapping is a correspondence between two sets like whole numbers and
their squares.
1
2
3
4
1
4
9
16
bitmap is the correspondence between
the pixels and the bits in memory.
In Black & White (monochrome) graphics there is a simple 1 – 1 correspondence
between each pixel and each bit.
Every bit maps to
a pixel.
For colour we can use more than 1 bit per pixel (called the colour depth or bit depth).
There will still be a mapping, but now 1 pixel will map to more than 1 bit.
If we have 4 colours you will need 2 bits per pixel, possibly like this:
00
01
Now the bit map for this:
10
11
will be this:
01001011
00010000
11110010
10000100
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Higher Systems
Data Representation
If you use n bits you will get 2n different colours or shades, n is called the colour
depth or bit depth. So:
No. of
Bits
1
2
4
8
16
24
n
No. of Colours
2
4
16
256
65 536
16 777 216
2n
24 bit colour depth is also called true colour as that is the maximum number of
shades the human eye can distinguish.
The actual bitmaps are stored in the VRAM (Video RAM) on the graphics card. The
amount of VRAM on the card determines the maximum colour depth and resolution
(number of pixels) your screen can display.
You can easily calculate the storage requirements for a screen using the formula:
No. of BITS = Pixels across x Pixels down x bit depth.
Example 1: A 1280 by 800 screen using 8 bit colour depth needs:
1280 x 800 x 8 BITS
= 8192000 bits
/8 = 1024000 bytes
/1024 = 1000 Kbytes
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Higher Systems
Data Representation
Example 2: Calculate the storage required for a screen using 1620 by 1280 pixels with
65 536 colours.
Bits
= 1620 x 1280 x 16
(because 216 = 65 536)
= 33177600 bits
/8
= 4147200 bytes
/1024 = 4050 Kbytes
/1024 = 3.96 Mbytes
Note how the number of bits per pixel increases the file size. 24 bit colour produces a
file 3 times larger than 8 bit colour.
VECTOR GRAPHICS
The second way of storing graphics is vector or object orientated graphics. Here
graphics are stored by their objects and their attributes.
In a graphics package you have a set of drawing tools. You can choose rectangle,
circle, polygon etc. You can also choose colours for the lines, fill patterns, line
thickness and so on.
The objects are rectangle, circle etc. The attributes are line colour, line thickness, fill
patters etc.
Here in Fireworks I
have drawn a circle
choosing the object
here.
The attributes (or
properties) can be
changed here.
Word uses vector graphics, Paint uses bitmap.
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Higher Systems
Data Representation
Vector (or object orientated) stores graphics by objects and their attributes. This is
just a list of numbers:
3, 100,200,250,3,4,2,1 where :
3 could be a circle
100,200 its centre
250 the radius
3 the line thickness
4 line colour
2 fill pattern
1 the layer
CAD uses vector graphics
You should get a feel yourself for the differences between vector and bitmap by trying
Appleworks where Draw is vector, Paint is bitmap or the Paint program on a PC
compared to Word graphics.
You can also examine the size of any file you save by right clicking and choosing
properties. Bitmaps are always big files.
Differences between bitmap and vector:
First of all you need to understand the effect of enlarging a graphic:
Scaling a bit map in a painting package is done by applying a scale factor to each
pixel. For an enlargement, there is the same number of pixels as in the original, each
pixel just gets bigger.
A screen’s resolution might be 100 dpi. If you print that on a printer with 600 dpi then
each pixel is scaled by a factor of 6, i.e. it gets larger and appears ‘blocky’.
With vector, the computer is using a sort of formula to draw each object, it can use all
600 dots on the printer so in fact the graphic actually gets finer and improves.
Bitmap is called resolution dependent and vector is resolution independent.
On the next page are printouts to show the differences, both of these graphics looked
the same on screen.
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Higher Systems
Data Representation
Advantages of bit mapped graphic representation
A bit mapped image can be manipulated at the pixel level. Thus a designer may apply
particular colour values to a selected pixel area to produce shading or texture effects.
You can use spray cans, you can use an eraser.
It is possible to create a wider range of irregular shapes and patterns by simply
deleting pixels or adding pixels anywhere on the image.
Disadvantages of bit mapped representation
Requires large amounts of storage space; image becomes ‘blocky’ when scaled;
does not take advantage of resolutions that are higher than the resolution of the image.
Advantages of vector graphic representation:
Requires less storage space than a bit mapped image;
They can be edited at the "object" level, thus allowing the user to reposition, scale and
delete entire objects, or groups of objects, with ease;
Objects can be grouped to form larger objects that can then be manipulated as a
single image;
Objects can be layered.
Images are resolution independent meaning that they can use the full quality of the
display or print device.
BITMAP
FOR
Edit individual
pixels.
Can create
irregular shapes
and lines.
Use brush and
spray can for paint
effects.
AGAINST
Large amount of
storage required
Resolution
dependent, loses
quality on printer.
VECTOR
FOR
Less storage
required.
Edit individual
objects, e.g. move,
scale, change
colour, layer.
Resolution
independent, can
use full quality of a
printer.
AGAINST
Cannot edit pixels
Do not have eraser
or spray can.
Because bitmaps take up so much storage space you often need to compress files for
saving on digital camera storage cards or displaying on the Web. Specialised
compression result in images that are indistinguishable from the original as far as the
human eye is concerned, but much smaller file sizes. Two standards that will produce
compressed image files are GIF (Graphic Interchange Format), better suited to
drawings and cartoons that have only a few colours in them, and JPEG (Joint Pictures
Expert Group) which can compress as much as 10 times more than GIF and is also
more suitable for photo-realistic images.
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Higher Systems
Data Representation
EXERCISE 4
1. What does a ‘bitmap’ mean?
2. How many colours can be displayed using a colour depth of 8 bits?
3. Calculate the storage required for a black and white (monochrome) screen
using 800 by 600 pixels.
4. Calculate the storage required for a screen using 2048 by 1600 pixels with true
colour.
5. How much storage would be required for a scan of a 6” by 4” photograph at
400 dpi using 65 536 colours?
6. What does the term resolution independent mean?
7. Name two operations you can carry out on a vector graphic that cannot be
done on a bitmap.
8. If a systems analyst has to work out how much backing storage to have for a
system which uses a great number of graphics, why might they prefer to know
that only bitmaps are used on the system.
9. Give one advantage and one disadvantage of bitmap graphics.
10. Give one advantage and one disadvantage of vector graphics.
11. An animated gif has 12 frames each 40 by 40 pixels using 256 shades of grey.
Calculate the total storage for the gif animation.
Summary
You should know / understand:
Data representation as positive binary numbers and range of values (using up to 32
bits) and negative binary numbers (using Two’s Complement with up to 8 bits);
Conversion between binary and decimal numbers;
Representation of numbers using floating point technique and accuracy/range
depending on size of mantissa and exponent;
Conversion of binary numbers to and from bit, byte, Kbyte, Mbyte, Gbyte, Tbyte;
Unicode and limitations of ASCII when representing character sets;
Bit map methods of graphic representation including greyscale and colour;
Relationship of bit depth and colour (up to 24 bits);
Vector graphic methods of graphic representation (using objects);
Relative advantages and disadvantages of bit mapped and vector graphics;
Relationship between the bit depth and file size;
The need for data compression.
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Higher Systems
Computer Structure
Topic 2
Computer Structure

Detailed description of the purpose of the ALU and control unit

Description of the purposes of registers: to hold data being processed, instructions
being executed, and addresses to be accessed

Description of the function of the data bus and the address bus

Description of the read, write and timing functions of the control lines

Identification of other control lines, including reset and interrupt lines

Simple description, referring to the appropriate buses and control lines, of the steps in
the fetch-execute cycle

Description of the following elements of computer memory: registers, cache, main
memory, backing storage

Distinction between the above elements of memory according to function and speed
of access

The concept of addressability

Description and evaluation of the following measures of performance: clock speed,
MIPS, FLOPS, and application based tests

Description of the effect the following factors have on system performance: data bus
width, use of cache memory, rate of data transfer to and from peripherals

Description of current trends in computer hardware, including increasing clock
speeds, increasing memory and backing storage capacity
The main aim is to understand how binary programs (machine code) are stored and
executed. Also you should understand what factors make one computer faster / more
powerful than another computer.
A British mathematician called John von Neuman in the 1940s first described how a
computer should be designed. This revolved around the idea of the ‘stored program
concept’ where a list of instructions was loaded into memory and a processor would
fetch an instruction, decode it and execute it; fetch the next instruction and so on. This
‘von Neuman architecture’ is exactly the way computers operate today.
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Higher Systems
Computer Structure
Computer Organisation
A computer system is made up of Input / Output devices, backing storage, a processor
and memory.
Backing
Storage
Memory
Input
Devices
Processor
(CPU)
Output
Devices
The input / output / storage devices are peripherals that plug into the system unit. We
are going to take a closer look at the processor and memory.
First of all there are three main parts to the processor:
The ALU (Arithmetic & Logic Unit) which performs all the calculations and logical
comparisons. A logical comparison is deciding whether one thing equals another or if
it is bigger or smaller (e.g. IF answer$ = “PARIS” THEN ….)
The Control Unit manages the fetching, decoding and executing of instructions.
Registers which are temporary storage locations inside the processor.
The CPU communicates with the Memory by BUSES.
A bus is a set of lines working in parallel to carry bits back and forward.
Here is an 8 bit bus carrying a byte from the CPU to RAM:
P
R
O
C
E
S
S
O
R
0
1
0
0
1
1
1
0
M
E
M
O
R
Y
The byte will have come from a register in the processor and is going to a storage
location in RAM.
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Higher Systems
Computer Structure
There are three buses:
The DATA BUS: this carries actual data back and forward between memory and the
processor. It links to the Memory Data register (MDR) inside the CPU.
The ADDRESS BUS: this carries the address of the actual storage location that data is
being read from or written to. It links to the Memory Address Register (MAR) inside
the CPU.
The CONTROL BUS: this is not an actual bus but a series of lines that carry out
different functions. One of these is the READ line that tells memory that the data has
to come out of that storage location. Another line is the WRITE line that tells memory
the data is to go into that storage location. There are 3 others that we’ll deal with later.
So this is how the CPU communicates with memory:
A Memory READ:
The address is put on the address bus
The read line is set on the control bus
The contents of that location are put on the data bus and so transferred to the CPU.
A Memory WRITE:
The data is put on the data bus
The address is put on the address bus
The write line is set on the control bus
The contents of the data bus go into that storage location.
Each step in each of these operations has to be synchronised. There is another line on
the control bus called the clock. The clock pulses billions of times a second (e.g. a
Pentium IV 3 Ghertz microprocessor has a clock speed of 3 billion pulses per second).
Each time a clock pulse is received on the clock line, the processor goes onto the next
step.
Registers inside the processor have an important role in all of this. For instance for a
memory write:


The data is put in the MDR which connects to the data bus.
The address is put in the MAR which connects to the address bus.
Likewise for a memory read except the data coming in goes from the data bus to the
MDR.
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Higher Systems
Computer Structure
THE FETCH _ EXECUTE CYCLE.
Fetch
Going back to the Von Neuman stored
program concept, all a processor is
doing over and over again is fetching
an instruction, decoding it and
executing it. This is called the fetchexecute cycle.
Decode
The cycle involves doing a memory
read (to get the instruction), decoding
the instruction and then carrying it out
(executing).
Execute
The actual execution of the instruction
could be just one step or many steps
depending on what the instruction is.
So the fetch – execute cycle involves:





Put the address on the address bus
Set the READ line
The instruction comes in on the data bus
The instruction goes to the Instruction Register where it is decoded
The instruction is executed
All of this is synchronised by the clock line on the control bus.
Summary of what you need to know so far:
You should know the purpose of the ALU is to perform calculations and make logical
comparisons. The control unit manages the fetch – execute cycle by clock pulses to
synchronise each step, by decoding instructions and managing their execution.
Registers are storage locations inside the processor, they hold data that is being
processed, the address that is to be accessed, the instruction that is to be decoded.
There are three buses connecting the processor with memory:
The data bus carries the actual data back and forward, the address bus carries the
address of the storage location that is to be accessed.
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Higher Systems
Computer Structure
The control bus is not really a bus as its lines do different things. Examples of lines
on a control bus are:
READ: tells the storage location being accessed to put its contents on the data bus.
WRITE: tells the location being accessed to take in the contents of the data bus.
CLOCK: synchronises all operations by regular pulses.
RESET: sets the processor back to its start up state.
INTERRUPT: carries a signal to tell the processor to stop what it is doing and deal
with a peripheral.
NON-MASKABLE interrupt. The interrupt line can be masked by programmers so
that the CPU does not ‘see it’. However some things are too important to be masked
– e.g. a power failure when the computer has a few seconds to save anything before
RAM discharges.
The fetch – execute cycle consists of





Putting address of next instruction on the address bus
Setting the read line
The instruction comes into the processor
The instruction is decoded
The instruction is executed
EXERCISE 5
1. What is meant by the stored program concept?
2. Name the three main parts of the CPU.
3. Give two examples of the purposes of registers.
4. Name the three buses that connect the processor to memory.
5. Why is the control bus not really a bus?
6. Give two different examples of the lines on the control bus.
7. What does it mean to say an interrupt can be masked?
8. Give an example of a non-maskable interrupt.
9. What is the purpose of the address bus?
10. Explain why the data bus is two way and the address bus is one way.
11. List the steps involved in performing a memory write.
12. By referring to the buses and control lines involved, state the steps in the fetch
& execute cycle.
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Higher Systems
Computer Structure
MEMORY
We are now going to take a closer look at memory and all its different types.
You will certainly know RAM and ROM. This forms the computer’s MAIN
MEMORY.
Read Only Memory is permanent; it is ‘fixed’ in the factory by blowing fuses inside
the memory module. If we could look inside we would see circuits like this:
1
0
0
1
1
1
0
0
So ROM doesn’t need electricity to hold its data and cannot be altered.
Random Access Memory is volatile, it need power to hold its data.
There are two main ways of doing this:
Dynamic RAM (DRAM) uses capacitors, this is a
cheap form of memory and they can be organised in
large modules (SIMMs or DIMMs) in groups of 256
Mbytes for instance. However the charge is miniscule
and capacitors ‘leak’, so they have to be constantly
refreshed. This slows down access to DRAM.
SIMM, acronym for single in-line memory
module, a small circuit board that can hold a
group of memory chips. Typically, SIMMs hold
nine RAM chips. On PCs, the ninth chip is often
used for parity error checking.
The bus from a SIMM to the actual memory
chips is 32 bits wide. A newer technology, called
dual in-line memory module (DIMM), provides
a 64-bit bus. For modern Pentium
microprocessors that have a 64-bit bus, you must
use either DIMMs or pairs of SIMMs.
Static RAM (SRAM) uses transistors, this is more
expensive and you cannot organise it in large
quantities, however it is perfectly stable and has very
fast access.
RAM in a computer uses DRAM, cheap and large scale modules.
Because DRAM is slow to access, computer manufacturers try to speed things up by
adding another kind of memory between the CPU and RAM called cache.
Cache memory uses SRAM. There are two kinds called L1 and L2. Level 1 comes
with the processor and is attached to it. Level 2 is just off the processor on the
motherboard. Cache usually comes in 512 Kbytes or nowadays 1 Mbyte size.
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Higher Systems
Computer Structure
The idea of cache is that once data is fetched from RAM it is stored in cache, then, if it
is needed again it can be recovered very quickly.
Although rare nowadays, computers can run out of main memory when running
programs. If this happens they use an area of hard disk to store data. This is called
VIRTUAL MEMORY. Obviously you can get a huge amount of Virtual Memory (or
VRAM), however it is very slow (excruciatingly slow!) and it is to be avoided.
So there is a hierarchy of memory available to the processor:
Type of memory Description
Registers
RAM
Locations inside the
processor
DRAM right next to
processor
SRAM on motherboard
Virtual Memory
Using the hard disk.
Cache
Speed of
access
Lightening
Very fast
Slow (ish)
Snail’s pace
Cost / Availability
Expensive, very limited
amount.
Dear, limited amount.
Fairly cheap, large
amount.
Dirt cheap, huge amount.
To get an idea of the relative differences, fetching data from memory could be
described like this:
If the processor is the teacher sitting at his desk in room D10a and the data is a sheet
of paper that he needs:




A register is when the paper is lying on his desk.
Cache is when the paper is in the filing cabinet.
RAM is when the paper is in Mr. Cairns’ room.
VRAM is when the paper is in the office.
Clearly more registers will speed up a computer, but they are
numbered in hundreds. More cache will speed up a computer,
but that comes in limited amounts.
It is essential to have enough RAM in your computer because
if your machine has to resort to VRAM it will slow down very
noticeably.
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Higher Systems
Computer Structure
Whatever type of memory is being used it is divided into locations which hold a set of
bits. The number of bits in each location is called the WORD size. This is equal to
the number of lines on the data bus. So:
Word size = lines on data bus = bits in each storage location.
A word is the number of bits that can be processed in one cycle.
When a computer is described as a 16 bit machine it means the word size is 16 bits,
there are 16 lines on the data bus, there are 16 bits in each memory location.
A Nintendo 64 games machine had 64 bits in each storage location, a 64 bit data bus,
its word size was 64.
Each storage location in memory has its own unique address. This address is used (via
the address bus) to pinpoint whichever location the processor wishes to access.
The width of the address bus will determine the amount of memory that a processor
can access. Imagine an address bus had 2 lines on it, then the number of unique
addresses it could carry would be 4.
0
0
0
1
1
0
3 lines could carry 8 different addresses,
n lines: 2n unique addresses.
The number of lines on the address
bus determines the number of unique
locations you can have in memory.
If the address bus has ab lines then the
CPU can access 2ab different storage
locations.
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1
1
Higher Systems
Computer Structure
Question1: So if an address bus has 12 lines, what is the maximum number of storage
locations it can access?
Answer: 212 = 4 096 different storage locations.
Now in order to work out the amount of memory in bytes, we need to know how much
is stored in each location (the computer’s word size).
Question2: How much memory can a computer have if the address bus is 24 bits wide
and each storage location holds 16 bits?
Answer: BITS = 2ab x No. of bits in each location (where ab is the number of lines on the address bus)
= 224 x 16 BITS
= 16 777 216 x 16
= 268 435 456 bits
/8
= 33 554 432 bytes
/1024 = 32 768 Kbytes
/1024 = 32 Mbytes
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Higher Systems
Computer Structure
EXERCISE 6
1. What does it mean to describe ROM as non-volatile?
2. Explain the difference between Dynamic RAM and Static RAM.
3. What is cache memory?
4. What is Virtual Memory?
5. Why does using cache speed up the operation of a computer?
6. Why do we not uses a lot more SRAM instead of DRAM for main memory?
7. What is a word?
8. A computer has a 32 bit address bus with a 16 bit word size. Calculate the
maximum amount of memory the processor can access.
9. A computer has an 8 bit data bus and a 12 bit address bus. Calculate the
maximum memory it can use.
10. (hard!)A 32 bit computer can have a maximum memory of 16 Gbytes. How
many lines are there on the address bus?
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Higher Systems
Computer Structure
COMPUTER PERFORMANCE
By performance we mean how quickly can it carry out instructions. However this is
not as simple as it sounds, there are many factors to take into account and the
instructions themselves vary a great deal, so measuring the speed of a computer is very
difficult. Because of this there are many different ways of trying to measure the speed
of a computer:
1. Clock speed. This is a prime factor, the clock pulses synchronise the steps in
the fetch & execute cycle, the faster the clock, the quicker it will get through
the cycle. A processor is always quoted with its clock speed (the number of
pulses per second) given in Gigahertz. E.g. a Pentium IV processor 2.5 Ghz,
an AMD Athlon 3400+ and so on. However it doesn’t take in to account any
other factors like data bus width, amount of cache and so on. Secondly,
different processors have different machine codes and this can greatly affect
their speed, clock speed can only properly be used to compare the same make
of processor
2. MIPS: Millions of Instructions Per Second. The more instructions that are
fetched and executed per second then clearly the faster the computer will be.
This is better than just clock speed, however, it is not as straightforward as
that. Again, different types of processor use different instructions, some can be
simpler and faster, some longer.
3. FLOPS: Floating Point Operations Per Second. Floating Point arithmetic is a
good measure of a processor because the steps carried out are common to all
types of processor. However it doesn’t tell you how the processor deals with
other types of instructions.
4. BENCHMARKS: The idea of a benchmark is to give the computer a specific
task to do and time it. Examples are the Whetstone test which is a series of
arithmetic functions. The Dhrystone test uses a series of very common
programming statements and string comparisons. The benchmarks are coded
in the machine code of that processor and timed from start to finish. Using a
series of benchmarks you can get a good measure of a system’s overall
performance.
You can also just a run a program like Excel with a very complex spreadsheet
and time it to recalculate.
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Higher Systems
Computer Structure
What are the main factors that affect a computer’s performance in terms of speed of
processing?
1. A faster processor. A Pentium IV 3 Ghz processor will be a lot faster than a
Pentium IV 2 Ghz processor. However as we saw above, you cannot readily
compare the speed of two different makes of processor, but that doesn’t change
the general fact that faster processors make for faster computers.
2. Data bus width. The more lines on the data bus, the larger the word size, the
more bits that are processed each cycle. A 32 bit data bus will give you a
much faster computer than one with a 16 bit data bus. (N.B. the address bus
affects the amount of memory, not the speed)
3. Amount of cache cuts down on the amount of accessing slow RAM, this
speeds up the operation of the computer. More registers could help, but as they
are numbered in hundreds they are not going to have a big effect. 1 Mbytes of
cache can have a big effect on a computer’s speed.
4. Speeding up the accessing of peripherals will make a computer appear a lot
faster. Loading and saving, reacting to key presses or mouse clicks if these are
speeded up by faster interfaces then the computer will be faster. (more on
interfaces later).
The trend in computers since the
very first PC was brought out nearly
30 years ago has been for more
powerful, faster processors, more
main memory, more backing store.
Some years ago a computer would
come with an 8 Mhz processor, a
floppy disc drive, a 40 Mbyte hard
drive and 16 Mbytes of RAM.
Nowadays a 4 Ghz processor (500 x
faster), floppy, CD and DVD writers,
a 200 Gbyte hard drive (5 000 times
larger) and 1 Gbyte of RAM (64
times larger).
What is even more dramatic is the new computer is much cheaper than the old one!
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Computer Structure
Someone once said if cars had progressed as much as computers over the last 25 years
then a Rolls Royce would have a top speed of Mach 2, it would have a turning circle
of 10cm and it would cost £5.
Someone else said if cars had progressed the same way as computers over the last 25
years then simple warning lights like fuel low would be replaced by ‘system error
32xx450, please send us a report’, it would randomly crash twice a week and the
airbag would ask ‘are you sure’ before activating.
However the trends are continuing, faster and faster processors, more RAM to handle
all the overblown software we use and masses of hard disk space to store all our
videos, MP3s, photos etc.
EXERCISE 7
1. Name four ways of measuring the speed of a computer.
2. Create a table for your four methods and outline the advantages and
disadvantages of each.
3. How does the data bus width affect the speed of a computer?
4. Describe one other method of increasing computer speed.
5. How would you describe the trends in the power of computers?
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Higher Systems
Peripherals
Topic 3
PERIPHERALS

Description of the use and advantages of buffers and spooling

Description of a suitable selection of hardware, including peripherals, to
support typical tasks including production of a multimedia catalogue, setting
up a LAN in a school, development of a school website

Justification of the hardware selected in terms of appropriate characteristics
including resolution, capacity, speed, cost and compatibility

Description of the features, uses and advantages of solid state storage devices
including flash cards

Description of the development trends in backing storage devices

Description of the following functions of an interface: buffering, data format
conversion (serial to parallel and analogue to digital), voltage conversion,
protocol conversion, handling of status signals

Distinction between parallel and serial interfaces

Description and explanation of the current trends towards increasing interface
speeds and wireless communication between peripherals and CPU
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Higher Systems
Peripherals
Buffering and Spooling
The processor is very fast, peripherals are
relatively slow. It is impossible therefore for
the processor to deal directly with a
peripheral or else the processor would be
sitting idle for exceedingly long times. There
are two possible ways of dealing with this
which we shall illustrate by considering a
printer:
Buffer: A buffer is an area of memory. The laser printer in the corner of the room has
a 4 Mbyte buffer, i.e. its own 4 Meg of RAM. When anyone in the room prints, the
processor sends the pages to the buffer, which it can do reasonably quickly. The pages
are then printed from the buffer which is fairly slow because printing takes time.
Meanwhile the processor gets on with other jobs.
Spooler: A spooler uses backing storage to store the print job. In a busy office
network where a hundred print jobs might go to the printer at the same time, a print
server is used. The print job goes to the print server which ‘spools’ it to the hard disk.
The server then sends the pages from the hard disk to the printer while the network
computers get on with other things. (In the old days a tape spool was used for backing
store).
So buffering is using an area of memory, spooling is using backing storage. Both
allow the processor to send the print job quickly and then let it get on with something
else.
Print jobs spooled to
the server’s hard disk.
Print jobs sent to
the printer’s buffer
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Higher Systems
Peripherals
As we have seen, the processor is too fast to deal directly with a peripheral, it also uses
different values or methods for storing data (e.g the processor might use 8V for a 1,
3V for a 0 while a keyboard uses 15V for 1 and -15V for 0).
If you plug 15V into the motherboard then you are going to blow it.
For a peripheral to communicate with the processor, it
has to connect through an interface.
The peripheral will plug into a port, usually at the back of the computer. Behind this
port is a card of electronics. This is the interface. An interface has a number of jobs
to do including:





buffering,
data format conversion (serial to parallel and analogue to digital),
voltage conversion,
protocol conversion,
handling of status signals
Buffering is an area of memory built into the interface which stores data temporarily
before it is sent on to the processor (or vice versa). A keyboard has a buffer that holds
256 characters, when you press return the contents are processed.
Data conversion could include serial to parallel. Data that comes in on a serial port
(e.g USB – Universal Serial Bus) is coming in one bit at a time. This data will go onto
the data bus for transferring to the processor. The data bus is parallel, so the interface
buffers the bits coming in until it has enough to send them on the parallel bus.
1
1
0
0
1
1
0
1
Serial to Parallel
--1----0----1----1----0----0----1----1
Some interfaces are parallel (most computers still have the big parallel printer
interface), however nearly all communication with peripherals nowadays is serial. The
reason is that parallel is only reliable over a short distance. With longer distances the
bits get out of sync and this causes errors. These 4 bits are out of sync :
0
1
1
0
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Higher Systems
Peripherals
Analogue to digital conversion is necessary for any analogue device which must have
its data converted to digital for the computer and vice versa. Video input is an
example of analogue to digital.
Voltage conversion: as mentioned before, a peripheral might use different voltage
values for ones and zeros compared to the processor.
Protocol conversion: protocols are agreed rules used in data transmission about how
data is formatted, timing, error checking etc. A network interface will have to handle
protocol conversion to ensure data is sent out according to the rules of that network.
Status signals: this could be something as simple as the printer letting the interface
know it is on and ready. If the printer is switched off or out of paper, when you print
the interface will inform you of that fact.
As with all aspects of computing there has been a great increase in the speed of
interfaces. USB 2 transfers data at 480 Mbits per sec., compared with USB 1.1 which
had a speed of 12 Mbits per sec. (USB is Universal Serial Bus). Before USB, SCSI
(scuzzy – Small Computer System Interface) was popular. It was parallel and
transmitted at 5 Mbits per sec. Then ultra SCSI transferred at 20 Mbits p/s then ultra–
wide SCSI (16 bit) at 40 Mbits p/s.
Firewire was a successor to SCSI, but is serial. It can transfer data at 800 Mbits per
sec. It was developed by Apple and is particularly useful for connecting video
cameras. It is also used on all iPods. It can be used for printers / cameras / scanners
etc. and devices can be daisy chained through one port. The main reason USB is used
rather than firewire is because it adds a couple of pounds to a systems cost.
Another trend is towards wireless communication with peripherals. Infra red is quite
common for keyboards and mice. Laptops can communicate with printers via infra
red. WAP (Wireless Application Protocol) can be used to govern how data is sent
wirelessly. Bluetooth will possibly be very common in the future especially in setting
up a WPAN (a Wireless Personal Area Network) where all your personal devices;
phone, laptop, pda, computer, printer, camera etc. can all communicate.
Page -33-
Higher Systems
Peripherals
EXERCISE 8
1. Why does it improve computer performance if the attached printer has a
buffer?
2. What method do print servers on networks use instead of buffering?
3. The difference between buffering and spooling could be summarised by simply
stating (copy out):
Buffering is using ________, spooling is using ________ _______ to take data
quickly from a computer to let it get on with other jobs while the peripheral takes
the data from the buffer/spooler in its own time.
4. What is the difference between a port and an interface?
5. What does buffering mean for an interface?
6. Why might an interface have to convert analogue to digital?
7. Give another example of data format conversion.
8. What is a network protocol?
9. Give an example of a status signal.
10. Which type of interface is very fast and suited to video transfer?
11. Why is wireless connection as with keyboards and mouse becoming popular.
12. State a drawback with wireless connection.
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Higher Systems
Peripherals
Backing Storage
There has been a great deal of development in storage devices over the last 10 years
and this is continuing at a great rate.
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.
These 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.
Hard disks and floppy disks have been around for 20 years in PCs but
lately the floppy disk has almost become extinct. It only holds 1.4
MB and for anything other than text files that is no use.
The storage capacity of hard drives has increased
enormously with 200 Gigabytes normal. Hard disks can
spin at 7 200 rpm and transfer data at 50MB per sec., this
depends a lot on the interface: IDE or SCSI being the most
common.
All discs are divided into tracks and sectors when they
are FORMATTED. This divides the disc into blocks
and the track and sector gives co-ordinates for each
block so that the data can be found.
Hard disks contain more than one disk (called platters).
When a file is saved the name, the track, sector and
length of the file are recorded in a table called the FAT
(File Allocation Table). When a file is to be loaded,
the computer looks up the FAT to find where it is.
Magneto-optical storage devices combine magnetic and
optical technologies to read and record data. With a magnetooptical disk, a laser beam and a magnetic field is used to write
the data. Only the laser is used to read the data. ZIP discs are
an example, but rewriteable CDs and DVDs and now flash
memory has made them almost redundant.
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Higher Systems
Peripherals
OPTICAL STORAGE
CD drives which both Read and Write and now DVD – R/W drives are common place.
CDs store about 650 MB and DVDs store 4.5 GB (there are dual layer which store
8.5GB and double sided is possible).
These also have different rates of data transfer. The basic CD
transfer rate is 0.15 MB per second. A 32X CD transfers data
at 4.8 MB per second. A DVD’s original speed is 1.4 MB per
second, an 8X DVD can transfer at 11 MB per second.
CDs have probably reached their maximum speed at 52X,
DVDs will probably not go higher than 16X.
DVDs suffer from format differences in that you get DVD-R and DVD+R and not all
drives play both.
Tape storage
Storing data on tapes used to be the only solution to backing up hard disks of large
capacity. Now, with the advent of large, removable magnetic disks and optical CD RRW or DVD technology, this is no longer the case. However, removable storage
media is comparatively expensive, with overall costs up to ten times that of tape.
Tape, therefore, still has the edge in this market.
Tape is read and written on a tape drive. This
drive winds the tape from one reel to the
other causing it to move past a read/write
head. Data is written to tape in blocks with
inter-block gaps between them. The tape runs
continuously and a single operation writes
each block
Capacity
Magnetic tapes have large capacities, reaching up to
several gigabytes and come in a variety of sizes and
formats.
DAT tape is now the most popular for backups of
servers.
Access
Tapes are sequential access devices which means that to get to a particular block of
data on the tape, it must go through all the preceding blocks of data. Accessing data
on tapes is therefore much slower than accessing data on disks with direct access.
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Higher Systems
Peripherals
SOLID STATE STORAGE means there are no moving parts, everything is done
electronically. It is non-volatile (i.e. does not need power) and so is called ROM,
however the stored data can be changed as often as you wish when it is attached and
power is going into it. (technically it should be called EEPROM – Electrically
Erasable Programmable Read Only Memory!)
This solid state storage comes in the shape of flash drives or USB sticks, SIM cards
and various memory cards in cameras.
This solid state storage is at the 1 Gbyte stage in 2005, but this will increase. In fact a
lot of research is going into this area and they could replace hard drives in a few years.
PERSONAL INVESTIGATION
www.webopedia
and
www.whatis.com
are always useful sites.
Investigating Magnetic Tape Technologies
Using the Internet or current technical magazines, investigate the cost, capacity and
access times of tape media
You may wish to look at the following manufacturers in your search:
Athana, Gigatek, Hewlett-Packard, Imation, Verbatim.
Hard disk
You should use the Internet or current technical magazines to investigate the
following characteristics of hard disks:
 Capacity
 Speed
 Cost
Using textbooks or the Internet, find out how accuracy of stored data is achieved.
Keywords you might like to explore are cyclic redundancy checks and error
correction codes.
Optical Storage: find out how data is stored (pits and lands), capacity, speed, cost for
CDs and DVDs.
Solid State Storage: find out about EPROM, EEPROM, flash ROM, capacities, cost.
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Higher Systems
Peripherals
Peripherals
Here we look at the hardware devices required to carry out typical tasks using a
computer system. For example, to set up a LAN, or to develop a multimedia catalogue
or a school website. When considering devices we specifically look at

speed,

cost,

resolution,

capacity and

compatibility.
Cost is self explanatory, compatibility is concerned with whether the peripheral will
work with your system, e.g. does it have the right interface? Speed, resolution and
capacity do not always apply, for some peripherals speed might be important, for
others irrelevant.
Scanners
The flat-bed scanner is like a photocopier where you put a sheet
or photo up to A4 size flat on the screen. Light is reflected onto
photo cells called CCDs (charged coupled devices) that detect
the light and the values are digitised (analogue – to – digital
converter) to form the pixels of the bitmap.
You can get hand scanners that you
drag across the sheet / photo.
Accuracy is determined by resolution and bit depth. The
resolution is how many ccds there are per inch (dpi) and
bit depth is how many bits are used to record the colour of
each pixel.
Capacity does not apply to scanners, but scans (bitmaps) take up a lot of storage on
disc.
Speed does not really come into consideration, there is little difference in the time
taken to scan, but it could be a factor. The transfer rate from scanner to computer
would also come under speed.
Compatability would include type of interface and does the software run on your
computer.
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Higher Systems
Peripherals
Digital Cameras
Like scanners, the digital camera uses millions of
photosensitive diodes called charge coupled devices
(CCDs), to record the intensity of light in an image.
These analogue values are then converted to digital
using an A-D-C. Digital photographs are bitmaps,
made up of thousands or millions of pixels with values
to represent image brightness and colour.
Accuracy
As in scanners, accuracy refers to how well the computer representation of
the image matches the original. This will depend on the resolution and bit
depth.
Resolution
This is measured in megapixels and can also be given as, say, 2560 x 1960.
Bit-Depth
The number of bits per pixel determines the number of colours that can be
represented.
Speed does not apply at all to cameras. For compatibility you might take into
account ‘pictbridge enabled’, type of storage card as well as interface.
Capacity is the number of photos that can be stored. Most cameras come with a very
low capacity card, say 32 Mbyte. To be useful you need at least a 256 Mbyte card.
(It might seem a 5 megapix camera using true colour (3 bytes per pixel) would need
15 Meg to store 1 photo, however the photo is compressed, usually jpeg to about 3
Meg.)
Printers
Liquid ink-jet
Also known as bubble-jet, this device operates by squirting tiny droplets of ink onto
the page. The ink is first heated by a 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 (measured in picolitres).
Resolution is typically 600 to 1200 dots per inch. They support the printing of text
and graphics, colour and a range of shades.
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Higher Systems
Peripherals
Speed is pretty slow with a range of 4 pages per minute to
8 or maybe 12 pages per minute, depending upon the
model.
Cost is relatively cheap, though the cost of ink can be
high.
Photoquality ink jets are becoming popular with digital
cameras and there are small dedicated photo printers.
These can have very high resolutions.
Laser Printers
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 that are transferred to
the page and heated to set the image.
Resolution is typically 600 to 1200 dpi, although higher resolutions are available if
you are prepared to pay the price. They print a complete page at a time to a predefined maximum page length and width. Colour has now become affordable for laser
printers
Speed ranges between 4 pages per minute and 40 pages per minute.
Capacity could include buffer size, important for network printers.
Cost can be form £50 to thousands of pounds.
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Higher Systems
Peripherals
Standards of peripherals are changing all; the time and the only way to keep up to date
is for you to carry out you own investigation.
Try to compare features of an expensive model and a cheaper one. The main features
for all devices are: speed, cost, resolution, capacity and compatibility.
For compatibility mainly look at the interface (eg USB) and possibly the software..
Speed will not apply to some devices like cameras, but will for printers. Capacity
could be a storage card, a buffer or not apply as in a scanner. Resolution revolves
around dpi and colour depth. For cost, take into account possible running costs as
well.
For each peripheral take into account other obvious factors. For a printer the
‘footprint’ is important (the size on a desk). For a camera, the quality of lens, does it
take short video, the size of the view screen.
Exercise
1. Using the Web find two different digital cameras (not too close in price).
Compare the features of both cameras (in particular the characteristics
mentioned above). Write a short report comparing them and recommend
which one to buy, justifying your reasons.
2. As with one, this time search for two scanners.
3. Imagine you have a budget of £2 500 for buying a complete computer system
with software and peripherals for producing a school magazine. Investigate
the cost of a computer that would be up to the task plus the cost of software.
Decide what essential peripherals will be required. Justify your choices
particularly in terms of speed, cost, resolution, capacity and compatibility
P. S. I.
Peripheral
Scene
Investigation.
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Higher Systems
Networking
Topic 4
NETWORKING

Comparison of LANs, WANs, Intranet and Internet work in terms of transmission
media, bandwidth, geographical spread and functions

Distinction between a mainframe with terminals and a network of computers

Descriptive comparison of peer-to-peer networks and client server networks

Description of the functions of file, print and web servers

Description of a node and a channel

Description of bus, star, ring and mesh topologies using the terms node and
channel

Description of the consequences for each of the above topologies of node and
channel failure

Simple description of the functions and uses of a hub, switch and router

Identification of the need for a network interface card (NIC)

Description and explanation of the trends towards higher bandwidth and wireless
communications

Description of the following technical reasons for the increasingly widespread use
of networks:


advances in computer hardware, including processors, main memory capacity,
backing storage, data transfer rates
improved network related software, including browsers and network operating
systems

Description of the misuse of networks for the following illegal purposes:
breaching copyright, hacking and planting viruses

Description of the application of the Computer Misuse Act, the Copyright Designs
and Patents Act and the Data Protection Act to the misuse of networks
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Networking
Definitions:
Bandwidth is the speed of a connection, the bits per second that can be transmitted.
Transmission media is what is used to transport the bits. It could be wireless or
satellite, it could be copper phone lines or network cabling.
Network cabling can be like TV aerial cables called coaxial or twisted
pair. Twisted pair can be shielded from electrical interference, so you
get STP (shielded twisted pair) or much more commonly UTP
(unshielded twisted pair).
UTP is by far the most common and is what is used in this
school. There are various standards but by far the most
common is Category 5 which transmits at 100 Mbits per
second (compare to Broadband – 512 Kbits per second or
even fast broadband at 2 Mbits per sec).
This cable is often just called Cat 5.
You can also get fibre optic cable which is very expensive but transmits over long
dstances and has very high bandwidth.
Comparing LAN / WAN / Intranet / Internet
LAN: Local Area Network, in one room or building or
site. Cabling and hardware usually owned by the
company. High bandwidth (100 Mbits per sec), used
for sharing peripherals e.g. printers and sharing data
and files. Can also have application server which saves
having programs installed in each machine.
WAN: Wide Area Network, covers a large
geographical area, can be private or open. The
cabling is not opened by the company as they
will use phone lines (bandwidth from 56 Kbits to
2 Mbits per sec), the hardware might be owned
by one company as in a private WAN, or it could
be like the web which does not have individual
ownership. Can be used for file sharing, data
sharing, emailing, instant messaging or chat.
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Networking
Intranet: This is a network (either LAN or WAN)
that uses web technology to create a private web
for use by only that company. So within their own
network they can display information in the form
of web pages, they can download (or upload) files,
have email, instant messaging or chat. It is a
private web.
Internet: The internet is the actual hardware, the servers, the cabling, the modems, the
routers and so on. There are 5 applications that run on the internet:
o
o
o
o
o
The Web (http) - what most people call the Internet.
File downloading (ftp)
Chat (IRC)
Email (SMTP, POP)
Newsgroups
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Higher Systems
Networking
Comparison of LANs, WANs, Intranet and Internet work in terms of transmission
media, bandwidth, geographical spread and functions:
LAN
Transmission Cat 5
(twisted pair)
media
Owned by
company
Very high
Bandwidth 100Mbps
Geographical One site
spread
Sharing
Functions peripherals,
files, email
WAN
Intranet
Internet
Phone lines Possible mixture of Phone lines
cat5 and phone
lines
56Kbps to
2Mbps
Externally probably
1Mbps, internally
100Mbps
Up to
From one office to
Worldwide Worldwide
Sharing
Sharing files,
files, email email. Central
access to up-to-date
documents
Varies, 56Kbps to
2Mbps
Worldwide
Web / information /
shopping / email /
chat etc.
NETWORKS
Original networks were multi-access systems where you had a central mainframe with
lots of terminals attached. These were usually ‘dumb terminals’ in that they did not
have any processing power of their own; they ran the programs off the mainframe and
stored all their data on the mainframe.
You can still get these types of
system today, though they are called
‘thin client’ nowadays rather than
dumb! They need a very powerful
central mainframe but are easy to
manage as all upgrades etc. are done
on the mainframe rather than
servicing hundreds of terminals.
The terminals are very cheap and are
easily replaced.
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Higher Systems
Networking
Most networks however are not ‘thin client’ but use proper, quite powerful PCs as the
terminals and each PC has its own software installed. There are two types of such
networks:
Peer to Peer: (don’t mix this up with Kazaa and its like, though it is the same idea).
In P2P networks there is no central server, each PC is equal on the network, each
machine runs its own software and saves its own data. However the PCs are joined
together and can share data, send messages and share peripherals. Each user decides
which files on their machine can be shared and whether they are read only or
read/write. If a user is not logged on then their files are not available to everyone else.
There is no central manager and each user is responsible for their own backups.
Security is very low level.
On this course by P2P we mean a LAN in an office. Kazaa, Morpheus etc have just
taken that idea and turned it into a WAN by running software on a server on the Net.
Client server: Here there is a central file server that controls access to the network
through logins and passwords. Files are stored centrally on this file server (your Hdrive). Files can be made available for sharing (e.g. My Network Places). You can
have internal / external email, file downloading and peripheral sharing. Backups are
managed centrally from the file server.
SUMMARY
Peer to Peer:






Every node equal.
Share peripherals / send messages.
Users make files available for
sharing.
Have to be logged on for files to be
available.
Each user responsible for backups
Low security.
Client / Server





Central server controlling access.
Share peripherals / send messages.
Files available on server.
Backup organised centrally.
High level of security.
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Higher Systems
Networking
For client / server you must have a Network Interface Card fitted to your computer
(NIC). Each NIC has a unique number (the MAC1 address) that the server uses to
identify your computer.
1
Medium Access Control
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Higher Systems
Networking
SERVERS
The file server stores everyone’s data and manages access.
You can also have a print server (though we don’t) to
spool print jobs and manage the printing, this could be just
a basic PC.
There can be application servers for thin client systems,
email servers and web servers. Web servers transmit and
store web pages, they are sometimes the same as proxy
servers or can be separate.
A proxy server connects a LAN to the Internet. All
requests for web pages go through the proxy server and as
far as the Internet is concerned there is only one machine
connected although there might be hundreds of PCs on the
LAN. A firewall usually runs on the proxy server.
SUMMARY:



File Server, stores data centrally and handles logins and passwords.
Print server, manages printing on a network, spools and schedules print jobs.
Web servers transmit and store web pages.
Exercise
1. List 3 differences between the characteristics of a LAN and a WAN from
transmission speed, geographical spread, functions and bandwidth.
2. What is an Intranet and what are its main functions?
3. What are the main functions of the Internet?
4. What is the difference between a mainframe with terminals and a network of
computers?
5. Describe the functions of :
a. A file server
b. A print server
c. A web server
6. Compare and contrast peer to peer networks with client server.
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Higher Systems
Networking
There are various designs for the layout of networks. In these designs a terminal is
called a node and the wire (or wireless!) is called a channel. These designs are called
the network topology.
You should be able to describe (or draw) each of these 4 topologies and explain what
would happen if there is a node or channel failure.
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Networking
BUS
A typical bus network uses the Ethernet standard. It has a single channel to which all
the nodes are attached. It can use co-axial cable or cat 5 and transmits at 10 – 100
Mbits per sec. It needs terminators at the ends to kill any messages that are not picked
up by the nodes as only one message can be on the channel at one time. Cable is
usually limited to 100 metres maximum, but repeaters can extend this.
This is a very simple network and cheap to set up, easy to extend and simple to add or
remove nodes. If a node is down it has no effect on the rest of the network (unless it is
the file server). If the main channel (backbone) goes down then the whole network is
down.
STAR
For a LAN this will also use Ethernet protocols and tends
to be twisted pair or coaxial cable.
All data passes through a central hub which usually has a
file server attached.
This set up uses more cable
than a bus. The whole
network will be down if the
hub or the central server fails. Node failure will not affect
the rest of the network. It is fairly simple to add or remove
nodes.
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Networking
RING
A monitor station is required for a ring to remove any
signal that is not being picked up by any node.
If the main cable goes then the whole network will be
down. If a node is down, then the network might go down
but you can have a ‘bypass’. Nodes can only transmit
when the ‘token’ is available. The token is a message
container and only one message can be on the ring at one
time.
They can be large networks because each node retransmits the token as it goes round.
MESH
This is a fully connected mesh, a mesh does
not need every node connected to every other
node like here. There will always be multiple
routes from node A to node B.
This is the most reliable of networks as
channel failure or node has no effect on the
network, however it is the most expensive to
cable.
Exercise
1. On a network what is meant by :
a. A node
b. A channel
2. Draw a labelled diagram of a bus network.
3. What is the effect of node failure on a star network?
4. Why is a mesh network the most reliable?
5. What is the effect of node failure on a ring network?
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Higher Systems
Networking
HARDWARE
We have already come across a hub as the centre of a star network
It can also act as a repeater, extending the
distance of UTP cable.
A switch, or switched hub also
reads addresses on the network
traffic and only sends the data to
the correct node. An ordinary hub
broadcasts the data to every node.A
switch will also effectively divide
the network up into a series of
different segments, thus reducing
the likelihood that a cable fault will
bring the whole network down.
A router is used to connect
networks (internetworks). It uses
IP addresses to determine which
route or path data should take to get
to its destination.
A Network Interface Card (NIC) is the circuit
board (interface) behind the Ethernet port at the
back of a computer. You cannot connect to a
LAN without an NIC. Every NIC has a unique
MAC address that identifies the node on a
network.
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Higher Systems
Networking
Summary:
A Network Interface Card is essential for connecting a node to a network.
A hub is where all the nodes connect together in a star network. When a message
comes in for a node the hub transmits the message to every node, the message has an
address on it and the node it is for picks it up.
A switch is an ‘intelligent’ hub, when a message comes in it looks at the address and
only transmits it to that node.
A router is for forwarding data through an internetwork. It uses the IP address to
determine the best route to send the data on its way.
TRENDS
Networks are becoming faster, there has always been a trend towards higher
bandwidth on LANs and WANs due to the amount of data that has to be transferred,
particularly multimedia data, music and video which all have large files.
LANs are normally 100Mbits per sec, compared to 10 Mbits per sec.
Modems were originally 14 Kbits per sec., quickly superseded by 28 Kbps, then after
a while came 56 Kbps. More recently we have ISDN and broadband.
Home ADSL was 512 Kbps, recently
that has become 2 Mbits per sec. for
those that want it. Faster speeds will
still be sought so that eventually the
Internet could provide video on
demand, personal TV scheduling and
so on.
Another recent trend is towards
wireless connections and home
networks and this will inevitably
continue.
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Higher Systems
Networking
Technical reasons for the increasing use of networks include advances in
processors, main memory, storage and transfer rates.
Processors have become much more powerful and capable of handling all the
processing concerned with network traffic both for servers (which often use multiple
processors) and for clients.
The Web is multimedia and with LANs moving to intelligent clients then huge
amounts of RAM are necessary. However large quantities of DRAM are readily and
cheaply available for modern computers allowing them to handle these applications.
Huge amounts of storage is required for client server networks as well as web
servers. Also client PC need enormous storage for multimedia files. Again this has
become readily available with even cheap PCs coming with 200 Gbytes of storage.
As stated on the previous page, transfer rates have increased enormously, home
broadband of 2Mbps is readily available and this has transformed the web in terms of
multimedia content which needs large bandwidth. For LANs UTP used to be 10
Mbps and this has increased to 100 Mbps being normal nowadays for cat5 cable. This
allows larger more data intensive LANs to operate.
So:




faster and more powerful processors,
cheap and large quantities of RAM,
cheap, large capacity hard discs and
increasing bandwidth
have all been important hardware factors in the rising use of networks.
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Higher Systems
Networking
In addition to hardware advances, there have been great advances in software.
A browser is a program that lets you view pages and interact with the World Wide
Web. These have advanced a great deal since their inception and all a user has to do
nowadays is enter an address in the address line or click a link to go to a site. Various
plugins can be added that greatly enhance their capabilities. Also software like flash,
real player and so on have all integrated with browsers giving us the present
multimedia web that we all know.
Operating Systems have also
advanced greatly. Since Windows
’95, peer to peer networking has
been possible. Windows NT,
updated to Windows 2000 Pro and
now Windows XP Pro allows client
server networking. (NT stood for
New technology). All client
computers must have a Network O/S
on their machines to connect to the
network.
These Operating systems have
become more reliable and more feature rich and so are another technical reason for
the increasing use of networks. Also security has been greatly enhanced which is
very important nowadays with hacking and viruses.
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Higher Systems
Networking
MISUSE OF NETWORKS
Hacking is illegal access to a network.
Controlling access to the network
The network operating system is responsible for security on the network. The most
obvious example of this is when a user logs on. The user must supply an identity and
a password. The operating system compares the
data entered with the identities and passwords in
its database and if the two do not match up then
it will not allow that user any access to the
resources on the network.
If the identity and password do match, then the
resources which the user has access to will depend on
the level of access that user has been given by the
network manager.
The access a user has to resources depends on that
user’s level of permissions.
There will always be a network manager who has access to everything, everybody’s
files and passwords. They are able to trace anyone misusing the system, they organise
the permissions for different categories of users, allocate ids and passwords to new
users, remove old users from access.
The easiest way for a hacker to operate is to find out
a users login name. This is often freely available or
can be found out from emails. Then find their
password.
This might seem fairly impossible, but can often be
remarkably easy:
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Higher Systems
Networking
1. Bribery (everyone has their price!)
2. By knowing about them e.g. family names, football team, favourite singer etc.
3. By a ‘con’ e.g. phone them saying you are from the helpdesk and need their
password to check something.
4. Set up a dummy website (‘phishing’) which seems like a real one where they
have to enter their password.
5. Planting software on their system that records keystrokes.
6. Simple burglary and find it written down in their desk drawer.
7. Actual hacking using ‘backdoors’ and faults in the Windows software.
Hackers often work for the imagined kudos rather than to do anything destructive.
However there are also plenty of vandals out there who get a kick out of disrupting or
destroying. Then there are professional criminals and Eastern Europe have some very
clever unemployed Computer Scientists working on hacking into financial systems.
To keep yourself safe you must have the latest version of your Operating System
installed and make sure you get automatic updates. You must have an effective
firewall, you must run anti-adware programs regularly. Be aware of the cons in the
list above.
Viruses
A virus is a piece of programming code that causes some unexpected and usually
undesirable event in a computer system. They are often designed so that they
automatically spread to other computer users on a network. Viruses can be transmitted
as attachments to an e-mail, as a download, or be present on a disk being used for
something else.
Some viruses take effect as soon as their
code takes residence in a system whilst
others lie dormant until something triggers
their code to be executed by the computer.
Viruses can be extremely harmful and may
erase data or require the reformatting of a
hard disk once they have been removed. Up
to date virus protection software is essential
on any computer connected to the Web.
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Networking
Copyright
P2P software like Kazaa, Morpheus or Bit torrent has led to a huge market in free
downloading of music, videos and software. Revenge of the Sith was on the Web
before it was released to cinemas. Anything made available to a file sharing network
can be downloaded a million times within a day and it is quite socially acceptable to
steal in this way, nobody thinks anything of it, though court cases are arising as music
companies try to strike back.
One of the main problems is in the copyright
laws themselves. It is illegal to make a copy
of anything (text, picture, music etc.) in
electronic or any other form. Now when you
access a website a copy is ‘cached’ on your
hard disk. So technically it is illegal to access
most websites. This means the Prime
Minister, the Chief Constable and everyone
else is breaking copyright. If you keep a video
recording of a program, you are breaking the
law. Showing a video to friends is illegal. It
all becomes a bit absurd, so downloading
some mp3 files doesn’t seem very wrong
either.
Computer Misuse Act
In the United Kingdom, the Computer Misuse Act (1990) covers using computers to
damage or steal data. The Computer Misuse Act covers crimes such as breaking into
computer systems or networks to destroy or steal data and propagating viruses that
destroy or damage information or computer systems.
Data Protection Act
In the United Kingdom, the Data Protection Act (1998) describes the duties and
responsibilities of those holding data on individuals. It also describes the right of these
individuals. In general, it is the duty of those holding data on individuals to register
with the Data Protection Registrar, to keep the information secure, make sure it is
accurate, and to divulge it only to those persons who are authorised to view it. It is the
right of an individual who has data stored concerning them to view that information
and to have it changed if it is inaccurate. There are a number of organisations that
may be given exemption from this act -namely the Police, Customs, National Security
and Health Authorities.
Copyright Designs and Patents Act
This protects anyone’s rights to anything they have ‘created’ whether it is an essay, a
song, a piece of artwork etc. It makes it illegal for anyone to make a copy, never
mind sell it or anything else.
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Higher Systems
Networking
Exercise
1. What is the main difference between a hub and a switch?
2. What is the job of a router?
3. Explain 2 technical reasons for the growth of networks.
4. One user of a network gets access to all files and folders, another user is
restricted to only some files. How is this enabled?
5. Give some examples of how hackers can gain access to a network.
6. What is a virus?
7. Which law makes the spreading of viruses illegal?
8. Why has downloading mp3s become socially acceptable?
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Higher Systems
Software
Topic 5
SOFTWARE
 Description of the function of a bootstrap loader
 Description and exemplification of the main functions of a single user
operating system:
o
o
o
o
o
o
interpreting users commands,
file management,
memory management,
input/output management,
resource allocation,
managing processes
 Definition of a utility program
 Description of utility programs (including virus checker, disk editor and
defragmenter)
 Description of the standard file formats for graphics files: jpeg, gif, TIFF
 Description of a suitable selection of software to support typical tasks
including production of a multimedia catalogue, setting up a LAN in a school,
development of a school website
 Description and exemplification of software compatibility issues (including
memory, storage requirements, and OS compatibility)
 Classification of viruses by type of file infected: file virus, boot sector virus,
macro virus
 Description of the following virus code actions: replication, camouflage,
watching, delivery
 Distinction between a virus, a worm and a Trojan horse
 Description of anti-virus software detection techniques:
o
o
o
o
use of checksum,
searching for virus signature,
heuristic detection and
memory resident monitoring
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Higher Systems
Software
There are two kinds of software – Applications and System
Software. Applications are what we run in order to do
something useful on the computer e.g. Microsoft Office.
System Software is the set of programs that let our
Applications access the hardware, control the
running of the computer, protect the system and
perform household tasks like tidying up disks. E.g.
Windows and utilities.
Systems software
The operating system is part of the system
software.
The purpose of an operating system is to
provide the user with a means of accessing the
computer hardware and operating the system
without knowing anything technical. The
operating system can be viewed as providing a
layer of software between the user
applications and the underlying hardware of
the machine.
Nowadays as the O/S is so large, it is held on hard disc. However the computer
cannot do anything without a program, so there must be something in ROM that is
there as soon as the computer is switched on. This program checks the hardware, the
RAM, peripherals etc. The main task however is to load the O/S from disk. This
ROM based program is called the bootstrap2 loader. Starting a computer is often
called ‘booting’.
The Americans have an expression ‘Pull yourself up by your bootstraps’ meaning to get yourself
organised. Bootstraps are laces.
2
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Higher Systems
Software
As the O/S is a very complex program it usually consists of a many various programs
working together. It can be best described as having six functions:
1
2
3
4
5
6
interpreting user commands;
file management;
input and output;
memory management;
resource management;
managing processes.
1. Interpreting user commands is undertaken by the
‘Command Language Interface’. This name comes
from old command driven Operating Systems,
nowadays it takes the commands from mouse clicks
or menus. However the object is the same, take
commands from the user, interpret them and pass
them to the appropriate layer.
2. File management is about organising
files on the disc and maintaining the
hierarchical file structure. It does this by
maintaining tables called the FAT. These
File Allocation Tables contain the names
of all the files, where they are on disc, the
length etc.
3. Input / Output. The BIOS in a PC is the
Basic Input Output System. This layer deals
with all the peripherals and their interfaces.
It sends data to the printer, loads data from a
disc and so on. All the device drivers are
part of the Input / Output System.
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Software
4. Memory Management. Computers can run more than one program at once.
Each program will take up an area of memory and it is the Memory
Management’s job to allocate space in RAM to the different programs.
However as programs run they produce data which must also be stored in the
RAM, now it is very important that this data does not overwrite the memory
space of another program, else that program will crash. This is the most
important job, ensuring that one program does not interfere with the memory
allocated to another program. In particular it must ensure that no program
interferes with the O/S.
Possible Memory Map for a PC running I.E. and Word:
Windows
0
Word
100Meg
Internet
Explorer
125Meg
165Meg
256Meg
If Word is creating data and it runs out of space in its block from 100-125Meg,
then there are locations free between 165 and 256Meg, however if Windows
has also been using that area to store data, then this would crash the PC.
5. Resource Allocation. This could be needed in a multi-tasking environment. When
a computer is running more than one program it cannot have two programs printing
at the same time. Resource management ensures this does not happen. It is
certainly needed in a multi-access environment where many users might be
accessing a peripheral at the same time.
6. Managing processes (the Kernel). A process is a
running program. Even when you think nothing
is running on your computer, just check the Task
Manager to see all the processes going on. The
CPU can only be used by one program at a time,
the job of the kernel is to schedule access to the
processor by all the processes that are running.
The simplest way to do this is ‘round robin’
where it just goes round each process in turn
giving it say 0.01 seconds access to the processor
then switching to the next process and so on.
However it usually more complicated than that as
a system of priority operates.
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Software
All these functions of an Operating System work together as you use a computer.
Imagine you start up a PC, load Word, Load a file and print it. Here is what will be
going on:
1. Switch on
The bootstrap loader in the ROM BIOS checks your PC, what memory you have,
what peripherals are attached and then loads Windows from your hard disk.
2. Load Word
The Command Language Interpreter takes your command to Load Word and
passes it to File Management which has the location of Word on the hard disk. Once
located on disk, the Input / Output System takes in the data from the disk and passes
it to Memory Management which positions word in its own block of RAM.
3. Load a File
Exact same as for loading Word, though the file goes in a separate area of RAM.
4. Print the File
The Command Language Interpreter takes your command and passes it to the
Input / Output system, which communicates with the interface and send the data to
the printer.
In addition you have the kernel, Process Management. It switches all these
programs in and out of the processor so that the Command Language Interpreter is
running, then Process Management removes that program from the CPU and brings in
File Management etc. etc.
Also the Input / Output system would be
showing all this on the monitor.
Resource Management would handle any
conflict in use of disks, printers etc. if you
were multi-tasking (i.e. playing a CD while
doing your work).
So, although you click Print or Load file in
Word, the program just passes this command
to the Operating System which handles all the
processes involved.
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Software
Utility programs
Utility programs perform housekeeping tasks on a computer. Some come with the
Operating System, others can be added.
Most operating systems will include:
1
2
3
disk cleanup;
disk defragmentation tools:
disk formatting programs.
1. Disk cleanup (disc editor)
This frees up space on your hard disk by:
• Removing temporary Internet files.
• Remove Windows temporary files.
• Remove optional Windows components that you
are not using.
• Remove installed programs that you no longer use.
2. Disk defragmentation tools
Disk defragmentation tools are used
to combat the problem of files being
split up on a disc. When there has
been a lot of deleting on a disc,
empty space is all over the disc
instead of just at the end. When a
large file is saved the O/S will save
part in one free space then another
part in free space elsewhere and so
on. A ‘defragger’ finds these files
and puts them together. In doing so
it also squeezes all the files together
so that the free space goes to the
end.
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Higher Systems
3. Disk
Software
formatting programs
Disk formatting programs are used to prepare the surface of
a disk for use. The process often involves laying down
’markers’ for tracks and sectors. Formatting deletes any
data on a disk..
Third party utilities
Third party utilities are tools provided by others than the supplier of the operating
system, like WinZip. These programs can often be free and include:
Anti-virus software
Anti-virus software is used to prevent the spread of small, usually malignant
programs that spread amongst machines. As programs, they need to be executed
before they can be effective, so viruses often attach themselves to program files.
Another way is as email attachments often masquerading as something else, e.g.
AnnaKournikova.jpg was an infamous virus which caught out millions throughout
the world though luckily it was not harmful, just sent an email to everyone in your
address book and directed them to a webpage. Others are not so lucky and can
have all their data destroyed.
Commercial anti-virus programs provide regular
updates for their products, and given the speed
with which viruses can propagate through the
system, it is a good policy to update virus
protection as often as possible.
There is more on viruses later.
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Higher Systems
Software
Exercise
1. What is the function of the bootstrap loader?
2. What is the job of file management?
3. What goes on in the Process management layer of the Operating System?
4. When you save a file to disc, outline the layers of the Operating system that
will be involved.
5. What is a utility program?
6. What does defragging a disc mean?
7. Give an example of a disc editor.
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Higher Systems
Software
SOFTWARE
When you buy a program it comes with a System Specification3.
This will list the requirements the software needs to run on your computer.
Examples are:
Memory: A minimum amount of RAM will be required e.g. 128Mb.
Processor : A minimum processor or a certain processor type e.g. Pentium III
800Mhz or better.
Hard Disk: A minimum amount of free space on your disk drive, e.g. 50 Mb free.
These are hardware requirements.
Also there is usually a software requirement:
e.g. Operating System, Windows ME or later.
For your computer, try and answer these questions.

What operating system is installed?

What is the available RAM?

What is the processor speed?

What is the disk capacity?

What peripherals are available?
Exercise:
1. Go to www.dabs.com and under software, Graphics and Media > Illustration
& Drawing find Photoshop. Under ‘Specification’ scroll down to the bottom 3
and check to see if this program will run on your computer.
2. Try to find Adobe premiere Pro video editing and check to see if that will run
on your computer.
While in DABS software, have a look at Utilities for sale.
Question 3 over the page
Always take system specs with a pinch of salt. They’ll say it needs Windows XP and 128Mb RAM, I
wouldn’t run Notepad on that system, it will be using VRAM the whole time. They just want to sell
software and not put you off thinking you need a new computer. If it says 128Mb RAM, then 256Mb
MINIMUM will be really needed, likewise for processor, hard disk space etc.
3
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Higher Systems
Software
3. The iPod Nano is on sale at the apple store (Google iPod nano). PC
requirements are given on the bottom right of the page. Does your PC meet
these requirements? Are there any hardware requirements??
Various tasks a computer is used for have certain software requirements.
Newsletter
Word plus some graphics / photo editing software. Possibly a Desk Top Publishing
program like Microsoft Publisher.
Website Development
A web authoring package like Dreamweaver, graphics software -- Fireworks, possibly
photo editing, Animation (possibly Fireworks for gifs) or Flash.
4. Using the web find a list of software for the above tasks with their costs and
system requirements.
IN SUMMARY
The main software compatability issues are:



Memory requirements
Storage requirements
Operating Systems requirements
A program that requires 256 Mb RAM will not run on a machine with 128Mb RAM.
A program that needs 2 Gbytes of space to install on a hard disk won’t install if your
disk is full.
A program that runs on Windoews XP will not work on a Windows ME computer.
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Software
STANDARD GRAPHIC FILE FORMATS
A standard file format means any program of that type can load the file. In Word
Processing , Microsoft Word cannot load Appleworks files and vice versa. However
they can both save and load a file as RTF (Rich Text Format), so RTF is called a
standard file format for Word Processsing.
JPEG
JPEG (Joint Photographic Experts Group) is group of experts that develop and
maintain standards for compression algorithms for computer image files.
JPEG processing makes image files small by removing detail. This is called lossy
compression. This will reduce the number of colours used to store the image and
avoid unnecessary repetition of bit patterns.
GIF
The Web also supports GIF (Graphics Interchange Format) images. These images are
based on a compression algorithm that creates a codebook or dictionary of particular
bit patterns. These in turn, are then substituted resulting in a smaller file. When
decoding, the algorithm uncompresses the file to generate the original image.
This is lossless compression lossless compression.
An algorithm that allows compression to be decompressed is called a CODEC
COmpress – DECompress)4
TIFF
Tagged Image File, can be any resolution, any number of colours. Used for bitmaps
especially scanner images.
4
LOSSLESS COMPRESSION using a CODEC
Here is a simple sentence. There are simply too many ways from here to there to
impel one to go there by the impossible route.
Our CODEC is:
Here replaced by X
Imp replaced by Y
Our sentence becomes:
X is a sYle sentence. TX are sYly too many ways from X to tX to Yel one to go
tX by the Yossible route.
Not counting spaces we have replaced 102 characters by 79, about 23% reduction.
The full message can be reassembled as we know the codec.
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Higher Systems
Software
Viruses
A virus is a programming code that causes some undesirable and unexpected event to
happen in a computer. Viruses are usually disguised as something innocent and are
designed so that they automatically spread between computer systems. Viruses can be
enter a system as attachments to an e-mail, a download from the Web, or from on a
disk or CD. Some viruses take effect as soon as their code is executed and others can
wait until circumstances cause their code to be executed by the computer e.g. a certain
date. Viruses can be quite harmful and erase data or close down a system.
Virus types
Viruses are classed by three main types:
�
File virus;
�
Boot sector virus;
�
Macro virus.
File virus
A file virus can be attached to a program file (.exe) so that when you load the
program, you load the virus.
A file viruses can also take the form of a complete program attached to
something else, e.g. an e-mail. They then take up residence in the computer
ready to cause havoc. These are often disguised as something else.
Boot sector virus
These viruses infect the boot sector on disks where programs are executed when the
Operating System starts. Every time your computer starts up, the virus is loaded in
with the O/S.
Macro virus
Macro viruses are fairly common viruses, but
tend to do the least damage. Macro viruses
infect applications and typically cause a
sequence of actions within the application e.g.
inserting unwanted words or phrases in a
document. These also often come attached to
documents in an email.
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Software
Virus code actions
Viruses don’t all follow the same course of action. They can, and do, use a
combination of the following actions:
�
Replication;
�
Camouflage;
�
Watching;
�
Delivery.
Replication
Like a biological virus they can spread quickly and are can be difficult to control.
They can attach themselves to almost any type of file and spread as files copied and
sent between computer users. A virus programmed to operate on a certain date can
have a long time to replicate itself before activation happens. This gives it time to be
spread over many computers before being discovered.
Camouflage
It is possible for a virus to avoid detection by taking on the characteristics that
detection software is programmed to look for and ignore. However, detection
software has evolved to prevent this happening.
(Today’s anti-virus software does much more than simply check particular
characteristics (or signature) of a virus. They also check the virus code and even
checksum the virus code to identify it. With these cross-checks it would be extremely
difficult for a virus to camouflage itself and get past detection. This is dealt with later)
Watching
A virus can lie in wait and ambush a computer when something routine is carried out
e.g. opening a particular application. The damage routines will activate when certain
conditions are met. A certain date or when the infected user performs a particular
action may trigger the virus.
Delivery
Infected disks brought in from the outside used to be the main source of viruses until
e-mail provided the ideal delivery vehicle. Downloads from Peer –2–Peer sites are
another common source
Once delivered the virus will wait for the trigger to wreak its havoc.
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Software
Worm
A worm is a self-replicating code
that does not alter any files but takes
up residence in the computer’s active
memory and duplicates itself. They
only become noticeable once their
replication consumes the memory to
the extent that the system slows
down or is unable to carry out
particular tasks. Worms tend to use
the parts of the computers operating
system that is not seen by the user -until it’s too late.
Trojan horse
A Trojan horse is a program where harmful code is contained inside another code
that can appear to be harmless. Once the apparently harmless code is in the
computer, it releases the malicious code to do its damage. Trojan horses may even
claim to be anti-virus in order to get the user to install it.
Anti-virus techniques
The best protection against a virus is to know that each file you open from an e-mail,
disk or from the Web is free from any virus. This requires anti-virus software that can
screen e-mail attachments or Web downloads, and checks all of your files from timeto-time removing any viruses that are found.
Techniques used by ant-virus software to detect a virus include:
�
Checksum;
�
Signature;
�
Heuristic;
�
Memory monitoring.
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Higher Systems
Software
Checksum
In checksum detection, the binary of all the machine code for the key files
(particularly boot files) is added up as numbers and stored in the system. When these
files are called to execute the checksum is calculated and compared with what it
should be. If there are any anomalies, then the file about to be run could have been
infected and a warning given.
Virus signatures
A virus signature is a unique pattern of bits within a
virus. Once known, the anti-virus software uses the
virus signature to scan for the presence of malicious
code and removes it. This is why anti-virus software
has to be regularly updated, known signatures are
added and then they can be easily detected.
Heuristic detection
Heuristic detection describes the technique of approaching a problem through
previous experience. The technique is used to find unknown viruses that have not yet
been identified by their signatures by looking for characteristics in a file that have
previously been associated with a known virus.
Heuristics can also detect a virus that has disguised its signature, by recognising a
particular sort of behaviour. For example, if a file attempts to access your address
book then that might be suspicious. If the same file includes code that checks a date,
then the suspicion rises. There will come a point when a warning is issued on the
possibility of a virus.
Memory resident monitoring
Some anti-virus software is memory resident and is loaded on start up. It actively
monitors the system for viruses whilst the computer is switched on and checks
programs for infection every time they run. This will include the boot files on start up
checking any disk as it is accessed, checking any files accessed during operation, and
checking any files being loaded on the hard drive.
The price to be paid with memory resident programs is that they can cause delays in
program loading and execution whilst the checks are being carried out.
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Higher Systems
Software
Using a virus information library
Activity
Literally thousands of viruses have been detected and catalogued using a Virus
Information Library (VIL). One such VIL can be found at:
http://vil.nai.com/vil/default.asp
Using this or another source, find the details of at least one of each type of virus:






file
boot
macro
worm
Trojan
A hoax5
Use the search box to enter your criteria although this won’t always work. Also use
Google for Virus Information Library or just general searches like ‘boot virus’.
For each one, make a note of the following:
1
2
3
4
5
6
name:
type:
symptoms:
date discovered (try to find one from this year):
delivery (e.g. email, website):
cure (if any):
5
Another problem with viruses is that some of them are hoaxes (and some of the hoaxes are
themselves hoaxes). A simple example that has become quite common is for an email to arrive warning
that such and such is a terrible virus and avoid at ball costs. The email goes into great detail and
exhorts you to pass this urgent message on. Victims (suckers?) immediately spread this news to
everyone in their office, who spread it on and so on. The whole thing of course is just a load of
rubbish. I have known many people in Computing and IT who have passed such messages to me.
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Higher Systems
Software
Exercise
1. Give two examples of hardware requirements that would have to be
considered when purchasing software.
2. Give one example of a lossy graphic compression and one example of lossless.
3. In simple terms, how can compression be accomplished losslessly?
4. List three virus types.
5. Which type of virus starts up with your computer?
6. Which type of virus could be found in a Word document?
7. What does it mean to say a virus is ‘watching’?
8. What is a worm?
9. Why are some viruses called Trojans?
10. How does checksum detection work?
11. What is a virus signature?
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