MARR
Computer
Systems
MARR
This set of notes is matched with the Computer Systems Workbook
S3
What is a computer?
Have you ever wondered what the word “computer” means? If you look it up in a dictionary,
you will find something like:
“an electronic device which runs a program to process data at great speed”
We will come to the words program and data soon; however, the word “computer” means
“something that computes”.
So what does “compute” mean? Well, it means to calculate or work out.
The very first computers were actually people. They did sums – all day,
every day!
From around the 1700s until the 1950s, teams of these “computers” used to carry out
complex calculations (and check each other’s work).
Representing Information
A computer is a machine that processes information (data) using instructions given to it by
a human. Computers are electronic machines, so the instructions and information they
work with is represented electronically – by switches.
Switched on
No matter what kind of computer you are using – a desktop PC, smartphone, games console
or embedded computer – it is doing one thing: processing information (data). Since a
computer is made up of switches, then the information it processes is represented by the
positions of switches.
How is this possible? Well, think about an ordinary switch. It has two positions – OFF and
ON – but we could also call these positions (or make them stand for):
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YES or NO
BLACK or WHITE
0 or 1
If we had two switches, there would be four possible combinations of these.
Look at the table below that shows the four combinations of two switches and examples of
the way that you could represent different kinds of information by these.
Switch
positions
Numbers
Colours
Letters
OFF / OFF
0
Black
A
OFF / ON
1
Blue
B
ON / OFF
2
Red
C
ON / ON
3
White
D
Now consider if there were millions of switches – you can see how there could be almost
endless combinations of OFF and ON for all of these switches.
A modern computer does contain millions (even billions) of tiny switches and they are used
to represent all such information.
The silicon chip
As you have seen, a computer is basically a collection of
electronic switches.
Over the years, the technology used for this has changed from old
electronic valves used in Colossus to the silicon chip.
Incredibly, a modern silicon chip can contain over a billion
switches (called transistors) on a wafer of silicon smaller than
your fingernail!
Binary: The language of computers
No matter what you are using your computer for – watching a movie, listening to music or
surfing the web – the data (information) that the computer is processing is represented by
the positions of millions of switches. Computing scientists like to call the positions 0 and 1
instead of OFF and ON.
This is known as binary. It may seem strange, but it is easier than you might think.
Bits and Bytes
Computers can sense when a signal being sent is on or off. This is represented by a ‘1’ (on)
or a ‘0’ (off). Each individual 1 or 0 is called a binary digit or bit and it is the smallest piece of
data a computer system can work with.
Eight bits are grouped together to make one byte.
One byte provides enough codes (256) to represent all of the characters that appear on a
standard keyboard. A byte is the basic unit used to measure computer memory size.
Main memory
A running program is stored along with its data in the main memory of the computer. We
will cover this in more detail.
Main memory is divided up into storage locations each with its own address.
A single storage location can hold one or more bytes of information.
Main memory size
This is the amount of memory allocated to a computer system. Most modern computers
come with a minimum of 1 gigabyte of RAM.
Stored program
All programs and instructions that are stored in main memory are called stored programs.
Concept of machine code
Machine code is the computer's own language. It consists of only the numbers 0 and 1. This
is the binary system.
Representation of positive whole numbers
We can represent any number, however large, in binary. Remember we can only store
numbers between 0 and 255 in one byte. This is obviously rather restrictive since it's not
dealing with large integers, negative numbers or decimal numbers.
A table exemplifying the representation of numbers in binary
Storing text (ASCII)
Representing Text
Each character on the keyboard has a special one byte (8-bits) code to represent it. A
standard for representing these characters is the ASCII set.
ASCII stands for the American Standard Code for Information Interchange.
Characters like letters a to z and A to Z and the digits 0 to 9 are each given a standard code
which is the same on all computer systems.
A table showing different representations of a list of characters in binary, and decimal.
The whole range of characters recognised by a computer system is known as the character
set of that computer.
Bitmapped Graphics
Calculation of storage requirement of black and white bit-map
You may need to calculate the storage required for a black and white graphic.
To do this you:
multiply the width by the height to get the number of pixels (dots), this equals the numbers
of bits divide by 8 to get the number of bytes divide by 1024 to get the number of kilobytes
divide by 1024 to get the number of megabytes
Inside every computer, there is a processor. This is a chip containing digital electronic
circuits. These circuits work with tiny pulses of electricity and electronic components. The
pulses of electricity can be represented by the digits 1 and 0. Every item of data and every
instruction for the processor is represented by a group of these binary digits.
Processors only 'understand' these binary digits. The only inputs you can make to a
processor are groups of binary digits. The only output that a processor can make is a group
of binary digits.
Instructions and commands made for processors in this binary digital form are known as
machine codes. Here is an example of machine code.
These two programs both print the letter "A" 1000 times on the screen.
Machine Code
169 1 160 0 153 0 128 153 0 129 153 130 153 0 131 200 208 241 96
BASIC (A High Level Language)
5 FOR I=1 TO 1000: PRINT "A";: NEXT I
There are several problems with machine code:
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Machine codes for different processors are different
They are very hard for humans to understand and use
They take up a lot of space to write down
It is difficult to spot errors in the codes.
Machine code is an example of a low-level language. To get around these difficulties
computer scientists invented high-level languages.
High Level Languages
High-level languages are similar to human languages. Instead of
binary codes they use normal English words. For example VB6
uses words like IF, WHILE, PRINT, RAW_INPUT, INT and so on.
So with high level languages using English words it makes the
code easier to understand, easier to spot errors and more
readable.
.
Translators
High-level languages have some great advantages compared to machine code however
there is one major problem – processors don’t understand high-level language at all. To get
round this problem computer scientists have developed translator programs that translate
high-level languages (written by humans) into machine code (understood by processors).
Translator Program
Compiler or Interpreter
10001001 11001001 1001000
11100001 10001101 1000111
Interpreter
An interpreter takes each line of high-level language and translates it into machine code
and passes it to the processor to carry out that instruction. It then works its way through the
program one line at a time in this way.
Advantages
Disadvantages
A program will run even if it is not
finished.
No copy of the machine code is saved.
Meaning the source code has to be
translated every time taking longer.
Easy to spot errors during the
translation.
The process of translating the program
slows down the running of it.
Program will run as soon as the first line
is translated.
You will need to have a translator
program or you cannot run it.
Compiler
A compiler takes your high level language and translates the whole program into machine
code once. The machine code can then be saved and kept and does not need to be
translated before it is run. This is an example of .exe file. Software that you buy, such as a
games or an application, will have been compiled into machine code before being
distributed and sold.
Advantages
Disadvantages
The machine code is saved so the program
only needs to be translated once.
You have to wait until the code is complete
and the errors have been fixed before the
translation can be finished and the machine
code is run.
The user does not need a translator
program to run the machine code therefore
the program runs quicker.
Each time the program is changed it needs
to be re-translated.
Computer Architecture
A simple computer consists of the following components as represented in this block diagram:
 Processor;
 Memory;
 Input/output device;
 Backing Storage
 Communication channels
Input Devices
An input device is a piece of hardware that lets you put information onto or into a
computer to be processed.
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Keyboard
Mouse
Microphone
Digital Camera
Scanner
Webcam
Touch Screen
Output Devices
An output device is a piece of hardware that provides the user with information.
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Monitor
Printers
Loudspeakers
Storage Devices
When we talk about ‘storing’ data, we mean putting the data in a known place. We can later
come back to that place and get our data back again.
‘Writing’ data or ‘saving’ data are other ways of saying ‘storing’ data. ‘Reading’ data,
‘retrieving’ data or ‘opening’ a file are ways of saying that we are getting our data back from
its storage location.
Magnetic
Magnetic
Capacity
Hard Drives
Magnetic Tapes
Floppy Disk
Zip Drive
500Gb – 1Tb
1.44Mb
750Mb
Hard-drives have a very large storage capacity. They can be used to store vast amounts of
data, sometimes used to store all types of films, including huge files such as movies. Data
access speeds are very fast.
Optical
Optical
Capacity
CD-ROM
CD-R
CD-RW
DVD-ROM
DVD-R
DVD-RW
Blu-Ray
700Mb
700Mb
700Mb
4.7Gb
4.7Gb
4.7Gb
25-50Gb
Read-Only storage devices such as CD-ROM, DVD-ROM and Blu-Ray have data written onto
them when they are manufactured. This data cannot be changed.
CD-ROMs are used to distribute all sorts of data: software (e.g. office applications or
games), music, electronic books (e.g. an encyclopaedia with sound and video.)
DVD-ROMs are used in the same way as CD-ROMs but, since they can hold more data, they
are also used to store high-quality video.
Blu-Ray discs are used in the same way as DVD-ROMs but, since they can hold more data,
they are also used to store very high-quality, high-definition (HD)
video.
Writeable storage devices such as CD-R and DVD-R can be written
to. For example you could burn some music onto a CD-R or write
your holiday video to a DVD-R.
Rewriteable storage devices such as CD-RW and DVD-RW allows
you to re-write data to the disk multiple times. This means you
could burn a CD-RW with some music then a few months later use
the same disc to rewrite new songs onto it.
Portable Hard Drive
A portable hard-drive is one that is placed into a small case along with some electronics that
allow the hard-drive to be accessed using a USB or similar
connection.
Portable hard-drives allow very large amounts of data to
be transported from computer to computer.
Processor Type & Speed
Processors were originally developed with only one core. There are
different types of processors in today’s markets. Dual-core and
Quad-core are the most common in many products today. If a
processor has multiple cores it can run multiple instructions at the
same time, increasing overall speed for programs.
Dual-core processor has two cores (e.g. AMD Phenom II X2, Intel
Core Duo)
Quad-core processor contains four cores (e.g. AMD Phenom II X4,
Intel's quad-core processors, see i3, i5, and i7 at Intel Core)
Processors are measured in Hz (Hertz). In most modern computer systems processors are in
GHz (GigaHertz).
Below is an example of a typical processor type and speed.
Apple iMac
Processor
2.9GHz quad-core Intel Core i5 processor
(Turbo Boost up to 3.6GHz) with 6MB L3
cache
Memory: RAM & ROM
People often get confused between main memory and backing storage, they are not the
same. Main memory is located inside the computer system. It can either be RAM or ROM.
Backing storage is outside the main processor, e.g. Hard Drives, CD/DVD drives, USB Flash
Memory (Pen Drives). Main memory in today’s computers is on average around 4-6 Gb of
RAM. Backing Storage is much bigger with average computers having around 500 Gb or
more.
ROM (Read Only Memory)
ROM is used to store a small part of the operating system called the bootstrap loader.
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Data is stored permanently in ROM,
Data is not lost when the power goes off
Data in ROM cannot be changed
RAM (Random Access Memory)
This is where the operating system is stored; it also holds all
programs and data. You can purchase additional RAM chips and
install them in your desktop computer, which normally speeds up
multi-tasking.
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The processor can write to and read from RAM at high speed
Data held in RAM can be changed
All data in RAM is lost when the power is switched off
Operating System
An operating system is a large program which manages the hardware
and software of the computer system. Windows 7, Windows 8, OS X,
Android and iOS are examples of operating systems.
To run a certain program you may need to have a certain operating system. This kind of
information is normally stated in the documentation with the program what operating
system is required.
Networking
What is a network?
A collection of computers interconnected by communication channels that allow sharing of
resources and information.
Stand Alone or Networked
Most computer systems today are connected to a network most of the time or at least some
times. If a computer is just stand alone then it has no access to share resources and
communicate with other computers.
Local Area Network (LAN)
A group of linked computers in close proximity to each other, such as in an office building,
a school or at home.
LANs are useful for sharing resources such
as files, printers or other applications.
Wide Area Network (WAN)/Internet
A WAN covers a large geographical area.
Most WANs are made from several LANs
connected together.
The Internet is a WAN
Transmission Media
In a network data is sent from one computer to another in various forms. We are going to
look at three of these, wired, optical and wireless.
Wired
Cat 5 cable is used in structured cabling for computer networks such as Ethernet.
The network in the school uses this type of cable
to connect the computer to the server.
You may also use this type of cable at home if you do not use Wi-Fi.
Wireless
Wireless network refers to any type of computer
network that is not connected by cables of any kind.
Some businesses will use wireless as it saves money
not having to lay down all the cable.
Wireless lets you connect portable devices such as
tablets and smartphones and can make a network
feel more portable.
Security Risks
Virus
A computer virus is a piece of program code that, like a
biological virus, makes copies of itself by attaching itself to
another program. The virus can waste the host's resources,
and sometimes destroy or change files.
While viruses are usually malicious - destroying data or
crashing network services - many do nothing or are just annoying (for example, displaying a
message to the user). Many viruses are made to wait a long time before doing anything,
usually destroying data on a certain day, like a holiday.
Viruses are usually spread by a computer network, by e-mail, or by removable media, like a
memory stick.
Worms
A computer worm is a computer program that makes lots of
copies of itself, like a computer virus. The main difference
between the two is that a computer virus attaches itself to
another computer program, but a worm works by itself.
As well as copying itself, a worm can be made to do all sorts of things, like delete files on a
computer, or send e-mails to everyone the owner has in their address book
Trojans
A type of 'malware' software.
A Trojan seems to do an useful task and so the user will use it. But
in the background, the Trojan is also carrying out its real purpose
which is hidden and unknown to the user.
For example, you download a free game, it's great! You keep on using it. But meanwhile the
Trojan may also install a permanent keylogger that now tracks every keystroke you make
and sends it off over the Internet to whomever created the Trojan.
Anti-virus Software
This is referring to software that can detect the presence of a virus within a computer file
and warn you there is a problem.
The anti-virus software may then offer to
'quarantine' the file so it cannot be used to spread
the virus.
Many packages will also try to remove the virus, but this is not always successful.