Page 1 of 131
Computer Science (9618)
CAIE AS Level
complete notes
No.
Chapter
1 Number System, Multimedia and File Compression
Page
4
2 Communication and Network
12
3 Input/Output Devices & Memory/Storage
40
4 CPU Architecture & Mnemonics Tables
53
5 System Software
70
6 Security, Privacy and Data Integrity
82
7 Ethics and Ownership
94
8 Databases
99
Page 4 of 131
Chapter 1: Number System, Multimedia and File Compression
Binary to Denary to Hexadecimal:
Hexadecimal:
Binary:
0 1 1 0 1 0 1 12
8 4 2 1
Binary:
6B16
6
11
Denary:
1x20 + 1x21 + 1x23 + 1x25 + 1x26
1 + 2 + 8 + 32 + 64
10710
Hexadecimal:
16 107 Remainder
16 6
11
16 0
6
16 0
0
= 6 11
= 6B16
Denary:
6B16
6
11
6 x 161 + 11x160
96 + 11
10710
Binary Subtraction:
127 – 90 = 37
Value 1:
Value 2:
Inverse Value 2:
Add 1:
New Value 2:
Carry:
Add New Value 2 and 1:
8 4 2 1
0 1 1 0 1 0 1 12
(4+2) (8+2+1)
6
11
6B16
8 4 2 1
8 4 2 1
(4+2) (8+2+1)
0 1 1 0 1 0 1 12
Binary:
To Denary:
➢ Multiply by power of
original base
From Denary:
➢ Divide by the base of
the answer required
2 107 Remainder
2 53 1
2 26 1
2 13 0
2 6
1
2 3
0
2 1
1
2 0
1
= 0110 1011
=0 1 1 0 1 0 1 12
0 1 1 1 1 1 1 1 (127)
- 0 1 0 1 1 0 1 0 (90)
1010 0101
1
1010 0110
1111 11
1| 0 0 1 0 0 1 0 1 (37) Ignore Overflow
Binary 2’s Complement Subtraction:
-33 - -61 = 28
Value 1:
1 1 0 1 1 1 1 1 (-33)
Value 2:
- 1 1 0 0 0 0 1 1 (-61)
Inverse Value 2:
0011 1100
Add 1:
1
New Value 2:
0011 1101
Carry:
1111 111
Add New Value 2 and 1:
1| 0 0 0 1 1 1 0 0 (28) Ignore Overflow
Negative Denary to Binary:
-116
↓
116 =
0111 0100
Carry =
11
Inverse =
1000 1011
Add 1 =
1
Answer =
1000 1100
OverFlow:
9th bit is overflow error – the result of
carrying out a calculation that produces
a value that is too large for the
computer’s allocated word size (8-bit,
16-bit, 32-bit etc.)
4
Page 14 of 131
Network has LAN (Local Area Network) and WAN (Wide Area Network):
Local Area Network (LAN):
A network connecting computers in a single room or in a single building or devices connected over a small
geographical area
Company-owned infrastructure // Uses dedicated infrastructure // No leasing external infrastructure /
transmission media // does not use internet to transmit within the building
Wireless LANs (WLANs) :
Similar to LANs but there are no cables, wireless communications over fairly short distances (upto 100m)
using radio or infrared signals instead of cables
Wireless Access Points (WAPs):
❖ Hardware component that provides radio
communication from the central device
to nodes on the network (and vice versa)
❖ Allowing devices to connect to the LAN
instead of using a cable
❖ Are connected into the wired network at
fixed locations
❖ Because of the limited range, most
commercial LANs (airports etc.), use
several WAPs to permit uninterrupted
wireless communication
❖ WAPs either use spread spectrum
technology (wideband radio frequency with high range) or infrared (short range, easily blocked) and
therefore has limited use
❖ The WAP receives and transmits data between the WLAN and the wired network structure
❖ End users access the WLAN through wireless LAN adapters built into the devices or as a plug-in
module
❖ Easy to move a device to different location
Wide Area Network (WAN):
A network connecting computers on different sites, possibly thousands of kilometers apart
If a number of LANs are joined togethers using a router or a modem, they can form a WAN
The network of ATMs is an example
A typical WAN will consist of end systems
and intermediate systems:
The orange are the end systems and purple
are the intermediate systems
Metropolitan Area Network (MAN):
Network which is larger than a LAN but smaller than a WAN, which can cover several buildings in a single
city, such as a university campus
Personal Area Network (PAN):
Network that is centered around a person or their workspace
WAN: 100 km to over 1000 km
MAN: 1 km to 100 km
LAN: 10 m to 1000 m
PAN: 1 m to 10 m
14
Page 24 of 131
Transmission Media:
Wired:
Advantages:
❖ More reliable and stable network (wireless may be subject to interference)
❖ More secure less interference
❖ Data transfer rates tend to be faster with no ‘dead spots’
❖ Cheaper overall
Disadvantages:
❖ Devices not mobile
❖ Tripping hazards
❖ Overheating of connections, can cause fires
❖ Downtime during routine office cleaning or rearrangement of cables
❖ Permission from authorities and land owners may be required before laying cables underground
Copper Cables:
Twisted Cable:
❖ It is used mostly in LANs and for connecting
telephones to telephones lines, one cable with
four twisted pairs
❖ It has the lowest data transfer rate and suffers
the most from external interference (such as
electromagnetic radiation)
❖ Can be shielded (commercial use) or
unshielded (residential use)
Benefits of Copper Cable:
❖ Installation cost is low
❖ Easy to set up
❖ More flexible
❖ Easier to terminate
❖ Vast knowledge as it has been around for
years
❖ Robust
Coaxial Cable:
❖ It is most commonly used in MANs and by cable
television companies
❖ The cost of coaxial cables is higher than
twisted pair cables but they offer a better data
transfer rate (x80 of twisted cables) and are
affected less by external interference
Drawbacks of Copper Cable:
❖ Less bandwidth rates
❖ Need repeaters
❖ Higher interference
24
Page 34 of 131
For ex., the devices in the Admin and finance department might have hostIDs of 1, 8, 240, 35, 67, 88, 134, and
so on, with similar spreads for the other seven departments.
It would be beneficial to organise the netIDs and hostIDs so that the network was a lot less complex in
nature
With sub-netting, the hostID is split as follows:
000 00000, where the first 3 bits are netID expansion and the last 5 bits are the hostIDs.
Thus, we have eight sub-nets with the same range of hostIDs:
The devices in the Admin and finance department will have IP addresses:
192.200.20.000 00001 to 192.200.20.000 11110
The Humanities department will have IP addresses:
192.200.20.001 00001 to 192.200.20.001 11110
And so on for the other departments…
To obtain the netID from the IP address we can apply the AND mask (recall that 1 AND 1 = 1, 0 AND 0 = 0 or
1 AND 0 = 0)
Thus, if a device has an IP address of:
11000000.11001000.00010100.011 00011
We can apply the AND mask:
11111111.11111111.11111111.111 00000
Which results in the netID value:
11000000.11001000.00010100.011 00000 (or 192.200.20.03)
Hence, we know this computer is from Science department
34
Page 54 of 131
Register:
Temporary component in the processor which can be general or specific in its use that holds data or
instructions as part of the fetch execute cycle
Status Register (SR) (Special Register):
❖ Status register is interpreted as flags for different reasons, such as overflow in addition calculation,
or result of a calculation is zero
❖ Used when an instruction requires some form of arithmetic or logical processing
❖ Contains bits which can be set or cleared depending on the operation
❖ It is a special register
Flag:
❖ Indicates the status of a bit in the status register, for example, N = 1 indicates the result of an
addition gives a negative value.
❖ Each bit is known as a flag
❖ Most systems have 4 flags
➢ Carry flag (C) set to 1 if the answer does not fit in the given register // goes to the 9th bit
➢ Negative flag (N) is set to 1 if the answer is a negative value (leftmost bit 1)
➢ Overflow flag (V) set to 1 if the answer is more than 7 bits and a carry goes to the 8th bit, disturbing
the sign bit
➢ Zero flag (Z) set to 1 if the answer is zero on all bits
➢ Other flags include parity flag, half-carry flag and interrupt flag
Program Counter (PC) (Special Register):
❖ Program Counter is used to store the address of the next current instruction to
be fetched. It is incremented by one as soon as the address is fetched into MAR
❖ (also known as instruction address register – IAR)
❖ A register used to store the address of the instruction which is currently being executed
CIR (Current Instruction Register) (Special Register):
A register used to contain the instruction which is currently being executed or decoded
IX (Index Register) (Special Register):
Used when carrying out index addressing operations (assembly code)
Stores values that can be added to an address to give another address
Interrupt Register (Special Register):
Used for interrupt management
MAR (Memory Address Register) (Special Register):
Stores the address of the memory location currently being read from or written to
MDR/MBR (Memory Data/Buffer Register) (Special Register):
Stores data which has just been read from memory or data which is about to be written to memory
(sometimes referred to as MBR)
ACC (Accumulator) (Special/General/Temporary Register):
A register used when carrying out ALU calculations, stores temporary result of calculation
Core:
A unit made up of ALU, CU and registers which is part of a CPU. A CPU may have multiple cores
54
Page 64 of 131
Directive:
❖ An instruction that directs the assembler to do something
❖ A directive is not a program instruction
❖ It is information for the assembler
❖ For example: State the start address for the program // tell the assembler to set aside space for
variables // include an external file etc. // Opcode in second pass (instruction set)
Instruction Groups:
Input and output of data
Takes an input from the user // outputs the character of the binary number (IN, OUT, OUTCH)
Arithmetic operations
Perform addition and subtraction (ADD, SUB, INC, DEC)
Unconditional and conditional instructions
Move to another instruction (identified by a label) (JMP, JNE, JPN)
Compare instructions
Compare the result to another value (CMP, CMI)
Data Movement:
LDM, LDI, LDX, LDD, LDR
64
Page 74 of 131
Defragmentation Software:
Utility that reorganizes the sectors on a hard disk so that files can be stored in contiguous data blocks
Re-organizes the disk contents
Moves split files so they are continuous
Creates a larger area of continuous free space
❖ As an HDD becomes full, blocks used for files will become scattered all over the disk surface (in
potentially different sectors and tracks as well as different surfaces)
❖ This happens as files are deleted, partially-deleted, extended and so on
❖ The consequence is slower data access time: the HDD read-write head requires several movements
just to find and retrieve the data making up the required file
❖ It would be advantageous if files could be stored in contiguous sectors, considerably reducing HDD
head movements
❖ Note that, due to their different operation when accessing data, this is less of a problem with SSDs
➢ Consider the following example using a disk with 12 sectors per surface
➢ We have three files (1, 2 and 3) stored on track 8 of the disk surface:
➢ File 2 is deleted by the user and file 1 has data added to it
➢ However, the file 2 sectors which become vacant are not filled up straight away by new file 1 data
since this would require ‘too much effort’ for the HDD resources
➢ Hence, we get the following:
➢ File 1 has been extended to write data in sectors 10 and 11
➢ Now, suppose file 3 is extended with the equivalent of 3.25 blocks of data
➢ This requires filling up sector 9 and then moving to some empty sectors to write the remainder of
the data, the next free sectors are on track 11:
➢ If this continues, the files just become more and more scattered throughout the disk surfaces
➢ It is possible for sectors 4, 5 and 6 (on track 8) to eventually become used if the disk starts to fill up
and it has to use up whatever space is available
A disk defragmenter will rearrange the blocks of data to store files in contiguous sectors wherever
possible; however, if the disk drive is almost full, defragmentation may not work
➢ Assuming we can carry out defragmentation, then track 8 now becomes:
➢ This allows for much faster data access and retrieval since the HDD now requires fewer read-write
head movements to access and read files 1 and 3
➢ Some defragmenters also carry out clean up operations
➢ Data blocks can become damaged after several read/write operations (this is different to bad
sectors)
➢ If this happens, they are flagged as ‘unusable’ and any subsequent write operation will avoid writing
data to data blocks which have become affected
74
Page 84 of 131
Anti-Spyware Software:
•
•
Software that detects and removes spyware programs installed illegally on a user’s computer system
The software is either based on rules (it looks for typical features associated with spyware) or based
on known file structures which can identify common spyware programs
Antivirus Software:
Running antivirus software in the background on a computer will constantly check for virus attacks
Although different types of antivirus software work in different ways, they all check software or files
before they are run or loaded on a computer
They compare possible viruses against a database of known viruses
They carry out heuristic checking (check software for behavior that could indicate a virus, which is
useful if software is infected by a virus not yet on the database)
They quarantine files or programs which are possibly infected and
▪ allow the virus to be automatically deleted, or
▪ allow the user to make the decision about deletion (it is possible that the user knows that the file
or program is not infected by a virus – this is known as a false positive and is one of the drawbacks
of antivirus software)
Antivirus software needs to be kept up to date since new viruses are constantly being discovered
Full system checks need to be carried out regularly (once a week, for example), since some viruses
lie dormant and would only be picked up by this full system scan
Biometrics:
✓ Use of unique human characteristics to identify a user (such as fingerprints or face recognition)
✓ In an attempt to stay one step ahead of hackers and malware writers, many modern computer devices
use biometrics as part of the password system
✓ Biometrics rely on the unique characteristics of human beings
✓ Examples include fingerprint scans, retina scans (pattern of blood capillary structure), face recognition
and voice recognition
✓ Mobile phones use biometrics to identify if the phone user is the owner
Fingerprint Scans:
❖ Images of fingerprints are compared against previously scanned fingerprints stored in a database
❖ If they match then access is allowed
❖ The system compares patterns of ‘ridges’ and ‘valleys’ which are fairly unique (accuracy is about 1
in 500)
Retina Scans:
➢ Retina scans use infra-red to scan the unique pattern of blood vessels in the
retina (at the back of the eye)
➢ It requires a person to stay still for 10 to 15 seconds while the scan takes place
➢ It is very secure since nobody has yet found a way to duplicate the blood
vessels patterns’ (accuracy is about 1 in 10 million)
Auditing:
❖ Logging all actions / changes to the system
❖ In order to identify any unauthorized use
Application / OS security:
➢ Applying regular updates to all software and ensuring latest security patches are being used
➢ Finding, fixing and preventing security vulnerabilities in any installed application
Physical Measures:
–
–
–
–
Locks
Guards / Employees
Sealed Room
CCTVs
84
Page 94 of 131
Chapter 7: Ethics and Ownership
Legal: relating to, or permissible by, law
Morality: an understanding of the difference between right and wrong, often founded in personal beliefs
Ethics: moral principles governing an individual’s or organization’s behavior, such as a code of conduct
Culture: the attitudes, values and practices shared by a group of people/society
Intellectual property rights: rules governing an individual’s ownership of their own creations or ideas,
prohibiting the copying of, for example, software without the owner’s permission
Privacy: the right to keep personal information and data secret and for it to not be unwillingly accessed or
shared through, for example, hacking
Plagiarism: the act of taking another person’s work and claiming it as one’s own
Legal, moral, ethical and Cultural Implications:
The following definitions are important when considering ethical behavior:
➢ Legal covers the law, whether or not an action is punishable by law
➢ Morality concerns questions of right and wrong, and is more often thought of in relation to personal or
individual choices
➢ Ethics also concerns questions of right and wrong, but is more often used in a professional context
➢ Culture refers to the attitudes, values and practices shared by a society or group of people
✓ Anything which breaks the law is termed illegal
✓ Examples include copying software and then selling it without the permission of the copyright holders
•
•
•
•
•
•
Immoral does not mean something is illegal (and vice versa)
Creating a fake news website, for example, is not illegal, but it may be considered immoral if it causes
distress to others
If the creator tried to obtain personal and financial data, then it would be become an illegal act
Similarly, hacking is generally regarded as immoral, but not illegal
However, it becomes illegal if it compromises national security, or results in financial gain, or reveals
personal information, for example
In short, there is a fine line between an immoral act and an illegal act
❖ Unethical behaviour is the breaking of a code of conduct
❖ For example, if somebody works for a software company and passes on some ideas to a rival company,
this would be regarded as unethical behaviour
❖ If the software is related to national security or is formally copyrighted, then it is also illegal
❖ It is essential to be clear whether any law has been broken
❖ The importance of culture is less tangible
❖ When writing computer games, for example, programmers need to be careful that they do not include
items which some cultures would find offensive or obscene
❖ Again, this may not be unethical or illegal, but could still cause distress
❖ It is important to realise that boundaries can easily be crossed; in some countries making fun of religion,
for example, is illegal
Computer Ethics:
A set of principles set out to regulate the use of computers
3 factors are considered:
❖ Intellectual property rights, for example, copying of software without the permission of the owner
❖ Privacy issues, for example, hacking or any illegal access to another person’s personal data
❖ Effect of computers on society, for example, job losses, social impacts, and so on
➢ Internet use has led to an increase in plagiarism – this is when a person takes another person’s idea
or work and claims it was their own
➢ While it is fine to quote another person’s idea, it is essential that some acknowledgement is made so
that the originator of the idea or work is known to others
➢ This can be done by a series of references at the end of a document or footnotes on each page where
a reference needs to be made
➢ Software exists that can scan text and then look for examples of plagiarism by searching web pages
on the internet
94
Page 104 of 131
Name and describe levels of schema of database:
•
External Schema
❖
•
Conceptual Schema
❖
❖
•
Describe the views which users of the database might have
Describes the data as seen by the applications marking use of the DBMS
Logical Schema
❖
❖
❖
❖
❖
❖
•
The individual’s view of database
Describe how the relationships will be implemented in the logic structure of the database.
the overview of a database structure
models the problem / situation
... by using methods such as an ER diagram
independent of any particular DBMS
a data model for a specific database that is independent of the DBMS used to build that database
Physical / Internal Schema
❖
Describes how the data will be stored on the physical media
The ways developer interface of a DBMS will help teacher set up the database
- Reports can be made to output data, e.g marks of a student
- Forms can be set up to input data
- Objects such as drop down can be used to make input easier
- A menu can be added to select options for different actions
Developer Interface
To create user friendly features e.g., forms
To create outputs e.g. reports for a given date
To create interactive features eg. dropdowns
104
Page 114 of 131
Queue:
❖ A list containing several items operating on the first in, first out (FIFO) principle
❖ Items can be added to the queue (enqueue) and removed from the queue (dequeue)
❖ The first item added to a queue is the first item to be removed from the queue
Features:
❖
❖
❖
❖
❖
❖
Each queue element contains one data item
A Pointer to the front / start of the queue
A Pointer to the back / end of the queue
Data is added at back / end and removed from front / start // works on a FIFO basis
May be circular
When front and end pointer have same value, there is only one data item in queue
2 Operations and their checks:
Operation: Add an item / Enqueue
Check: There are unused elements in the array // The queue is not full
Operation: Remove an item / Dequeue
Check: There are items in the array // The queue is not empty
➢
➢
➢
➢
➢
➢
Items are removed from the front (front pointer) and added from the back (rear pointer)
Value of the front pointer changes after dequeue
Value of the rear pointer changes after enqueue
A queue can be implemented using an array and a set of pointers
As an array has a finite size, the queue may become full and this condition must be allowed for
Position of a queue changes after manipulation so it is a circular queue
Manipulation:
❖ Before adding, location of point is changed, then item is stored
❖ For removing, first item is deleted, then location of pointer is changed
114
Page 124 of 131
Structure Chart:
❖ A modelling tool used to decompose a problem into a set of sub-tasks
❖ It shows the hierarchy or structure of the different modules and how they connect and interact with
each other
❖ Module relationships / hierarchy / selection / repetition // how a problem is broken down
❖ The parameters that are passed between the sub-tasks / modules // whether a module is a function or
a procedure
❖
❖
❖
❖
A filled black arrow means the program is a function with a Boolean return
A non-filled black arrow means the program is function with a different return
A double sided arrow means By Reference Procedure
Repeatedly means a u-turn arrow around the procedure
124