AD Computer Science Flashcards
1
Q
What is a digital computer?
A
a computer that accepts and processes data that has been converted into binary numbers.
2
Q
What is binary and why do we use it?
A
These are transistors. They are tiny electrical components that form the basis of all digital computing devices. They have two distinct states: 1 - On 0 - Off
3
Q
What kind of data do we store?
A
Text, image, audio, numbers.
4
Q
Bits
A
A bit is the smallest unit of information that can be processed inside a computer.
A bit can hold one of two symbols, 0 or 1.
`
5
Q
How many patterns?
A
2ⁿ
1 bit - 2 values = 0, 1.
3 bits - 8 values = 000,001,011,100,110,010,111,011.
6
Q
Nibbles
A
A set of 4 bits is called a nibble.
A single nibble can store 16 unique values (2⁴) - the denary numbers 0 - 15.
7
Q
Bytes
A
Two nibbles make a byte.
A byte is 8 binary digits.
Is the standard unit of measurement in digital computing.
Represents 256 values (0-255).
8
Q
Most significant bit
A
Refers to the leftmost bit.
Has the largest place value.
9
Q
Rules for binary addition
A
0 + 0 = 0
0 + 1 = 1
1 + 1 = 0 carry 1
1 + 1 + 1 = 1 carry 1
10
Q
Overflow error
A
Overflow error means that the result of the calculation is too large for the space allocated.
The way a computer handles this will depend on how its set up:
It could crash and report an overflow error.
It could truncate the answer.
It could wrap the number around back to 0.
11
Q
Sign and magnitude
A
The most significant bit is the ‘sign bit’.
1= minus
0 = positive
12
Q
Problems with sign and magnitude
A
Waste one binary code.
Both 1000 0000 and 0000 0000 represent 0.
Addition doesn’t always work.
13
Q
Two’s complement
A
The MSB is a minus number as well as a sign bit.
The largest positive number that can be represented is +127 (0111 1111)
The largest negative number that can be represented is -128 (1000 0000)
If it’s positive, make sure to write the + sign.
14
Q
Denary to binary (negative numbers)
A
Convert the unsigned equivalent into 8 bit binary Flip the bit (each 1 becomes a 0 and vice versa) Add 1 (arithmetically, not appending)
15
Q
Left shifting
A
Logical left shifting
MSB is shifted out of the pattern.
Each remaining bit is shifted left.
A new 0 is placed into all vacated spot
Arithmetic left shifting
Same as logical left shifting
16
Q
Right shifting
A
Logical right shifting
A 0 is always inserted into the vacated MSB.
A 0 is always placed into all vacated spots
Arithmetic right shifting
The MSB is always maintained and copied into all vacated spaces.
17
Q
Binary to Hexadecimal
A
Binary to Hexadecimal
0110 - 1111
6 - 15
6 - F
18
Q
Hexadecimal to Binary
A
B - E
11 - 14
1011 - 1110
BE - 1011 1110
19
Q
Denary to hexadecimal
A
Denary to Hexadecimal
58₁₀
16 - 1
3 - 10
3 - A
20
Q
Hexadecimal to denary
A
Hexadecimal to denary
27₁₆
16 - 1
2 - 7
(32) - (7)
39₁₀
21
Q
ASCII
A
One way to encode binary to store text is to use ASCII.
ASCII is the American Standard Code for Information Interchange.
Why is it necessary?
Imagine if everyone used a different encoding scheme for text.
They would not be able to communicate.
ASCII enables computing devices to communicate with another and to translate their communication into identical information.
Bit patterns
ASCII uses 7 bits.
128 patterns.
A - 65 - 0100 0001
a - 97 - 0110 0001
22
Q
Limitations of ASCII
A
ASCII represents 128 unique characters.
ASCII is suitable for English (and languages that use the same characters) but it cannot represent languages that have additional different characters.
French, German and Mandarin are examples of languages that do not fit into ASCII.
For these languages Unicode was created.
The first version is able to represent 120,000 characters such as:
÷ x ᵀᴹ ©
23
Q
Von Neumann Architecture
A
Every computer is made up of:
A processing unit that contains an ALU and processor registers.
A control unit that contains an instruction register and program counter register,
Memory that stores both data and instructions.
External data storage.
Input and output mechanisms.
24
Q
Fetch
A
CPU fetches data and instructions from RAM.
CPU stores in its own memory called registers.
CPU uses address bus.
The memory address of the next item in the CPU wants is put onto the address bus.
Data from this memory travels from RAM to CPU on the data bus.
25
Decode
The CPU works out what the instruction it just fetched actually means.
The control unit decodes the instruction and gets things ready for the next step.
It does this by looking up the instructions from the instruction set.
This is the full list of operations that a microprocessor can carry out.
26
Execute
Data processing happens.
Instructions are carried out on the data,
Some instructions are carried out by the ALU.
27
Fetch - Decode - Execute summary
Instructions and data are stored in memory.
Instructions are fetched one at a time into the processor.
The instructions are decoded by the control unit.
The instructions are executed, sometimes using the ALU.
28
Explain why programmable computers were considered a huge step forward in digital computing.
Mechanical computers could only do one task.
29
The term ‘stored program concept’ is used to describe modern computers. State why this term is accurate.
Modern computers allow data and instructions to be stored in the same memory.
30
Every processor has a unique instruction set. Define what is meant by an ‘instruction set’.
Set of instructions that can be recognized and executed by the CPU.
31
Give three examples of operations that would be executed in the ALU.
Addition, subtraction, multiplication and division.
32
State what goes onto the address bus during the fetch-decode-execute cycle.
Memory address of location in RAM.
33
State two items that can go onto the data bus, during the fetch-decode-execute cycle.
Data and instructions
34
Fetch - decode - execute
Fetch - The CPU fetches data and instructions from the RAM.
decode - CPU working out what the instructions it fetched actually means.
execute - Execute
Data is processed
35
Central Processing Unit
The CPU is the brain of the computer.
It is the hardware component responsible for all the processing that the computer carries out.
Its job is to process data.
Processing means: searching, sorting, calculating and decision-making.
36
Control Unit
The control unit receives signals from other parts of the computer system and sends signals to them.
It is responsible for handling hardware interrupts.
It is responsible for fetching, decoding and executing instructions.
It has the most responsibility.
It is the powerhouse of the CPU.
37
Arithmetic logic unit
```
Performs actual operations on data.
Arithmetic: + / = *
Logic: AND, OR, NOT
It is able to compare numbers against 0.
It can test if two numbers are equal.
It uses logic gates in combination to perform operations.
```
38
Registers
Also known as immediate access storage.
Registers are a type of memory that is extremely fast.
Each type of processor has different registers that are designed to hold different information.
Most processors have:
An instruction register - holds instructions currently being executed by the CPU.
An accumulator - holds the accumulated result of operations carried out by the ALU.
A program counter - holds the memory address of the next instruction to be executed.
39
Clock
A clock chip determines the speed of the CPU.
This chip uses vibrating crystals that maintain a constant rate of vibration.
The speed of the clock is measured in Hertz (Hz), which is the amount of cycles per second.
Current computers have a clock speed of 3GHz, which means 3 billion cycles per second.
Each ‘tick’ means that one part of the fetch,decode,execute cycle can be carried out.
40
pipelining
Pipeline is a method of keeping all the components busy to improve overall CPU performance.
Pipelining attempts to keep every component busy at all times.
Instructions flow through the CPU in stages.
This means that an instruction can be completed every clock cycle.
This increases CPU throughput allowing overall performance to be outputted.
A CPU is fully ‘pipelined’ if an instruction can be fetched every clock cycle.
41
Clock Definition
An electronic device that ‘ticks’ at regular intervals and is used to synchronise the actions of the other components of the CPU.
42
Hertz Definition
A measure of clock speed. One Hertz equals one cycle per second.
43
Pipelining definition
A way of improving the performance of the CPU. The stages of the fetch-decode-execute cycle are overlapped. While the CPU is executing one instruction, it is decoding a second and fetching a third one from memory.
44
Instructions Definition
An operation that the CPU carries out.
45
Fetch Decode Execute Definition
The cycle followed in the CPU to retrieve an instruction or data from memory, look it up in an instruction set to determine its meaning, and then enact the operation.
46
Control Unit
The unit that decodes the instruction and coordinates all the actions of the other components.`
47
Arithmetic logic unit
Carries out arithmetic operations (add, shift) and Boolean logic operations (AND, OR, NOT)
48
RAM Definition
The memory where the instructions and data are stored for programs.
49
Instructions and Data
In a computer, everything is represented in binary.
This includes the instructions that the CPU needs to carry out.
Instructions and data are stored in RAM until they are needed by the CPU.
Each instruction and item of data is a location in memory.
Each element of the memory has a unique address.
This means that each instruction has an address.
The first memory element that contains the instructions is the address of the instruction.
50
Buses
The CPU is connected to other components using a physical connection called a bus.
They are known as buses as they carry a number of bits simultaneously.
51
Data bus
Carries binary data from component to component.
For example an instruction from RAM that is being transferred to the CPU.
Bidirectional
Two way data transfer is required when reading and writing data to memory.
52
Control Bus
Carries signals that control the CPU components.
For example a signal to start fetching the next instruction
Bidirectional
It needs to carry status information to devices and to carry the status information signals that devices send back.
53
Address bus
Carries the address of a memory location.
For example the address of an instruction being fetched from memory.
Unidirectionality
The CPU can point to the addresses in memory or to other devices, however those hardware components cannot point to an address in the CPU.
54
Address bus
32-bit processors are cheaper, but can only use a limited amount of RAM.
The ‘bandwidth’ of these CPUs is related to the size of the address bus.
Bandwidth - the capacity of information that can be transferred in a second.
It refers to the number of unique memory addresses that can be accessed.
A CPU with a 4 bit bandwidth
4 bit addresses range from 0000 to 1111 inclusive.
1111 in denary is 15, which gives 16 addresses (including 0).
2⁴ = 16 addresses
55
Primary storage (memory)
A 32-bit CPU can access 4,294,967,296 unique memory locations.
Each memory location consists of a row of cells.
If the row is 8 cells long, each 4,294,967,296 different addresses points to a row of 8 bits (1 byte).
That’s why a 32 bit CPU can only use 4GiB of RAM which means it can hold 4 billion memory locations (addresses).
56
Memory (RAM)
Random Access Memory is volatile memory.
This means its contents are erased when its power supply is turned off.
RAM is often referred to as ‘primary storage’ as it sits close to the CPU.
It's necessary to have somewhere to store the program and data when the power is switched off.
The non-volatile storage required for this job is most often a hard disk.
Hard disks are often referred to as ‘secondary storage’ to make them distinct from ‘primary storage’.
57
Control, address and data bus
Control bus
Transports signals that control the CPU and other parts of the computer system.
Bidirectional
It needs to carry status information to devices and to carry the status information signals that devices send back
Address bus
Carries the address of memory locations used to store data and program instructions.
Unidirectionality
The CPU can point to the addresses in memory or to other devices, however those hardware components cannot point to an address in the CPU.
Data bus
Transfers the binary data and instructions around the computer.
Bidirectional
Two way data transfer is required when reading and writing data to memory.
58
Give one reason why RAM is called ‘primary storage’.
RAM sits very close to the Central Processing Unit (CPU). It is very fast and can therefore store running programs and data.
59
Give one reason why a hard disk is called ‘secondary storage’
A hard disk sits further away from the CPU than RAM. It is not as fast as RAM and is used for storing programs and data that are not in execution.
60
General purpose devices
You can use the device for multiple purposes.
Most of the computers we use are general purpose.
New software can be downloaded and installed to give additional functionality.
61
Secondary storage
Making sure you can store and receive data.
In everyday computer use, you need a way to write data and store data when your computer is switched off.
You also need a way to retrieve data the next time you switch it on.
62
Magnetic storage
Magnetic media are coated with a substance that can be magnetised.
Magnetic storage technology works by magnetising parts of this substance as north and south poles to represent binary 1s and 0s.
Examples include: hard disk drive, tape drive and cassette.
63
Optical storage
Optical media consists of a platter with a flat reflective surface.
Optical storage technology works by using a laser to burn ‘pits’ into the flat surface.
The reflective areas between pits are known as ‘lands’.
A laser is directed onto the medium’s surface.
When it hits land, a beam reflects (this is read as binary 1).
When it hits a pit, the beam does not reflect (this is read as binary 0).
Examples: CD and DVD.
64
Solid state / flash drive
Solid state media consists of silicon chips that feature a special kind of transistor called NAND flash.
This type of media is called ‘solid state’ because it has no moving parts.
Solid state technology works by using a large electric current to force electrons through a barrier trapping them in positions called pools.
A full pool - 0.
An empty pool - 1.
Examples include: SSD card, USB, SD memory card and flash storage.
65
Advantages and disadvantages of optical storage
ADVANATGES
Cheap to produce
Thin and lightweight
Portable
DISADVANTAGES
Slow to access
Low capacity
Prone to scratches
66
Advantages and disadvantages of magnetic media
ADVANTAGES
High capacity
Quick to access,
Providing the disk is defragmented periodically
DISADVANTAGES
Has moving parts that will eventually fail
67
Advantages and disadvantages of solid state storage
```
ADVANTAGES
Quick to access
No moving parts so very reliable.
Low power requirements
Quite
No need to defragment
```
DISADVANTAGES
Expensive.
Has a limited number of read write cycles
68
Explain why secondary storage is necessary.
RAM is volatile. It loses its contents when the power is switched off. Secondary storage provides non-volatile storage that can be used to write and rewrite data, so that it does not vanish when the power is turned off.
69
Define the term ‘general purpose device’.
A computing device that can have different application programs loaded into it to provide different or enhanced functionality. For example, a smartphone can also be used as a calculator and a camera.
70
Capacity, portability and speed
Capacity: how much data can the device store?
Portability: can the device be moved around without damaging it or the data it contains?
Speed: how fast can data be read from and written to the device?
71
Units
```
bit
byte
kibibyte
mebibyte
gibibyte
tebibyte
```
```
B (byte)
K (thousand)
M (million)
G (gazillion)
T (trillion)
```
72
Media students have taken part in a national competition. Each has produced a one-hour high-definition video of an original screen play. The judges for the competition are in different parts of the country and each will do a pre-screening of a subset of entries. They are willing to accept any type of media. The competition sponsors have asked that all submissions be sent through the post, directly to one of the judges.
Name and give a justification for a way to send the submission by physical post to the judge.
The file size of the video is likely to be about 3-4 GiB. The capacity of a DVD is more than sufficient to hold such a video. DVDs are cheap to buy and strong enough to be sent through the post. Alternatively, students could use a high-capacity flash drive (memory stick). The advantage of using a flash drive is that there would be no need to have a dedicated peripheral device to record/view the video. However, using a memory stick for this purpose would be considerably more expensive than using a DVD. There is also a greater likelihood of a memory stick being damaged in transit.
73
A group of students is working on a monthly college newsletter. Due to other activities, four of the group did not have time to complete their sections at school, so must complete them at home. Unfortunately, the college does not allow access to students’ file storage from home.
Name and give a justification for a way that the students can carry the newsletter files and finish their sections of the newsletter at home.
A flash drive (memory stick) would be the best way to take the newsletter files home. Flash drives are lightweight and portable and would have sufficient capacity to hold the files. Most laptops, even old ones, have USB ports where a USB drive can be inserted, so no other device is required.
Files can then be worked on either directly on the USB drive or uploaded to the hard disk of each student’s laptop. Files can be copied back to the USB drive when they have been updated and they need to be taken back to school.
74
What makes a computer
What makes a computer
- Software programs are made up of data and instructions.
- Software programs are stored in secondary storage which is persistent but slow to read and write.
- Programs are copied into the memory (which is volatile) but have a high read and write speed.
- Then the CPU performs the fetch decode execute cycle to carry out each instruction.
75
Software
- Two types: system and application
- System software - programs that enable a computer to function. They include the operating system and utility programs.
- Application software - programs that the user interacts with most.
76
OS
The Operating System
-Its main function is to provide a platform for other software, so that this software can interact with the computer hardware.
What is an operating system?
Its job is to provide an interface between the hardware and other software, and the user.
77
Operating system responsibilities
1) Process management: organising instructions in memory so that they can be processed.
2) File management: organising the contents of secondary storage.
3) User management: providing a user interface and access levels.
4) Peripheral management: interfacing with device drivers to control physical devices.
78
OS definition
An operating system (OS) is system software that enables a computer’s hardware to communicate effectively with its software.
It provides a platform on which application software can run and an interface for users to interact with a computer.
79
Heiracgy
Hierarchy
- Most operating systems use a hierarchy of directories to organise all of the stored files.
- A directory refers to a folder containing other items- those can also be directories (sub-directories) or files.
80
Allocating space
- The OS is responsible for allocating space on the hard disk.
- Files are broken into blocks.
- Each block of a file is placed into an empty sector. Blocks may not be placed into contiguous sectors.
- If the whole sector is not filled, then wasted (slack) space is left.
81
Q: A file is 5.4MiB.
The block size of the disk is 512 bytes.
How many blocks are needed to store the
file?
A: ⌈ (5.4 × 1024 × 1024) ÷ 512 ⌉
82
Q: A file is stored in 137 blocks.
The block size of the disk is 2048 bytes.
What is the maximum size of the data in
the file in kibibytes?
A: (137 × 2048) ÷ 1024
83
Q: A file is 1.2 GiB.
The block size of the disk is 1KiB.
How many blocks are needed to store the
file?
A: ⌈ (1.2 × 1024 × 1024 × 1024) ÷ 1024 ⌉
84
Q: A file is stored in 83 blocks.
The block size of the disk is 1KiB.
What is the maximum size of the data in
the file in bytes?
A: 83 × 1024
85
Processes
- When a program is run, it is copied from the secondary storage into the memory so that it can be executed by the CPU.
- Programs in the memory that are being executed (or waiting) are called processes
86
Multitasking
Multitasking
- Modern CPUs and OS are able to make multiple processes available at the same time.
- The OS is responsible for ensuring that all processes are able to execute by allocating a fair share of time with the hardware.
- A CPU with one core - allows one instruction in one process at a time.
- A CPU with multiple cores - allows one instruction in one process at a time on each core.
87
Memory
Memory
-The CPU is not the only hardware a process accesses.
-The other major component that processes need to use is the computer’s memory.
-The size of any memory is finite, so the OS needs an efficient way of using it.
- When a process is loaded into memory, the OS assigns it a series of addresses to use.
These blocks of addresses are called pages.
-Management of these pages is often done with a paging algorithm
88
Virtual memory
- When many processes are running a computer's RAM may become full.
- When this happens, the OS needs to temporarily move some of the contents of the RAM to a space on the hard drive.
- The part of the hard drive designated as an extension to the main memory is called virtual memory.
- The OS moves the pages of memory associated with a process in RAM to the hard disk therefore freeing up memory.
- When the process is ready to be executed again, the OS moves the page from virtual memory back into RAM.
89
First In First Out
This processes the queue in the order that items arrive.
ADVANTAGES
- Low overhead - task switching only happens when a process has been completed.
- Processes are never starved of CPU time.
DISADVANTAGE
- Throughput may be low as longer processes may cause wait time to be high.
- Response and completion times depend on the state of the queue.
- There is no prioritisation of tasks.
90
Shortest Jib First
The process which is closest to being completed goes first.
ADVANTAGES:
- Average wait time is low.
- Response time for short processes is very fast.
- Lower overhead as there may be less switching.
DISADVANATGES
- Longer processes may be starved of processing time.
- Execution time must be known ahead of time.
91
Round Robin
Processes are allocated a time slice.
If the process is not complete at the end of its time, it returns to the queue.
Higher priority tasks are given more time slices.
ADVANTAGES
- Balanced throughput (number of processes completed over time).
- Average response time is good if the number of processes is high.
- Processes can never be starved of access to hardware.
DISADVANTGES
- There is some overhead (wasted time) involved in switching between processes.
- Processes may have to wait a long time to complete, especially if there are many waiting.
92
peripheral device
- The word peripheral means ‘on the edge’ or ‘secondary to something else’.
- Peripheral devices are additional hardware connected to a computer.
- Peripherals can be inside or outside the computer case itself.
93
Devices speaking the sam elanguage
- Devices have specialist pieces of software called drivers that are installed on the hard disk.
- The driver provides an interface between the peripheral device and the OS so they are able to communicate.
94
Graphical user interface (GUIs)
Most GUIs use the WIMP format.
Windows - programs that open in frames that can be moved around and overlaid.
Icons - pictures to represent programs you can execute or actions you can do.
Menus - lists of options to choose from
Pointer - using a mouse to click on items.
95
Command line interfaces (CLIs)
-CLIs are commonly used by people who need to control the function of a computer, server or network.
-There are no icons or pointers in a CLI.
The user can only interact with the computer by typing in commands to be executed.
-CLIs give the user more control over the function of the computer.
-It's quicker.
96
GUI
Features - Windows, icons, menus, and pointers.
Advantages - Intuitive, as icons (pictures)
represent applications or tasks.
Little if any interaction with the keyboard, so the user does not need to know how to type.
Protects the machine itself by
limiting what the user has access
to.
Disadvantages -Competes for system resources such as memory and CPU.
97
CLI
Features - User types keyword commands
on a blank screen.
Advantages -Can access complex system-
level commands.
Allows greater productivity, as a user can normally input more quickly typing than with a mouse.
Uses fewer system resources than GUI.
Disadvantages - User needs to learn the key commands.
98
Utility software
- Adds functionality to a computer, improves its performance, or helps maintain its hardware or other software.
- Performs specific management tasks such as backup and recovery, disk cleanup, compression or archiving.
99
File repair/recovery software:
for when files may have become corrupted due to malfunction if cyberattack.
100
Backup/recovery software:
to save a copy of the system in another place in cases of hardware failure or attack, so that the most recent state of the system can be recovered later
101
File compression software
to reduce file sizes in order to save space on a hard drive or for transmission.
102
Disk defragmentation software
to move fragmented files into contiguous space on the disk in order to free up larger blocks of continuous space and speed up file access.
103
When opening a file, the application shows a message indicating that the file might be
corrupt.
Repair utility – used to recover as much of the information from the corrupt file as is
possible.
104
A teacher accidentally deleted the master copy of the student database this morning.
Recovery utility – used to restore a file from a backed-up copy or from the computer’s rubbish bin.
105
A group of students is working on a group programming project.They want to ensure that none of their work is lost.
Backup utility – so that copies of their work are saved frequently and can be recovered in the event of loss or corruption.
106
A student needs to email coursework, consisting of word-processed documents and data in spreadsheets, to a teacher.
Compression utility – so that the size of the files can be reduced. This will allow the files
to transmit more quickly and will ensure they meet any restrictions on the size of
attachments that can be sent with emails.
107
When opening a slide presentation, the hard disk light flashes for a long time before the presentation is ready to edit.
Disk defragmentation utility – so that the file blocks for a single file can be moved closer to each other. This should make it quicker to access them.
108
File repair
To repair and
recover data from a
file that has
become corrupted.
```
It scans the damaged
file, extracts as much
data from it as possible
and stores it in a new
usable file.
```
109
Backup and recovery
To backup data and recover it in the event of data loss or damage
Backup copies of files are made at regular intervals and stored on a separate
device in a different location to the live working environment – possibly in the cloud.
110
Data compression
To reduce the size of a file so as to free up storage space and speed up data transfer across networks.
Repackages (lossless) or removes(lossy) some of a file’s data to reduce its size.
111
Disk defragmentation
To speed up disk access and improve performance.
Files stored on a hard drive are rearranged so that they are stored in contiguous blocks rather than being
spread out across the disk.
112
file encryption
To improve the security of a file so that it is impossible for unauthorised people to decode and read its contents
Uses an algorithm to
scramble the data in a
file. Only a recipient
with the correct key is able to unscramble it.
113
antimalware
To protect computer systems and data from
damage caused by malware
Scans for malware using a database of known malware definitions (signatures).
If it detects a file that matches a definition in its database, it deletes or quarantines it.
The database has to be kept up-to-date so that it can detect new malware that is released.
113
antimalware
To protect computer systems and data from
damage caused by malware
Scans for malware using a database of known malware definitions (signatures).
If it detects a file that matches a definition in its database, it deletes or quarantines it.
The database has to be kept up-to-date so that it can detect new malware that is released.
114
Cyberattack
Cyberattacks are malicious acts in which a computer system comes under attack by unauthorised persons (hackers).
115
malware
Malware is any software that is hostile, malicious or intrusive.
116
anti malware
- This is software designed to detect, quarantine and destroy potentially dangerous programs.
- When a file enters the system, anti- malware software will scan the file for known malicious code.
- If it finds a problem, the malicious program is quarantined so that it cannot affect the system.
- The user is then notified and can decide where to let it in or delete it.
117
Ransomware
- Form of malware that encrypts a victims files.
- The attacker demands money to restore access to the data.
- Users are shown instructions on how to pay a fee to get the decryption key.
118
Describe how anti-malware works with signature databases.
- Any file coming into the computer is scanned.
- The file contents are compared to a database of virus signatures.
- If any of the signature patterns are found in the file, it's identified as a virus.
119
Static heuristic analysis
This involves reverse engineering (decompiling) the code and inspecting the source code.
- This is compared to the code of known viruses.
- If enough of the code matches, it's identified as a virus.
120
Dynamic heuristic analysis
- The program is run in an isolated, controlled environment known as 'SANDBOX'.
- The analysis looks at each command that the program runs.
- It it identified as suspicious behaviour common to viruses, then the program is identified as a virus.
121
Explain why heuristic analysis is so valuable for the identification of viruses
Heuristic analysis can catch very new viruses because it does not rely on updating a signature database.
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Heuristic analysis may not be 100% reliable
Heuristic analysis may not be 100% reliable
May identify a false positive or may not identify a threat at all.
In static heuristic analysis:
-A valid program may contain binary code that just happens to match part of the virus.
-The valid program will be identified as a virus.
In dynamic heuristic analysis
-A virus may delay actioning its malicious intent.
- It may therefore be let out of the sandbox before it has executed the part of its code that infects the machine.
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Hackers
A hacker is a person who attempts to gain or gains unauthorized access to a computer system.
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Motivations
```
Money
Desire to damage an organisation
Political motivation
Spying or corporate espionage
Help an organisation
Just for fun
```
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Patches
Patches are security fixes for existing software. The vulnerabilities that are fixed are often common knowledge or easily discovered.
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Firewalls
Firewalls are software.
They are designed to monitor and block incoming and outgoing connections to a network.
Firewalls protect a system from hackers.
If a hacker is already inside the system, they may be prevented from doing extra damage by the firewall blocking outgoing traffic
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ethical hacking
Hacking in order to help organisations protect themselves from malicious hackers.
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penetration testing
Ethical hackers may be employed to break into systems in order to identify security weaknesses..
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Social engineering
Social engineering is practised on people, not computers.
| Hackers (social engineers) exploit human behaviour, fallibility and goodwill to gain confidential information.
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Protecting against social engineering
AUP - Acceptable Use Policy
An AUP is a collection of rules and procedures that employees are required to follow in order to protect their organisation's system and networks.
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Common AUPs
limits on access to the internet
preventing user downloading files or installing software
preventing user plugging in external devices (USB sticks )
training users to recognise phishing emails
multi-factor authentication
training users not to give out confidential information over the phone
preventing user from removing files from the premises.
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Phishing
Phishing uses an email and fake website to trick users into giving away personal data. A phishing email pretends to be a real message from a known business or organisation.
It entices the recipient to visit a fake website. Once on the website, they’ll be asked to enter their personal details.
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Pretexting
Pretexting (blagging)
The attacker invents a pretext for contacting an employee over the phone, pretending to be from a known organisation.
They will engage the employee in conversation in order to find out personal information about them or sensitive information about a customer.
They later use this information to gain something or sell it on to other criminals. These types of attack may be targeted using research done through social networking sites.
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Baiting
Baiting attacks exploit people’s natural curiosity to gain information.
They may offer something free or exclusive to manipulate the user.
Often, the attack results in the user
downloading malware that is then run on their machine.
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Quid pro quo
A quid pro quo attack exploits a person’s belief that they can get something of value for free.
Malware is presented as a freebie, a giveaway, or as some other kind of reward.
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Encryption
Encryption is the process of converting data into an illegible, scramble format so that it cannot be understood by unauthorised parties.
Data may still be accessed and read by the hacker, but will be understandable.
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Backup and recovery
Backing up involves taking a copy of the data and storing it in a different location.
Recovery is the process of using a back-up copy of data to restore a database or system to the correct state when a failure occurs.
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Encryption and decryption
Encryption
Scrambling or changing the original data so that only the
intended recipient can decode and read it.
Decryption
Unscrambling encrypted data so that it is readable and understandable.
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Key
Key
| The secret password for unlocking an encrypted message.
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Symmetric encryption
| Asymmetric encryption
Symmetric encryption
Both the sender and the receiver share the same key to
encrypt and decrypt the data.
Asymmetric encryption
A public key, which can be given to anybody, is used to
encrypt the data.
A private key, which only the recipient knows, is used to
decrypt the encrypted message.
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Backup
Backup
This is a copy of data or files. Backups are stored on drives or servers far from the original.
Full backup
This type of backup makes a copy of everything on the
system, regardless of when it was created or if it has
changed since the last backup.
Incremental backup
This type of backup only makes a copy of new files and
files that have changed since the last incremental or full
backup was made.
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Recovery
Recovery
Recovery is the process of copying backed-up data onto
the live system.
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Access control
Access control
Determines the facilities that each user can engage with
or access.
A network manager restricts each user’s access so they are only allowed to see what they need to see.
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Physical security
This involves actually restricting physical access to parts
of the computer network. Servers are usually kept in
locked rooms. These rooms sometimes have key cards or
biometric access systems.
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Vulnerabilities
Vulnerabilities are flows in code that introduce security issues into software.
Hackers use these vulnerabilities to break into computer systems
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```
Bad practises
Poorly planned design
Quick fixes for problems
Not using industry coding standards
Poorly structured code
Insufficient testing
```
```
Bad practises
Poorly planned design
Quick fixes for problems
Not using industry coding standards
Poorly structured code
Insufficient testing
```
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Fixing bad practises
Code reviews can reduce some of these vulnerabilities.
Automated reviews can be used to ensure programmers adhere to consistent standards.
Programmers can review each other's code to make sure it doesn't have any flows.
Audit trails improve accountability by keeping track of who made what changes during the development process.
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Define the term ‘robust software’.
-Software that is designed to handle untoward occurrences without crashing or producing unexpected output.
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Explain how hackers exploit a codes vulnerability
-Vulnerabilities introduce security flows.
-Hackers are able to use these flows to break into computer systems.
They can insert their own code to do this.
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Explain how code reviews and audit trails help to identify vulnerabilities.
- Allow flaws to be monitored and tracked back to where they were introduced.
- Reviewing code can catch vulnerabilities before a program goes live.
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Give a definition of a code review.
When a developer has finished working on a piece of software, a different developer
looks at the code to identify potential vulnerabilities or poor programming practices.
The reviewer asks a series of questions to ensure that the code is of a good quality
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Give four questions that might be asked in a code review.
Are there obvious errors in the code?
Are all the requirements fully implemented?
Are existing (automated) tests enough for this new code or should new ones be
written?
Does the code conform to the existing style guidelines?
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Give a definition of an automated code review.
Automated software checks source code to make sure it meets a predefined set of
rules for best practice.
It can detect bugs and security issues.
The review generates a list of warnings and may suggest ways to fix them.
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State one reason why automated code reviews are used in preference to code reviews by another developer.
They can be done faster and more efficiently. They can be done by one developer, thereby saving time and cost. They can be used as a first pass check, which can then be followed by a review by another developer.
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Give a definition of an audit trail.
A record of what has been done, who did it, and when it was done.
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Explain the purpose of version control software.
Helps teams manage changes to documents, programs, websites, and other
resources. It keeps track of changes and the order in which they were done
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Give one reason why version control software is important to software developers.
It allows them to keep the code for different releases of the software isolated. This
allows developers to regenerate an old version if bugs are reported in the new
version.
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Describe two approaches to version control.
One approach is to only allow a single developer to work on a file at any one time.
Another allows multiple users to make simultaneous edits.
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How are devices connected to each other
Physical objects that can connect with each other and other systems via the internet.
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What benefits are there to using a connected computer?
Not expensive
Boosts storage capacity.
Makes file sharing easier
Enhances communication
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Categorising a network
Networks can be classified based on their geographical coverage.
Networks that are contained within a single building or site are known as Local Area Networks (LANs).
Networks that are connected over a larger geographical area are known as Wide Area Networks (WANs).
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Why do businesses have WANs?
Why do businesses have WANs?
Big organisations may have their own WANs.
The organisation owns all of the WANs hardware and has full control of the network.
Others cannot use its infrastructure.
This means the network can be very secure.
This is a very expensive option because the infrastructure has to be installed and maintained.
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Virtual Private Network (VPNs)
An organisation can stimulate its own WAN over the internet by using a Virtual Private Network.
A VPN uses encrypted communication channels over the infrastructure of the internet.
A VPN allows an individual to appear to be connected directly to their organisation’s network.
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Connecting globally
Protocols are sets of rules that govern the communication between devices.
For two devices to communicate, they must use the same protocol.
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Finding the device you need.
Another key element needed for communication is an addressing system.
Each device on a network needs a unique address so that connections can be made to it.
IP addresses are used on the internet.
MAC addresses are used on LANs.
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IP addresses
IP addresses
In a WAN, an IP address is used to identify an individual device or network access point.
There are two types, version 4 and version 6.
IPv6 addresses are longer so they can address a greater number of unique devices.
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LAN
Local area network
| a network that covers a relatively small geographical area, often a single site.
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WAN
wide area network
| a network that covers a large geographical area and connects together multiple LANs.
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Network
an arrangement of computers and other digital devices connected together to share resources and data.
170
interent
the worlds largest public WAN.
| it is a global system of interconnected networks, linking together billions of digital devices worldwide.
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Protocol
A set of rules which allow devices to communicate
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Protocol
A set of rules which allow devices to communicate
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What does speed mean?
The term ‘download speed’ refers to how much data can be downloaded per second over a connection.
There are a number of factors that can affect the speed of a connection.
The most important two are the bandwidth and latency.
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bANDWIDTH
Bandwidth is the maximum amount of data that can be transmitted over a connection per second.
It is a measurement of capacity.
It is measured in bits per second (bps).
(Amount of information transferred in a second).
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Latency
Latency is the time taken for a transmission to reach its destination.
It is measured in milliseconds.
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units (bits per second)
Units (per second)
Abbreviation
Bits per second
Bits
bps - 1
Kilobits - Kbps- 1,000
Megabits - Mbps - 1,000,000
gigabits - Gbps - 1,000,000,000
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Calculate transmission times
Time = file size (in bits) / network speed (in bps)
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How long will it take to download a 20MiB file over a 12 Mbps connection?
How long will it take to download a 20MiB file over a 12 Mbps connection?
Convert the file size into bits
20 x 8 x 1-24 x 1024
Convert the speeds to bits per second
12 x 1000 x 1000
Arrange the size and speed expression
(20 x 8 x 1024 x 1024) / (12 x 1000 x 1000)
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Transmitting data
Two methods used to transmit data between digital devices:
Serial
Parallel
Serial data transmission sends data bits one after another over a single channel
Parallel data transmission sends multiple data bits at the same time over multiple channels.
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What is a channel
A channel is used to convey an information signal, for example a digital bitstream, from one or several senders (or transmitters) to one or more received.
A channel has a certain capacity for transmitting information, often measured by its bandwidth or its data bits per second.
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The time estimate for downloading a 22GiB file over a 54Mbps connection
Time = (22 x 8 x 1024 x 1024 x 1024 ) / (54 x 1000 x 1000 )
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The time estimate for downloading a 45KiB file over a 4Gbps connection.
Time = (45 x 8 x 1024) / (4 x 1000 x 1000 x 1000)
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The maximum speed for uploading a 25MiB file in 4 seconds.
Speed = (25 x 1024 x 1024 x 8) / 4
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The maximum file size (MI) that can be uploaded at 12Mbps in 30 second.
Size = time x speed
(30 x 12 x 1000 x 1000) x (8 x 1024 x 1024)
b
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Wired vs wireless
Wired vs wireless
All methods of transmitting data are either wired or wireless.
The difference is simple:
Wired transmission involves making a physical connection from one device to another (USB or ethernet cable).
Wireless transmission uses no physical connection. Instead it used radio waves (Bluetooth, Airdrop and NFC).
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cables
Copper is more common as it was invented over 150 years ago.
| Fibre-optic cables are a modern alternative that has vastly higher bandwidth and lower latency.
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Broadband
Broadband is a type of high-capacity internet connectivity that is all over the country.
FFTC (fibre to the cabinet) - fibre optic cable runs to green street-cabinets. Copper cables then link homes to the cabinets (a short distance).
FTTP (fibre to the premises) - used for businesses. In this approach, the final stretch from the cabinet to the business uses other fibre optic cables.
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Copper cables and fibre optic cables
Copper cables : data is transmitted as electrical pulses.
| Fibre optic cables : data is transmitted as pulses of light.
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Copper cables
```
Copper cable
Four twisted pairs of copper wires enclosed in a protective case.
range - 100 cm
speed - 10 - 1000 mbps
latency - 13 - 14.2 microseconds
usage - LAN
```
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Fibre optic cables
```
Light travels down a fibre optic cable by bouncing off the walls of the cable repeatedly.
range - 2,000 metres
speed - 10 Gbps / 25-300 Mbps
latency - 3.33 microseconds / km
usage - Internet / WAN
```
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A bit of science
Microwaves and radio waves are frequently used to transmit data.
This involves converting the binary data into an electromagnetic wave using a specific protocol.
Examples of the protocols used are Wi-Fi and Bluetooth
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limitations of wireless data
Wireless data transmission has its limits:
Generally the range is limited to a few metres.
Easily interrupted by other signals or physical obstacles.
Has much lower bandwidth than wired connections which makes transfer slow.
Security is a concern as wireless signals can be intercepted by others.
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Wireless access point
In a WLAN, there will be a wireless access point which sends and receives wireless transmissions to and from the other devices.
A WAP is usually part of a box you call the broadband router.
The router will be connected to an internet access point in order to connect the WLAN to the internet.
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Describe how devices are connected on a wireless network.
Wi-Fi uses radio waves to transmit data to and from a wireless access point.
The router, connected to the wireless access point, distributes signals to the devices.
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Organising a network
The way that a LAN is organised is known as its topology.
Bus
Star
Mesh (partial and full)
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Topology security
Bus - not very secure as all devices ‘see’ all the data traffic.
Star - very secure as each transmission is sent directly to the intended recipient.
Mesh - reasonably secure as the traffic takes the shortest route to its destination.
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the internet
The internet's cables (long distance and high bandwidth) travel across the globe are known as the backbone.
Each country generally has its own backbone, which can be linked to others using Network Access Points (NAPs).
Networks are connected to the internet using a Point of Presence (PoPs) provided by an Internet Service Provider.
ISPs are organisations that ‘rent’ bandwidth from telecommunications companies such as BT.
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Bus topology
How it works
•All the nodes are joined to one cable, called ‘the bus’.
•A terminator is fitted at each end to stop signals reflecting back
Advantages
•It’s easy to install extra devices because you only need a connector for each device.
•The cable is inexpensive because there is only a single length.
Disadvantages
•It’s not reliable because when the cable is down, all the devices are down.
•It does not perform well under load, because the more devices that are added the more chance there is of data collisions.
•It may represent a security risk because every node on the bus receives every transmission.
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star topology
How it works
Each node is connected to a central device, either a ‘hub’ or a ‘switch’, which acts as a central point. All communications go through this central point.
A hub broadcasts messages across the whole network, whereas a switch sends the message to the intended recipient only.
Advantages
Failure of one node or its cable does not affect any of the other nodes, because each is separately connected.
It is easy to add new devices because you simply connect them to the central point (switch/hub).
Star networks tend to have higher performance than a bus topology, because each message is passed only to its intended recipient.
Disadvantages
If the central point (switch/hub) fails, then the whole network fails because every device is connected to it.
A wired star topology may be expensive to install because it can require a lot of cable.
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Mesh topology
How it works
There is no central connection point.
Each node is connected to one or more other nodes.
Each node can send and receive messages.
Each node acts as a relay, passing on a message towards its final destination.
Advantages
Wireless mesh networks are becoming more common because they are usually simple and cheap to connect (as they use radio signals).
Messages may be received quickly because the route to the destination can be very short.
Messages should always get through because there are many potential routes to the same destination.
No node should be unreachable because each node should have more than one connection to it.
Each node can transmit to and receive from more than one node at the same time because of its multiple connections.
Disadvantages
Wired mesh networks are relatively uncommon, because connecting all nodes to all other nodes is expensive and impractical.
Fully connected mesh networks can be impractical to set up because of the high number of connections required.
Maintenance requirements may be high because of the large number of connections.
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