IGCSE Computer Science: Exam Study Guide

• This study guide is for the Cambridge IGCSE and O Level Computer Science syllabuses (0478/0984/2210).
• It is designed to help students revise for their Computer Science examinations.
• It follows the new Computer Science syllabus, with first exams in June 2023.
• The guide includes sample questions, answers, and exam-style practice questions and answers.
• Sample practice papers are available at the end of the book and online at https://www.hoddereducation.co.uk/cambridgeextras.
• The author has also provided an online glossary for key words that appear in red bold in the book.

Study Tips

• Find a copy of the IGCSE Computer Science syllabus to use with this text.
• A list of topics will act as a revision checklist.
• Stop and learn something if you don’t understand it early, while you still recall the topic.
• Anything in the syllabus could be in a question.
• Understand the instruction type given by the questions of what action is required (complete, describe, explain, give, state, suggest).
• Make sure an answer relates to a specific scenario given.
• Give more than a single-word answer to questions.
• Use correct technical terms and avoid trade names.
• Allocate enough time for the number of marks awarded to a question.
• Ensure spellings are correct for computer science terminology.
• Learn the definitions of key terms and show understanding by providing examples.
• Don't ignore syllabus items not understood or with no notes; research items to address gaps in knowledge.
• Avoid cramming facts without understanding.
• Instead, revise to make it interesting, rather than just reading and re-reading.
• Write things down to better recollect important facts.
• Do exam-style questions while timing responses to avoid running out of time in the actual exam.
• Revision cards are useful for last-minute checks, such as for binary arithmetic.
• Questions asked in exams will be within the syllabus; guidance is given to call on already known facts.
• Read each question carefully, understand the work being done, and make sure of the facts.
• Sensibly use allocated times based on allocated marks.

Getting Ready for the Exam

• Get enough sleep and a decent lunch break before tests.
• Stay calm and formulate ways to relax before starting.
• Read the whole paper first to gather thoughts.
• Then answer the questions that are familiar or easy to complete.
• Be sure to read each question thoroughly before answering.
• Highlight key words in the question to avoid missing something.
• Use clear English and handwriting.
• Do not use correction fluid; cross out unwanted work neatly.
• Show clearly where an alternative answer is found on a different page.
• Attempt all questions; you might get credit by writing something that seems relevant.

Command Words

• Calculate: Work out from given facts, figures, or information.
• Compare: Identify/comment on similarities and/ or differences.
• Define: Give precise meaning.
• Demonstrate: Show how or give an example.
• Describe: State the points of a topic/give characteristics and main features.
• Evaluate: Judge or calculate the quality, importance, amount or value of something.
• Explain: Set out purposes or reasons; make the relationships between things evident; provide why and/or how and support with relevant evidence.
• Give: Produce an answer from a given source or from memory.
• Identify: Name/select/recognize.
• Outline: Set out the main points.
• Show: Provide structured evidence that leads to a given result.
• State: Express in clear terms.
• Suggest: Apply knowledge and understanding to situations where there are a range of valid responses in order to make proposals/put forward considerations.

Assessment Overview

• The guide supports the Cambridge IGCSE and O Level Computer Science syllabuses (0478/0984/2210) for examination from 2023, as well the following:
• Paper 1 Computer systems: 1 hour 45 minutes, 75 marks, 50% of overall marks, Syllabus topics 1-6
• Paper 2 Algorithms, programming and logic: 1 hour 45 minutes, 75 marks, 50% of overall marks, Syllabus topics 7-10

Data Representation

Number Systems

• The basic building block in all computers is the binary number system as it consists of 1s and 0s only which correspond to ON and OFF states in the computer system.

• The binary number system is based on the number 2 and it can only use the two values 0 and 1 (these are referred to as bits).

• The binary heading values are 20, 21, 22, 23 etc.

• If an 8-bit system is being used the headings are: 128 (27), 64 (26), 32 (25), 16 (24), 8 (23), 4 (22), 2 (21) and 1 (20).

• The denary system is a base 10 number system with column headings: 100 (1), 101 (10), 102 (100), 103 (1000) and so on.

• Converting from binary to denary is accomplished by simply adding together all the heading values where a 1-value appears.

• Converting from positive denary to binary, it is necessary to carry out successive divisions by 2 until a zero value results.

• The hexadecimal number system is based on the number 16.

• The 16 digits are represented by the numbers 0 to 9 and the letters A to F (representing 10 to 15).

• The hexadecimal headings are 160, 161, 162, 163, and so on.

• To convert a binary number to a hexadecimal number, it is first necessary to split the binary number into 4-bit groups starting from the right-hand side.

• If the final (left-most group) doesn't contain four binary digits, then infill by zeros is done and each 4-bit group is then assigned a hexadecimal digit.

• To convert from hexadecimal to binary, it is necessary to write the 4-bit binary code for each hexadecimal digit.

• To convert a hexadecimal number to denary, it is necessary to multiply each hexadecimal digit by its heading value and then add them together.

• To convert from denary to hexadecimal, it is necessary to carry out successive divisions by 16 until zero value results.

Uses of the Hexadecimal System

• The hexadecimal number system is used by computer programmers and designers because it is easier to deal with, for example AF01, than the binary equivalent of 1010111100000001.
• Error codes refer to memory locations; automatically generated by the computer.
• A Media Access Control (MAC) address identifies a device on a network via the NIC (NN-NN-NN-DD-DD-DD, first six digits are the manufacturer code and the last six digits are the serial number).
• Internet Protocol (IP) address is given to a device when it joins a network.
• Hypertext mark-up language (HTML) colour codes; pixel represented by a hex code.

Addition of Binary Numbers

• Binary addition involves a carry and a sum for each of the 2 or 3 bits being added: 0+0+0 = 0 (carry), 0 (sum); 0+0+1 = 0 (carry), 1 (sum); 0+1+0 = 0 (carry), 1 (sum); 0+1+1 = 1 (carry), 0 (sum); 1+0+0 = 0 (carry), 1 (sum); 1+0+1 = 1 (carry), 0 (sum); 1+1+0 = 1 (carry), 0 (sum); 1+1+1 = 1 (carry), 1 (sum)
• If the result of a calculation is too large for the allocated word size (for example a word size of 8 bits can represent a maximum value of 255) then overflow occurs.
• Logical shifts involve shifting bits to the left or the right, if the result results in a loss of 1-bits, then this would result in an error.
• With shifting, any gaps created by the shift operation can be filled by zeros.
• Shifting a binary number left is the same as multiplying by factors of 2.
• Shifting binary numbers right is the same as dividing by factors of 2.

Two's Complement Binary Numbers

• To allow for the possibility of representing negative integers we make use of two's complement notation.
• Converting denary numbers into binary in two's complement format involves placing 1-bits in the appropriate position remembering that the right-most bit now represents –128.

Text, Sound and Images

• All keyboard characters are represented by 7-bit American Standard Code for Information Interchange (ASCII code) or 8-bit Extended ASCII code character set.
• Use Unicode characters in order to represent non-Western languages, which supports up to 4 bytes per character (that is, up to 32 bits per character).
• Sound is analogue data, to store in a computer, it is necessary to convert the analogue data into a digital format.
• To convert sound to digital, the sound waves must be sampled at regular time intervals.
• The amplitude (loudness) of the sound uses a number of bits to represent the range (for example, 0 to 15 bits).
• The number of bits per sample is called the sampling resolution and the sampling rate is the number of sound samples taken per second.

Bitmap Images

• Bitmap images are made up of pixels (picture elements). An image is made up of a two-dimensional matrix of pixels.
• Each pixel can be represented as a binary number, so bitmap images are stored as a series of binary numbers.
• A black and white image only requires 1 bit per pixel (1 if white and 0 if black).
• A pixel represented by 2 bits has 4 possible values (00 = 0, 01= 1, 10 =2, and 11 = 3) so that four colors are represented.
• The number of bits to represent each possible colour is called the colour depth. Image resolution refers to the number of pixels.

Data Storage Measurement

• A bit refers to each binary digit and is the smallest unit; four bits make up a nibble, and eight bits make up a byte.
• 1 KiB (kibibyte) = 210 bytes
• 1 MiB (mebibyte) = 220 bytes
• 1 GiB (gibibyte) = 230 bytes
• 1 TiB (tebibyte) = 240 bytes
• 1 PiB (pebibyte) = 250 bytes
• 1 EiB (exbibyte) = 260 bytes

Image File Size and Sound File Size

• Image file size is calculated by: image resolution (number of pixels) × colour depth (in bits)
• Sound file size is calculated by: sample rate (in Hz) × sample resolution (bits) × length of sample (secs)

Data Compression

• Files are compressed to save storage space, reduce streaming and downloading or uploading times, reduce the bandwidth requirements and reduce costs.

Lossy compression

• Lossy file compression algorithms eliminate unnecessary data.
• The original file cannot be reconstructed once it has been compressed.
• The files are smaller than those produced by lossless algorithms, for example MPEG and JPEG.

Lossless compression

• Data from the original uncompressed file can be reconstructed following compression.
• No data is lost following the application of the lossless algorithms
• The most common example is Run length encoding (RLE).
• Run length encoding (RLE) works by reducing the size of a string of adjacent, identical data items, the repeating unit is encoded into two values (first value represents number of identical data items, and second value represents ASCII value of data item).

Data Transmission

Data Packets

• Data is split up into packets before it is transmitted, and those data packets are split up into what follows:
• packet header (containing the IP address of the sending station and receiving station, the sequence number of the packet, so it can be reassembled, and packet size, to ensure the receiving station knows that the whole packet has been received)
• payload (the actual data), and
• packet trailer (containing cyclic redundancy check (CRC) error check and a way of identifying the end of the data packet).
• Data packets allow data to be sent in manageable chunks to enable the most efficient route from A to B with routers used known as nodes.
• This uses called packet switching.
• Hop numbers are added to packet headers to ensure packets don't 'bounce around' from router to router and get lost.

Benefits of Packet Switching

• There is no need to tie up a single communication line.
• It is possible to overcome failed, busy or faulty lines by simply re- routing packets.
• It is relatively easy to expand package usage.
• A high data transmission rate is possible.

Drawbacks of Packet Switching

• Packets can be lost and need to be re-sent.
• The method doesn't work well with real-time streaming (for example, live sporting event being transmitted over internet).
• There is a delay at the destination whilst the packets are being re- ordered.

Modes of Data Transmission

• Simplex
  • Data can be sent in one direction only.
• Half Duplex
  • Data can be sent in both directions, but not at the same time.
• Full Duplex
  • Data can be sent in both directions at the same time.

Serial Data Transmission

• Transmits data one bit at a time down single wire or channel (e.g. USB).

Parallel Data Transmission

• Transmits several bits of data down multiple wires or channels, with each wire or channel transmitting each bit (e.g. the internal circuits of a computer).

Features of Serial Transmission

• Has less risk of external interference.
• Provides more reliable transmission over longer distances.
• Transmitted bits won’t have the risk of skewing.
• Is used if the amount of data being sent is relatively small, since transmission rate is slower than parallel.
• Is used to send data over long distances.
• Is less expensive than parallel due to fewer hardware requirements.

Features of Parallel Transmission

• Has a faster rate of data transmission than serial (preferred method where speed is needed).
• Works well over short distances.
• Data can become skewed over long distances (no longer synchronised).
• Provides easier input/output operations.
• It requires more hardware, making it more expensive to implement than serial ports.

Universal Serial Bus (USB)

• A form of serial data transmission known as the industry standard that automatically detects the device and loads the appropriate device driver.

USB Benefits

• Devices are automatically detected and device drivers are loaded, and connection types prevent incorrect connections from being made.
• USB has industry standard, has a number of transfer rates, +5V power in the cable, and re-transmits data if its corrupted.
• Easy to attach more ports using hubs and they're backwards compatible.

USB Drawbacks

• Standard USB only supports a maximum cable length of 5 m; beyond that, USB hubs are needed to extend the cable length.
• Very early USB standards of even standard versions are backwards compatible on the latest computers.
• Even the latest version 3 (V3) and version 4 (V4) USB-C systems have a data transfer rate which is slow compared with, for example, Ethernet connections.

Error Detection

• Data may be corrupted, lost, or even gained.
• Error conditions range from electrical interference, packet switching, and data skewing.

Parity Checks

• Parity checks are a method to check if data has changed or corrupted following data transmission set to even parity or odd parity.

Checksums

• Calculated by the sending computer and sent to the receiving computer for comparison.
• Differences in checksum means data alteration.

Echo Check

• Sends data back to the sending computer to be compared with the data originally sent.
• Data is re-sent if differences are found.

Automatic Repeat Requests (ARQ)

• A method to check data following data transmission, commonly used by mobile phone networks to guarantee data integrity.
• ARQs use positive and negative acknowledgements and timeout.

Check Digits

Check digits are used to identify any errors following data entry. Check digits can detect incorrect digits entered, transposition errors, omitted or extra digits in the number, and phonetic errors.

Encryption

• Encryption protects data by altering it into an unreadable. The original data is called plaintext and data which is encrypted is called ciphertext.

Symmetric Encryption

• Uses a single encryption key to encrypt and decrypt data.

Asymmetric Encryption

• Uses two keys, a public key that is public and a private key that is known only to one user.

Hardware

Central Processing Unit (CPU)

• Responsible for execution and processing of all instructions and data in a computer.
• It consists of: a control unit (CU), an arithmetic and logic unit (ALU), registers, and buses.

Microprocessor

• An integrated circuit (microchip) that contains the ALU and control unit to execute all instructions.
• A microprocessor contains the system clock and primary memory.

Von Neumann Achitecture

Introduced the ideas of:

• Central processing unit
• Processor access to memory
• Computer memories store data and programs.
• Stored programs made up of instructions executable in sequence.

Arithmetic Logic Unit

• The ALU allows arithmetic (such as +, -) and logic operations to be carried out.
• Computers can have more than one ALU.
• The ALU allows multiplication and division using shifting operators.

Control Unit

• Reads instructions from memory, and synchronises data flow and program instructions throughout the computer.
• Signals are generated during the fetch-decode-execute cycle to control all components of the computer.

Registers

• They are high speed memory, and may be for general or special purpose.
• Stored is small amounts of data, address of next instruction to be executed, or current instruction being decoded.
• include: current instruction register , accumulator, the memory address register, memory data or buffer register, program counter.

System Clock

• Clock that synchronises all functions, changes to the clock speed improve performance but at the risk of system crashes.
• Generates timing signals on the control bus to ensure all functions are synchronised

System (IO) Busses

• The transfer data and control signals, and use parallel data transfer; width of a bus varies according to computer architecture or the number of bits used. The three bus types are: - Address Bus - Data Bus - Control Bus

Memory

Random Access Memory, or RAM, holds all data, programs, or any of the operating system currently being used. The Address indicates the location for data or instructions, and is unique to every aspect of information.

EEPROM

• Flash Memory chip that stores the BIO, and its task is to tell the CPU where the operating system can be found.

Fetch, Decode, Execute Cycle

• Carries out stored instructions
• Fetch: CPU requests data from memory and stores the instructions/data in the appropriate registers.
• Memory (MAR) has address of the next action is copied from it to memory through an address bus.
• Data passes along the data bus. If not sufficient room, more data is read along a buffer.
• This processes everything because each instruction needs to be decoded and fully enacted to perform everything correctly. All data also goes through an address bus to make everything possible.

Computer Performance Core Elements

• Caches & Internal Clock - The system clock defines the clock cycle, timing signals are transmitted on the control bus. - Speed comes from clock speed. More speed or width increases processing.

Instruction Set

• A set of oerations for the processor encoded in sequence of an Opcode and Operand

Embedded Systems

• An electronic system can become an electromechanical computer or microprocessor on a chip that becomes more specialized over time.

• Embedded systems contain sensor input, mechanical components, actuators, and software.

• Benefits include being small, low-cost, dedicated, low power and able to be controlled remotely. Concerns include that the devices are not easy to upgrade or repair, they can use simpler interfaces and can lead to environmental issues.

Input/Output Devices

• Barcode Readers - These consist of special shapes, letters, and more, and use lasers and LED lights to get a correct reading by a sensor and a key field.

• QR Codes - Carry more power than barcode, can provide advertising products and gain automatic access to websites through camera from smart devices.

Touchscreens

• Allow use with bare fingers or Stylus -Capacitive technology is used with an electrostatic field where a change in electricity is generated, allowing a unique reading. These are easy to read and durable. -Infrared technology is durable and has glass screens and infrared transmitters with good screen durability -Resistive is good for dust and water, but less sensitive; made up of plymers and glass with high water resistance.

Output Devices

• Actuators -Used an an output component for processes so as to start or stop operations.

-Light Projectors - These output data and show the results on screens or Interactive Whiteboards through beams.

Types of Projectors

• Digital Light/LCD

• Digital Light Projectors have higher quality contrast, but can be better suited for dusty enviroments, uses single microchip and have fewer grey elements.

• Liquid Crystal Displays have limited contrast and require better colour definition, generating fewer image problems.

• Lasers

  • Printers operate using powder, and are used to produce solid objects on a computer with additive printing and direct printing.

Input-Output Devices

-Speakers -Sound from a PC needs a digital to analogue converter (DAC) which turns them into sound with cones. -Sound/Lights -Sensors, are input devices that constantly send signals that are in the range of parameters (pH, temp) - Temperatures is read by sending signals by electricity - Moistures are read by water -Humidity can read water - Lights provide an output of sound while working in connections.

Memory

• RAM and ROM, and has Primary vs Secondary status. The main aspects of information are as follows: -Primary- Used for RAM and Direct operations being made -RAM comes in DRAM with lower speeds and SRAM at higher speeds and is more costly vs disc -Secondary, however is non-volatile and is not a volatile type of storage. -It is best at data preservation. These external factors may be CD to DVD-ROM or hard discs. HD discs are used for sectors and tracks, storing different quantities of bytes and data in their sectors. Information read needs to be in contiguous fashion for processing. - SSD which controls electron movement and are reliable with chips.

Media

• CD &DVD need light to work the system and read with data and can allow reading, are dual layering types, blue rays are used and with this speed allows for the usage of new systems and formats and allow encryption. CD is single layered system but with a spiral for track information, and also uses different lasers for various operations.

Memory

• Virtual
  • This is RAM when files are exceeded is when virtual memory in file data starts to be used more effectively. In short its used all areas with data and makes up for the best of information possible because its constant and is used effectively.
• Cloud
  • Used by lots of computer systems, is a service where most data is provided off or in one server and is used easily. Server types exist on these kinds of systems, or on storage levels of clouds if on more quantity.

The Internet and Its Uses

Main Objectives

• To cover use with the internet, its objectives, the uniform resource tools and the means to access content and see the web. It also touches the internet and its devices and security and currency.

Internet

• The ability to use or deliver data from email devices online with protocols; provides audio and video transfer to other elements.
• World Wide Web, used to access data for all devices, uses HTML coding and requires web browsers to work.

URLs

Web pages (often simply referred to as 'browsers') are software that allow users to access and display web pages or play multimedia from websites on their devices.

Transfering HTTP/HTTPS

is an aspect for following a series of rules such that files can be moved and work and be secured over network access.

Browsers

Operate and interpret HTML coding and also support the use of storing fav websites (or store history). Browsers can open many webpages as well during any operation.

Retrieval

Web server or local address to find its IP addresses and access pages, as a result computers can be found and be coded during runtime.

Cookies

These have access on browser activity. They then have two options - session cookies are temporal and don't obtain identity, with persistent cookies offering to have log-ins and more for more usage and access of specific items. They come in many options as well, often being online transactions or preference types.

What is a cookie? are smaller data or information for all users. It then has a variety of types and more based on time and usage.

Security

For digital types, security for devices and more for different systems and means of security to attack different attacks, for example:

• Attacks using brute force method and data intercepting to be of note.

• Attacks using DDoS in such actions to lead to phishing for social usage and hacking that could spread with Malware to effect a key database. Mitigation to secure the system. In short, all should be reviewed in terms of firewall access.

Malware

This can contain multiple viruses that are known to spread and needs codes just as trojan horses effect and effect security, it also exists with spyware so security is a must have. Encryption to the codes.

Pharming attacks for more security is used, as its also very useful when determining any malicious actions on hand can be mitigated when the use of protection of passwords is a must. Biometric features can then become an authentication.

Automatic updating occurs when things are activated over the PC during checks for those things so nothing wrong can carry the devices.

Firewalls Proxies

Check or Examine "traffic" and block certain issues while being sure to stop things such as hacking. Proxies may see when a user first makes a request.

Proxies can be set to filter website blocks while keeping user access separate. To secure devices more for various elements, Secure sockets layer, has better encoding during the process when its run with HTTPS or the green padlock being used.

It then has a digital authentication and the secure connections are run on everything.

Privacy

Finally, control over safety is had with use levels for users and better account integrity. With malware prevention for systems and better firewalls, it would stop unwanted code or programs over computer systems and its related code to secure and guard all aspects of each connection at whatever level you make something for an ethical standpoint.