IGCSE & O Level Computer Science Revision

Cambridge IGCSE and O Level Computer Science Study and Revision Guide Second Edition

• This book helps revise for Computer Science examinations and is designed to accompany the Cambridge IGCSE and O Level Computer Science Second Edition Student's Book.
• Covers all key content and includes sample questions, answers, and exam-style practice questions, aligned with the new Computer Science syllabus first examined in June 2023.
• Practice papers are available online with an online glossary for key terms.

How To Use This Book

• The book provides study tips such as understanding questions, improving knowledge, making sure the answer makes sense.
• Be able to calculate binary arithmetic, read carefully, and ensure understanding, know allocation.
• Help yourself by getting enough sleep, staying calm, reading whole paper, use clear English and good handwriting, attempt all questions.

Command Words

• Some keywords need a definition for answering questions:
  • 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/recognise.
  • 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

• Information is from the Cambridge IGCSE and O Level Computer Science syllabuses for examination from 2023 (0478/0984/2210).
• There are two examination papers: Paper 1 Computer systems and Paper 2 Algorithms, programming and logic.
• Both papers are 1 hour 45 minutes and 75 marks
• Both account for 50% of overall marks and include Syllabus topics examined.

Data Representation

• This chapter reviews Number systems, the binary, denary and hexadecimal number:
  • Conversion of numbers between all three number systems
  • Use of the hexadecimal (hex) number system
  • Binary addition and Overflow error
  • Logical shifts, Two's complement format for negative and positive binary numbers
  • text, sound and images, ASCII and Unicode character sets
  • Representation of sound in a computer
  • Sampling rate and sample resolution
  • Image representation, including resolution and colour depth
  • Data storage and file compression
  • Calculation of file sizes, The need for data (file) compression
  • Lossy and lossless compression

Binary Represents Data

• The basic building block in all computers is the binary number system, using 1s and 0s, corresponding to ON and OFF states in the computer system.

Conversion of Number Systems

• The Binary System is based on the number 2, the denary system is a base 10 number system, and hexadecimal number system is based on the number 16.
• To convert from binary to denary, add together all the heading values where a 1-value appears.
• To convert from positive denary to binary, carry out successive divisions by 2 until a zero value results, reading remainders from bottom to top.
• To covert to Hexadecimal number: Split the binary number into 4-bit groups starting from the right-hand side
• To convert a hexadecimal number to denary, it is necessary to multiply each hexadecimal digit by its heading value and then add them together.

Addition of Binary Numbers

• Binary addition involves a carry and a sum for each of the 2 or 3 bits being added.

Overflow

• Overflow occurs if the result of a calculation is too large for the allocated word size.

Logical Shifts

• Logical shifts involve shifting bits to the left (multiplying by 2 for each shift) or the right (dividing by 2 for each shift).
• When shifting a binary number, any gaps created by the shift operation can be filled by zeros.

Two's Complement

• Two's complement notation allows the possibility of representing negative integers with 1-bits in the appropriate position.

Text, Sound and Images

• All keyboard characters are represented using 7-bit ASCII code or 8-bit Extended ASCII code character set.
• Sound is analogue data. To store sound in a computer, it is necessary to convert the analogue data into a digital format by sampling at regular time intervals.
• Bitmap images are made up of pixels (picture elements); each pixel can be represented as a binary number

Text, Sound and Images: Data Storage and File Compression

• Each data type must me measured and saved into memory which has various level of measurement: kibibyte, mebibyte, gibibyte, tebibyte, pebibyte, exbibyte.
• Image file size uses: Image resolution (number of pixels) x colour depth (in bits)
• Sound file size uses: Sample rate (in Hz) x sample resolution (bits) x length of sample (secs)

Data Compression

• Files are often compressed to save storage used, reduce streaming and downloading/uploading times, reduce the bandwidth requirements and reduce costs.
• Two common types of (file) compression are lossy and lossless.
  • Lossy: File compression algorithms eliminate unnecessary data, but the original file cannot be reconstructed once it has been compressed. Files are smaller than those produced by lossless algorithms, and examples include MPEG and JPEG.
  • Lossless: Data from the original uncompressed file can be reconstructed following compression; no data is lost, and the most common example is RLE (Run Length Encoding).

Data Transmission

• This chapter reviews methods of data transmission - data packets and structure and error detection, and serial, parallel, simplex, half-duplex and full-duplex and also USB.

Methods of Data Transmission

• The modes of data transmission are:
  • 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 sends data one bit at a time down a single wire/channel.
• Parallel data transmission sends several bits of data down several wires/channels at the same time.
• The universal serial bus (USB) is a form of serial data transmission.

Data Packets and Switching

• Data is broken up into packets before it is transmitted: packet header, payload, packet trailer
• Routers controp a data packet's path from sending station to receiving location.
• Packet switching sends manageable chunks along an efficient route with 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.
• Packets can be lost and need to be re-sent in real-time streaming which causes delay at the destination whilst the packets are being re-ordered.

Methods of Error Detection

• Electrical interference can corrupt data.
• Packet switching can lead to data being lost or out of synchronisation.
• Skewing of data – bits arrive at their destination no longer synchronised.
• Parity checks, Checksum, and Echo checks

Error Detection

• A parity check is a method used to check whether data has been changed or corrupted following data transmission. Parity can be even or odd.
• A checksum is calculated by the sending computer, from the block of data, using an agreed algorithm.
• An echo check requires data to be sent back to the sending computer where it is compared with the data originally sent.
• Check digits are used to identify any errors following data entry by calculating with some methods, and is added to the end of the number.
• Automatic Repeat Request (ARQ) uses positive and negative acknowledgements and timeout.
• Positive and negative acknowledgements and timeout

Symmetric and Asymmetric Encryption

• Uses Encryption and encryption
• Symmetric encryption uses a single encryption key, which has a Public key (known to everyone) and a private key (known to one user only).
• Asymmetric encryption needs both types of key are needed to encrypt and decrypt messages.

Harware

• This chapter reviews:
  • Computer architecture
  • Input and output devices
  • Data storage
  • Network hardware

Computer Architecture

• The central processing unit (CPU) has the responsibility for execution and processing of all instructions and data in a computer and consists of: control unit (CU), arithmetic and logic unit (ALU), and registers and buses.

Von Neumann Architecture

• Introduced the idea of the stored program computer, which:
  • Has a concept of a central processing unit (CPU or processor)
  • Able to access the memory directly, and memories could programs as well as data
  • Stored programs were made up of instructions which could be executed in sequential order.

Typical CPU components

• Arithmetic and Logic Unit (ALU): The ALU allows all arithmetic and logic operations to be carried out.
• Control Unit: The control unit synchronises data flow and program instructions throughout the computer.
• Registers: High speed areas of memory within a microprocessor/CPU - used to store small amounts of data and address of next instruction to be executed.
• System Clock: The system clock is used to produce timing signals on the control bus to ensure all functions are synchronised.
• System Buses: Buses transfer data and control signals throughout a computer.
• Memory: The random access memory (RAM) holds data, programs and the part of the operating system currently in use.

Fetch-Decode-Execute Cycle

• Steps to carry out a set of instructions within the CPU and: fetches data and instructions from memory, stores them in registers, decodes and executes.

Cores, Cache, Internal Clock, Instruction Set

• The system clock defines the clock cycle used to synchronise all computer operations for an amount of processes per second with correct data bus and address bus widths.
• caches store frequently used instructions and data, allowing quad-core processors to execute instructions.
• Instructions are a set of operations that need to be decoded in sequence of one opcode and one oprand with Fetch-Decode-Execute cycle controls.

Embedded Systems

• A combination of hardware and software designed to carry out a specific task, which can be programmable or non-programmable.
• Hardware can be electronic, electrical or electro-mechanical. Embedded systems can be based on a microcontroller, microprocessor or system on a chip (SoC).

Uses of Embedded Systems

• Security systems
• Set-top box
• Lighting applications
• Vending machines
• Washing machines
• Motor vehicles

Input and Output Devices

• Input devices read data from the outside into memory
• Output devices writes memory to the outside

Main Input Devices

• barcode reader
• QR code reader
• digital camera
• keyboard
• mouse
• 2 and 3D scanner
• Touch screen

Output Devices and Actuators

• Actuator: a starter or stopper of a process to do with a system
• Light projectors: cast data into space for information or data reasons.
• LCD and LED screens cast an image for consumption and viewing
• Speakers: use DAC to produce sound
• Common Sensors: temperature, moisture, humidity, light, infrared (active), infrared (passive), pressure, acoustic/sound, gas, and pH.

Data Storage

• Data storage in a computer is based with memory, which is split into to Main and Secondary:

Main memory:

• Random access memory which controls data (RAM) for performance data, and read-only memory (ROM) containing static data.
• Computers need both RAM and ROM. RAM is the working area of the computer where data currently being used is temporarily stored. ROM contains data that can't be changed and needs to be stored permanently.
• RAM which can be two types: dynamic RAM (DRAM) or static RAM (SRAM).

Secondary and Offline Storage:

• Includes non volatile devices and applications/files (music, and files)

Storage types

• Magnetic, optical and solid-state storage:
  • Magnetic is with HDD and can be removed. With sectors, tracks, heads which has latency and suffers Fragmentation as it reaches capacity. Solid-state by contrast has no moving part, operates in NAND matrix with limited of write cycles and a controller chips with NAND. All from memory sticks, flash memory and SSD. Opticals have lasers (blue, red) read/write. CD, DVD and Blu Ray and double sides to increase more memory. Virtual is a cloud storage that has ram+swap from hdd, with paging but has thrashing of SSD and HDD for memory processes. Has benefit to reduce costs to install hardrives
• Memory. Cloud is online storage and servers

Network Hardware

• Network interface cards, Media Access Control addresses, and Internet Protocol addresses all needed to ensure safe data and internet usage

Network Interface Cards

• A network interface card (NIC) is needed to allow a device to connect to a network.
• The NIC contains the Media Access Control (MAC) address, generated at the manufacturing stage
• Wireless NICS (WNICs) can also be used; these plug into the USB port or can be part of an internal integrated circuit

Media Access Control

• Identify a device connected to a network and Made up of 48 bits written in groups of six hex digits and addresses can be Universally Administered or Locally Administered.

Internet Protocol

• Protocol (IP) addresses are to assign the device an Internet Protocol (IP), uniquely based in the DHCP server, with a static or dynamic configuration.

Routers

• Enable data packets to be routed between different networks and convert data to a network (A and B) to connect.

4. Software

• Software has
• Types of programs and interrupts
• Language
• Translators

Types and Interrupts

• Software comes as System software and with Applications.

System Software

• System software has
  • Operating
• Drivers
• Utilities Whereas application is for daily usages. All need kernel for boot-up so hardwares runs well.

Operating System

• Hardware operation
• Input/Output operation
• Users to read and write data with memory

Types of Language and Compilation/Interpreter

• Complication gives executable files
• Easier distribution
• Interpration has to check code to run. But easy edit and debug
• Low-Level Language
• HLL gives high understanding, readability, and easier usage
• Assembly, and Machine

Interrupt

• Signal
• Process
• Priority and Buffer

Cyber Security

• Cyber security threat, that prevent computer and data from bad entities.

Security Risks

• Brute Force Attack
• tapping
• cyberattack

Security Prevention

• Firewall: data check, data protection, data direction
• anti virus software for scan and kill.
• Password
  • Authentication: Something we know, some we have, or some we can identify.
• Encryptions