Computer science

Computer science

1. 1.1

   • 1.1.1
     • Processor Components and Their Roles
       • Arithmetic and Logic Unit (ALU) Performs all arithmetic (e.g., addition, subtraction) and logical operations (e.g., AND, OR, NOT). It is the part of the CPU that actually processes data.
       • Control Unit (CU) Manages and coordinates the CPU. It decodes instructions, sends control signals, and ensures everything happens in the correct order.
       • Registers (Small, fast memory locations inside the CPU)
         • Program Counter (PC) Holds the address of the next instruction to be executed
         • Accumulator (ACC) Stores intermediate results of calculations performed by the ALU.
         • Memory Address Register (MAR) Holds the address in memory that the CPU wants to read from or write to.
         • Memory Data Register (MDR) Holds the actual data being transferred to or from memory.
         • Current Instruction Register (CIR) Stores the current instruction after it has been fetched from memory.
       • Buses (Communication pathways)
         • Data Bus – Carries data between CPU, memory, and I/O.
         • Address Bus – Carries memory addresses from the CPU to memory.
         • Control Bus – Carries control signals (e.g., read/write, interrupt signals).
     • The Fetch–Decode–Execute Cycle
       • This is the continuous cycle the CPU performs to run programs.
       • Fetch
         • The PC sends the address of the next instruction to the MAR.
         • A read signal is sent via the control bus.
         • The instruction is fetched from memory into the MDR.
         • The instruction is then copied into the CIR.
         • The PC increments to point to the next instruction.
       • Decode
         • The Control Unit interprets the instruction in the CIR.
         • It identifies which operation is required and which data or registers are involved.
       • Execute
         • The instruction is carried out.
         • Examples:
           • ALU performs a calculation
           • Data is moved between registers
           • A jump/branch updates the PC
           • A memory read/write occurs
       • During this cycle, register values change depending on the instruction being processed.
   • 1.1.2
     • Reduced Instruction Set Computers (RISC)
       • In these processors, there is a small instruction set. Each instruction is approximately one line of machine code and takes one clock cycle.
     • Complex Instruction Set Computers (CISC)
       • In these processors there is a large instruction set. The aim is to try and accomplish tasks in as few lines of assembly code as possible. These instructions are built into the hardware. Early on these processors were used as the standard, however with time they got replaced by RISC design. Now they are used more in microcontrollers and embedded systems.
     • graphics processing unit (GPU)
       • a co-processor which has lots of independent processors working in parallel making it very efficient at tasks such as image processing, and machine learning.
     • Multi-core CPUs
       • multiple independent cores that can complete instructions separately which results in higher performance. Parallel systems accomplish a similar task however instead of requiring multiple cores they can complete tasks with a single core, by using threading. Overall, in larger projects, multi-core systems perform better than parallel systems
   • 1.1.3
     • Examples of inputs include:
       • Keyboard
       • Mouse
       • Microphone
       • Scanner
       • Touch screen
     • Examples of outputs include:
       • Headphones
       • Monitor
       • Printer
       • Speakers
       • Phone torch
     • Examples of sensors include:
       • Speed sensors
       • Infra red sensors
       • Ultra violet sensors
     • Virtual storage is where you use the internet to access a storage device like the cloud.

2. 1.2

   • 1.2.1
     • OS
       • An operating system (OS) is system software that acts as an interface between the user, application software, and hardware.
         • functions
           • Resource management: allocates CPU time, memory, storage, and I/O devices
           • Memory management: allocates and deallocates memory
           • Device management: controls peripherals via device drivers
         • reasons for use
           • Users require a consistent and simple interface
           • Hardware alone cannot be used efficiently by users or programs
           • Multiple programs need controlled access to limited resources
     • memory management
       • paging
         • Memory is divided into fixed-size pages (logical) and frames (physical)
         • advantages
           • Efficient use of memory
           • No external fragmentation
         • disadvantages
           • Internal fragmentation
           • Overhead from page tables
       • segmentation
         • Memory divided into variable-sized segments based on logical structure
         • advantages
           • Matches program structure
           • Easier protection and sharing
         • disadvantages
           • External fragmentation
           • More complex memory management
       • Virtual Memory
         • Uses secondary storage (e.g. disk) as an extension of RAM
         • advantages
           • Programs larger than RAM can run
           • Better memory utilisation
         • disadvantages
           • Slower access than RAM
           • Excessive page faults reduce performance
     • interupts
       • Sources of interrupts
         • Hardware (keyboard, mouse, I/O devices)
         • Timers
         • Software (system calls, errors)
       • Role of Interrupts
         • Allow the CPU to respond to events efficiently
         • Prevents constant polling
         • Enables multitasking
       • Interrupt Service Routine (ISR)
         • A small program that handles a specific interrupt
         • Stored at a known memory location
         • Executes, then returns control to the interrupted process
     • Scheduling Algorithms
       • First Come First Served (FCFS)
         • Processes executed in arrival order
         • advantages
           • Simple
           • efficent when responding
         • disadvantages
           • Long waiting times
           • Poor response time
       • Round Robin
         • Each process gets a fixed time slice
         • advantages
           • Fair
           • Good for time-sharing systems
         • disadvantages
           • Overhead from context switching
           • complicated to code
       • Shortest Job First (SJF)
         • advantages
           • Minimises average waiting time
           • high response time
         • disadvantages
           • Requires accurate time estimates
           • Starvation possible
         • Executes process with shortest estimated execution time
       • Shortest Remaining Time
         • Pre-emptive version of SJF
         • advantages
           • Efficient CPU usage
           • short response times
         • disadvantages
           • High overhead
           • Starvation risk
       • Multi-Level Feedback Queues
         • Multiple queues with different priorities
         • advantages
           • Flexible
           • Prevents starvation
         • disadvantages
           • Complex to implement
           • long wait times for low priority operations
     • Types of Operating Systems
       • Distributed Operating Systems
         • Manage a group of independent computers
       • Embedded Operating Systems
         • Designed for specific hardware
       • Multi-tasking Operating Systems
         • Allow multiple processes to run seemingly simultaneously
       • Multi-user Operating Systems
         • Support multiple users at the same time
       • Real Time Operating Systems (RTOS)
         • Must respond within strict time constraints
         • types
           • Hard real-time (missed deadline = failure)
           • Soft real-time (performance degrades)

3. 1.3

   • 1.3.1
     • A: programing constructs
       • Sequence
         • Instructions are executed in order, one after another.
       • selection
         • Different paths are chosen based on conditions.
         • examples
           • IF
           • IF…ELSE
           • CASE / SWITCH
       • iteration
         • Repeating a block of code.
         • examples
           • FOR (count-controlled)
           • WHILE (condition-controlled)
           • REPEAT-UNTIL (post-condition)
     • B: data types
       • Integer
       • Real / Float
       • Boolean
       • Character
       • String
       • Used to define how data is stored and manipulated.
     • C. Variables and Constants
       • Variables store data that can change
       • Constants store fixed values
     • D. Arrays
       • Store multiple values of the same data type
       • Can be one-dimensional or multi-dimensional
       • Indexed using integers
     • E. File Handling
       • Reading from files
       • Writing to files
       • Appending data
       • Used for persistent storage.
     • F. Subroutines
       • Includes procedures and functions.
       • Improve modularity
       • Reduce code repetition
       • Allow abstraction
       • Functions return a value; procedures do not.
     • G. parameter passing
       • By value: copy passed, original unchanged
       • By reference: original value can be modified
     • H. Validation and Verification
       • Validation: checks data is sensible
       • Verification: checks data is entered correctly
   • 1.3.2
     • Waterfall Model
       • stages
         • Requirements
         • Design
         • Implementation
         • Testing
         • Maintenance
       • features
         • Structured
         • Documentation heavy
         • Testing at the end
       • advantages
         • Easy to manage
         • Clear milestones
       • disadvantages
         • Not flexible
         • Errors found late
     • Agile
       • definition
         • iterative development in small sections
       • features
         • Develop in cycles
         • Flexible to change
         • Regular client feedback
       • advantages
         • Adapts to changes
         • Bugs found early
       • disadvantages
         • Harder to predict cost/time
         • Requires regular customer input
     • Iterative Development
       • definition
         • System developed in repeated cycles.
       • the cycle
         • Improves previous version
         • Adds features
         • Fixes issues
       • why is it used
         • Reduces risk
         • Early error detection
     • Maintenance
       • Types
         • Corrective → Fix bugs
         • Adaptive → Adapt to new systems
         • Perfective → Improve features
         • Preventative → Prevent future issues
   • 1.3.3
     • Types of Testing
       • Iterative Testing
         • Testing carried out continuously during development.
         • Errors are fixed as they are found.
         • Reduces debugging complexity later.
       • Final/Terminal Testing
         • Occurs after development is complete.
         • Ensures system meets requirements before release.
     • Testing Methods
       • Black Box Testing
         • Tests functionality without viewing internal code.
         • Tester does not need programming knowledge.
         • Focus: Inputs → Outputs.
       • White Box Testing
         • Tests internal program structure and logic.
         • Requires knowledge of code.
         • Ensures all paths, branches, and statements are executed.
     • Test Data Types
       • Normal Data
         • Expected, valid inputs.
         • Should produce correct output.
       • Boundary Data
         • Data at the edge of valid range.
         • Tests limits of validation.
       • Erroneous (Invalid) Data
         • Incorrect type or format.
         • Should trigger validation error.
       • Extreme Data
         • Highest/lowest possible values.
         • Tests system robustness.

4. 1.4

   • 1.4.1
     • A
       • the 5 primitive data types are:
         • Integer
         • Real/Float
         • Boolean
         • Character
         • String
       • definition
         • Basic built-in data types used to store single values.
     • B
       • the 3 composite data types are
         • Array
         • Record
         • Set
       • definition
         • Data types made from multiple values grouped together.
     • C
       • definitions
         • A static data structure has a fixed size that cannot change after it is created, and memory is allocated before the program runs.
         • A dynamic data structure can change size during program execution, and memory is allocated when needed.
       • static
         • Fixed size
         • Allocated at compile time
       • dynamic
         • Size changes during runtime
         • Allocated at runtime
   • 1.4.2
     • Arrays
       • definition
         • Ordered collection of elements stored in continuous memory
       • uses
         • Storing marks, Lists of numbers
     • linked lists
       • types
         • Singly linked
         • Doubly linked
         • Circular
       • definition
         • A dynamic data structure made of nodes connected by pointers.
     • records
       • definition
         • A data structure that stores a group of related fields, where each field can have a different data type.
       • uses
         • Student information
         • Employee data
         • Database records
     • stacks
       • definition
         • A data structure that uses Last In, First Out (LIFO).
       • uses
         • Function calls
         • Undo operations
         • Recursion
     • Graphs
       • definition
         • A structure made of nodes (vertices) connected by edges.
       • Types
         • Directed
         • Undirected
         • Weighted
       • Uses
         • Maps
         • Social networks
         • Computer network
     • queues
       • definition
         • A data structure that uses First In, First Out (FIFO)
       • types
         • Linear queue
         • Circular queue
         • Priority queue
       • uses
         • Printer queues
         • CPU scheduling
         • Network systems
     • trees
       • definition
         • A hierarchical data structure made of nodes connected in levels.
       • key terms
         • Root → top node
         • Parent → node above
         • Child → node below
         • Leaf → node with no children
   • 1.4.3
     • search algorithms
       • linear search
         • definition
           • A search method that checks each element one by one until the value is found or the end of the list is reached.
         • advantages
           • Works on any list
           • Simple
         • disadvantages
           • Inefficient for large datasets
           • Means time increases directly with number of items
       • binary search
         • definition
           • A search method that repeatedly divides a sorted list in half to find a value.
         • advantages
           • Very efficient
           • Fast for large datasets
         • disadvantages
           • complicated to code
           • Requires sorted data
     • Sorting Algorithms
       • Bubble Sort
         • definition
           • Repeatedly compares adjacent elements and swaps them if they are in the wrong order.
         • Advantages
           • Very simple
           • efficent for small lists
         • disadvantages
           • Very slow for large lists
           • checks even if 2 data points are already sorted
       • Insertion Sort
         • definition
           • Builds a sorted list by inserting each new element into the correct position.
         • advantages
           • Efficient for small datasets
           • Easy to implement
         • disadvantages
           • Slow for large datasets
           • can be complicated to code
       • Merge Sort
         • definition
           • Uses divide and conquer by splitting the list, sorting parts, and merging them.
         • advantages
           • Very efficient
           • Good for large datasets
         • disadvantages
           • Uses extra memory
           • complicated to code
       • Quick Sort
         • definition
           • Uses a pivot value to divide the list into smaller and larger values.
         • Advantages
           • Very fast in practice
           • efficent for small lists
         • Disadvantages
           • less efficent as list size increases
           • difficult to code
     • Big O Notation
       • definition
         • A way of describing how algorithm performance changes as data size increases.
       • Common Big O Values
         • O(1) – Constant
           • Time does not change with size
           • Example: accessing array index
         • O(log n) – Logarithmic
           • Very efficient
           • Example: binary search
         • O(n) – Linear
           • Time increases directly with size
           • Example: linear search
         • O(n log n) – Efficient sorting
           • Example: merge sort, quick sort
         • O(n²) – Quadratic
           • Slow
           • Example: bubble sort
         • Why Big O is important
           • Allows comparison of algorithms
           • Helps choose most efficient algorithm
     • Recursion
       • definition
         • A programming technique where a function calls itself to solve a problem.
       • Two Required Parts
         • Base case
           • Stops the recursion
           • Prevents infinite loop
         • Recursive case
           • Function calls itself with smaller problem
       • Uses of Recursion
         • Tree traversal
         • Merge sort
         • File system navigation
       • advantages
         • Simplifies complex problems
         • easy to use
       • disadvantages
         • Uses more memory
         • Can be slower than iteration

5. 1.5

   • 1.5.1
     • Data Protection Legislation
       • definition
         • Laws that govern how personal data is collected, stored, and used.
       • Key Principles
         • Lawful processing
           • Data must be collected legally and fairly.
           • Organisations must have permission or a valid reason.
         • Data minimisation
           • Only the minimum necessary data should be collected.
           • Reduces risk if data is leaked.
         • Accuracy
           • Data must be correct and kept up to date.
           • Incorrect data must be corrected or deleted.
         • Security
           • Data must be protected using methods such as encryption, passwords, and access controls.
           • Prevents unauthorised access.
       • Purpose
         • Protects individuals’ privacy
         • Prevents misuse of personal data
         • Ensures organisations are responsible
     • Computer Misuse Act (Unauthorised Access)
       • definition
         • Law that makes it illegal to access or modify computer systems without permission.
       • Offences
         • Unauthorised access to a device
           • Accessing a computer, account, or network without permission.
         • Unauthorised access with intent to commit offences
           • Accessing systems to commit crimes such as fraud or data theft.
         • Unauthorised access with intent to cause harm or damage
           • Creating malware, Deleting or altering data, Damaging systems
       • purpose
         • Protects computer systems
         • Prevents hacking and cybercrime
     • Copyright Legislation
       • definition
         • Protects intellectual property, including software, music, videos, and written content.
       • What it prevents
         • Illegal copying of software
         • Software piracy
         • Sharing copyrighted material without permission
       • Education and research use
         • Limited copying allowed for educational purposes
         • Must not be used for commercial gain
       • purpose
         • Protects creators’ rights
         • Encourages innovation
     • Investigatory Powers and ISP Monitoring
       • explination
         • Websites visited can be logged.
         • This information can be accessed by authorities with legal permission.
       • Police access
         • Request ISP data during investigations
         • Monitor devices if authorised by a warrant
       • Purpose
         • Prevent crime
         • Investigate illegal activity
         • Protect national security