Computer science mock

Purpose of the CPU

To fetch, decode and execute instructions (the process data / execute programs)

Von Neumann architecture — MAR

Memory Address Register: holds the address of the memory location currently being read from or written to

Von Neumann architecture — MDR

Memory Data Register: holds the data/instruction being transferred to or from memory

Von Neumann architecture — Program Counter (PC)

Holds the memory address of the next instruction to be fetched

Von Neumann architecture — Accumulator

Register that stores the results of calculations performed by the ALU

Role of the ALU

Arithmetic Logic Unit: performs arithmetic (e.g. +, -) and logical (e.g. AND, OR, comparisons) operations

Role of the CU

Control Unit: manages/coordinates the fetch-decode-execute cycle and controls the flow of data in the CPU

Role of Cache

Very fast memory built into/close to the CPU that stores frequently used instructions/data, reducing time spent waiting on RAM

Role of Registers

Tiny, extremely fast storage locations inside the CPU used to hold data/instructions currently being processed

Fetch stage

The next instruction is fetched from memory using the address in the Program Counter, and the PC is incremented

Decode stage

The Control Unit decodes the fetched instruction to work out what it means / what needs to happen

Execute stage

The instruction is carried out (e.g. by the ALU), and results may be stored or output

Clock speed

The number of instructions a CPU can process per second, measured in Hz; higher clock speed = more instructions processed per second = faster performance

Cache size (effect on performance)

A larger cache means more frequently used data/instructions can be stored close to the CPU, reducing time spent fetching from slower RAM, increasing performance

Number of cores (effect on performance)

More cores allow more instructions to be processed simultaneously (multiple pipelines), increasing performance, especially for multitasking — but only if software is written to use multiple cores

Embedded system

A computer system built into a larger device to perform a specific, dedicated function

Examples of embedded systems

Washing machine controller, microwave, traffic lights, car engine management system, digital watch, satnav

Characteristics of embedded systems

Usually have a dedicated/single function, limited processing power, small memory, low cost, low power consumption

RAM

Volatile memory that temporarily stores data/programs currently in use; contents are lost when power is off; read AND write

ROM

Non-volatile memory that permanently stores the startup instructions (e.g. BIOS); contents cannot be changed; read only

Why does a computer need RAM?

To store the operating system, currently running applications, and data currently in use, since it's much faster to access than secondary storage

Virtual memory

An area of secondary storage (hard disk) used as an extension of RAM when RAM is full, allowing more programs to run than physical RAM would allow — slower than RAM

Why might virtual memory slow a computer down?

Reading/writing to secondary storage (e.g. HDD) is much slower than reading/writing to RAM

Primary storage

Memory directly accessible by the CPU (RAM and ROM); fast but often volatile and limited in size

Secondary storage

Non-volatile storage used to hold data/programs permanently, e.g. hard disk, SSD, optical disc

Optical storage

Uses a laser to read/write data as pits and lands on a reflective disc (CD/DVD/Blu-ray); cheap, portable, but slow and easily scratched/damaged

Magnetic storage

Uses a spinning disk coated in magnetic material and a read/write head (HDD); large capacity, relatively cheap, but slower and has moving parts that can fail

Solid state storage

Uses flash memory with no moving parts (SSD, USB); fast, durable, silent, low power, but more expensive per GB than magnetic

Advantage of SSD over HDD

Faster read/write speeds, more durable/reliable (no moving parts), silent, lower power consumption

Advantage of HDD over SSD

Cheaper per GB, larger capacities widely available

Bit

The smallest unit of data in a computer, either a 0 or a 1

Byte

A group of 8 bits

Data unit order (smallest to largest)

Bit → Nibble (4 bits) → Byte (8 bits) → Kilobyte → Megabyte → Gigabyte → Terabyte → Petabyte

How to calculate file size (basic)

File size = number of pixels/characters/samples × bits per pixel/character/sample (converted to bytes by dividing by 8)

Lossy compression

Reduces file size by permanently removing some data/detail, so the file cannot be restored exactly to the original — results in smaller files but lower quality

Lossless compression

Reduces file size by removing redundancy without losing any data — file can be restored exactly to the original, but compresses less than lossy

Advantage of lossy compression

Produces much smaller file sizes, saving storage space and reducing time to transmit over a network

Disadvantage of lossy compression

Some quality/detail is permanently lost and cannot be recovered

Why compress files at all?

Smaller files take up less storage space and take less time to transfer over a network/internet

Run-length encoding (RLE)

A lossless compression technique that replaces sequences of repeated data with a single value and a count (e.g. 5 white pixels → "5,white")

LAN

Local Area Network: computers connected together over a small geographical area, e.g. one building or site

WAN

Wide Area Network: computers connected together over a large geographical area, e.g. connecting sites in different cities/countries (the internet is the largest WAN)

Star topology

All devices connect individually to a central switch/hub; if one cable fails only that device loses connection, but the whole network fails if the central device fails

Advantage of star topology

If one cable/device fails, the rest of the network keeps working; good performance as data doesn't pass through other devices

Disadvantage of star topology

Requires more cabling than other topologies; if the central switch/hub fails, the whole network goes down

Mesh topology

Every device is connected to every other device (full mesh) or to several others (partial mesh); no single point of failure

Advantage of mesh topology

Very reliable — no single point of failure, and if one connection fails data can be rerouted

Disadvantage of mesh topology

Expensive and complex to set up due to the large amount of cabling/connections needed

Client-server network

A central server manages resources/security and provides services to client computers on the network

Advantage of client-server

Easier to manage security, backups and resources centrally; better performance for large networks

Peer-to-peer network

All devices have equal status and share resources directly with each other, with no central server

Advantage of peer-to-peer

Cheaper and simpler to set up (no server needed); no reliance on a single central point

Router (function)

Forwards data packets between different networks, e.g. connects a home network to the internet, and directs data to the correct destination

Switch (function)

Connects devices within the same LAN and forwards data only to the specific device it is intended for, using MAC addresses

Network Interface Card (NIC)

Hardware (built-in or plug-in) required to connect a device to a network, giving it a MAC address

Wireless Access Point (WAP)

Allows wireless devices to connect to a wired network using radio signals (Wi-Fi)

Factors affecting network performance

Bandwidth, number of devices/users on the network, interference, transmission media used, latency, server/hardware performance

Bandwidth

The maximum amount of data that can be transferred over a network in a given time (measured in bits per second)

IP address

A unique numerical address assigned to a device on a network (logical address), used to identify and route data across networks; can change

MAC address

A unique physical address permanently assigned to a device's network interface card by the manufacturer; used to identify devices within a LAN and does not change

Difference between IP and MAC address

IP addresses are logical, assigned by a network/router and can change depending on the network; MAC addresses are physical, hard-coded by the manufacturer and never change

DNS (Domain Name System)

Translates human-readable domain names (e.g. www.bbc.co.uk) into IP addresses that computers use to locate servers

Packet switching

Data is broken into packets, each sent independently (potentially via different routes) across a network, then reassembled in the correct order at the destination

Protocol

An agreed set of rules that governs how devices communicate/exchange data

TCP/IP

TCP controls how data is split into packets, transmitted, and reassembled in order; IP handles addressing and routing of packets to the correct destination

HTTP

HyperText Transfer Protocol: used to transfer web page data between a web server and browser (not encrypted)

HTTPS

HTTP Secure: same as HTTP but encrypts the data being transferred, providing security/authentication

FTP

File Transfer Protocol: used to transfer files between computers on a network

Email protocols

SMTP (sends email), IMAP/POP3 (receive/retrieve email from a mail server)

4-layer TCP/IP model

Application layer, Transport layer, Internet layer, Link layer

Application layer

Provides network services directly to applications and encodes/decodes data according to the protocol used (e.g. HTTP, FTP)

Transport layer

Splits data into packets, adds sequence/port information, and manages reliable delivery (TCP)

Internet layer

Handles addressing, packaging and routing of packets between networks (IP addresses)

Link layer

Handles the physical transmission of data over the network hardware/media (uses MAC addresses)

Wired network

Devices connected via physical cables (e.g. Ethernet); advantage: faster, more reliable, less interference

Wireless network

Devices connected via radio waves (Wi-Fi); advantage: greater mobility/flexibility, easier to add new devices, no trailing cables

Encryption

Scrambling data using a key/algorithm so that it is unreadable to anyone who intercepts it without the correct key to decrypt it

Malware

Malicious software designed to disrupt, damage, or gain unauthorised access to a computer system (e.g. virus, worm, trojan, spyware, ransomware)

Virus

Malware that attaches itself to files/programs and replicates when the infected file is run, potentially corrupting or deleting data

Trojan

Malware disguised as legitimate software that, once run, allows unauthorised access or performs malicious actions

Ransomware

Malware that encrypts a victim's files and demands payment (a ransom) to restore access

Phishing

Sending fraudulent emails/messages that appear to be from a trustworthy source to trick users into revealing personal/sensitive data

Social engineering

Manipulating people (rather than technology) into breaking security procedures, e.g. through phishing, shouldering, or pretexting

Brute force attack

Repeatedly and systematically trying every possible password/key combination until the correct one is found

Denial of Service (DoS) attack

Flooding a server/network with excessive traffic/requests to overload it and make it unavailable to legitimate users

SQL injection

Entering malicious SQL code into an input field (e.g. a login box) to access, damage, or manipulate a database

How to prevent SQL injection

Input validation/sanitisation, using parameterised queries, restricting database permissions

Penetration testing

Authorised, simulated attacks on a system carried out to find security vulnerabilities before real attackers can exploit them

Anti-malware software

Software that detects, quarantines and removes malicious software from a computer system

Firewall

Hardware or software that monitors and controls incoming/outgoing network traffic based on security rules, blocking unauthorised access

MAC address filtering

Restricting network access to only devices whose MAC addresses are on an approved list

User access levels

Restricting what different users can view/edit/access based on their role, limiting the damage a compromised account can do

Physical security

Measures like locks, biometrics, or ID badges used to physically prevent unauthorised people accessing hardware

Operating system — memory management

Allocates and manages RAM between the OS, running applications and background processes, and handles virtual memory

Operating system — multitasking

Allows multiple processes/programs to run seemingly at the same time by rapidly switching the CPU between them

Operating system — peripheral management

Manages communication between the computer and peripherals (e.g. printers, keyboards) using drivers

Operating system — user management

Manages user accounts, permissions and access levels to keep the system secure

Operating system — file management

Organises, names, stores and provides access to files and folders on secondary storage

Utility software

Software that helps maintain/optimise the computer system, e.g. antivirus, backup software, defragmentation, compression software

Defragmentation software

Reorganises fragmented files on a hard disk so that each file's data is stored in contiguous blocks, improving read/write speed