Computer Science (UK GCSE)
Flashcards covering the UK GCSE Computer Science curriculum (AQA/OCR, Years 10-11): systems architecture, memory and storage, data representation, networks, security, systems software, ethics, algorithms, programming, Boolean logic, translators and testing.
Ämne: Teknik · Nivå: Högstadium (13–15) · 438 kort
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- The CPU (Central Processing Unit) is the component that processes data and instructions. It is often called the 'brain' of the computer.
- The purpose of the CPU is to fetch, decode and execute instructions stored in memory in order to process data.
- In the von Neumann architecture, both program instructions and data are stored together in the same main memory.
- The three stages of the fetch-decode-execute cycle are: fetch the instruction from memory, decode it in the control unit, and execute it.
- The ALU (Arithmetic Logic Unit) carries out arithmetic calculations (e.g. addition) and logical operations (e.g. AND, OR, comparisons).
- The Control Unit (CU) coordinates the activities of the CPU. It directs the flow of data, decodes instructions, and sends control signals.
- Registers are small, extremely fast storage locations inside the CPU that hold data and instructions currently being processed.
- The Program Counter (PC) is a register that holds the memory address of the next instruction to be fetched.
- The MAR (Memory Address Register) holds the address of the memory location that is about to be read from or written to.
- The MDR (Memory Data Register) temporarily holds the data or instruction that has been fetched from, or is to be written to, memory.
- The Accumulator is a register that stores the intermediate results of arithmetic and logic operations carried out by the ALU.
- Clock speed is measured in hertz (Hz). It is the number of fetch-decode-execute cycles a CPU can perform per second. 1 GHz = one billion cycles per second.
- A CPU core is an individual processing unit. A multi-core CPU (e.g. dual-core, quad-core) can process multiple instructions simultaneously.
- Cache is a small amount of very fast memory located inside or close to the CPU. It stores frequently used instructions and data to speed up processing.
- The three main factors that affect CPU performance are clock speed, number of cores, and cache size.
- An embedded system is a computer built into a larger device to perform a specific, dedicated task — e.g. a washing machine controller or a car engine management system.
- Cache memory closer to the CPU (Level 1) is faster but smaller; Level 2 and Level 3 cache are progressively larger but slower.
- Buses are sets of parallel wires that transfer data between CPU components. The main three are the address bus, data bus and control bus.
- An instruction set is the complete list of all the commands a particular CPU can understand and execute.
- Embedded systems usually have low power consumption, low cost, and dedicated software (firmware) stored in ROM that rarely changes.
- RAM (Random Access Memory) is volatile main memory that temporarily holds the data, programs and parts of the operating system currently in use. Its contents are lost when power is off.
- ROM (Read Only Memory) is non-volatile memory that stores the bootstrap/BIOS used to start up the computer. Its contents are not lost when power is off.
- Volatile memory loses its contents when power is removed (e.g. RAM). Non-volatile memory keeps its contents without power (e.g. ROM, SSD, hard disk).
- Virtual memory is a portion of secondary storage (hard disk/SSD) used as an extension of RAM when RAM becomes full. Inactive data is swapped to disk.
- Virtual memory is slower than RAM because moving data between RAM and disk (swapping) takes time, and disk access is much slower than RAM access.
- Flash memory is a type of non-volatile solid state storage with no moving parts, used in USB sticks, SSDs and memory cards.
- Magnetic storage (e.g. hard disk drives, magnetic tape) stores data using magnetised regions on spinning platters or tape. It is cheap with high capacity but has moving parts.
- Optical storage (CD, DVD, Blu-ray) stores data as pits and lands on a disc, read by a laser. It is cheap and portable but has low capacity and slow access.
- Solid state storage (SSD) uses flash memory with no moving parts. It is fast, durable and silent but more expensive per gigabyte than magnetic storage.
- Secondary storage is non-volatile and is needed to store programs and data permanently, because RAM is volatile and loses data when powered off.
- A bit (binary digit) is the smallest unit of data in a computer and can hold a value of 0 or 1.
- A nibble is 4 bits. A byte is 8 bits.
- The order of data units from smallest to largest is: bit, nibble, byte, kilobyte (KB), megabyte (MB), gigabyte (GB), terabyte (TB), petabyte (PB).
- Using decimal (SI) prefixes: 1 KB = 1000 bytes, 1 MB = 1000 KB, 1 GB = 1000 MB, 1 TB = 1000 GB. (AQA uses this 1000-based convention.)
- Computers use binary (base 2) because the two states 0 and 1 can be represented reliably by electronic components being off or on (low or high voltage).
- The place values of an 8-bit binary number, from left to right, are: 128, 64, 32, 16, 8, 4, 2, 1.
- The largest denary (decimal) number that can be stored in 8 bits is 255 (binary 11111111). This gives 256 different values (0 to 255).
- Overflow occurs when the result of a binary addition is too large to be stored in the available number of bits (e.g. a 9-bit result in an 8-bit register), causing the carry bit to be lost.
- In binary addition the four rules are: 0+0=0, 0+1=1, 1+1=10 (carry 1), and 1+1+1=11 (write 1, carry 1).
- A left binary shift multiplies a number by 2 for each place shifted; a right binary shift divides by 2 for each place shifted. Bits shifted off the end are lost.
- Hexadecimal is a base-16 number system using the digits 0-9 and the letters A-F, where A=10, B=11, C=12, D=13, E=14, F=15.
- One hexadecimal digit represents exactly 4 binary bits (one nibble), so two hex digits represent one byte.
- Hexadecimal is used because it is shorter and easier for humans to read and write than long binary strings, and it is easy to convert between hex and binary.
- Hexadecimal is commonly used to represent colours (e.g. #FF0000 for red), MAC addresses, and memory addresses.
- A character set is a defined list of characters a computer can recognise, where each character has a unique binary code (e.g. ASCII, Unicode).
- ASCII uses 7 bits to represent 128 different characters. Extended ASCII uses 8 bits to represent 256 characters.
- In ASCII the codes for letters are sequential: 'A' is 65, and uppercase letters come before lowercase ('a' is 97). The difference between a letter's upper and lower case code is 32.
- Unicode uses more bits per character than ASCII, allowing it to represent over a million characters including alphabets from many languages and emoji.
- ASCII is backward compatible with Unicode: the first 128 Unicode code points are identical to the ASCII codes.
- The number of bits needed for a character set determines how many characters it can hold: n bits can represent 2ⁿ different characters.