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A-LEVEL COMPUTER SCIENCE —
KEY TERMS (OCR H446) STUDY
GUIDE (2026) GRADE A+
CPU - Correct Answer- Central Processing Unit
"The main part of the computer, consisting of the registers, ALU and control unit."
Clock Speed - Correct Answer- "Measured in Hertz,
the clock speed is the frequency at which the internal clock generates pulses. The higher the clock rate, the faster the computer may work. The "clock" is the electronic unit that synchronises related components by generating pulses at a constant rate."
Cores - Correct Answer- "A part of a multi-core
processor. A multi-core processor is a single component with two or more independent actual CPUs, which are the units responsibly for the fetch-decode-execute cycle."
Cache - Correct Answer- "A part of the main store
between the central processor and the rest of the
memory. It has extremely fast access, so sections of a program and its associated data are copied there to take advantage of its short fetch cycle."
Pipelinning - Correct Answer- "The concurrent
decoding of two or more machine instructions. While part of one instruction (for example, an address field) is being decoded, another part of a second instructions (for example, an operation code) may also be decoded."
Von Neumann Architecture - Correct
Answer- "Traditional computer architecture that forms
the basis of most digital computer systems. A single control unit manages program control flow following a linear sequence of "fetch-decode-execute""
Harvard Architecture - Correct Answer- "A computer
architecture with physically separate storage and signal pathways for instructions and data. These early machines had data storage entirely contained within the central processing unit, and provided no access to the instruction storage as data."
Contemporary Architecture - Correct Answer- "Any
modern set of disciplines that describes the functionality, the organisation and the implementation of computer systems."
Parallel Processor System - Correct Answer- "The
simultaneous use of several processors to perform a single job. A job may be split into a number of tasks each of which may be processed by any available processor."
Input Device - Correct Answer- "Any peripheral
device that can accept data, presented in the appropriate machine-readable form, decode it and transmit it as electrical pulses to the CPU"
Output Device - Correct Answer- "Any peripheral
device that translates signals from the computer into a human-readable form or into a form suitable for reprocessing by the computer at a later stage."
Storage Device - Correct Answer- "Any medium
(optical, magnetic, solid state and even paper which holds data or programs"
Magnetic Storage - Correct Answer- "Storage
medium which uses surfaces coated with a layer of magnetic material on which data can be stored by magnetically setting the arrangement of the magnetic material. This is done by electromagnetic read/write heads."
Flash Storage - Correct Answer- "Collection of
memory chips that is controlled by its own software to make the collection of chips act like a disk drive."
Optical Storage - Correct Answer- "Storage medium
that uses plastic discs on which the data is stored as patterns on the surface in pits and lans."
RAM - Correct Answer- Random Access Memory
"Volatile main memory. Access times are very fast. Often referred to as Main Memory, although RAM can be used in main areas of computing and computing storage. When used as main memory RAM typically can be thought of as containing the Operating System, programs in use and the data those programs are using while the computer is running."
ROM - Correct Answer- Read Only Memory
"Memory for which the contents may be read by cannot be written to by the computer system. Software in ROM is fixed during manufacturing. It typically holds system boot up instructions. ROM is non-volatile. More modern types of ROM can be written to in limited ways, these include, PROM, EPROM and EAROM."
program by carving up memory logically rather than physically."
Virtual Memory - Correct Answer- "Used when
sufficient immediate-access store is not available. Part of a disk drive is allocated to be used as if it were main memory. This is very slow, and the software will attempt to use the immediate-access store if possible."
Interrupt - Correct Answer- "A signal generated by a
source such as an input or output device or a systems software routine that causes a break in the execution of the current routine. Control passes to another routine in such a way that the original routine can be resumed after the interrupt."
ISR - Correct Answer- Interrupt Service Routine
"A software routine that hardware invokes in response to an interrupt. ISRs examine an interrupt and determine how to handle it."
Scheduling - Correct Answer- "The method by which
central processor time is allocated in a multi-access system."
RR - Correct Answer- Round Robin
"A scheduling algorithm that deals with each user or tasks to be processed in turn."
FCFS - Correct Answer- First Come First Serve
"A scheduling algorithm that deals with each user or task in the order in which they arrive, this can be thought of as a queue."
MLFQ - Correct Answer- Multi-Level Feedback
Queue "A complex scheduling algorithm that deals with tasks based on a set of priorities and rules across different league tables, jobs in a certain table get promoted up or down their table based on these rules and can then end up in totally different tables if they relegated."
SJF - Correct Answer- Shortest Job First
"A scheduling algorithm that deals with each user or task based on the getting the smaller ones out of the way."
SRT - Correct Answer- Shortest Remaining Time
"A scheduling algorithm that deals with each user or task based on an calculating a estimated time remaking to complete."
Real-Time OS - Correct Answer- "An operating
system intended to serve real-time application process data as it comes in, typically without buffering delays."
BIOS - Correct Answer- Basic Input / Output System
"Part of the operating system that handles the input and output of the computer. It enables the operating system to use the particular features of the hardware being used."
Device Drivers - Correct Answer- "A computer
program that operates or controls a particular type of device that is attached to a computer."
Virtual Machine - Correct Answer- "An emulator of a
particular computer system. Virtual machines operated based on the computer architecture and functions of a real or hypothetical computer, and their implementations may involve specialised hardware, software, or a combination of both."
Intermediate Code - Correct Answer- "Half-way type
of code which is standard across machine types. Runs on a virtual machine."
Application - Correct Answer- "Any program, routine
or procedures (together with their associated documentation) that can be run on a computer system."
Utilities - Correct Answer- "A systems program that
performs some specific task in the operation of the computer, for example file backup, virus checking or a compression program."
Open Source - Correct Answer- "Software for which
the original source code is made freely available and may be redistributed and modified. - Open source file types are often able to be opened on many different types of applications."
Closed Source (Propriety) - Correct Answer- "Often
called Proprietary is a type of computer program for which the software code is not shared with the public for anyone to look at or make changes to - Closed / Proprietary file types are often only able to be opened if you own a version of the software they were originally made in."
Source Code - Correct Answer- "Original code typed
in by the programmer in the native language. This is the code as it appears before it is compiled or interrupted."
symbol table. The lexical analyser also remove unnecessary charters such as spaces, tabs and coder comments."
Syntax Analysis - Correct Answer- "The stage in
complication where language statements are checked against the rules of the language, errors being reported if a statement is not valid."
Code Generation - Correct Answer- "The stage in
complication that produces a machine-code program that is equivalent to the source program."
Optimisation - Correct Answer- "The stage of
compilation that ensures that the executable program if optimised at least as much as the compiler can predict. e.g. removing reference to unused variables or routines."
Linker - Correct Answer- "Software tool that allows
already compiled object code files or modules to be combined with the compiled program."
Loaders - Correct Answer- A program that copies an
object program held on backing store into main store ready for execution."
Libraries - Correct Answer- "A collection of pre-
compiled routines which can be incorporated into a program."
SDLC - Correct Answer- Systems Development Life
Cycle "The various stages in designing and implementing a new computer system. These stages - typically analysis, design, implementation, testing and installation - are part of the systems life cycle performed by the main design team. Not be to confused with the Software Development Cycle which is part of the implementation stage of the systems life cycle."
Waterfall Model - Correct Answer- "An earlier
abstract description of the system lifecycle where each identified stage of development flows from the previous one and down to the next one. Feedback from each to the previous takes place independently of the forward flow."
Agile Methodologies - Correct Answer- "Describes
the responsive development of a system made of small software modules (often web-based applications) by a group of collaborators who work concurrently and closely under a leader who ensures engineering best practice and delivery of the customer requirements."
Paradigm is a style or way of programming. E.g. Low- Level languages, High-Level languages, Declarative languages are all examples of different programming paradigms."
Procedural Language - Correct Answer- "Any high
level language in which program statements can be grouped in self-contained blocks called procedures and functions. These procedures have their own variables, not accessible outside the procedure."
Assembly Language - Correct Answer- "A language
which is related very closely to the computer's own machine code. "
Machine Code - Correct Answer- "Set of all possible
instructions made available by the hardware design of a particular processor. Closest to pure binary."
Low Level Language - Correct Answer- "A language
which is close to machine code. Related closely to the design of the machine. A one-to-one language"
High Level Language - Correct Answer- "A language
designed to help a programmer express a computer program in a way that reflects the problem that is being
solved, rather than the details of how the computer will produce the solution. One-to-many language."
LMC - Correct Answer- Little Man Computer
"An instructional model of a computer, created by Dr. Stuart Madnick in 1965. The LMC is generally used to teach students, because it models a simple von Neumann architecture computer - which has all of the basic features of a modern computer. It can be programmed in machine code or assembly code"
Immediate Addressing - Correct Answer- "Modifies
the address (either a direct or an indirect address) in the address field by the addition of a number held in a special-purpose registers, called an index register, before the address is used. Index registers are quickly and easily altered providing an efficient way of accessing a range of memory l9ocations, such as in an array."
Direct Addressing - Correct Answer- "Uses the data
in the address field without alteration. This is the simplest method of addressing and also the most common."
CU - Correct Answer- "The part of the CPU that
manages the execution of instructions. The control unit fetches each instruction in sequence, and decodes and synchronises it before executing it by sending control signals to other parts of the computer."
Register - Correct Answer- "Tiny areas of extremely
fast memory located in the CPU normally designed for a specific purpose, where data or control information is stored temporarily."
PC - Correct Answer- Program Counter
"A register in the control unit which holds the address of the next instruction to be executed."
ACC - Correct Answer- Accumulator
A special register within the ALU. It is used to hold the data currently being processed by the central processor. Any data to be processed is stored temporarily in the accumulator, the results ending up back in the accumulator being stored in the memory unit."
MAR - Correct Answer- Memory Address Register
"A register in the CPU that stores the address of the memory location currently in use. In the fetch phase, this would be the address of the instruction being loaded; in
the execute phase, it would be the address of the data being used."
MDR - Correct Answer- Memory Data Register
"A register in the CPU that stores data being transferred to and from the immediate-access store. It acts as a buffer, allowing the central processor and memory unit to act independently without being affected by minor differences in operation. A data item will be copied to the MDR ready for use at the next clock pulse, when it can either be used by the central processor or be stored in main memory."
CIR - Correct Answer- Current Instruction Register
"A register in the control unit that stores the address of the next instruction currently being executed and decoded."
Buses - Correct Answer- "A common physical
pathway shared by signals to and from several components of a computer."
Data Bus - Correct Answer- "A common physical
pathway shared by signals to and from several components of a computer."
