OCR Computer Science A
Level Definitions 2026 |
Key Terms, Study Guide,
Practice Questions,
Answers & Exam Review |
Complete Ethics Study
Guide
Updated 2026 Questions and Answers
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Included
,Arithmetic and Logic Unit (ALU) The part of the CPU where data is processed and manipulated. This processing and
manipulation normally consists of arithmetic operations or logical comparisons
allowing a program to make decisions.
Control Unit (CU) 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 Tiny areas of extremely fast memory located in the CPU normally designed for a
specific purpose, where data or control information is stored temporarily.
Program Counter (PC) A register in the control unit which holds the address of the next instruction to be
executed.
Accumulator (ACC) 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.
Memory Address Register (MAR) 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.
Memory Data Register (MDR) A register that contains the data to be stored in the computer storage, or the data
after a fetch from the computer storage. It acts like a buffer and holds anything that
is copied from the memory ready for the processor to use it.
Current Instruction Register (CIR) A register in the control unit that stores the address of the instruction currently being
executed and decoded.
Buses A common physical pathway shared by signals to and from several components of a
computer.
Data Bus The part of the bus which carries the actual information.
Address Bus The part of the bus which carries identification about where the data is being sent.
Control Bus This bus carries command and control signals to and from every other component
of a computer.
, Fetch Decode Execute The complete process of retrieving an instruction from store, decoding it and
carrying it out. Also known as the instruction cycle.
CPU The main part of the computer, consisting of the registers, ALU and control unit.
Clock Speed 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 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 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.
Pipelining 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 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 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 Any modern set of disciplines that describes the functionality, the organisation and
the implementation of computer systems.
Complex Instruction Set Computer (CISC) A design that produces a complicated and expensive integrated circuit capable of
performing a large variety of complex instructions. Complex instructions can be
executed with few machine cycles.
Reduced Instruction Set Computer (RISC) A design that produces a simple, cheap integrated circuit with a basic range of
machine instructions. Relies on speed as complex instructions take many machine
cycles.
Graphics Processing Unit Designed to rapidly manipulate and alter memory to accelerate the creation of
images in a frame buffer intended for output to a display.
Modern GPUs have a highly parallel structure that makes them more effective than
CPUs for algorithms where processing of large blocks of data is done in parallel.
Level Definitions 2026 |
Key Terms, Study Guide,
Practice Questions,
Answers & Exam Review |
Complete Ethics Study
Guide
Updated 2026 Questions and Answers
100% Verified Exam Prep and Comprehensive
Rationales
Included
,Arithmetic and Logic Unit (ALU) The part of the CPU where data is processed and manipulated. This processing and
manipulation normally consists of arithmetic operations or logical comparisons
allowing a program to make decisions.
Control Unit (CU) 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 Tiny areas of extremely fast memory located in the CPU normally designed for a
specific purpose, where data or control information is stored temporarily.
Program Counter (PC) A register in the control unit which holds the address of the next instruction to be
executed.
Accumulator (ACC) 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.
Memory Address Register (MAR) 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.
Memory Data Register (MDR) A register that contains the data to be stored in the computer storage, or the data
after a fetch from the computer storage. It acts like a buffer and holds anything that
is copied from the memory ready for the processor to use it.
Current Instruction Register (CIR) A register in the control unit that stores the address of the instruction currently being
executed and decoded.
Buses A common physical pathway shared by signals to and from several components of a
computer.
Data Bus The part of the bus which carries the actual information.
Address Bus The part of the bus which carries identification about where the data is being sent.
Control Bus This bus carries command and control signals to and from every other component
of a computer.
, Fetch Decode Execute The complete process of retrieving an instruction from store, decoding it and
carrying it out. Also known as the instruction cycle.
CPU The main part of the computer, consisting of the registers, ALU and control unit.
Clock Speed 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 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 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.
Pipelining 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 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 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 Any modern set of disciplines that describes the functionality, the organisation and
the implementation of computer systems.
Complex Instruction Set Computer (CISC) A design that produces a complicated and expensive integrated circuit capable of
performing a large variety of complex instructions. Complex instructions can be
executed with few machine cycles.
Reduced Instruction Set Computer (RISC) A design that produces a simple, cheap integrated circuit with a basic range of
machine instructions. Relies on speed as complex instructions take many machine
cycles.
Graphics Processing Unit Designed to rapidly manipulate and alter memory to accelerate the creation of
images in a frame buffer intended for output to a display.
Modern GPUs have a highly parallel structure that makes them more effective than
CPUs for algorithms where processing of large blocks of data is done in parallel.
