IGCSE Computer Science Paper 1 Definitions Worksheet: 3.1 Computer Architecture (Modules 218–227)
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Date:
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Key terms
Write what each term means, in your own words.
1
register
2
Program counter (PC)
3
MAR
4
MDR
5
CIR
6
Accumulator (ACC)
7
bus
8
address bus
9
data bus
10
control bus
Answer Key
IGCSE Computer Science Paper 1 · Modules 218–227 · Student wording will vary for definitions
1register — A very small, very fast storage location inside the CPU (not in RAM) that holds one item of data or one address, such as the PC, MAR, MDR, CIR or ACC.
2Program counter (PC) — A register in the CPU that holds the address of the next instruction to be fetched. It increments after each fetch and changes when the program jumps.
3MAR — Memory Address Register: a CPU register that holds the address of the memory location about to be read from or written to. It holds an address, not data, and sends it along the address bus.
4MDR — Memory Data Register: a CPU register that temporarily holds the data or instruction just fetched from RAM, or data about to be written to RAM. It holds data, not addresses.
5CIR — Current Instruction Register: holds the instruction copied from the MDR while the control unit decodes it and the CPU executes it. It stores the instruction itself, not an address.
6Accumulator (ACC) — A register in the CPU that holds data being worked on and the results of ALU calculations, storing intermediate results before they are saved back to memory or output.
7bus — A set of wires that carries data, addresses or control signals between the CPU, RAM and other components. More wires means more bits can be carried at once.
8address bus — A one-way bus that carries the address of a memory location from the CPU (sent from the MAR) to RAM. It carries addresses, not data.
9data bus — A set of wires that carries data and instructions in both directions between the CPU and RAM. It connects to the MDR.
10control bus — A set of wires that carries control signals, such as read and write, from the control unit to other components so they work together. It carries signals, not data or addresses.