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Computer Science IGCSEPDF 22
142 pagesrevision 46930
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Each register serves a specific function, and there are
five main types.
Name
Acronym
Purpose
Program Counter
PC
Keeps track of address of next instruction to b
executed, increasing by 1 with each cycle of
fetch-decode-execute process.
Memory Address
Register
MAR
Holds the address in memory from which data o
instruction will be fetched.
Memory Data
Register
Stores the data or instruction that has just bee
fetched from memory.
MDR
Current Instruction
Register
CIR
Stores the current instruction being decoded o
executed by the CPU.
Accumulator
ACC
Stores the result of calculations performed by t
Arithmetic Logic Unit (ALU).
○ Buses
A bus is a set of parallel wires through which data /
signals are transmitted from one component to
another.
There are 3 types of buses:
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