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Computer Science IGCSEPDF 23
142 pagesrevision 46930
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→ Address Bus – One-way; it transfers address
information, showing where data should be read
from or written to.
→ Data Bus – Two-way; it carries data or instructions
to and from components.
→ Control Bus – Two-way; it sends control signals
and commands to tell components when to carry
out read or write operations, etc.
Fetch-Decode-Execute Cycle
The Fetch-Decode-Execute Cycle is the process CPU performs
billions of times each second to operate a computer.
A computer receives input, processes it, and produces an output
for the user.
Input: Pressing a key on the keyboard
Process: The CPU processes the keypress by referencing the
program’s instructions
Output: A letter appears on the screen in a text editor
Fetch Stage
○ During the fetch stage of the cycle, the program counter
(PC) contains the address of the next instruction to be
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