FETCH · DECODE · EXECUTE
HOW A CPU RUNS CODE
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How Does the CPU Run an Instruction?
CPU
PC
IR
ACC
CU
ALU
RAM
0:
LOAD 5
1:
ADD 3
2:
STORE
3:
HALT
bus
step
instruction
accumulator
1. Machine Cycle
1
Fetch
2
Decode
3
Execute
2. Who Does What?
CU: control · ALU: calculations
Registers: fast, small memory
3. Remember
3 GHz = 3 billion cycles
every second!
COMPUTER SCIENCE · COMPUTER SYSTEMS · PUNJAB BOARDS
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