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The CPU and the fetch-decode-execute cycle
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The CPU is the brain of the computer. It runs programs by repeating one simple cycle billions of times a second.
Von Neumann architecture
In the von Neumann model, instructions and data are stored together in the same memory.
The CPU fetches each instruction from memory and carries it out.
The CPU fetches each instruction from memory and carries it out.
Parts of the CPU
Control unit (CU): decodes instructions and controls the other parts.
Arithmetic logic unit (ALU): does calculations and logical comparisons.
Registers: tiny, very fast stores inside the CPU.
Clock: sends regular pulses that keep everything in time.
Arithmetic logic unit (ALU): does calculations and logical comparisons.
Registers: tiny, very fast stores inside the CPU.
Clock: sends regular pulses that keep everything in time.
Fetch, decode, execute
Fetch: the next instruction is copied from memory into the CPU.
Decode: the control unit works out what the instruction means.
Execute: the instruction is carried out, for example by the ALU.
The cycle then repeats.
Decode: the control unit works out what the instruction means.
Execute: the instruction is carried out, for example by the ALU.
The cycle then repeats.
A CPU has a clock speed of 3 GHz. How many cycles does it perform each second?
- Giga means 109, or one billion.
- 3 GHz = 3 × 109 cycles per second.
- That is 3 billion cycles a second.
Answer: 3 billion
Von Neumann: instructions and data share memory. CPU parts: control unit, ALU, registers, clock. It repeats fetch, decode, execute.
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