Multiple choiceQuestion 1 · Recall
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By the end you can describe the job of the Arithmetic Logic Unit (ALU) and the Control Unit (CU), and follow one instruction through both of them.
The CPU does exactly two kinds of work, and each has its own unit:
| Unit | Job |
|---|---|
| Arithmetic Logic Unit (ALU) | The doer: arithmetic (, , , ) and logic (AND, OR, NOT, comparisons) |
| Control Unit (CU) | The director: fetches instructions, decodes them, and tells everything else when and how to act |
The ALU produces the numbers. The CU decides which numbers, when, and where the result goes.
Arithmetic Logic Unit (ALU)
The component of the CPU that performs arithmetic operations (, , , ) and logical operations (AND, OR, NOT and comparisons such as less than). It takes its inputs from registers and writes the result back to a register, setting flags in the status register as a side effect.
Control Unit (CU)
The component of the CPU that controls and coordinates all the other parts: it fetches instructions, decodes them, and issues control signals — timed electrical pulses that tell memory, the registers and the ALU what to do and when.
Worked example: What happens to ADD 7
The CIR holds ADD 7 while the accumulator holds 5. Follow the instruction through
the CPU.