📚 Knowledge Library — Topic 3.4 — Hardware

Execute Stage

Understand how the CPU carries out a decoded instruction and stores the result before the Fetch–Decode–Execute cycle repeats.

Start LearningWorked Example
1. Invitation

The CPU now carries out the instruction.

Fetch brought the instruction into the CPU. Decode worked out what it meant.

Execute is the stage where the instruction is finally carried out.

💡 Remember: execute means do the action.
Figure 1.1
FDE Journey
Fetch
↓
Decode
↓
Execute
2. Big Idea

The Control Unit sends the right signals.

Once the instruction has been decoded, the Control Unit sends control signals to the correct CPU components.

These signals tell the CPU what needs to happen next.

💡 The Control Unit coordinates the execute stage.
Figure 2.1
Control Signals
Control Unit
↓
signals
↓
ALU • ACC • RAM
3. FutureLogic Bridge

Think of the chef finally doing the task.

The chef has read the recipe card and understood the instruction.

Now they actually do it: chop, mix, add, compare, or store something for later.

💡 Bridge: execute is the moment the task happens.
Figure 3.1
Chef Model
Read task
↓
Understand task
↓
Do task
4. Worked Example

The ALU performs calculations and logic.

If the instruction needs arithmetic or logic, the ALU does the work.

ALU means Arithmetic Logic Unit. The result is stored in the ACC, the Accumulator.

Example instruction

Instruction
ADD
ALU
calculates
ACC
stores result
ALU does the calculation. ACC stores the answer.
Figure 4.1
ALU and ACC
ALU
+
calculation
↓
ACC stores result
5. Data Movement

Some instructions move data instead.

Not every instruction is a calculation. Some instructions load data from memory or store data back into memory.

The Control Unit sends signals so data moves between RAM, the MDR and the CPU.

💡 Execute can calculate, compare, load data or store data.
Figure 5.1
Moving Data
RAM
↔
MDR
↔
CPU
MDR = Memory Data Register.
6. Exam Tip

Separate ALU and ACC clearly.

The ALU performs arithmetic and logical operations.

The ACC stores the result of those operations.

🎯 Exam Tip: ALU performs the calculation. ACC stores the result.
Figure 6.1
Different Jobs
ALU
= does work

ACC
= stores answer
7. Common Mistake

The Accumulator does not carry out calculations.

Students often write that the accumulator calculates the answer.

That is not correct. The accumulator stores the result after the ALU has done the work.

⚠️ Common Mistake: ACC stores results. It does not perform calculations.
Figure 7.1
Avoid the Trap
Calculation
✓
ALU

Result storage
✓
ACC
8. Summary

The execute stage in one screen.

During execute, the Control Unit sends signals to carry out the decoded instruction.

The ALU may perform arithmetic or logical operations, and the ACC stores the result. Then the FDE cycle repeats.

💡 Key idea: execute is where the instruction becomes action.
Figure 8.1
Execute Summary
Control Unit signals
↓
ALU may calculate
↓
ACC stores result
↓
Cycle repeats