Trace a given simple assembly language program

4.2 Assembly Language – Tracing a Simple Program

Assembly language is like a recipe for the computer’s CPU. Each line tells the CPU exactly what to do, step by step. In this lesson we’ll trace a tiny program to see how the CPU changes its registers and memory as it runs.

Key Concepts

  • Registers – tiny storage spots inside the CPU (e.g., R1, R2).
  • Memory – a big list of addresses where data can be stored.
  • Instruction Pointer (IP) – tells the CPU which line to execute next.
  • Opcode – the operation to perform (e.g., ADD, SUB, LOAD, STORE).
  • Operands – the data the opcode works on (registers, immediate values, or memory addresses).

Example Program

Line Instruction
1 LOAD R1, 10
2 LOAD R2, 20
3 ADD R1, R2
4 STORE R1, 100
5 HALT

Tracing the Program

  1. Initial State:
    • IP = 1
    • R1 = 0, R2 = 0
    • Memory[100] = 0 (uninitialised)
  2. Line 1 – LOAD R1, 10
    • R1 now holds the value 10.
    • IP increments to 2.
  3. Line 2 – LOAD R2, 20
    • R2 now holds 20.
    • IP increments to 3.
  4. Line 3 – ADD R1, R2
    • R1 = R1 + R2 = 10 + 20 = 30.
    • IP increments to 4.
  5. Line 4 – STORE R1, 100
    • Memory[100] now contains 30.
    • IP increments to 5.
  6. Line 5 – HALT
    • Program stops.
  7. Final State:
    • R1 = 30, R2 = 20
    • Memory[100] = 30
    • IP = 5 (HALT)

Analogy: Think of LOAD as “pick up a number from the shelf”, ADD as “mix two numbers together”, and STORE as “put the result back on the shelf”. The HALT instruction is like a stop sign that tells the kitchen to close for the day.

Exam Tips

✔ Remember the order of execution: The CPU always follows the IP, so trace line by line.
✔ Keep a table: Write down register values and memory contents after each instruction – it’s the safest way to avoid mistakes.
✔ Watch for side effects: Some instructions change the IP (e.g., jumps). If you see a JMP or CALL, note the new IP.
✔ Use emojis for memory: 🗂️ for memory, 🧠 for registers, ⏱️ for IP – they help you visualise the flow.

Common Mistakes to Avoid

  • Confusing LOAD R1, 10 with LOAD R1, [10] – the first loads an immediate value, the second loads from memory address 10.
  • Forgetting that ADD R1, R2 does not change R2 – only R1 changes.
  • Assuming the program ends automatically after the last line – you must see a HALT or similar stop instruction.

🎓 Practice Tip: Write your own 3–5 line program that uses SUB and MUL, then trace it. The more you practice, the easier it becomes to spot errors in exam questions.

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