Understand a CPU instruction set
Describe an instruction set as the machine-code commands a CPU can process.
This sequence follows Cambridge IGCSE Computer Science 0478, 2026–2028 syllabus Version 6. It covers subject content sections 3.1–3.3 only. The separately published 2029–2031 syllabus is not mapped here. Practice is original, self-checked and does not award an official Cambridge grade. Full timed papers and human subject-teacher review remain outstanding.
Before you begin
- No programming language is required. Be comfortable with place value and multiplication; each lesson introduces the computer-science notation it uses.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What kind of code does a CPU process directly?
What you will learn
- Define an instruction set
- Explain that its commands are machine code
- Connect supported instructions to CPU compatibility
A CPU has a defined command set
An instruction set is the list of machine-code commands that a CPU can process. Instructions specify operations such as moving data, arithmetic, comparisons or branching.
Apply and check the rule
A program must be translated into commands the target CPU can execute. Different processor architectures may use different instruction sets, so software compatibility can depend on the processor or a translation layer.
Pause and explain
What is a likely reason an executable may be incompatible with another CPU architecture?
Worked example
Why might a program built for one processor not run directly on a different processor?
Show the worked solution
- The processors may support different instruction sets.
- The program's machine-code commands may not be understood by the other CPU.
- It may need to be rebuilt or translated for that architecture.
Answer The target CPU may not understand the program's machine-code instruction set, so the software may need recompilation or translation.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Define an instruction set in one sentence.
Give me a hint
Include commands and which component processes them.
Compare my reasoning
- A CPU processes a defined set of machine-code commands.
- That list is its instruction set.
An instruction set is the list of machine-code commands a CPU can process.
Look for these in your work
- Machine code named
- CPU connection made
A program contains a machine-code command that the target CPU does not support. What is the compatibility problem?
Give me a hint
Compare the command with the CPU's supported set.
Compare my reasoning
- The command is not in the target instruction set.
- The CPU cannot execute that instruction directly.
- The program needs a compatible build or translation.
The CPU cannot directly process a command outside its instruction set; the program must be rebuilt or translated for the target.
Look for these in your work
- Unsupported command identified
- Execution consequence
- Possible remedy
A laptop and a microcontroller both have CPUs. Explain why this does not mean the same machine-code program will run on both.
Give me a hint
The CPU label describes a role, not a common instruction set.
Compare my reasoning
- Each CPU has an instruction set it can process.
- Different processor architectures can use different machine-code commands.
- Software may need a separate build or translation for each target.
Both CPUs process instructions, but their supported instruction sets may differ. A program must use commands the target processor understands.
Look for these in your work
- CPU role stated
- Architecture difference stated
- Compatibility implication
Common mistakes
- Calling the instruction set a list of applications
- Confusing an instruction set with the CPU's clock speed
What can you explain now?
What does an instruction set list?
Compare with the explanation
The machine-code commands a CPU can process
It defines the commands supported by that processor.
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Make it stick
Why does a program's target architecture matter to machine-code compatibility?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.