CPU Instruction Sets (Cambridge International IGCSE Computer Science)
Exam board Cambridge International · IGCSEUpdated 2026-01-05
Spec point —
3.1.4· "4 Understand the purpose and use of an instruction set for a CPU"
CPU Instruction Sets
What is an Instruction set?
- An instruction set is a list of all the commands that can be processed by a CPU
- Each command has a binary code which is called machine code
- The table below shows an example instruction set
- Each instruction has a mnemonic that indicates what the instruction does alongside an example binary code
- After an instruction is decoded, the CPU matches it to an instruction in its instruction set
- It then knows what operation to perform when executing the instruction
| Instruction | Mnemonic | Binary code | Command |
|---|---|---|---|
| Add | ADD | 10100001 | Adds a value to the value currently stored in the accumulator (ACC) |
| Subtract | SUB | 00100010 | Subtract a value from the values stored in the accumulator |
| Load | LDA | 10111111 | Load the value stored in a memory location into the accumulator |
| Store | STA | 01100000 | Store the value in the accumulator in a specific location in memory |
| Stop | HLT | 00000000 | Stop the program |
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Instruction lists are machine-specific
- A program created using one computer’s instruction set would not run on a computer containing a processor made by a different manufacturer
- For example, a computer program created using Intel’s instruction set would not run on a device containing an ARM processor
Worked Example Using the instruction set in the table above, what operation would be performed if the instruction was 00100010 00000010?
[1]
Answer
Either of:
- The operation would be SUB [1] - This instruction subtracts a value from the value stored in the accumulator [1]
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