CPU Instruction Sets (Cambridge (CIE) O Level Computer Science)
Revision Note
Written by: James Woodhouse
Reviewed by: Lucy Kirkham
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 binary code is made up of an operation code (opcode) and an operand
The opcode is the operation to be performed
The operand is the location where the operation is to be performed in
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 into an opcode and an operand, the CPU finds the opcode in the processor’s 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 |
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 would be the operation if the instruction was 00100010 00000010?
[1]
Answer
Either of:
The operation would be SUB [1]
If the operand was raw data the complete instruction would be to subtract 2 from the value in the accumulator [1]
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