CPU Instruction Sets (Cambridge (CIE) O Level Computer Science)

Revision Note

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Expertise

Computer Science

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

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]

to absolutely everything:

the (exam) results speak for themselves:

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