A machine instruction is a unique bit string that a device can identify and execute.
An Instruction refers generally to the words of the language understood by the CPU but other device (such as the GPU or printer) has also one. See Device - Programmable Device
The CPU requires a fixed number of clock ticks (or clock cycles) to execute each instruction. See Instruction - Cycle
Instruction that involves a memory address:
An instruction:
The instruction format is device dependent but show always this structure:
There is no grammar that defines what is a valid sequence of machine instructions. The CPU reads the instruction bytes, and if it is a valid machine instruction, it will be executed. Otherwise, typically an “invalid instruction” hardware exception will be generated.
Instructions are usually written in assembly with the help of mnemonic. See Assembly - Instruction
This group of instructions are general-purpose instructions that perform:
They operate on:
Each instruction is implemented via a circuit.
You can see them as function name.
The instruction pointer holds the location of the next instruction, and increments itself after every instruction.
Read the contents of memory at location X into register Y
With logic gates, it's possible to build a machine that perform the following instructions when the following code input are received.
Instruction Code
"add a number to another number" 00000001
"subtract a number from another number" 00000010
Example on the x86 ISA architecture: