Machine code

Computer Science · Machine code

Same four lines. Two processors.

Both processors here, and their instruction sets, are invented for illustration. In each, the left four bits pick the action and the right four give a number when one is needed. Both start with a running value of 0.

Step through processor A first. When processor B takes over, stop and predict: same four lines, same result?

 1  0001 0101
 2  0010 0011
 3  0011 0000
 4  0000 0000

What you need to know

• Machine code is a language written entirely in binary: every instruction is a pattern of 1s and 0s.
• A processor executes machine code. Binary instructions are the form it carries out directly.
• Have a goSam says: 'Machine code is just English commands, but in capital letters.' One thing Sam has wrong?

  Machine code is written in binary, not in words.

  Every machine code instruction is a pattern of 1s and 0s, and the processor executes those patterns.

• That makes machine code a low-level language: it sits close to the processor, in the processor's own instructions.
• Every type of processor has its own machine code instruction set: the instructions it can carry out.
• Each instruction in that set is a particular binary pattern that this type of processor is built to recognise.
• Have a goProcessor X is built to recognise the pattern 1010. Processor Y is a different type. Does that tell you Y recognises 1010 too?

  No. It tells you nothing certain about Y.

  Each type of processor has its own instruction set, so what X recognises says nothing certain about what Y recognises.

• A different type of processor has a different instruction set, so the same pattern may mean something else, or nothing.
• So machine code is specific to a processor or family of processors, and a program is written for one family.
• Have a goMia says: 'It's all just 1s and 0s, so any computer can run it.' Is that safe to rely on?

  No. On a different type of processor it may not run.

  Binary is only how the instructions are written. What a pattern means depends on the processor's instruction set, so a program for one family may not run on another type.

• 'Low-level' is about closeness to the processor. It is not a measure of how easy or how old a language is.

The big picture

Machine code is the language a processor executes: every instruction is a pattern of binary. It is called a low-level language because it sits close to the processor. But each type of processor has its own machine code instruction set, so the same pattern can mean something different, or nothing, on another type. That makes machine code specific to a processor or family of processors.

⚠ Watch out

'It's all 1s and 0s, so any computer can run it.' Binary is only how machine code is written. Each pattern's meaning comes from the processor's instruction set, and a different type has a different one. And an instruction is a code, not a number to total.

Memory hook

One processor, one dictionary. The 1s and 0s only mean something inside that processor's instruction set, and a different type of processor brings a different dictionary. (The analogy has one crack: a processor doesn't look anything up. It is simply built to recognise its own patterns.)

Check yourself

Replay the hero in your head: the same four binary lines went to processor A, then to processor B. Say out loud what stayed the same, what changed, and what caused the change.