Low-level vs high-level languages

Computer Science · Programming languages

One line in. How many instructions out?

In Python you write c = a + b, with a = 3 and b = 4. Step through what a processor has to do for that one line. This is assembly language for an imaginary processor: real CPUs each have their own instructions.

Before you step: c = a + b is ONE line of Python. How many instructions do you think the processor needs?

 1  LDR R0, a      ; copy a from memory into register R0
 2  LDR R1, b      ; copy b from memory into register R1
 3  ADD R2, R0, R1 ; add R0 and R1, put the answer in R2
 4  STR R2, c      ; copy R2 back into memory as c

Computer Science · Language levels

Which kind of code is it?

Sort each item into machine code, assembly language or a high-level language.

6 of 6 still to sort.

You've just seen a high-level line and the low-level steps beneath it. Now commit: sort each one before you check.

Computer Science · Translators

Your Python can't run itself

Choose a translator to see where your high-level code ends up.

Translator

On High-level program (for example, Python). 2 branches to choose from.

The CPU can only run machine code, so something has to translate your high-level code first. Pick a translator and follow the route.

What you need to know

• A computer runs instructions exactly as they are written. It can't guess what you meant, and a wrong instruction is still carried out.
• Have a goSam's program has one wrong instruction in it. Sam says, 'It's fine, the computer will spot it and quietly fix it.' What will the computer actually do with it?

  Run it exactly as written, mistake and all.

  A computer can't make assumptions or guess what an instruction was meant to be, so an incorrect instruction is still carried out.

• Machine code is the lowest-level language: binary 0s and 1s, and the only language a computer can run directly.
• Different CPUs have different machine code instructions, so low-level code is written for one specific machine. Assembly language is low-level too.
• Even a simple calculation takes several lines of assembly language.
• Low-level languages give direct control over the hardware, which is why they can run quickly.
• Have a goYou need to talk to the hardware directly. Do you pick a low-level or a high-level language?

  Low-level.

  Low-level languages give direct control over the hardware. High-level ones hide the hardware's details behind abstraction.

• A high-level language, such as Python, is human-readable and uses abstraction to hide the hardware's complex details.
• One line of high-level code, such as c = a + b, is translated into many lines of machine code.
• Have a goc = a + b is one line of Python. Is the machine code it becomes one line, two lines, or many lines?

  Many lines.

  One line of high-level code is translated into many lines of machine code, because the processor works in small precise steps.

• High-level code must be translated into machine code before it can run, using a compiler or an interpreter.
• Low-level code is hard to read, learn, debug and maintain. High-level languages are generally easier to learn, read and use.
• Low-level languages are used where direct hardware control and efficiency are essential, such as operating systems, device drivers and embedded systems.

The big picture

A computer's processor can only run machine code. Low-level languages sit close to that and give direct control of the hardware; high-level languages like Python sit further away, are easier for people, and have to be translated first by a compiler or an interpreter.

⚠ Watch out

Thinking the computer understands your code the way a person would, so it will work out what you meant. It won't. The processor only runs machine code, exactly as written.

Memory hook

Low-level: low to the ground, close to the hardware. High-level: up in the clouds, with the hardware hidden far below.

Check yourself

Cover the page. Explain to a friend why c = a + b can't go straight to the CPU. Use: high-level, translated, machine code.