Assembler, compiler, interpreter: why translation is needed

A processor executes machine code — binary instructions. Nobody wants to write thousands of binary numbers, so we write in a form we can read and let software do the translation.

All three exist for one reason: the processor cannot run anything else.

An assembler translates an assembly language program into machine code.

Assembly language uses mnemonics such as LDR, ADD and STR instead of binary, and labels instead of addresses. Each mnemonic maps to one machine instruction, so the translation is close to one-to-one.

The assembler's jobs:

• Convert each mnemonic and operand into the corresponding binary instruction.
• Work out addresses for labels (the symbol table), which is why assembly programs are often assembled in two passes.
• Produce object code that can be loaded and executed.

Why bother with assembly at all? It gives precise control of the hardware and produces very efficient code — useful for device drivers and embedded systems.

A compiler translates a whole high-level program into machine code before it runs. The output is an executable file you can run again and again without the compiler.

An interpreter translates and executes the program one statement at a time while it runs. Nothing permanent is produced: every run needs the interpreter again.

Both check for errors, but at different moments:

• a compiler reports all the syntax errors it finds and refuses to produce an executable;
• an interpreter stops at the first error it reaches when that statement is executed.