Translators: Compilers, Interpreters, and Assemblers

This lesson covers the three types of translators used to convert programming languages into machine code. This is part of OCR J277 Section 1.5.3.

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Why Are Translators Needed?

CPUs can only execute machine code (binary instructions). However, most programmers write code in high-level or assembly languages. A translator is a program that converts source code into machine code so the CPU can execute it.

There are three types of translator:

1. Compiler
2. Interpreter
3. Assembler

The following diagram compares how each translator converts source code:

graph LR
    SC["High-level\nSource Code"] --> COMP["Compiler"]
    SC --> INT["Interpreter"]
    ASM["Assembly\nCode"] --> ASMR["Assembler"]
    COMP --> EXE["Executable\n(Machine Code)"]
    INT --> RUN["Executes line\nby line"]
    ASMR --> MC["Machine Code"]

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1. Compiler

A compiler translates the entire source code of a high-level language program into machine code all at once, before the program is run.

How a Compiler Works

Source code (e.g. Python, C++) → COMPILER → Executable machine code file

1. The programmer writes source code in a high-level language.
2. The compiler reads the entire source code.
3. It checks for syntax errors in the whole program.
4. If no errors, it produces an executable file (machine code).
5. The executable can be run independently — the compiler is no longer needed.

Characteristics of a Compiler

Feature	Detail
Translation	Translates the entire program at once
Output	Produces a standalone executable file
Error reporting	Reports all errors after compilation; program won't compile until all errors are fixed
Execution speed	Fast — the executable is already in machine code
Distribution	The executable can be distributed without sharing the source code
Re-compilation	Any change to the source code requires the entire program to be re-compiled

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2. Interpreter

An interpreter translates and executes high-level source code one line at a time, as the program runs.

How an Interpreter Works

Source code → INTERPRETER → (translates and executes line by line)

1. The interpreter reads the first line of source code.
2. It translates that line into machine code.
3. It executes that line immediately.
4. It moves to the next line and repeats.
5. If an error is found, execution stops at that line.

Characteristics of an Interpreter

Feature	Detail
Translation	Translates and executes one line at a time
Output	No standalone executable is produced
Error reporting	Stops at the first error found, making debugging easier
Execution speed	Slower — must re-translate the code every time the program runs
Distribution	Source code must be shared (and the interpreter must be available)
Development	Useful for testing and debugging during development