Translators

This lesson covers the three types of language translator — compilers, interpreters and assemblers — and the stages of compilation. The OCR H446 specification requires you to understand how each translator works, compare them, and describe the stages of the compilation process.

---

Why Translators Are Needed

Computers can only execute machine code (binary). Programmers write code in high-level languages or assembly language, which must be translated into machine code before the CPU can run it.

Source Language	Translator	Target
Assembly language	Assembler	Machine code
High-level language	Compiler	Machine code (executable file)
High-level language	Interpreter	Machine code (executed line by line)

---

Assemblers

An assembler translates assembly language into machine code.

Feature	Detail
Input	Assembly language source file
Output	Machine code (object file)
Translation	One-to-one — each assembly mnemonic maps to exactly one machine code instruction
Passes	Most assemblers are two-pass: the first pass builds a symbol table (mapping labels to addresses); the second pass generates machine code using the symbol table

Two-Pass Assembly

Pass	Action
Pass 1	Scan the source code. Record all labels and their memory addresses in the symbol table. Check for syntax errors
Pass 2	Translate each mnemonic into its machine code equivalent. Replace labels with addresses from the symbol table. Generate the final machine code

---

Compilers

A compiler translates the entire high-level source code into machine code before the program runs. The output is a standalone executable file.

Feature	Detail
Input	High-level source code (e.g. a .c, .java, or .cpp file)
Output	Machine code executable (e.g. .exe) or bytecode
When translation happens	Before execution — the entire program is translated at once
Error reporting	Reports all errors after compilation. The program will not compile if there are syntax errors
Execution speed	Fast — the executable runs directly as machine code with no further translation
Distribution	The executable can be distributed without the source code
Portability of output	The executable is platform-specific. Must recompile for different architectures

---

Interpreters

An interpreter translates and executes high-level source code one line (or statement) at a time, without producing a separate executable file.

Feature	Detail
Input	High-level source code
Output	No separate file — code is executed directly
When translation happens	During execution — each line is translated and executed immediately
Error reporting	Stops at the first error encountered, reporting it immediately. Good for debugging
Execution speed	Slower — each line must be translated every time it is executed (e.g. in a loop)
Distribution	Source code must be distributed (or an interpreter must be bundled)
Portability	The same source code runs on any platform with a compatible interpreter

---

Compiler vs Interpreter Comparison

Feature	Compiler	Interpreter
Translation	Entire program at once	Line by line during execution
Output	Standalone executable file	No separate file
Execution speed	Fast (pre-compiled)	Slower (translated at runtime)
Error detection	All errors reported after compilation	Stops at the first error
Debugging	Harder — must recompile after changes	Easier — can test changes immediately
Source code distribution	Not needed — only executable distributed	Source code required
Memory usage	Needs enough memory to compile the whole program	Only needs to hold one line/statement at a time
Re-translation	Only when source code changes	Every time the program runs

When to Use Each

Use a Compiler When...	Use an Interpreter When...
The program is complete and ready for distribution	During development and debugging
Execution speed is critical	Rapid prototyping and testing
You want to protect source code from users	The program must be portable across platforms
The program will be run many times	The program is a script run occasionally

Hybrid Approach (Bytecode)