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Main Concepts of the FORTH Programming Language

🔹 1. Stack-Based Language

FORTH uses a data stack to perform operations.
Operands are pushed onto the stack, and operators consume values from the top of the stack.

Example:
```
3 4 +   \ pushes 3 and 4, then adds them → stack contains 7
```

🔹 2. Postfix Notation (Reverse Polish Notation - RPN)

Expressions are written in postfix notation: operands first, then the operator.

Example:
```
2 5 *   \ equivalent to 2 * 5 in infix notation
```

🔹 3. Concatenative Language

FORTH is a concatenative language: programs are created by chaining words (functions or commands).
Each word operates on the implicit stack.

🔹 4. Defining Words (Functions)

The : keyword defines a new word (function):

Example:
```
: SQUARE ( n -- n² ) DUP * ;
```
- DUP duplicates the top of the stack.
- * multiplies the top two values on the stack.

🔹 5. Built-in Interpreter and Compiler

FORTH works in two modes:
- Interpretation mode: immediate execution.
- Compilation mode: defining new words.
Enables direct and interactive control of the system.

🔹 6. Minimalist and Extensible

The core of FORTH is small and minimal.
Users can define new words to build domain-specific languages.
Offers low-level control over the system.

🔹 7. Portability and Efficiency

FORTH is lightweight and fast.
Popular in embedded systems, and used by organizations like NASA.

🔹 8. Comments and Conventions

Comments are enclosed in parentheses:
```
( This is a comment )
```

✅ Complete Example

: CUBE ( n -- n³ ) DUP DUP * * ;
5 CUBE .   \ Outputs 125