# 4. Assembly Integration with Arduino IDE

To combine Assembly with Arduino C++ code, the `extern "C"` directive is used in the `.ino` file and the `.global` directive is used in the `.S` (Assembly) file.

### File Structure:

#### .ino File (C/C++):
```cpp
extern "C" {
  void start();    // Declaration of function defined in Assembly
  void loop_asm(); // Another function from Assembly
}

void setup() {
  start();         // Call Assembly function for initialization
}

void loop() {
  loop_asm();      // Call Assembly function for main loop
}
```

#### .S File (Assembly):
```assembly
#define __SFR_OFFSET 0x00
#include "avr/io.h"

.global start
.global loop_asm

start:
    SBI DDRB, 5      ; Set PB5 (Pin 13) as Output
    RET              ; Return to caller

loop_asm:
    SBI PORTB, 5     ; Turn on LED
    ; ... other code
    RET
```

### Directive Explanations:
- **`#define __SFR_OFFSET 0x00`**: Sets the offset for I/O registers to use symbolic names (DDRB, PORTB, etc.).
- **`#include "avr/io.h"`**: Includes register definitions for the AVR chip.
- **`.global`**: Makes label/function accessible from other files (exported symbol).
- **`RET`**: Instruction to return from subroutine to the calling program.