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++):

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):

#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.