Question

Hi StackOverflow community,

I am trying to program my old Arduino Duemilanove Board (Atmega 168V-10PU) in Assembler. I tried it a few times before but everytime the code was not executed. So i tried to program an equivalent test program in C, and it worked. Here it is:

// file led.c
#include <avr/io.h>

int main(void)
{

    DDRB = 0xFF;
    PORTB = 0xFF;

    while (1) {
        asm("nop\n");
    }

    return 0;
}

The asm dump of the compiler results in (shortened),

ldi r24,lo8(-1)  ;  tmp44,
out 0x4,r24  ;  MEM[(volatile uint8_t *)36B], tmp44
out 0x5,r24  ;  MEM[(volatile uint8_t *)37B], tmp44

which works and activates the LED at Arduino Pin 13 (AVR pin PB5).

But when I use this asm file,

// file led.S
#include "avr/io.h"

.global main

main:
    ldi r24, 0xFF
    out DDRB, r24
    out PORTB, r24

 end:
    jmp end

the compiler dump results in (shortened),

ldi r24, 0xFF
out ((0x04) + 0x20), r24
out ((0x05) + 0x20), r24

what might explain why nothing happens.

In addition here are the makefiles for the C version and the Assembler version

Thanks for helping!

EDIT: Here are also the full assembler dump files of the C version and the Assembler version

EDIT 2: I looked up the register addresses in the include file iom168.h, which references to iomx8.h, where it says #define PORTB _SFR_IO8 (0x05). The compiler follows the include chain

io.h -> iom168.h -> iomx8.h
io.h -> common.h -> sfr_defs.h

In sfr_defs.h is written:

#define _SFR_IO8(io_addr) ((io_addr) + __SFR_OFFSET)

A few more lines upwards the offset is defined:

#ifndef __SFR_OFFSET
/* Define as 0 before including this file for compatibility with old asm
sources that don't subtract __SFR_OFFSET from symbolic I/O addresses.  */
#  if __AVR_ARCH__ >= 100
#    define __SFR_OFFSET 0x00
#  else
#    define __SFR_OFFSET 0x20
#  endif
#endif

(Sorry for the formatting)
Any idea where this error comes from?

Answer 1

You should use the helper macros _SFR_IO_ADDR() and _SFR_MEM_ADDR() to access SFRs using i/o and memory instructions, respectively, because they have different addresses in the two namespaces. The default is apparently memory mapped, but don’t count on it.

As such your code could look like:

#include "avr/io.h"

.global main

main:
    ldi r24, 0xFF
    out _SFR_IO_ADDR(DDRB), r24
    out _SFR_IO_ADDR(PORTB), r24

 end:
    jmp end

Or, you can switch to memory mapped access:

#include "avr/io.h"

.global main

main:
    ldi r24, 0xFF
    sts _SFR_MEM_ADDR(DDRB), r24
    sts _SFR_MEM_ADDR(PORTB), r24

 end:
    jmp end