How can I call assembly function from C code?

Question

I use avr-as assembler. I want to use functions defined in assembly from a C file. How can I use assembly code in C code?

I am looking for solutions where the assembly source is in a separate source, i.e. not inlined into the C source.

Answer 1

Here's a simple example to get you started. Suppose you want to write a main loop in C and you want to call a function written in assembly to blink PB5.

The C source declares and uses (but doesn't define) blinkPB5:

/* main.c */
#include <avr/io.h>
#include <util/delay.h>

extern void blinkPB5();

int main ()
{
    DDRB |= _BV(DDB0);

    for (;;)
    {
        blinkPB5();
        _delay_ms(500);
    }
}

The assembly source defines blinkPB5. Note that .global is used to export blinkPB5:

;; blinkPB5.s
.global blinkPB5

.section .text

blinkPB5:       
        ldi r25, 0x01
        in  r24, 0x05
        eor r24, r25
        out 0x05, r24
        ret

.end        

The two can be compiled separately:

avr-gcc -c -O3 -w -mmcu=atmega328p -DF_CPU=1000000L main.c -o _build/main.c.o
avr-gcc -c -O3 -w -mmcu=atmega328p -DF_CPU=1000000L blinkPB5.s -o _build/blinkPB5.s.o

then linked together, and formatted into a .hex image:

avr-gcc -Os -Wl,--gc-sections -mmcu=atmega328p _build/main.c.o _build/blinkPB5.s.o -o _build/image.elf
avr-objcopy -Oihex -R.eeprom _build/image.elf _build/image.hex