Simple AVR ASM example

Projects Programming
1

I am learning AVR ASM alone at home. Have got two simple progs working already, but having trouble making a simple MUL example work. If someone could point out where in my code I am wrong I would be grateful!

In my INO file:

extern "C" 
   {
   int MultTest();
   
   }

in my "S" file

#------------------------------------------

MultTest:
        LDI        R24,   7    
        LDI        R18,   9
        MUL       R24,   R18
        RET
#-----------------------------------------

Thank you all in advance!

2

Sorry, forgot to mention:

In the INO is a call for Serial.println( MultTest() );

3

Where do you think the result goes? (Hint: it is not r24.)

4

got it to work!

Apparently in must be written this way:

MUL R24, R16
MOVW  R24, R0
RET

why? What is special about R24? If I replace R24 with any other register it does not work...