[avr-gcc-list] AVR GCC R0 and asm

[Craig D Stewart]

Hi,

Im trying to use FMUL to give an approximation of floating point multiply for 
gain control for a lo-fi DSP. So if value = 120 and amplitude/gain = 0.5 ( or 
64 if treated as a uchar) then result would be the predictable 60. 

Unfortunately the FMUL instruction dumps the result in r0 and r1. My 
understanding was that as long as i include R0 and R1 in my used registers at 
the end of my inline assembler thent he complier would deal with pushing the 
values of the registers on to the stack before the asm and returning the values 
after the asm. However the r0 is not getting reset.    

The following code demonstrates the problem with ampcounter and valuecounter 
being set to what ever is left in ro.. and not 0...

#include <avr/io.h>
#include <avr/interrupt.h>

unsigned char tasm(unsigned char value, unsigned char amp)
{
unsigned char result=0;
asm volatile (
"fmul %1,%2" "\n\t"
"mov %0,r1"
:"=&r" (result)
:"a" (value),"a" (amp)
:"r0", "r1"
); 
 return result;
}
volatile unsigned char address[4] = {1,2,3,4};
volatile unsigned char values[10] = {10,20,30,40,80,120,140,160,180,200};
volatile unsigned char amplitude[4] = {32,64,96,128};

 unsigned char temp = 0;
 unsigned char tamp = 0;
 unsigned char tvalue = 0;
 unsigned char result = 0;
 
 unsigned char ampcounter = 0;
 unsigned char valuecounter = 0;

int main(void)
{
 for(;;)
 {
 tvalue = values[valuecounter];
 tamp = amplitude[ampcounter];
 temp = tasm(tvalue,tamp);
 valuecounter++;
 ampcounter++;
 if (valuecounter >9)
 {
  valuecounter = 0;
 }
 if (ampcounter >3)
 {
  ampcounter = 0;
 }
 }
}


Of course i can just set R0 to be zero after my asm instructions, but surely 
this is a minor issue that should be 'fixed' ?? or im i misunderstanding 
something here?

Thanks 

Craig

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