Those examples are not really useful in my case as they assume handwriting new functions with the assembler coming with the SDCC package.

I want to call object code in memory that has been prepared with another assembler - e.q. a routine in FLASH in my case!

However I'll try with the DPL one again. The last time I got errors on that one, e.q. the compiler complained that I didn't use my declared variables or more severely complained that my function was lacking the return() statement because I tried the following:

unsigned char myfunc()
{
  _asm
  ljmp MY_FUNCTION
  mov dpl,a      ; put my return value where SDCC expects it
  _endasm;
  
}

Note the missing return(...) - and SDCC complains on that (ofcourse!)
So I need to either pass this to the return() function or make sure that SDCC detects that I implicitly add a return() statement.

cheers,
Matthias