Re: [cc65] assembler/C stack issue

Hi Oliver,

On Fri, Feb 11, 2011 at 05:54:22PM +0100, Oliver Schmidt wrote:
> Hi Gábor,
> 
> >> Assembler funcions with 8-bit return value need to zero the X register on exit.
> 
> > [...] if I have - let's say - $20 in X and in A
> > too on exit, what the "C code will think", return value is $2020? It's quite
> > strange, since the return value of the prototype is "char", so for unsigned
> > it cannot be more than $FF. [...]
> 
> As far as I know it isn't important as long as you i.e. directly
> assign the return value to an 8-bit variable. If you however use the
> return value in an expression then according to the type promotion
> rules all values in the expression are promoted to an 16-bit value.
> The compiler makes the presumtion that the promotion of function
> return values is a nop - and this presumption breaks with assembler
> functions returning arbitrary values in X...

Ahaaa ... Interesting. For sure, I now that promoted stuff, but I always
imagined that unsigned char as a byte return value is always a byte, if it's
needed to promoted to an 16 bit value, X can be loaded with 0 there (if X
holds the high byte of the value), I haven't known that it's already assumed
that a function return with 16 bit value in same way (I mean: high byte is
zero) even if the return type is only 8 bit. Thanks for the explanation!

> 
> That's at least what I made up myself - we'll see how much of it is
> left when Uz tells the truth ;-)

:) :)
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