Local variables (even if designated as xdata) are in code space

Thanks Hans,
I don't believe it's an issue with misunderstanding the location of the pointer versus the target of the pointer. I've verified that I actually get incorrect behavior (pulling 0s rather than the expected data) when I declare the pointer within a function rather than as a global.

Thanks,
Chris

Thanks Hans,
I don't believe it's an issue with misunderstanding the location of the pointer versus the target of the pointer. I've verified that I actually get incorrect behavior (pulling 0s rather than the expected data) when I declare the pointer within a function rather than as a global.

Thanks,
Chris

But the pointer you've shown is, as already noted, uninitialised!
Therefore its behaviour is undefined, which means that anything goes - there is no such thing as "incorrect" here.

I didn't post the actual code, which does initialize the pointer. I still don't know why the compiler would place the location of the actual pointer variable (NOT the target) in code space. As I've mentioned, using a global produces the correct behavior while a local does not. Here is, essentially, the actual code:

#define BASE 0x1000

void blah (unsigned char Offset)
{ unsigned char xdata *Address; Address = (BASE + Offset);

DATA1 = *(Address+3); DATA2 = *(Address+2); DATA3 = *(Address+1);
}

"essentially" is meaningless!

Unless it's the real code, copied & pasted from a genuine, compilable source, who knows what apparently "insignificant" changes you might have introduced in the translation...?!

Please note the instructions on how to post source code:
www.danlhenry.com/.../keil_code.png
and remember to check it in the 'Preview'

I verified that the code I posted reproduces the issue. It's not the code from my program, as I can't post that code here. Reposting with the proper formatting...

#define BASE 0x1000

void blah (unsigned char Offset)
{
        unsigned char xdata *Address;
        Address = (BASE + Offset);
        SPXB_MSTR_DATA1 = *(Address+3);
        SPXB_MSTR_DATA2 = *(Address+2);
        SPXB_MSTR_DATA3 = *(Address+1);
}

.

There you go again erik. Looking into that assembly.

.

(I was being sarcastic... remember? from a thread where some ppl didn't think looking into the underlying assembly was 'needed')

I knew that, but the eaters of fried pigeons that flew in through the window would not

Erik

It appears to be an issue with pointer + fixed offset. If I actually modify the pointer instead, I get the correct behavior. Though I still can't inspect the pointer or its target. I think that's due to optimizations, which is fairly reasonable. I'll respond again if I find out more.

Thanks all.

So, have you looked at the generated assembler code?

A lot of compilers would ignore your pointer variable, and just evaluate the real address for the individual memory accesses - all depending on how good the processor is at doing pointer manipulation.

In this case, it is porbably as cheap - or cheaper - for the compiler to join (BASE plus a constant offset) in the individual accesses (or as an decremented temp variable) and then add the Offset parameter before making the memory access.