Re: [avr-gcc-list] one-byte stack bug in tiny26

[Peeter Vois]

would

void func(void)
{
        static unsigned char array[4];
        array[0] =1;
        array[1] =2;
}

help? if inside the scope variable declaration is not
static the C compiler will reserve memory inside stack.

Ühel ilusal kenal päeval Tue, 27 Apr 2004 09:42:23 +0200
"Svein E. Seldal" <address@hidden> kirjutas:

> Hi,
> 
> I'm using tiny26 with gcc (3.3.2). As we all know, the tiny26 has only
> one byte stack. My problem arises because of this code:
> 
>       void func(void)
>       {
>               unsigned char array[4];
> 
>               array[0] = 1;
>               array[1] = 2;
>       }
> 
> When the function is initialized, gcc copies SPL and SPH (which is
> reserved) into YREG in the function prolog.
> 
> In the tiny26 the SPH register is reserved; It contains bogus data, 
> making YREG into an invalid pointer. When the array is allocated on
> top of the frame, using the YREG pointer, accesses to or from the
> array data is invalid becuase of the corrupted YREG framepointer.
> 
> The -mtiny-stack option, does not help either. SPL and SPH is still 
> loaded/unloaded in the function's prologues and epiloges.
> 
> I found this:
> 
> http://www.avr1.org/pipermail/avr-gcc-list/2001-October/000895.html
> 
> where it is stated that the loading/unloading of the two stack bytes
> in the function prologue is intentional, despite the usage of the 
> -mtiny-stack option.
> 
> According to the datasheet, the register of SPH is reserved. This 
> implies that we cannot predict the contents of that register, thus gcc
> 
> should never read SPH on this target. (Even if its done intentional.)
> 
> 
> Svein
> 
> 
> 
> 
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