Re: [avr-gcc-list] Compile Error

[Galen Seitz]

lgp <address@hidden> wrote:

> Source file is:
> 
> int main( void ){
>       unsigned char a;
>       a++;
>       while(1);
> }
> 
> 
> List file is:
> 
> int main( void ){
>   8e: ce e5           ldi     r28, 0x5E       ; 94
>   90: d4 e0           ldi     r29, 0x04       ; 4
>   92: de bf           out     0x3e, r29       ; 62
>   94: cd bf           out     0x3d, r28       ; 61
>       unsigned char a;
>       a++;
>   96: 89 81           ldd     r24, Y+1        ; 0x01
>   98: 8f 5f           subi    r24, 0xFF       ; 255
>   9a: 89 83           std     Y+1, r24        ; 0x01
>       while(1);
>   9c: ff cf           rjmp    .-2             ; 0x9c
> 
> 
> 
> 
> Is it right??
> 
>       a++;
>   96: 89 81           ldd     r24, Y+1        ; 0x01
> >>  98:       8f 5f           subi    r24, 0xFF       ; 255 ??????
>   9a: 89 83           std     Y+1, r24        ; 0x01
> 

At first glance it looks correct to me.  0xff is a -1 in two's complement
form.  Therefore:

 r24 - 0xff  =  r24 - -1  =  r24 + 1

If you look at the Atmel doc for the AVR instruction set, you will see
that there is no 8 bit Add Immediate.  SUBI and Add Immediate are 
equivalent.

galen