Re: [avr-gcc-list] Howto I/O in asm instructions?

[Wouter van Gulik]

Ruud Vlaming schreef:
> Hi,
>
> There are a couple of ways to use i/o address in assymbly.
> Below i used a some:
>
> #define _EEARL_ "0x1E"
> uint8_t portFSReadByte(unsigned char * pAddress)
> { uint8_t result;
>   asm volatile ( \
>    "in   r26, __SREG__      \n\t"  
>    "cli                     \n\t"  
>    "out  %2, %A0            \n\t" 
>    "out  "_EEARL_", %B0     \n\t" 
>    "sbi  __EECR__, 0        \n\t"  
>    "in   %A0, 0x1D          \n\t" 
>    "out __SREG__, r26       \n\t" 
>    "" :"=r" (result) :"0" (pAddress), "I" (_SFR_IO_ADDR(EEARH)) : "r26" );
>   return result; }
>
> (1) You can define a constant, like i did for _EEARL_ 
>   This is nice but no so portable.
> (2) You can use the asm paramter list, like for EEARH
>   Also nice, but errorprone, since i keep making mistakes in 
>   numbering, especially when you have many parameters and 
>   have to change something.
> (3) The best thing to have would be something like __EECR__,
>   resembling the definition for __SREG__, but that does not
>   compile right now.
>
> Is the latter possible somehow? Or are there other solutions?

You could go for option 2 if you use syntax like this (took this from 
eeprom.h)

    __asm__ __volatile__ (
        "/* START EEPROM WRITE CRITICAL SECTION */\n\t"
        "in        r0, %[__sreg]           \n\t"
        "cli                               \n\t"
        "sbi       %[__eecr], %[__eemwe]   \n\t"
        "sbi       %[__eecr], %[__eewe]    \n\t"
        "out       %[__sreg], r0           \n\t"
        "/* END EEPROM WRITE CRITICAL SECTION */"
        :
        : [__eecr]  "i" (_SFR_IO_ADDR(EECR)),
          [__sreg]  "i" (_SFR_IO_ADDR(SREG)),
          [__eemwe] "i" (EEMWE),
          [__eewe]  "i" (EEWE)
        : "r0"
    );

This makes the code far more readable IMHO. For more info on the %[ ] 
notation take look at: 
http://gcc.gnu.org/onlinedocs/gcc-4.3.0/gcc/Extended-Asm.html

HTH

Wouter