Re: [avr-gcc-list] Using PSTR()

[David Kelly]

On Mar 29, 2005, at 2:05 AM, Bjarne Laursen wrote:

> >                 return( PSTR("01234567890ABCDEF")[u] );
>
> PSTR puts the string in flash, but the compiler knows nothing about 
> that when using the pointer. You must the use pgm_read_byte().
>
> something like (not tested):
> return( pgm_read_byte(PSTR("01234567890ABCDEF")+u));

Sorry about that, I had mail problems which dropped my original reply 
to myself, and then blocked some other mail until last night.

Quite often I've found once possibly making a fool of myself in public 
asking such a question, that after sleeping on it I wake up with the 
correct solution. The disassembly of what I'm using is essentially what 
Bjarne said altho I typed a goof (extra zero) in the string Bjarne 
copy/pasted from my original message:

Problem solved like this:

0000093c <ntohex>:

//
//      Binary Nybble to Hex ASCII
//      A bit convoluted for efficiency on AVR
//
unsigned char
ntohex(unsigned char n)
{
        return( pgm_read_byte(PSTR("0123456789ABCDEF")+(n&0xf)) );
 93c:   99 27           eor     r25, r25
 93e:   8f 70           andi    r24, 0x0F       ; 15
 940:   90 70           andi    r25, 0x00       ; 0
 942:   fc 01           movw    r30, r24
 944:   e4 57           subi    r30, 0x74       ; 116
 946:   ff 4f           sbci    r31, 0xFF       ; 255
 948:   84 91           lpm     r24, Z
}
 94a:   99 27           eor     r25, r25
 94c:   08 95           ret

--
David Kelly N4HHE, address@hidden
========================================================================
Whom computers would destroy, they must first drive mad.