AVR Freaks

Program memory constants are a weak area of AVR. The obvious didn't work compile: error: initializer element is not computable at load time

const __flash char *const __flash wizards[] = { "Harry", "Ron", "Hermionne" };

That's wrong - I think the initialiser element IS computable at load time.

The best I could do was:

const char *const __flash wizards[] = {
    "Harry",
    "Ron",
    "Hermionne"};

char getWizardInitial (uint8_t wizard)
{
    return wizards[wizard][0];
}

The actual strings end up in .rodata and the tables end up in .progmem. Which isn't good.
 

  26                   .LFE1:
  28                   .global    wizards
  29                       .section    .rodata.str1.1,"aMS",@progbits,1
  30                   .LC0:
  31 0000 4861 7272         .string    "Harry"
  31      7900
  32                   .LC1:
  33 0006 526F 6E00         .string    "Ron"
  34                   .LC2:
  35 000a 4865 726D         .string    "Hermionne"
  35      696F 6E6E
  35      6500
  36                       .section    .progmem.data,"a",@progbits
  39                   wizards:
  40 0000 0000              .word    .LC0
  41 0002 0000              .word    .LC1
  42 0004 0000              .word    .LC2
  43                       .text
  44                   .Letext0:

>and access them?

Assuming you want strings in flash, and you want to have the array of string (char) pointers also in flash, then I think your only choice is to split out the strings-

#include <stdint.h>
#include <avr/io.h>

__flash const char str0[] = "element1";
__flash const char str1[] = "element2";
__flash const char str2[] = "element3";

__flash const char* //store a __flash const char* in
__flash const arr[] = { str0, str1, str2 }; //a __flash const array

//remove the second __flash if you want the array of flash pointers in ram

void print(__flash const char* str){ //access str address via lpm
    while( *str ) PORTB = *str++;
}

//indirection so compiler cannot optimize and we can then see the flash address

//being read from the const char* array (in flash)
void printStr(uint8_t i){
    print( arr[i] );
}

int main(){

    //compiler will have to get char* address from flash (just to show is all correct)

    for(uint8_t i = 0; i < 3; i++) printStr( i );

    //compiler knows the address, so no flash access needed for the string addresses in the array

    //this will be the normal method used, and compiler most likely does not end up generating code

    //like above where it first needs to look up the arr element for the string address

    for(uint8_t i = 0; i < 3; i++) print( arr[i] );

    //or can just access the string you want

    print( str0 );
    print( str1 );
    print( str2 );

}