Re: [avr-libc-dev] RFC: index-redirection with PROGMEM and linker symbol use

[Tero Sinervo]

Stu Bell wrote:
> I have an interesting idea/problem and I'm looking for ideas.

> I would like to place et_ERR_NO_ERROR and et_ERR_TIMEOUT in high flash
> (easy), BUT I then want to place the index dereferenced from .proghigh
> (where all of these constants are placed) in ErrorTextTable[].

Here's my suggestion. Not fancy because you have to give the section 
start address and define by hand but what's being done is a hack anyway.


This is given to compiler (byte address):
-DEXTPROGMEM_START=0x10008UL

And this to linker (byte address too):
-Wl,-section-start=.extprogmem=0x10008


#include <avr/pgmspace.h>

const char et_ERR_NO_ERROR [] __attribute__((section (".extprogmem"))) = 
"No Error";
const char et_ERR_TIMEOUT  [] __attribute__((section (".extprogmem"))) = 
"Timeout";


#define FARTONEAR(a)    ((uint16_t) ((a) - EXTPROGMEM_START))
#define NEARTOFAR(a)    ((uint32_t) (a) + EXTPROGMEM_START)


uint16_t ErrorTextTable[2] PROGMEM = {
    FARTONEAR(et_ERR_NO_ERROR),
    FARTONEAR(et_ERR_TIMEOUT)
};


int main(void)
{
        volatile uint32_t farp = NEARTOFAR(pgm_read_word(&ErrorTextTable[1]));
        volatile char c = pgm_read_byte_far(farp);
}




-Tero