Re: [avr-gcc-list] Fixing function locations

[Dave Hansen]

From: "E. Weddington" <address@hidden>
[...]
> On 7 Jan 2004 at 16:36, Tom Maughan wrote:
>
> > Is it possible to fix the location of a function to a specific flash 
> address?
> >
> > I'm writing some bootloader code which no doubt will get changed in the
> > future but I'd like to fix certain externally accessable functions to
> > specific address locations such that if I run maincode designed for a
> > newer version of the bootloader it won't die horribly.
> > Many thanks,
>
>
> In brief, place your function(s) in it's own named section and then 
> relocate
> that section to the desired address.

There's something else you might want to do from a design standpoint.  I've 
never done it on an AVR, but I would guess the mechanics are similar to what 
Eric has described.

In your bootloader, define an array of pointers to functions.  If some of 
the functions require parameters, pass them through a void pointer.  That 
way you can declare a struct to pass whatever parameters you require, and 
even use the same struct for return values.  E.g.:

  void (*API_function[NUM_API_FNS])(void *) = {<function list>};

Locate this array in a fixed location in flash (probably using a mechanism 
similar tto that described by Eric).

Your application code can define the same array (extern), along with some 
symbols to index the array and the parameter structures.  Then to call the 
functions, you would used something like:

  #include "bootloaderapi.h"

  PARM_API_START_BOOTLOADER start;
  PARM_API_GET_VERSION ver;

  API_function[GET_VERSION](&ver);
  if (ver.major < 1) {
     error("Test Version of Bootloader");
  }else{
     start.mode = MODE_API_COLD_BOOT;
     API_function[START_BOOTLOADER](&start);
  }

The advantage to doing it this way is maintenance: Even if the functions 
change size or must move, the pointers don't.  If you need to add API 
functions, you simply extend the array.  The scheme is similar to the 
software interrupt BIOS interface.

HTH,
  -=Dave

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