Fix memory position for a function at compilation

Question

Is it possible to set the memory position a function start from? I want to know if it is possible to do so at compilation, linking or even in code.

Also I'm working with FreeRTOS, is it possible too?

Answer 1

What toolchain do you use? Some compilers have special syntax for this (e.g. with word "absolute"), but if you are using GCC you should edit a linker file (.ld). Its syntax is rather cryptic at first sight, but it have a docs where everything is explained. In short, you should mark your function that it belongs to a section like this:

__attribute__((section(".mysection"))) void reflashable_function(...

and then add following text to your MyProject.ld file after all existing sections:

.mysection 0xS0MEADDRESS:
{
  *(.mysection)
  *(.mysection*)
}

Also, add a key like "-Map=output.map" to your makefile to generate mapfile and then seek a name of your function in output.map to check if it has correct address. There may be additional difficulties with e.g. inlining a function or inlining something in it, so i suggest putting it to the separate .c file. You can also addg reflashable memory to MEMORY section of the .ld file, in that case you won't need to add address, but need to add memory name like this:

  .mysection:
{
  *(.mysection)
  *(.mysection*)
  ./sources/dir/reflashable_file.o(.text) /*this is to add a full file, not just one function */
  ./sources/dir/reflashable_file.o(.rodata)
} >BOOTFLASH

You can also add an EXCLUDE_FILE directive to other sections so it won't be included in other sections too. Check a .map file to see results.