Modes, stack setup for "no-OS" ARM app

[Toralf Lund]

I've been wondering about how I should handle processor modes and set up 
stack pointers etc. for my newlib-based ARM application running on a 
"bare" system (no OS, no debug monitor.) Unlike the default crt0, mine 
will definitely have to set up stack pointers for more than one mode, 
since I use interrupts actively. Also, I'm not quite sure what mode the 
application itself ought to run under. I need to access "privileged" 
registers etc. during the init phase at least, so some bits have to run 
in Supervisor or System, but do I want to do *everything* in one of 
those modes?

Some issues related to this (in no particular order):

   1. Once User mode is entered, there is no easy way back, as far as I
      know, so I can't simply switch to that mode e.g. to set up the
      stack, if I want SVC or System later.
   2. System is ARM version 4 and above, so perhaps I shouldn't use that
      in my crt0, which I intend to be very general.
   3. As far as I can tell, there is no way to access the user mode
      stack pointer e.g. from supervisor mode, at least when using gas.

Any thoughts?

- Toralf