[ECOS] ARM cpu modes

Hugo 'NOx' Tyson hmt@cygnus.co.ukx
Wed Apr 5 05:45:00 GMT 2000


Colin_Helliwell@Mitel.COM writes:
> Jesper said in a message yesterday that "[ARM] Application threads run in
> supervisor mode in eCos". I've heard/seen it said somewhere that the ARM's
> privileged Supervisor mode was intended for OS-type operations, whilst the
> non-privileged User mode was intended for application-level code. I was curious
> about the rationale behind the choice made in eCos, and whether it is felt that
> there are actually any pros and/or cons of running application threads in
> supervisor mode.

Those distinctions are only pointful if you do stuff like memory protection
(including of IO device) and therefore have a SWI interface to *all* system
services. 
"Big" OS's with processes (=> memory protection and VM) would do this.
"Small" OS's ie. eCos, with threads and one uniform memory space do not.

eCos apps are fully linked, there is no SWI interface, and for performance
reasons, we *want* appliction object files to contain "privilidged"
operations from eCos macros or inline functions.  Device driver code is not
"special" in any way.  For example, cache flushing or sync'ing is usually a
coprocessor operation, which you can only do in a non-User mode.  But it's
only a couple of instructions, so there's no need to put in a clunking
great function with mode changes surrounding it; just put it inline with
the app code.  So it all must run in Supervisor mode.

Actually, we have considered moving to System mode (User mode reg set with
SVC mode privilidges) but it's not available on all ARMs and there ain't
much to gain...

HTH,
	- Huge

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