RE: [ECOS] changing the stack ptr

["Dan Conti" <danc at iobjects dot com>]

> -----Original Message-----
> From: Gary Thomas [mailto:gthomas@redhat.com]
> Sent: Thursday, July 26, 2001 6:02 PM
> 
> On 27-Jul-2001 Dan Conti wrote:
> > Even with 'Use seperate stack for interrupts' enabled ?
> > 
> 
> Actually, you can probably do it safely.  However, the actual switch
> might need to be protected with the interrupts off.
> 
> The notion of a separate interrupt stack is that while ISR 
> and DSR routines
> run, it uses a dedicated stack which is separate from the 
> thread stacks.
> In this way, interrupt stack requirements (which could be 
> large) are not
> bourn by all threads.

Ok, i see the distinction. During the actual execution of the isr/dsr
routines the interrupt stack will be used.

> Fair enough.  Try changing the stack while the interrupts are 
> disabled.
> Note that this can be very tricky.  Anything can be on the 
> stack and the
> C compiler [at least] will assume that the stack looks the 
> way it thinks
> it should at all times.  Thus, after you change stacks, you have to be
> very careful about what you fiddle with, etc.
>
> Is this code in C or assembly?

It's in assembly (luckily) so i'm in full control of the stack usage.
I'll see about disabling interrupts and see if that helps out.
 
> > I feel like i'm missing something here. On this processor 
> the sp and lr
> > are banked in irq and fiq mode. When an irq occurs shouldn't it be
> > referencing the irq stack at that point?
> > 
> 
> That only happens for a few instructions.  We switch back to 
> supervisor
> mode for the majority of the interrupt handing code.  This 
> makes for much
> more uniform thread context handling all around.

That would be the something i am missing :)
As it turns out, my problems might be related to something else. I took
out the offending sp replacing code, and put a similarly timed chunk of
sp friendly code, and am still having some issues. But i'm glad i know
the sp story now, thanks gary.

-Dan