Lpc2000

> On Wednesday 21 September 2005 12:33, David Hawkins wrote:
> > > I guess you could do that, but that would add extra overhead to the
> > > SWI calls.
> > >
> > > AFAIK, the problem only applies to a few encodings of ldm/stm that you
> > > don't use in normal use anyway. Therefore I don't see much need to
> > > jump through hoops to accomodate it.
> >
> > Thats what I am trying to determine.
>
> >From a read of the ARMARM it would appear that the Bad Form only
> applies when
> you use stm(2)/ldm(2) in user/system mode because these provide a special
> "worm hole" to access the user/system registers - something that
> is pointless
> when you are already in sustem/user mode.
>
> >From what I can tell, this form would only be reasonably used
> when you are
> trying to access system/user registers when in a different
> protected mode.
> Example of this might be:
> 1) In an OS environment where you are using SWI as a trap to the OS.
> 2) Exception handler for data aborts/page swapping etc.
> 3) Perhaps as part of a scheduler in an RTOS.
>
> > > For calling APCS compilant (ie. C generated isrs) you do not have to
> > > save all registers, only r0..r3. The called function will save any
> > > other registers it needs itself.
> >
> > Right, but if you are running an RTOS, then the C generated ISR
> > code is insufficient, since a context switch may occur as a
> > consequence of the interrupt. So in uCOS-II you need to save
> > all processor context, service the interrupts, and then restore
> > to the highest priority task. Since you basically have to store
> > all registers, it would seem that the ldm/stm instructions would
> > be the desired ones to use.
>
> Unwrapped C generated code is definitely insufficient for many
> purposes, but perhaps you miss my point.
>
> The IRQ wrapper that calls a C function does not need to save all the
> registers. According to the APCS, the caller should preserve
> r0-r3,r12 (and
> r14) and the callee any others. Thus, the wrapper only needs to preserve
> r0-r3,r12,r14. The C function will save/restore any others itself.
>
> Of course if you trash other registers in the IRQ wrapper then
> you need to save those too.
>
> Of course, too, any context needed by the scheduler will need to save all
> registers.

Yep, understood. Thanks for the valuable discussion.

Cheers
Dave