Re: [avr-libc-dev] Interrupt vector redirection scheme

[Michael Schulze]

Dmitry K. wrote:
> On Monday 03 September 2007 19:59, Michael Schulze wrote:
> > Hi,
> >
> > > Very interesting idea. Though there is a discrepancy: function
> > > redir_func() keeps the SREG already after it is scratched (sbiw,
> > > eor).
> > I can't find the definition of that function. Where is it defined?
>
> Your file redirvec.S:
>
>    redir_func:
>         sbiw ZL, 0                      ; test for zero
>         ...
>         eor r1, r1                      ; clear zero register, as assumed to
>         ...
>         in r0, _SFR_IO_ADDR(SREG)       ; load status. SREG is a SFR macro
>         push r0                         ; save loaded status. Assumes irqs are
I can't see a discrepancy. I save the SREG and restore it at the end of 
the function, because of the called function could it change. However, 
the interrupt have to ensure that the state of all registers is the same 
like before.

> [...]
> > You have a very thin HAL or small operating
> > system which offers the possibility of loading and executing of
> > different applications (one at a time) depending on the actual state or
> > environment.
> [...]
>
> Aha, I have understood. Yes, it is an absolutely other level
> of program design. The simplest decision with the 'if' operator
> is not so convinient here.
Yes, correct. We have a system that executes different applications on 
top of a small HAL and middleware, which resides at the start of the 
flash. This region should not be flashed ever, because it can act as a 
boot loader or is used for recovery. That's the reason why we don't want 
to flash the lower parts of the flash. And if we want to support the 
application with ISRs, we have to redirect it.

Regards,
Michael