Date   : Mon, 23 Sep 2002 17:18:56 +0100
From   : Sprow <info@...>
Subject: Re: BBC B with edge-connectors instead of IDC connectors?

In article <70srouod72qm2vb9lgtlve23g3thdt33q4@...>,
   John Kortink <kortink@...> wrote:

[poking F4 before FE30 with no SEI]

> >> Depends. One possible scenario is that the interrupt handler
> >> needs to access the ROM that was selected when the interrupt
> >> occurred and needs to do so by explicitly switching to it at
> >> some point (which may be, e.g., if executes partially in a
> >> sideways ROM itself, or if it needs to access other ROMs).
> >> It will access the wrong ROM since ?&F4 has already been
> >> updated and no longer corresponds to the actual ROMSEL.
> >
> >In this situation it knows which ROM was paged in when the interrupt went
> >off since it's still selected by virtue of the foreground task not yet
> >poking FE30.

> No it doesn't. It doesn't know its number without reading ?&F4

I can't imagine a situation where this would be a requirement anyway,but
I'll agree that having swapped ROMs it no longer knows truly which ROM was
paged in at the instant of the IRQ.However,the key is that the machine
wouldn't crash since on exit the ROMSEL is updated with the ROM that
everyone wanted anyway.

To sum up,I think I'll carry on doing it the same way the OS does it and not
bother disabling interrupts,just to update the RAM copy first,
Sprow.