Re: Software for MS-DOS 1.25

Den Tue, 10 Oct 2017 14:07:29 +0200 skrev Michał Pleban
<lists@michau.name>:
> > In a PC this INT would handle the on board timer. The corresponding
> > IRQ0 comes from the on board 8253 or 8254. OK, that won't hurt us
> > you could assume. But the moment a piece of software hooks into INT
> > 8h because it thinks its is hooking into the timer interrupt, we
> > could run into trouble. Which on its turn means we cannot use any
> > software depending on hooking into any INT in the range from 8 to
> > 0Fh for the simple reason we don't have the hardware. Hmmm, just a
> > thought: can we direct a 6509 IRQ to the 8088 in one or another
> > way? If that is possible, we can support INT 0Ah en 0Bh for the COM
> > ports (read: 6551).
> 
> If you rewrote the 6509 part significantly, you could use INT 0Fh
> instead of 08h (I say significantly, because IR0 is negated coming out
> of the CIA while IR7 is not). But are there really many programs that
> hook to INT 08h?

One solution is to make the 6509 code count how many interrupts it has
done per time unit, and make sure that it averages out to about 18.2
times per second. That way most software should work.

As it seems like there is two IRQ's from 6509 to 8088 one could be used
for simulating the 18.2Hz timer combined with whatever communication
that could wait until the next 18.2Hz tick, and the other IRQ for actual
communication that cannot wait until a 18.2Hz tick.

I assume that DOS programs uses the IRQ0 timer to do some periodical
checking, update clock diskplay or similar, unless the software uses
polling and BIOS/DOS calls for all that stuff.



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