> >The problem is a basic philosophical decision made when the first Mac's
> >were designed:  The most important task of the machine was the user
> >interface, therefore the mouse is highest priority, etc, etc.  The timer
> >interrupt is lowest priority and you can't change it.

Eh? According to the docs for the original Mac II, the only interrupt sources
with unique CPU priorities are the SCC and the power/programmer switches, and
the two VIA's. All the other devices crowd into the VIA's and share their
interrupt levels (which do happen to be the lowest two of the five).

>  Is it possible that something's been missed?

I'd say it's more than possible; it's almost certain.

Macintosh VIA's are based on a stock part, the 6522. I happen to have a
data sheet for the Rockwell version here. VLSI Technologies made the Apple
enhanced version (as well as an enhanced SCC with a larger receive FIFO).
I've got one of their data books in a pile somewhere.

VIA-like interrupt hardware is quite common, and software techniques do
exist for simulating many "virtual" interrupt levels. The code looks
grossly complicated to anyone used to hardware-managed priorities, but
just like the 1-sec resolution of the clock chip, it's a damn sight better
than what we have now!

Todd Whitesel
toddpw @ best.com