ISA bus limitations (Re: binary Mach distribution for 386)

[Dick Dunn]

aland at informix.com (Colonel Panic) writes:
> [I wrote]
> >...Conventional wisdom (such as it is:-) says
> >that the PC/AT bus is seldom a limiting factor, unless you put memory out
> >there, which is rare for 386es...
...
> I wouldn't call this "rare" anymore.  AT&T (and others) have been
> shipping 386s that can support 40MB or more for several years.

You've got what I said turned around.  I'm saying that 386 boxes rarely
have memory on the ISA bus--it's only used as the I/O bus in 386 machines.
Certainly you won't have 40 Mb of memory on an ISA bus--it can't support
that much physical address space!

My point was that the bus load from putting memory on the ISA bus would be
disastrous, but "nobody" does that for 386es, so it's not a problem.

> You wouldn't think the ISA bus was so nifty if you were trying to do
> DMA over it (or use memory-mapped devices) on an ISA-bus machine with 
> over 16MB of memory on it...  

That's definitely a problem...although it really focuses on the DMA
controllers (wretched devices for various reasons) more than the I/O bus.
Somewhere along the way to moving memory off the ISA bus, someone should
have come up with a better DMA controller that also had a way to get to
more memory.
-- 
Dick Dunn     rcd at ico.isc.com -or- ico!rcd       Boulder, CO   (303)449-2870
   ...Mr. Natural says, "Use the right tool for the job."