Subject: Re: Multiple busses of the same kind
To: Nathan J. Williams <nathanw@MIT.EDU>
From: Bill Studenmund <>
List: tech-kern
Date: 07/30/2001 12:00:54
On 30 Jul 2001, Nathan J. Williams wrote:

> Wojciech Puchar <> writes:
> > > > The problem is basically that the ISA bus is attached twice because he
> > > > has two pcib's: one in his laptop, the other provided by his base
> > > > station.

Do either of these busses actually have ISA expansion slots in them? Also,
from looking at the attached devices, were you able to determine that
there really are ISA devices in both units?

> > > Err... do those isa bi (busses?) share their address space?

I doubt it. I suspect the BIOS has programmed one of them to bridge a
specific set of devices and nothing else.

> > can pci to isa bridges be programmed only to map isa address/port space
> > from 0?
> > if not we can have as much ISA busses as we need.
> In theory this could be done.
> In practice, I've never seen a PCI-ISA bridge that did it; I conclude
> that it's not a feature anyone wants. PCI-ISA bridges usually just do
> untranslated subtractive decoding of memory addresses below 16M and
> I/O addresses below 64k, plus decoding for whatever their onboard
> devices are.

Right, but you can really only have one subtractive-decoding bus in a
system. So one of these two isa busses has to be postitive-addressing

Take care,