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From: "Sebastian Herbszt" <herbszt@gmx.de>
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Subject: Re: Proper support for PCI-based option rom loading (was Re:
	[Qemu-devel] Re: qdev property bug?)
Date: Thu, 17 Dec 2009 22:14:03 +0100
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: "Michael S. Tsirkin" <mst@redhat.com>, glommer@redhat.com, qemu-devel@nongnu.org, Alexander Graf <agraf@suse.de>, Kevin O'Connor <kevin@koconnor.net>, Gerd Hoffmann <kraxel@redhat.com>

Anthony Liguori wrote:
> Sebastian Herbszt wrote:
>> What about isapc? Or (pci)pc with isa card?
> 
> I'm not sure how the bios enumerates isa option roms on bare metal.  

Let's give this a shot:

A typical isa expansion card seems to have a jumper to select a I/O port, IRQ and
boot rom location, e.g. 280, 10, cc00.
In its default configuration the PMC directs access to 0xc0000-0xdffff to pci and
doesn't respond to those. Those accesses are forwarded to isa. Now they can be
decoded by the expansion card. I assume isa option roms just appear to be there and
the bios only needs to scan for them. This seems to be exactly what qemu does right
now with "-option-rom".

- Sebastian