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Re: [Qemu-devel] Re: [PATCH 0/2] port over extboot from kvm


From: Avi Kivity
Subject: Re: [Qemu-devel] Re: [PATCH 0/2] port over extboot from kvm
Date: Tue, 08 Sep 2009 23:00:34 +0300
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On 09/08/2009 10:46 PM, Anthony Liguori wrote:
Avi Kivity wrote:
The bios exports the "bios drives". You can tell grub to load a kernel from the first partition of the second disk: (hd1,0) or even chain-load another boot loaded from another disk.

The whole point of device.map is that grub doesn't intrinsically know how to map from bios drives to real drives.

device.map is a wonderful thing, but unrelated to the bios exposing drive 0x81.

In fact, when you chain-load with grub, you often have to "swap" device mappings which has the effect of hooking int13 and faking the primary drive for another OS. See http://www.gnu.org/software/grub/manual/html_node/DOS_002fWindows.html

But grub still has to issue a real int 13 for drive 0x81. If we don't expose it, it can't. Until a real OS is loaded, the only way to access a drive is through int 13 and if we don't expose it, it's invisible.

All else aside, from a BIOS perspective, you can usually only boot from 0x80 and that can be mapped to different drives via BCV.

There things you can do with a drive other than boot it. For example, access it via int 13.

How will you access D:\ from DOS?

--
Do not meddle in the internals of kernels, for they are subtle and quick to 
panic.





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