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From: "Sebastian Herbszt" <herbszt@gmx.de>
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Date: Fri, 22 Aug 2008 00:47:44 +0200
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Subject: [Qemu-devel] Re: [Bochs-developers] BIOS, ACPI,
	CMOS and Windows EvenID: 4
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To: Gleb Natapov <gleb@qumranet.com>
Cc: bochs-developers@lists.sourceforge.net, qemu-devel@nongnu.org

Gleb Natapov wrote:
>> The Microsoft document suggests to copy the CMOS data to somewhere in memory
>> and access it there. I guess rombios space from 0xe000 to 0xffff could be a good
>> candidate.
> No need to copy the whole CMOS. Copying only memory size should be
> enough. Also putting it somewhere in first 4K should be better. You
> can't write into 0xe000-0xffff unless chipset shadows it in a memory.

bios_shadow_init() from rombios32.c does already support this for the 440fx.

>> The 440fx specs says "The top of memory is determined by the value written into DRB7."
>> We might get the proper value from there. But it also says "Note that the PMC supports
>> a maximum of 1 Gbytes of DRAM."
>>
> Yeah, I saw that my real HW takes top of the memory somewhere from the
> chipset and looked in 440fx specs if it has this info, but it turned
> out that we abuse 440fx and sometimes use much more memory that it
> supports. We can re-use DRB registers for our needs and store memory
> size there on POST (we can store 8 bytes there, so it should be enough)
> and read them from AML. Are all users of bochs BIOS uses the same
> chipset?

I think bochs, qemu, xen and virtual box emulate the 440fx.

- Sebastian