linux-kernel - Re: how to tell linux (on x86) to ignore 1M or memory

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Date:	Fri, 22 Jun 2007 15:16:09 +0200
From:	Rene Herman <rene.herman@...il.com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Bart Trojanowski <bart@...ie.net>, linux-kernel@...r.kernel.org
Subject: Re: how to tell linux (on x86) to ignore 1M or memory

On 06/22/2007 03:46 AM, H. Peter Anvin wrote:

> Rene Herman wrote:
>> On 04/19/2007 04:18 PM, Bart Trojanowski wrote:
>>
>>> I need to preserve some state from the bios before entering protected
>>> mode.  For now I want to copy it into some ram accessible by
>>> real-mode, say the last megabyte visible in real-mode.
>>>
>>> What's the easiest way to have linux ignore the megabyte starting at 15M?
>> Note that real-mode can only access the first megabyte (*) and not the
>> first 16. 16MB is the 16-bit protected mode (286) limit.
>>
> 
> No, 16-bit protected mode (on 386+) is not limited to 16 MB.

That all depends on one's definition of 16-bit protected mode. The "(286)" 
after mine meant I was talking about the definition in which descriptors 
have a 24-bit base (and 16-bit limit) field -- ie, real 286 and arguably, 
"real 16-bit protected mode".

Yes, I guess another valid definition is "code with a 16-bit address and 
operand size default" on a 386+ and sure, that's just flipping a bit away.
In the context of Linux I agree it's also a sensible definition, so, well, 
whatever. The point was that real mode could only access the first 1M, not 
the first 16... :-)

Rene.

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