Messages in this thread | | | Date | Fri, 22 Jun 2007 15:16:09 +0200 | From | Rene Herman <> | Subject | Re: how to tell linux (on x86) to ignore 1M or memory |
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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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