Re: Help about MBR and BOOT sectors

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Article #360

#360

From: b2a
Date: Tue, 14 Jul 1998 00:00

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I just tried this again, and found that John is correct; DOS requires a
valid partition table in MBR to boot.

First time through this, I made a stupid mistake: I have two drives in my
machine, and each has a bootable DOS installation.  If I load and start SBR
from image on disk 80H, after zeroing the partition table on MBR of same
disk, the SBR finds DOS on disk 81H and boots that, which made it appear
that DOS would boot w/o ptab in MBR.  Thankyou for the correction, John.

- Bruce

John S. Fine wrote in message <6ogea5$5nk$1@winter.news.erols.com>...
>b2a wrote:
>>
>> At least BSD and DOS both expect to be called with SI pointing to their
>> entry in the partition table....doesn't seem to matter where in memory
you
>> leave the table image, so long as the SBR code can see it.  If you do
this,
>> you don't actually need to leave the partition table in it's usual place
in
>> the MBR -- everything will boot ok
>
>  I don't know about BSD, but when I tested with DOS it need its
>partition
>entry BOTH in memory (pointed to by si) and in the MBR.
>
>  An early version of my multi-boot-mbr did not rewrite the modified mbr
>back to disk.  It just left it in RAM.  Unixware could be booted that
>way, but DOS required that its active bit be set in its partition entry
>on disk.
>--
>http://www.erols.com/johnfine/
>http://www.geocities.com/SiliconValley/Peaks/8600/

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From: "b2a" <blaing@efn.org>
Subject: Re: Help about MBR and BOOT sectors
Date: 1998/07/14
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