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Date:      Mon, 20 Nov 2000 12:30:36 -0800 (PST)
From:      "Rodney W. Grimes" <freebsd@gndrsh.dnsmgr.net>
To:        imp@village.org (Warner Losh)
Cc:        bfoz@glue.umd.edu (Brandon Fosdick), stable@FreeBSD.ORG, obrien@FreeBSD.ORG
Subject:   Re: Dangerously Dedicated
Message-ID:  <200011202030.MAA19474@gndrsh.dnsmgr.net>
In-Reply-To: <200011192214.eAJMEPG03693@billy-club.village.org> from Warner Losh at "Nov 19, 2000 03:14:25 pm"

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> In message <3A18304B.689C2CFE@glue.umd.edu> Brandon Fosdick writes:
> : Using what I consider to be a artifact of another operating system
> : on a machine that doesn't use that OS seems silly to me. Unless, of
> : course, that artifact has some useful feature(s) or
> : functionality. If it does, I'm all ears.
> 
> But it isn't an artifact of another OS.  It is an artifact of the
> BIOS.

Read other email.  The partition table is not an aftifact of the BIOS,
it is an actifact of PC-DOS's MBR, and hence forth MS's MBR code.

> : I'm a little confused here. Why are slices demanded by the Intel
> : arhictecture?
> 
> The BIOS demands that they are there.  At least some modern BIOSes
> don't do well when they aren't there.

Any BIOS that demands they are there is broken and violating IBM AT
compatibility.  Very few bioses actually have problems if they are not
there, just so long as 0xaa55 is there and the code in the sector runs
correctly.
> 
> It is the PC-AT architecture to be more specific.
No, it is not, see other email.

> : We've been successfully using DD mode for years now, if slices are "demanded"
> : what kind of voodoo have we been using? 
> 
> The problem is the bogus MBR that the DD writes confuses some BIOSes
> and causes your disks to be non-bootable.

This is true.  It is the _BOGUS_ mbr that is giving those bioses that violate
the specs and try to parse the partition table.  If you use a _VALID_
partition table starting at sector 0 these BIOS tend to work correctly with
a dangeriously dedicated disk.

> 
> : Is there some way or ways in which the 4-slot table is superior to DD-mode?
> 
> The 4 slot table already is there in DD mode.  It just happens to
> contain completely bogus data.

And that is the problem, not the DD mode, it is the bogus data.  Usually just
adjusting the end address of the bogus data to be cylinder alligned will fix
the non-boot problem on machines with a broken BIOS.


-- 
Rod Grimes - KD7CAX @ CN85sl - (RWG25)               rgrimes@gndrsh.dnsmgr.net


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