Path: utzoo!mnetor!uunet!husc6!bloom-beacon!mit-eddie!uw-beaver!cornell!rochester!pt.cs.cmu.edu!B.GP.CS.CMU.EDU!ralf
From: ralf@B.GP.CS.CMU.EDU (Ralf Brown)
Newsgroups: comp.sys.ibm.pc
Subject: Re: big disks on PC AT clones
Message-ID: <1570@pt.cs.cmu.edu>
Date: 30 Apr 88 02:23:30 GMT
References: <21346@amdcad.AMD.COM> <205@octopus.UUCP>
Sender: netnews@pt.cs.cmu.edu
Organization: Carnegie-Mellon University, CS/RI
Lines: 31

In article <205@octopus.UUCP> pete@octopus.UUCP (Pete Holzmann) writes:
}In addition, under many circumstances [seems somewhat random so far], many
}disk-partition-managers/DOS versions/??? require the boot partition on a
}large drive to be bigger than 16MB or you won't be able to boot from the
}hard drive. Has something to do with FAT table sizes. Someday I may understand
}this problem better, but for now suffice it to say that you may end up making
}the boot partition bigger than you would otherwise like.

The reason is that DOS FORMAT (and likely a lot of other programs) use 12-bit
FATs for drives <16M, and 16-bit FATs for drives >16M.  My 40M drive is split
30/10, and the 10M partition was using 4K clusters.  I fairly recently
reformatted it with 2K clusters, but to do so I had to 
	a) patch the boot sector by hand to show 2K clusters (FORMAT used 4K)
	b) build the FATs and root directory by hand
	c) patch the device driver to not assume 4K clusters for partitions
	   less than 16M

The reason for the switchover at 16M is that 12-bit FATs can only have about
4080 clusters, which works out to just over 16M at 4K/cluster.

}-- 
}  OOO   __| ___      Peter Holzmann, Octopus Enterprises
} OOOOOOO___/ _______ USPS: 19611 La Mar Court, Cupertino, CA 95014
}  OOOOO \___/        UUCP: {hpda,pyramid}!octopus!pete
}___| \_____          Phone: 408/996-7746

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