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List:       freebsd-fs
Subject:    Re: [Off topic] Trashed NT disk
From:       Yar Tikhiy <yar () freebsd ! org>
Date:       2003-07-24 16:25:06
Message-ID: 20030724162506.GA54954 () comp ! chem ! msu ! su
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On Thu, Jul 24, 2003 at 07:58:18PM +0400, Yar Tikhiy wrote:
> 
> On the dmesg vs. BIOS issue:  To a programmer's view, a modern drive
> is just a linear array of sectors.  However, software has stuck to
> the cylinder/head/sector (CHS) way of addressing.  So drives report
> some fictitious geometry, with traditional IDE values for the number
> of heads and sectors per track at 16 (or perhaps 15) and 63,
> correspondingly.

BTW, the nature of these numbers seems to be as follows:
16 heads is a limitation of the ATA (IDE) protocol.
63 sectors is a limitation of the old BIOS interface.
 
> Now BIOS seems to be the only consumer of the old, CHS, fields in
> MBR -- the rest of software uses the logical offset and size fields
> that were introduced to the MBR format at some moment.  That's why
> it's the BIOS geometry that you should care about when partitioning
> a drive.

After some reading, I'm afraid that the "logical" (or "relative")
fields were present in the original MBR format.  However they had
been miscalculated by MS-DOS fdisk and not used by anything else
until drives grew large enough to really reqire them for partitioning.

-- 
Yar

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