*BSD News Article 71750


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From: j@uriah.heep.sax.de (J Wunsch)
Newsgroups: comp.unix.bsd.freebsd.misc
Subject: Re: IDE Hard Disk questions
Date: 22 Jun 1996 11:50:24 GMT
Organization: Private BSD site, Dresden
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stesin@elvisti.kiev.ua (Andrew V. Stesin) wrote:

> : If the drive is FreeBSD and FreeBSD only, you can select any geometry
> : you want when using the ``dangerously dedicated'' mode.

> 	Is this also true for IDE drives?  I mean -- will just
> 	plain motherboard BIOS, not the SCSI HBA's one, live fine
> 	with this?

Of course, as long as your BIOS thinks that your drive has at least 15
sectors (the size of the primary bootstrap) per track.  I don't know
of any BIOS that fails this assumption (unless purposely mis-dictated
via the custom disk type of your setup).

Remember that the BIOS is used only, really *only*, to load the kernel
into memory.

FreeBSD's limits for a wd-style drive are c=65536, h=16, s=255 (127.5
GB).  Since FreeBSD must use the same h and s figures as the BIOS, the
above figures shrink for disks that must be shared with the BIOS
(e.g. boot disks) to c=65536, h=16, s=63 (31.5 GB), due to the int
0x13 limitation to only 6 bit sector numbers.  The other values are
mandated by the register interface of the hardware, in particular
there are only 4 bits to address the head number.

As you can see, the limits (31.5 GB or 127.5 GB) are quite a bit
larger than for messy DOS.  That's what you're getting for designing
(and using) a crippled and unextensible BIOS interface... :-)

-- 
cheers, J"org

joerg_wunsch@uriah.heep.sax.de -- http://www.sax.de/~joerg/ -- NIC: JW11-RIPE
Never trust an operating system you don't have sources for. ;-)