Date: Thu, 26 Oct 1995 09:11:02 +0100 (MET) From: J Wunsch <j@uriah.heep.sax.de> To: msmith@atrad.adelaide.edu.au (Michael Smith) Cc: terry@lambert.org, lenzi@cwbone.bsi.com.br, hackers@FreeBSD.org Subject: Re: boot disk.... Message-ID: <199510260811.JAA27021@uriah.heep.sax.de> In-Reply-To: <199510260347.NAA09839@genesis.atrad.adelaide.edu.au> from "Michael Smith" at Oct 26, 95 01:17:27 pm
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As Michael Smith wrote:
>
> Here's a question for any poor sucker with low-level BIOS experience :
>
> If I rewrite the BPT with a new geometry for the disk, can I assume that
^^^
> subsequent BIOS activity will honour the new geometry?
BPT == breakpoint trap? :-)
No. For IDE disks (without magic things in the way, like ``disk
managers''), the BIOS is looking up the disk values from the CMOS
and/or the built-in disk geometry tables, and it performs a simple
seek test to what it thinks is the very last sector of the drive. IDE
drives pick just this value during the POST, remember it and take it
as their geometry to re-calculate all externally given C/H/S numbers
into the internal block number. This way, IDE remained compatible
with the ST-506 disk handling in the regular BIOS, but gained the
ability to be ``self-learning'', and to use more sophisticated
geometry conceptions like ZBR.
--
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. ;-)
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