From owner-freebsd-hackers Thu Oct 26 02:02:50 1995 Return-Path: owner-hackers Received: (from root@localhost) by freefall.freebsd.org (8.6.12/8.6.6) id CAA27385 for hackers-outgoing; Thu, 26 Oct 1995 02:02:50 -0700 Received: from irz301.inf.tu-dresden.de (irz301.inf.tu-dresden.de [141.76.1.11]) by freefall.freebsd.org (8.6.12/8.6.6) with ESMTP id CAA27367 for ; Thu, 26 Oct 1995 02:02:20 -0700 Received: from sax.sax.de by irz301.inf.tu-dresden.de (8.6.12/8.6.12-s1) with ESMTP id KAA15566; Thu, 26 Oct 1995 10:02:08 +0100 Received: by sax.sax.de (8.6.11/8.6.12-s1) with UUCP id KAA10764; Thu, 26 Oct 1995 10:02:08 +0100 Received: (from j@localhost) by uriah.heep.sax.de (8.6.12/8.6.9) id JAA27021; Thu, 26 Oct 1995 09:11:03 +0100 From: J Wunsch Message-Id: <199510260811.JAA27021@uriah.heep.sax.de> Subject: Re: boot disk.... To: msmith@atrad.adelaide.edu.au (Michael Smith) Date: Thu, 26 Oct 1995 09:11:02 +0100 (MET) Cc: terry@lambert.org, lenzi@cwbone.bsi.com.br, hackers@FreeBSD.org Reply-To: joerg_wunsch@uriah.heep.sax.de (Joerg Wunsch) In-Reply-To: <199510260347.NAA09839@genesis.atrad.adelaide.edu.au> from "Michael Smith" at Oct 26, 95 01:17:27 pm X-Phone: +49-351-2012 669 X-Mailer: ELM [version 2.4 PL23] MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8bit Content-Length: 1086 Sender: owner-hackers@FreeBSD.org Precedence: bulk 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. ;-)