Date: Fri, 14 Jan 2000 10:25:28 -0800 (PST) From: Jim Shankland <jas@flyingfox.com> To: hackers@freebsd.org Subject: "very dangerously dedicated mode" is Message-ID: <200001141825.KAA18379@biggusdiskus.flyingfox.com>
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I was experimenting with disk formats yesterday, and wanted to see if the BIOS on one of my machines would boot a hard drive with a boot sector (like on a floppy), but no real MBR (i.e., the "AA55" magic number is in place at offset 510, but there is no partition table; more specifically, the "partition table" contains code). This is what Bruce Evans has called "very dangerously dedicated" (see http://www.FreeBSD.org/cgi/getmsg.cgi?fetch=102186+104290+/usr/local/www/db/text/1999/freebsd-current/19990808.freebsd-current). The BIOS on my machine couldn't deal with it, which didn't surprise me that much. The surprise came when I booted FreeBSD 3.2 off a floppy to try to fix my hard disk. The wd driver tries to read the garbage partition table, and wedges: wd0: error reading extended partition table reading fsbn 338137092 wd0s1c: hard error reading fsbn 1 (status 51<rdy,seekdon,err> error 4<abort>) wd0s2c: timeout waiting to give command reading fsbn 1 (status 0 error <abort>) with the last line being repeated at about 1-second intervals, ad infinitum. Now, this strikes me as a bug: sure, the partition table contains garbage, so go ahead and return ENODEV or ENXIO on access to wd0s1, but the whole system is unbootable, even if the kernel is on a floppy. By the way, I also struck out with DOS fdisk: it took one look at the garbage partition table, and wedged. I'll be trying a Linux rescue disk next. If that fails, too, then I seem to have generated a 1-Gigabyte hockey puck (you didn't think I was trying this with a new disk, did you)? Anyway, now you know, my friends, how "very dangerously dedicated mode" got its name :-). Jim Shankland NLynx Systems, Inc. To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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