Date: Fri, 11 Jun 1999 10:21:56 -0500 From: Constantine Shkolnyy <stan@osgroup.com> To: "freebsd-questions@FreeBSD.ORG" <freebsd-questions@FreeBSD.ORG> Subject: RE: MBR (Was RE: I just don't know...) Message-ID: <01BEB3F4.3C1D7C30.stan@osgroup.com>
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On Thursday, June 10, 1999 8:12 PM, Mark Ovens [SMTP:markov@globalnet.co.uk] wrote: > > It's my opinion that any software product should provide > > some means of "undoing" the changes it did to the computer system, > > whenever it's possible. Of course, it is not possible to restore the > > disk data that you agreed to overwrite with FreeBSD file system. > > It would be fine to let the user simply delete the FreeBSD partition. > > In the case of a boot manager, it would be nice if it could ask the > > user > > whether he wants to backup the existing MBR to some place, before > > re-writing it, so that the manager could be cleanly deleted in future. > > > > But why do you need to save a DOS/WIN3.x/WIN9x mbr? All it does in > effect is say "look for IO.SYS in the root dir of the first primary > partition, and if you find it, then run it" Actually, there is some term mixing here and I myself was too cautious yesterday when I wrote about dangers of fdisk /mbr, etc. The MBR is the sector 1 at track 0, head 0. This is what the BIOS reads and this is what the FreeBSD loader replaces. The "mbr" that looks for io.sys is called "boot sector" and there is one boot sector per each partition. The MBR loads one of them, that marked 'active" according to partition table info, or that that the user chooses at MBR's prompt. It is quite uncommon for software products to replace the MBR. They usually replace a boot sector, as NT does. If the original poster finds his Windows loading correctly after installation of FreeBSD loader, this means the original MBR did not contain any unusual functionality, so it can be safely reset with fdisk /mbr. What about a boot record, you should not touch it, since it may be NT's boot record, or a boot record that launches some encryption upon the partition by hooking int 13h. The correct way is what NT does - store the original boot sector, do your things in your replacement, then load and run the original boot record. To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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