Date: Mon, 12 Sep 2005 14:36:51 -0400 From: John Baldwin <jhb@FreeBSD.org> To: "Gary W. Swearingen" <garys@FreeBSD.org> Cc: doc-committers@FreeBSD.org, cvs-doc@FreeBSD.org, cvs-all@FreeBSD.org Subject: Re: cvs commit: doc/en_US.ISO8859-1/books/handbook/boot chapter.sgml Message-ID: <200509121436.52355.jhb@FreeBSD.org> In-Reply-To: <200509121514.j8CFEs9g054685@repoman.freebsd.org> References: <200509121514.j8CFEs9g054685@repoman.freebsd.org>
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On Monday 12 September 2005 11:14 am, Gary W. Swearingen wrote:
> garys 2005-09-12 15:14:54 UTC
>
> FreeBSD doc repository
>
> Modified files:
> en_US.ISO8859-1/books/handbook/boot chapter.sgml
> Log:
> Corrected implication that /boot/boot0 comes from MBR.
>
> Added detail about fdisk and /boot/mbr to head off reader assumption
> that FreeBSD's boot manager is FreeBSD-fdisk's default boot manager.
>
> Added detail to statement about 512 byte boot program
> to head off debatable claims that it's untrue, and also
> to inform the reader.
>
> Approved by: keramida
The first sentence has a mismatch between subject and verb ("The FreeBSD
MBR ... are"). Also, a better explanation is something more along the lines
that the MBR, strictly speaking, is just the 4-entry table at the end of the
first sector. The boot code prior to the MBR is not part of the MBR. The
way that booting off a disk works when using BIOS is that it loads the first
sector off of the device to a fixed memory offset (0x7c00 IIRC) and if it
ends with 0xAA55 it assumes it is valid and starts executing it at the
beginning. This is a bit more complicated in that some SCSI controllers
expect the first sector of a hard disk to contain an MBR so that the
controller's BIOS can guess at the geometry the BIOS is using for the disk.
Traditionally, slices are supposed to start and end on cylinder boundaries
meaning that the entire first cylinder (63 sectors on all but really old hard
disks) is available for use by the boot code (which is how boot0ext is two
sectors in length). I'm not sure if any BIOSes actually enforce that
requirement however.
--
John Baldwin <jhb@FreeBSD.org> <>< http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve" = http://www.FreeBSD.org
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