cvs commit: doc/en_US.ISO8859-1/books/handbook/boot chapter.sgml

[John Baldwin]

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 at FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve"  =  http://www.FreeBSD.org