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.\" $OpenBSD: boot_i386.8,v 1.5 2001/10/04 16:56:52 mpech Exp $
.\"
.\" Copyright (c) 1997 Tobias Weingartner
.\"
.\" All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\" must display the following acknowledgement:
.\" This product includes software developed by Michael Shalayeff.
.\" 4. The name of the author may not be used to endorse or promote products
.\" derived from this software without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
.\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.Dd September 4, 1997
.Dt BOOT_I386 8 i386
.Os
.Sh NAME
.Nm boot
.Nd
.Tn i386-specific
system bootstrapping procedures
.Sh DESCRIPTION
.Ss Power fail and crash recovery
Normally, the system will reboot itself at power-up or after crashes.
An automatic consistency check of the file systems will be performed,
and unless this fails, the system will resume multi-user operations.
.Ss Cold starts
The
.Tn "PC AT"
clones will perform a POST (Power On Self Test) upon being booted cold.
This test will find and initialize memory, keyboard, and other devices.
It will search for and initialize any extension ROMs that are present,
and then attempt to boot the operating system from an available boot
drive.
Failing this, older IBM systems came up in ROM BASIC.
.Pp
The newer
.Tn "PC AT"
clones attempt to boot off the drive specified in the BIOS setup, or
if it is an older BIOS, it will start with checking for a disk in floppy
drive A (otherwise known as drive 0) first, and failing that, attempt to
boot the hard disk C (otherwise known as hard disk controller 1, drive 0).
.Ss Warm starts
The BIOS loads the first block (at physical location: track 0, head 0,
sector 1) off the boot device into memory, and if the last two bytes in the
block match the signature 0x55AA, the BIOS considers the block a valid
bootable drive.
The BIOS then proceeds to call the machine code program in this block.
If the BIOS is current, it will also pass the boot drive
to the boot block in register %dl.
.Pp
There are two different types of boot blocks on devices.
There is the
MBR (master boot record) and the PBR (partition boot record).
A digression
into a little piece of history will quickly give light as to why this is so.
In the beginning, the PC
.Dq architecture
came with a single or dual floppy
drives, and no hard drives.
The only type of bootable sectors on any device were the PBRs.
They were responsible for loading the rest of the operating
system from the correct device.
When hard disks came out, it was felt that
such a huge space should be able to be partitioned into separate drives,
and this is when the MBR was invented.
.Pp
The MBR relocates itself upon being loaded and invoked by the BIOS.
Embedded within the MBR is a partition table, with four partition table
entries.
The MBR code traverses this table (which was loaded with the
MBR by the BIOS), looking for an active entry, and then loads the MBR or
PBR from the disk location specified by the partition table entry.
So in reality, the MBR is nothing more than a fancy chaining PBR.
.Pp
Note: The MBR could load another MBR, which is the case when you are booting
off an extended partition.
In other words, the first block of an extended
partition is really an MBR, which will then load the corresponding MBR or PBR
out of its extended partition's partition table.
.Sh GEOMETRY TRANSLATION
.Em WARNING :
This portion of the
.Dq PC BIOS Architecture
is a mess, and a compatibility nightmare.
.Pp
The PC BIOS has an API to manipulate any disk that the BIOS happens to
support.
This interface uses 10 bits to address the cylinder, 8 bits to
address the head, and 6 bits to address the sector of a drive.
This restricts any application using the BIOS to being able to address only
1024 cylinders, 256 heads, and 63 (since the sectors are 1 based) sectors
on a disk.
These limitations proved to be fine for roughly 3 years after
the debut of hard disks on PC computers.
.Pp
Many (if not all) newer drives have many more cylinders than the BIOS API
can support, and likely more sectors per track as well.
To allow the BIOS the ability of accessing these large drives, the BIOS would
.Dq re-map
the
cylinder/head/sector of the real drive geometry into something that would
allow the applications using the BIOS to access a larger portion of the
drive, still using the restricted BIOS API.
.Pp
The reason this has become a problem is that any modern OS will use its own
drivers to access the disk drive, bypassing the BIOS completely.
However,
the MBR, PBR, and partition tables are all still written using the original
BIOS access methods.
This is for backwards compatibility to the original IBM PC!
.Pp
So the gist of it is, the MBR, PBR, and partition table need to have BIOS
geometry offsets and cylinder/head/sector values for them to be able to
load any type of operating system.
This geometry can, and likely will,
change whenever you move a disk from machine to machine, or from controller
to controller.
.Em They are controller and machine specific .
.Sh FILES
.Bl -tag -width /usr/mdec/biosboot -compact
.It Pa /bsd
system code
.It Pa /usr/mdec/mbr
system MBR image
.It Pa /usr/mdec/biosboot
system primary stage bootstrap (PBR)
.It Pa /boot
system second stage bootstrap
.El
.Sh SEE ALSO
.Xr boot 8 ,
.Xr halt 8 ,
.Xr installboot 8 ,
.Xr reboot 8 ,
.Xr shutdown 8
.Sh BUGS
The
.Dq PC BIOS Architecture
makes this process very prone to weird and
wonderful interactions between different operating systems.
There is no published standard to the MBR and PBR,
which makes coding these a nightmare.
Somebody *please* write me a decent BIOS, and make them (the masses) use it!
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