1.\" $OpenBSD: boot_i386.8,v 1.14 2007/05/31 19:19:59 jmc Exp $ 2.\" 3.\" Copyright (c) 1997 Tobias Weingartner 4.\" 5.\" All rights reserved. 6.\" 7.\" Redistribution and use in source and binary forms, with or without 8.\" modification, are permitted provided that the following conditions 9.\" are met: 10.\" 1. Redistributions of source code must retain the above copyright 11.\" notice, this list of conditions and the following disclaimer. 12.\" 2. Redistributions in binary form must reproduce the above copyright 13.\" notice, this list of conditions and the following disclaimer in the 14.\" documentation and/or other materials provided with the distribution. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 17.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED 18.\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.Dd $Mdocdate: May 31 2007 $ 29.Dt BOOT_I386 8 i386 30.Os 31.Sh NAME 32.Nm boot_i386 33.Nd i386 system bootstrapping procedures 34.Sh DESCRIPTION 35.Ss Cold starts 36The 37.Tn "PC AT" 38clones will perform a POST (Power On Self Test) upon being booted cold. 39This test will find and initialize memory, keyboard, and other devices. 40It will search for and initialize any extension ROMs that are present, 41and then attempt to boot the operating system from an available boot 42drive. 43Failing this, older IBM systems come up in ROM BASIC. 44.Pp 45The newer 46.Tn "PC AT" 47clones attempt to boot off the drive specified in the BIOS setup, or 48if it is an older BIOS, it will start with checking for a disk in floppy 49drive A (otherwise known as drive 0) first, and failing that, attempt to 50boot the hard disk C (otherwise known as hard disk controller 1, drive 0). 51.Ss Warm starts 52The BIOS loads the first block (at physical location: track 0, head 0, 53sector 1) off the boot device into memory, and if the last two bytes in the 54block match the signature 0xAA55, the BIOS considers the block a valid 55bootable drive. 56The BIOS then proceeds to call the machine code program in this block. 57If the BIOS is current, it will also pass the boot drive 58to the boot block in register %dl. 59.Pp 60There are two different types of boot blocks on devices. 61There is the 62MBR (master boot record) and the PBR (partition boot record). 63A digression 64into a little piece of history will quickly give light as to why this is so. 65In the beginning, the PC 66.Dq architecture 67came with single or dual floppy 68drives, and no hard drives. 69The only type of bootable sectors on any device were the PBRs. 70They were responsible for loading the rest of the operating 71system from the correct device. 72When hard disks came out, it was felt that 73such a huge space should be able to be partitioned into separate drives, 74and this is when the MBR was invented. 75.Pp 76The MBR relocates itself upon being loaded and invoked by the BIOS. 77Embedded within the MBR is a partition table, with four partition table 78entries. 79The MBR code traverses this table (which was loaded with the 80MBR by the BIOS), looking for an active entry, and then loads the MBR or 81PBR from the disk location specified by the partition table entry. 82So in reality, the MBR is nothing more than a fancy chaining PBR. 83.Pp 84Note: The MBR could load another MBR, which is the case when you are booting 85off an extended partition. 86In other words, the first block of an extended 87partition is really an MBR, which will then load the corresponding MBR or PBR 88out of its extended partition's partition table. 89.Ss Geometry translation 90.Em WARNING : 91This portion of the 92.Dq PC BIOS Architecture 93is a mess, and a compatibility nightmare. 94.Pp 95The PC BIOS has an API to manipulate any disk that the BIOS happens to 96support. 97This interface uses 10 bits to address the cylinder, 8 bits to 98address the head, and 6 bits to address the sector of a drive. 99This restricts any application using the BIOS to being able to address only 1001024 cylinders, 256 heads, and 63 (since the sectors are 1 based) sectors 101on a disk. 102These limitations proved to be fine for roughly 3 years after 103the debut of hard disks on PC computers. 104.Pp 105Many (if not all) newer drives have many more cylinders than the BIOS API 106can support, and likely more sectors per track as well. 107To allow the BIOS the ability of accessing these large drives, the BIOS would 108.Dq re-map 109the 110cylinder/head/sector of the real drive geometry into something that would 111allow the applications using the BIOS to access a larger portion of the 112drive, still using the restricted BIOS API. 113.Pp 114The reason this has become a problem is that any modern OS will use its own 115drivers to access the disk drive, bypassing the BIOS completely. 116However, 117the MBR, PBR, and partition tables are all still written using the original 118BIOS access methods. 119This is for backwards compatibility to the original IBM PC! 120.Pp 121So the gist of it is, the MBR, PBR, and partition table need to have BIOS 122geometry offsets and cylinder/head/sector values for them to be able to 123load any type of operating system. 124This geometry can, and likely will, 125change whenever you move a disk from machine to machine, or from controller 126to controller. 127.Em They are controller and machine specific . 128.Ss Boot process options 129On most 130.Ox 131systems, booting 132.Ox 133from the BIOS will eventually load the 134.Ox Ns -specific 135i386 bootstrapping code. 136This versatile program is described in a separate document, 137.Xr boot 8 . 138Other bootstrapping software may be used, and can chain-load the 139.Ox 140bootstrapping code, or directly load the kernel. 141In the latter case, refer to your bootloader documentation to know which 142options are available. 143.Ss Abnormal system termination 144In case of system crashes, the kernel will usually enter the kernel 145debugger, 146.Xr ddb 4 , 147unless it is not present in the kernel, or it is disabled via the 148.Em ddb.panic 149sysctl. 150Upon leaving ddb, or if ddb was not entered, the kernel will halt the system 151if it was still in device configuration phase, or attempt a dump to the 152configured dump device, if possible. 153The crash dump will then be recovered by 154.Xr savecore 8 155during the next multi-user boot cycle. 156It is also possible to force other behaviours from ddb. 157.Sh FILES 158.Bl -tag -width /usr/mdec/biosboot -compact 159.It Pa /bsd 160default system kernel 161.It Pa /bsd.mp 162multi-processor capable kernel 163.It Pa /bsd.rd 164standalone installation kernel, suitable for disaster recovery 165.It Pa /usr/mdec/mbr 166system MBR image 167.It Pa /usr/mdec/biosboot 168system primary stage bootstrap (PBR) 169.It Pa /usr/mdec/boot 170system second stage bootstrap (usually also installed as 171.Pa /boot ) 172.It Pa /usr/mdec/pxeboot 173PXE bootstrap 174.El 175.Sh SEE ALSO 176.Xr ddb 4 , 177.Xr boot 8 , 178.Xr halt 8 , 179.Xr init 8 , 180.Xr installboot 8 , 181.Xr pxeboot 8 , 182.Xr reboot 8 , 183.Xr savecore 8 , 184.Xr shutdown 8 185.Sh BUGS 186The 187.Dq PC BIOS Architecture 188makes this process very prone to weird and 189wonderful interactions between different operating systems. 190.Pp 191There is no published standard to the MBR and PBR, 192which makes coding these a nightmare. 193.\" .Pp 194.\" Somebody *please* write me a decent BIOS, and make them (the masses) use it! 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