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BOOT(8)       OpenBSD System Manager's Manual (I386 Architecture)      BOOT(8)

     boot - system bootstrapping procedures

     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.

     Cold starts. The PC AT clones will perform a POST (Power On Self Test)
     upon being booted cold.  This test will find and initialize memory, key-
     board, and other devices.  It will search for and initialize any exten-
     sion 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

     The newer 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 con-
     troller 1, drive 0).

     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.

     There are two different types of boot blocks on devices.  There is the
     MBR (master boot record), and the PBR (partition boot record).  A digres-
     sion into a little piece of history will quickly give light as to why
     this is so.  In the beginning, the PC ``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 right device.  When hard disks
     came out, it was felt that such a huge space should be able to be parti-
     tioned into separate drives, and this is where the MBR was invented.

     The MBR relocates itself upon being loaded and invoked by the BIOS.  Em-
     beded within the MBR is a partition table, with four partitions table en-
     tries.  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 all
     reality, the MBR is nothing more than a fancy chaining PBR.

     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 correspond-
     ing MBR or PBR out of its extended partition's partition table.

     WARNING: This portion of the ``PC BIOS Architecture'' is a mess, and a
     compatibility nightmare.

     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 re-
     stricts 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.

     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 ``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 por-
     tion of the drive, still using the restricted BIOS API.

     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!

     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 con-
     troller to controller.  They are controller and machine specific.

     /bsd                system code
     /usr/mdec/mbr       system MBR image
     /usr/mdec/biosboot  system primary stage bootstrap (PBR)
     /boot               system second stage bootstrap

     halt(8),  reboot(8),  shutdown(8),  installboot(8),  boot(8)

     The ``PC BIOS Architecture'' makes this process very prone to weird and
     wonderfull interactions between differing operating systems.  There is no
     published standard to the MBR and PBR, which makes coding these a night-
     mare.  Somebody *please* write me a decent BIOS, and make them (the mass-
     es) use it!

OpenBSD 2.3                    September 4, 1997                             2

Source: OpenBSD 2.6 man pages. Copyright: Portions are copyrighted by BERKELEY
SOFTWARE DESIGN, INC., The Regents of the University of California, Massachusetts
Institute of Technology, Free Software Foundation, FreeBSD Inc., and others.

(Corrections, notes, and links courtesy of RocketAware.com)

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