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Boot process - 2nd stage from external device.
  Initial Program Load (IPL) of OS

  Access the appropriate boot device.

  Read (MBR) master boot record in sector 1 of primary drive - C=0,H=0,S=1.
    1 sector - 512 bytes.  (or LBA 1)

   (GUID Partition Table [GPT] - newer systems, backward compatible with MBR,
     much larger, friendlier to a variety of OSes, part of Unified 
     Extensible Firmware Interface, UEFI)

  Sector must contain certain values to indicate bootable device.

  If not found, 
    next boot device checked.

  If none found, 
    various boot error message displayed and system drops down to the 
    ROM monitor program or system hangs.

  Bios int 13 - provides chs access to drive.
    Bios must know how to access drive 1st sector[s] of boot device.

  Master boot record contains
    Flag that it is a boot record.

    Additional code to continue boot process (446 bytes).
      Read master partition table to know layout of partitions.

    Master partition table describing the partitioning of the drive (64 bytes).
      And which partition is active (where OS is).

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*  MBR - documents up to 4 primary partitions, only one of which should be
    flagged active at any time. Boot loader programs allow user to toggle
    which partition is active.

    One of the primary partitions can be marked as extended and that extended
      partition can further be divided into 24 logical partitions. Logical
      partition info kept in the volume boot record of that partition.

    MBR contains :

    446 byte bootstrap routine.

    4 16 byte records - one for each possible primary partition.
      Active (bootable) flag
      First sector in c:h:s format  c: 10 bit  h: 8 bit  s: 6 bit
      Partition type or id
      Last sector in c:h:s format
      LBA address of 1st sector. (4 bytes)
      Number of sectors in partion

    Disk signiture.

    Note two limits here : the c:h:s bit layout does not conform to current
      drive configurations. Max of 1024 cylinders, 256 heads, 64 sectors.

      Also total sector count is  2^32 * 512 = 2^41 bytes total.

*  GUID partition table - XP, NT, other newer osses.
    XP only supports GUID after OS is loaded (not on OS drive).
      Same with Vista, Windows 7 on PC bios.

    Vista, Windows 7 - on OS drive with Apple bios.

    Starts at LBA 1 (LBA 0 - protective MBR to fool software).
    Uses 32 sectors rather than 1
    Uses lba
    Duplicated at end of disk.
    Supports up to 128 partitions.
    Supported by several OSes. Linux, MacOS, FreeBSD, NetBSD, HP-UX,
       and Windows in a limited form.

    Used with UEFI (extensible firmware interface) which replaces the BIOS.
      UEFI and GPT are mostly found on 64-bit OS systems and/or systems with
        large (Terabyte range) drives, and Apple Macs.

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