Re: GRUB problem (long, description of BOOT)

From: Mike McCarty (mike.mccarty_at_sbcglobal.net)
Date: 09/28/05

  • Next message: Roberto C. Sanchez: "Re: how to add java support to mozilla in woody" 
    Date: Tue, 27 Sep 2005 21:06:35 -0500
To: Jeremy Merritt <brookvegas@yahoo.com>, debian-user@lists.debian.org

    Jeremy Merritt wrote:
    > I have been having a problem getting my GRUB
    > bootloader to return on boot. No matter what I do, it
    > keeps going to XP. I consulted with other people on
    > this list and got some good input. But have run into a
    > dead end again. Can someone analyze these steps and
    > tell me what I'm doing wrong, or what I need to do to
    > get the GRUB menu to return the way it was when I
    > first installed Debian:
    >
    > Steps so far:
    >
    > 1. Boot up with live CD (Knoppix)
    > 2. Activate shell and go root
    > 3. Mount /dev/hda5 as /mnt/hda5
    > 4. chroot /mnt/hda5 /bin/bash
    > 5. grub-install /dev/hda5 -- Reports successful
    > install but no results on bootup

    You installed GRUB as the boot record (BR) of one
    of your partitions (/dev/hda5). This will work fine, but it is
    not what the BIOS will load.

    I think you need a little more information about how
    boot is accomplished on IBM PC style computers. If
    you get that, then I think things will be much more
    clear. So, here is a brief tutorial on disc partitioning
    and how boot proceeds.

    Those who are familiar with this may either skip the
    rest of this message, or use it as a review, or
    criticize it for errors or omissions.

    Discs, to be used, must be formatted. The formatting
    takes place in levels. The lowest level (sometimes
    called low-level format) places tracks and sectors on
    each surface usable on the disc. Modern hard discs reserve
    one surface to hold tracking information alone, no
    data. Low-level formatting should not be done on modern
    hard discs except at the factory. Those of us who used to
    use MFM and RLL discs are glad for that.

    (Floppy discs have all three levels done at once
    by a single program, usually. This causes some
    confusion. The levels of formatting are actually
    done in passes even for floppies.)

    At the second level of formatting, the surface is divided into
    partitions (floppy discs have only one partition, so they
    have no partitioning to be done). Traditionally, the first
    record on the disc, the Master Boot Record (MBR) contains
    two parts: a small bootstrap program and a Partition Table
    (PT). Some consider the PT to be separate from the MBR.
    Also, traditionally, there may only be four (4) partitions defined.
    However, as time went by discs grew larger, and a percieved
    need for more partitions grew as well. So the concept of
    a Primary Partition and an Extended Partition was developed.
    There could be only four (4) Primary Partitions, but
    only one (1) Extended Partition. If an Extended Partition
    existed, it used up one of the entries, so only up to three
    (3) Primary Partitions could then be defined. The PT uses
    physical addresses (head, track [or cylinder], and sector).

    Within the Extended Partition, Logical Partitions (also
    called Logical Discs or Volumes) could be created. Primary
    Partitions also contain Volumes, but only one per partition.

    Each Partition has a type (OS, more or less) and a status.
    The status could be either Bootable (also called Active) or
    non-bootable (Inactive). Only up to one (1) partition may be in
    an Active state, and if so, it must be a Primary Partition.

    A floppy disc is a single Volume, hard discs
    may have up to one Volume per Primary Partition, and
    any number of Volumes (logical partitions or logical
    discs) in an extended partition. Each Volume has
    a Boot Record (BR) also called BIOS Parameter Block (BPB).
    Technically, the BPB is actually a part of the BR, in
    somewhat the same way the PT is part of the MBR.

    The BR contains a description of the logical layout of the Volume,
    like how many reserved sectors there are before the data area,
    how many logical sectors there are, etc. The BR uses logical disc
    addressing (logical sector number).

    The top level of format is the File System (FS). The
    file system uses allocatable units for addressing. Exactly
    what an allocatable unit is depends on what FS is being
    used. Usually the FS presents an Application Programming
    Interface (API) which uses File Addressing (directories,
    files, and records within file).

    >
    > fdisk -l information (hda5=Mandrake, hdb2=Debian,
    > system is booting from hda1=XP):
    >
    > Disk /dev/hda: 40.0 GB, 40020664320 bytes

    This is your first disc.

    > 255 heads, 63 sectors/track, 4865 cylinders
    > Units = cylinders of 16065 * 512 = 8225280 bytes
    >
    > Device Boot Start End Blocks Id System
    > /dev/hda1 * 1 2433 19543041 7 HPFS/NTFS

    Here is your primary partition which is active.

    > /dev/hda2 2434 4865 19535040 5 Extended

    This is your extended partition.

    > /dev/hda5 2434 3197 6136798+ 83 Linux
    > /dev/hda6 3198 3337 1124518+ 82 Linux swap
    > /dev/hda7 3338 4865 12273628+ 83 Linux

    These are volumes inside your extended partition.
    Each of them may be treated as a partition.

    > Disk /dev/hdb: 61.4 GB, 61492838400 bytes

    This is your second disc.

    > 255 heads, 63 sectors/track, 7476 cylinders
    > Units = cylinders of 16065 * 512 = 8225280 bytes
    >
    > Device Boot Start End Blocks Id System
    > /dev/hdb1 1 3188 25607578+ c W95 FAT32 (LBA)

    This is a primary partition, not bootable.
    (LBA = Linear Block Addressing, which means
    that your BIOS is translating logical disc
    addresses [sector number only] into physical
    addresses [head, cylinder, sector] for you.)

    > /dev/hdb2 * 3189 7298 33013575 83 Linux

    This is a primary partition, which is active.

    > /dev/hdb3 7299 7476 1429785 5 Extended

    This is an extended partition.

    > /dev/hdb5 7299 7476 1429753+ 82 Linux swap

    This is a logical volume inside your extended partition.

    >
    > Disk /dev/hdd: 30.0 GB, 30020272128 bytes

    This is your fourth disc. (Where is 3? Possibly
    your CD reader.)

    > 255 heads, 63 sectors/track, 3649 cylinders
    > Units = cylinders of 16065 * 512 = 8225280 bytes
    >
    > Device Boot Start End Blocks Id System
    > /dev/hdd1 * 1 3649 29310561 83 Linux

    This disc has only one partition, which is primary
    and active.

    Now, how does BOOT take place?

    After RESET your processor starts executing a program contained
    in Read Only Memory (ROM), which contains the Basic Input/Output
    System (BIOS), and other programs. Among these is the Power On
    Self Test (POST) program. During execution of the POST, a
    cursory check of basic functionality is done, like a memory test,
    test for video display, etc. Errors during execution of POST
    are normally signaled via "beeps codes". A successful POST is signaled
    by a single beep. Also during POST some codes are output to (IIRC)
    I/O port 0x80 to indicate the progress of the POST. One can purchase
    or build cards which display these, in order to do extended
    diagnosis of boot problems. Hardly anyone does this, because
    motherboards have become too cheap.

    After POST, the boot program starts looking for boot devices.
    Using BIOS settings (technically, these are not BIOS settings,
    they are boot program settings, but hardly anyone makes that
    distinction) the devices are searched in order. I am not familiar
    with the details of boot from USB, SCSI, or CDROM devices, so
    I do not deal with them here. Look elsewhere if you want that
    information.

    For a network boot, normally the Address Resolution Protocol
    (ARP) is used to acquire an IP address, and then start remote
    boot. I won't consider this any further, since it is not part
    of your problem.

    Anyway, the Boot Program searches in order some discs to use
    for boot. It does this by reading the absolute first sector
    of the disc. For a hard disc, this is the MBR (which is
    outside of any partition). It then looks for a Boot Marker
    (0x55, 0xAA) as the last two bytes of the first sector.
    (This is head 0, cylinder 0, sector 1 [sectors are numbered
    from 1].) For a floppy, this is the BR.

    If the marker is not found, then the next device in order
    is checked.

    If the marker is found, the sector is loaded to a fixed
    location in memory (I forget the exact address, but I
    believe it may be 07C0:0000). Then it is given control.

    For a hard disc, there is a small program which searches
    the PT for a Primary Active partition. If exactly one
    Primary Active partition is located, then the first sector
    in that partition is loaded. This is the BR for this partition.
    If no Primary Active partition is found, then an error message
    about no operating system being found is displayed. If more
    than one is found, then some other error message is displayed.
    In either of these cases, BOOT fails.

    At this point, the sequence for floppies and hard discs
    converges, for we now have a BR in memory. Or maybe we
    do. If the magic marker (0x55, 0xAA) is found in the last
    two bytes of the putative BR are found, then we are ok
    so far, otherwise a "missing operating system" message
    is displayed.

    At this point, we load the BR into memory. This contains
    the BPB and a boot program. This boot program (which
    is different for each OS) knows how to load the OS.
    If this program can't find the OS it's looking for in
    the partition, a "missing OS" message is displayed.

    Otherwise, the OS gets loaded, and hopefully completes
    the bootstrap.

    Now, you may install GRUB as the MBR boot program, or
    as a BR boot program. If it is installed into the MBR,
    then it manages the boot. If not, then it
    can be installed as the first sector of the partition
    you are using as /boot.

    To do the first, one must install it to /dev/hda, as
    an example. To do the latter, on installs it to
    /dev/hda5 or whatever partition is being used. Either
    will work. But if GRUB is installed in /dev/hda5, then
    there must be another boot program installed into the MBR.

    This is your current situation. You have a program in the
    MBR which is not GRUB. The program in the MBR knows how
    to load the Widows XP boot loader program (from the BR
    in the XP bootable partition). The XP boot program
    can be instructed to load GRUB if you like. This is the
    way I have my system set up. The XP boot manager either
    loads XP (not the default!) or it loads GRUB. GRUB then
    loads one of the versions of Linux I have installed.

    So you must either:

    (1) Install GRUB into the MBR, or
    (2) Inform the XP boot manager of GRUB's existence.

    I'd do the latter. But either will work. One can argue
    ad nauseum about which is better, but ultimately, either
    will work. I find that it is usually better to let Windows
    be loaded by a MicroSoft loader (*). YMMV.

    To do (1), you need to install GRUB into /dev/hda, not into
    /dev/hda5.

    To do (2), you need to use dd to lift the first sector of
    /dev/hda5 into a file, put that on the XP partition, and
    edit the boot configuration of XP to offer that as an option.

    (*) Some machines (like mine) will go into an "auto recovery"
    mode if the MBR is not what it expects.

    Mike 

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