Results 1 to 6 of 6

Thread: MBR & Boot Record

  1. #1
    Join Date
    Apr 2011
    Beans
    35

    MBR & Boot Record

    I'm a little stumped about this. I've search online and found nothing so I'm hoping I can get some help from here. I was recently looking at my MBR and Boot Record on my mechanical harddrive which is currently running 64-bit Ubuntu. The starting C/H/S for the MBR is 20 21 00 (80 20 21 00 07 FE FF FF 00 08 00 00 00 00 00 05). I was under the impression that the starting C/H/S was typically 01 01 00.

    Also, the total number of sectors between the MBR and the Boot Record have thrown me for a loop. The total sectors in the MBR is 00 00 00 05 (83886080) and the total sectors in the Boot Record is F0 FF FF 04 (83886064) which is a 16 sector difference. I always thought that there was a 1 sector difference between the two.

    MBR:

    80 20 21 00 07 FE FF FF 00 08 00 00 00 00 00 05

    Boot Record:

    EB 52 90 4E 54 46 53 20 20 20 20 00 02 08 00 00
    00 00 00 00 00 F8 00 00 3F 00 FF 00 00 08 00 00
    00 00 00 00 80 00 80 00 F0 FF FF 04 00 00 00 00
    00 00 0C 00 00 00 00 00 02 00 00 00 00 00 00 00

  2. #2
    Join Date
    Jun 2009
    Location
    Chicago Suburbs
    Beans
    Hidden!
    Distro
    Ubuntu 14.04 Trusty Tahr

    Re: MBR & Boot Record

    I not sure what the question or answer is.

    But drives have not used CHS since hard drives went over 8GB. They use LBA or large block allocation or large setting in BIOS.

    In fact we find issues from both Windows and testdisk which still internally use CHS and write extended partitions end beyond the last sector as they incorrectly round CHS values back.

    Testdisk uses CHS - formula for conversion to LBA
    http://en.wikipedia.org/wiki/Cylinder-head-sector


    Since drives became over 8GB CHS cylinders, heads, sectors has been obsolete and LBA is how drive is configured. For more compatibility with newer drives, 4k drives, SSDs and better configuration, partitions now start at sector 2048 rather than rounding to cylinders. With 512 bytes per sector that is 1MB at the beginning of the drive.
    Oldfred on vacation. Back Sept 9. Will not have much if any Internet access.
    For info on UEFI boot install & repair:
    http://ubuntuforums.org/showthread.php?t=2147295
    Please use Thread Tools above first post to close thread when/if answered completely.







  3. #3
    Join Date
    Apr 2011
    Beans
    35

    Re: MBR & Boot Record

    Okay, from what I understand now is that those bytes, located at the c/h/s offsets, are no longer being used, correct? If that's true, why would they have changed the way they did? I know that the new starting sector of a disk is located at 2048 instead of 63 but why is there a 16 sector difference between the total number of sectors on the volume instead of 1?

  4. #4
    Join Date
    Apr 2011
    Beans
    35

    Re: MBR & Boot Record

    Okay, I did some looking around and I found some links within your links that helped me with the c/h/s offsets and values. This link: http://thestarman.pcministry.com/asm...7MBR.htm#INTRO helped me tremendously on how the c/h/s offsets and values relate to the starting sector of the partition (2048). My other question still remains about the sector differences though.
    Last edited by nethompson; January 24th, 2013 at 07:23 PM. Reason: Mispell

  5. #5
    Join Date
    Jun 2009
    Location
    Chicago Suburbs
    Beans
    Hidden!
    Distro
    Ubuntu 14.04 Trusty Tahr

    Re: MBR & Boot Record

    Is it a new 4K drive?
    http://www.ibm.com/developerworks/li...-sector-disks/
    Post on 8-sector boundaries alignment by srs5694
    http://ubuntuforums.org/showthread.php?tit's 8-sector (4096-byte) alignment - post 8
    http://ubuntuforums.org/showthread.php?t=1768635
    Alignment issues on 4K drives
    http://ubuntuforums.org/showthread.php?t=1635018
    Oldfred on vacation. Back Sept 9. Will not have much if any Internet access.
    For info on UEFI boot install & repair:
    http://ubuntuforums.org/showthread.php?t=2147295
    Please use Thread Tools above first post to close thread when/if answered completely.







  6. #6
    Join Date
    Apr 2011
    Beans
    35

    Re: MBR & Boot Record

    No, this drive was made in 2008 and the sectors are 512 bytes each according to fdisk -l.

Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •