Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Master Boot Record

  1. Aug 5, 2009 #1
    I installed the Dreamlinux OS and chose the option to write grub to the Master Boot Record during installation and now the grub menu loads slower than it used to. If I were to delete the partition containing Dreamlinux, would that effect the grub bootloader? Also, I want to be able to edit the /boot/grub/menu.lst file from Linux Mint or Ubuntu (doesn't matter which) instead of having to log into Dreamlinux, so how would I go about doing this?
  2. jcsd
  3. Aug 5, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper

    Do you mean it takes longer to load the menu or the menu stays up for longer?
    There is a setting in the grub menu.lst to set the timeout

    If you delete the partition grub won't be able to run, there is only 256 bytes in the MBR, it's mostly just a pointer to the partition with the grub program installed.
    You can mount the existing partition and edit menu.lst from any linux boot disk
  4. Aug 6, 2009 #3


    User Avatar
    Homework Helper

    The MBR is just the first sector on the first "active" disk in the boot list. It's 512 bytes long. The BIOS just reads this into memory at location 7c00 (hex), and jumps to the code at 7c00.

    Microsoft partition sector code typically relocates itself to 600 (hex) in memory, scans partitions in it's list for a boot sector, then reads that in at 7c00 (hex), repeating the boot cycle. Eventually the code reads in some type of loader, Dos and Win9x loading MSDOS.SYS and IO.SYS, or for Windows 4.0 or later NTLDR.

    During install of NT 4.0 or later, any active 16-bit OS boot sector is backed up into a 512 byte file called something like BOOTSECT.DOS. On a multi-boot system, if the user chooses a Dos or Win9X parition to boot from, NTLDR will read in BOOTSECT.DOS into 7c00 and continue the boot process. Win98 will rename betwewn two versions of MSDOS.SYS and IO.SYS between DOS and Windows version if you use it's menu to reboot in MSDOS mode.
    Last edited: Aug 6, 2009
  5. Aug 6, 2009 #4


    User Avatar
    Gold Member

    Alternatively, you can use grub to chainload Mint or Ubuntu. Then you can just use Ubuntu's menu.lst file.
  6. Aug 6, 2009 #5
    The grub menu takes about 6-8 seconds to load, where it used to take 1-2 seconds. When I try to mount the partition and edit the menu.lst file, it says I need to have administration capabilities to edit it. I take that to mean I need to be logged in as root on the Dreamlinux partition.

    How can I edit the MBR to point to a different partition?


    I have no idea what chain loading is much less how to do it. Could you offer me a website or something that would give me a definition and instructions?
  7. Aug 6, 2009 #6


    User Avatar
    Science Advisor
    Homework Helper

    Yes you need admin to edit boot.lst, you could boot from a live CD and then mount the partition if you don't have the password.
    You can reset the MBR using either the tools in linux or there is a restore option in fdisk (something like fixmbr?)
    Alternatively you can use a windows CD, select rescue mode and there is a fixmbr command.

    Then you can set any partition to be active, in windows use the control-panel->admin tools->local disk admin, select a prtition and set active.
    In linux use fdisk /dev/sda (or whatever) and use the a command to toggle active paritions - note only one should be active.
  8. Aug 7, 2009 #7


    User Avatar
    Gold Member

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Master Boot Record
  1. Recording sounds (Replies: 2)

  2. Recording a webcam (Replies: 2)

  3. Partial boot? (Replies: 4)

  4. Dual booting (Replies: 6)