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Master Boot Record, Booting from USB etc.(8)
============================================
Name
----
mbr - Master Boot Record, Booting from USB etc.
Synopsis
--------
None. :)
[IMPORTANT]
This document is work in progress.
[NOTE]
We are using /dev/sdz as the typical device name. Adjust it to whatever
your device corresponds to of course, it's just to prevent any data loss due to
wrong copy/paste.
Layout
------
The Master Boot Record (MBR) is the first data block with 512 bytes on the x86
architecture providing a partition table and a bootloader. In the first 446
bytes is the bootcode, providing the bootloader (400 bytes), a disk signature (4
bytes) and NULL (2 bytes). The following 64 bytes are the partition table,
followed by 2 bytes which are the MBR signature.
Bootcode
~~~~~~~~
The bootcode consists of bootloader and disk signature:
Bootloader
^^^^^^^^^^
Windows: ntldr with boot.ini (Windows NT/XP) or bootmgr with \Boot\BCD (Windows Vista)
Disk Signature
^^^^^^^^^^^^^^
Operating systems like Windows identify the disk using the disk signature.
Running 'fixmbr' doesn't touch the disk signature but just the bootloader.
Running 'fdisk /mbr' touches the bootloader *and* the disk signature (it writes
a standard MBR, it won't touch the partition table though).
TODO: what about fixboot?
Partition table
~~~~~~~~~~~~~~~
Harddisk addressable: 8GB (Cylinder Head Sector, CHS), 2TB (Logical Block
Addressing with 32bit, LBA) or >2TB (GUID Partition Table, GPT)
MBR Signature
~~~~~~~~~~~~~
The 2 bytes 55 and AA (both hex), known as the Magic Number. This signature
tells the system that there should exist a valid MBR - otherwise you'll get
something like "No operating system" or "Non-Bootable Disk".
Backup and Restore
------------------
Clone whole MBR:
# dd if=/dev/sdz of=mbr_backup.dd bs=512 count=1
Restore the MBR again:
# dd if=mbr_backup.dd of=/dev/sdz bs=512 count=1
Clone MBR *without* partition table:
# dd if=/dev/sdz of=mbr_bootcode.dd bs=446 count=1
Restore MBR *without* partition table again:
# dd if=mbr_bootcode.dd of=/dev/sdz bs=446 count=1
Backup partition layout:
# sfdisk -d /dev/sdz > backup.sfdisk
Restore partition layout again:
# sfdisk /dev/sdz < backup.sfdisk
Write new, clean MBR:
lilo -M /dev/sdz -s /dev/null
USB modes
---------
* USB-HDD: usually the default and preferred booting mode.
* USB-ZIP: ??? - can be set up via:
# mkdiskimage -4 /dev/sdz 1 64 32 # device = 1GB
# mkdiskimage -4 /dev/sdz 0 128 32 # device >1GB and <=2GB
# mkdiskimage -F -4 /dev/sdz 0 255 63 # device >2GB and <=8GB
For devices above 8GB (taken from
http://www.knoppix.net/wiki/Bootable_USB_Key):
# mkdiskimage -F -4 /dev/sdz 1 255 63
# dd if=/dev/zero of=/dev/sdz bs=1 seek=446 count=64
# echo -e ',0\n,0\n,0\n,,C,*' | sfdisk /dev/sdz
USB-ZIP requires to have 64 heads and 32 sectors and less than 1024
cylinder count.
* USB-Floppy: ???
TODO
----
Check out the *real* difference between:
# mbr-install /dev/ice
# lilo -S /dev/null -M /dev/ice ext && lilo -S /dev/null -A /dev/ice 1
# cat /usr/lib/syslinux/mbr.bin > /dev/ice
# syslinux /dev/iceX
# syslinux -sf /dev/iceX
# mkdiskimage ... (USB-ZIP?)
# ...?
Partition stuff:
* How does a correct partition look like?
* The partition should start behind MBR at 1st cylinder and 63rd sector?
* fdisk -l -u /dev/sdz vs. fdisk -l /dev/sdz
Resources
---------
* http://de.wikipedia.org/wiki/Master_Boot_Record
* http://en.wikipedia.org/wiki/Master_boot_record
* http://michael-prokop.at/blog/2007/04/22/booting-from-usb-pen-troubleshooting-and-pitfalls/
* http://www.ata-atapi.com/hiwmbr.html
* http://thestarman.pcministry.com/asm/mbr/index.html
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