Junior Member
May 26, 2016
As i know MBR is created when we partition the disk. According the the doc:

MBR... contains a boot loader for the installed operating system and information about the drive’s logical partitions

And here is my confusion. How can MBR contains a boot loader for the installed operating system while when we partition the disk there isn't any system installed yet?


Diamond Member
Dec 22, 2012
What is the DOC from?
When you partition a disk and use the bootable/active flag it only writes information to the mbr that is important for the bios to be able to start from this disk,the disk will just boot into a flashing cursor or some other error message.
If you then install a OS it will write additional info to the mbr telling it where to find the OS loader.
If you partition a disk from a OS installation media than it will write all the info including the OS specific parts.


No Lifer
Aug 25, 2001
For DOS/MBR systems, the BIOS reads sector 0 off of the HDD into a certain memory address, and executes it. That sector, contains additional-stage bootloader info, to read the partition table (already loaded into RAM, as part of sector 0), finds a primary partition marked bootable, and then loads the OS bootloader / partition boot sector, and kind of chain-loads it, I think. Then, that sector, loads whatever it has to from that partition to boot (NTLOADER).

For UEFI, it's a bit different.