Editing Boot sector (section)

== Operation ==

{{details|Booting}}

On [[IBM PC compatible]] machines, the [[BIOS]] is ignorant of the distinction between VBRs and MBRs, and of partitioning. The firmware simply loads and runs the first sector of the storage device.<ref>{{cite web |author=Smith |first=Roderick W. |date=2010-04-14 |title=Migrate to GRUB 2 |url=http://www.ibm.com/developerworks/linux/library/l-grub2/index.html?ca=dgr-lnxw97Grub2dth=LX |access-date=2013-03-05 |publisher=Ibm.com}}</ref> If the device is a floppy or [[USB flash drive]], that will be a VBR.  If the device is a hard disk, that will be an MBR. It is the code in the MBR which generally understands disk partitioning, and in turn, is responsible for loading and running the VBR of whichever primary partition is set to boot (the ''active'' partition). The VBR then loads a second-stage bootloader from another location on the disk.

Furthermore, whatever is stored in the first sector of a floppy diskette, USB device, hard disk or any other ''bootable'' storage device, is not required to immediately load any bootstrap code for an OS, if ever. The BIOS merely passes control to whatever exists there, as long as the sector meets the very simple qualification of having the boot record signature of 0x55, 0xAA in its last two bytes. This is why it is easy to replace the usual bootstrap code found in an MBR with more complex loaders, even large multi-functional [[Booting|boot managers]] (programs stored elsewhere on the device which can run without an operating system), allowing users a number of choices in what occurs next. With this kind of freedom, abuse often occurs in the form of boot sector viruses.