Editing Boot sector (section)

== Partition tables ==

A disk can be partitioned into multiple partitions and, on conventional systems, it is expected to be. There are two definitions on how to store the information regarding the partitioning:

* A [[master boot record]] (MBR) is the first sector of a data storage device that has been [[partition (computing)|partitioned]]. The MBR sector may contain code to locate the active partition and invoke its volume boot record.
* A [[volume boot record]] (VBR) is the first sector of a data storage device that has ''not'' been partitioned, or the first sector of an individual partition on a data storage device that has been partitioned. It may contain code to load an operating system (or other standalone program) installed on that device or within that partition.

The presence of an IBM PC compatible boot loader for x86-CPUs in the boot sector is by convention indicated by a two-byte hexadecimal sequence <kbd>0x55 0xAA</kbd> (called the boot sector signature) at the end of the boot sector (offsets <kbd>0x1FE</kbd> and <kbd>0x1FF</kbd>). This signature indicates the presence of at least a dummy boot loader which is safe to be executed, even if it may not be able actually to load an operating system. It does not indicate a particular (or even the presence of) [[file system]] or operating system, although some old versions of DOS 3 relied on it in their process to detect [[File Allocation Table|FAT]]-formatted media (newer versions do not). Boot code for other platforms or CPUs should not use this signature, since this may lead to a crash when the BIOS passes execution to the boot sector assuming that it contains valid executable code. Nevertheless, some media for other platforms erroneously contain the signature, anyway, rendering this check not 100% reliable in practice.

The signature is checked for by most system BIOSes since (at least) the [[IBM Personal Computer/AT|IBM PC/AT]] (but not by the original IBM PC and some other machines). Even more so, it is also checked by most MBR boot loaders before passing control to the boot sector. Some BIOSes (like the IBM PC/AT) perform the check only for fixed disk/removable drives, while for floppies and superfloppies, it is enough to start with a byte greater or equal to <kbd>06h</kbd> and the first nine words not to contain the same value, before the boot sector is accepted as valid, thereby avoiding the explicit test for <kbd>0x55</kbd>, <kbd>0xAA</kbd> on floppies. Since old boot sectors (e.g., very old CP/M-86 and DOS media) sometimes do not feature this signature despite the fact that they can be booted successfully, the check can be disabled in some environments.
If the BIOS or MBR code does not detect a valid boot sector and therefore cannot pass execution to the boot sector code, it will try the next boot device in the row. If they all fail it will typically display an error message and invoke INT 18h. This will either start up optional resident software in ROM ([[ROM BASIC]]), reboot the system via INT 19h after user confirmation or cause the system to halt the [[bootstrapping (computing)|bootstrapping]] process until the next power-up.

Systems not following the above described design are:

* CD-ROMs usually have their own structure of boot sectors; for [[IBM PC compatible]] systems this is subject to [[El Torito (CD-ROM standard)|El Torito specifications]].
* [[Commodore 128|C128]] or [[Commodore 64|C64]] software on [[Commodore DOS]] disks where data on Track 1, Sector 0 began with a [[magic number (programming)|magic number]] corresponding to string "CBM".<ref>{{cite book|title=Commodore 128 Programmer's Reference Guide|url=https://archive.org/details/C128_Programmers_Reference_Guide_1986_Bamtam_Books_a|year=1986|publisher=Bantam Books|isbn=0-553-34292-4|pages=[https://archive.org/details/C128_Programmers_Reference_Guide_1986_Bamtam_Books_a/page/n453 446]–667}}</ref>
* IBM mainframe computers place a small amount of boot code in the first and second track of the first cylinder of the disk, and the root directory, called the [[Volume Table of Contents]], is also located at the fixed location of the third track of the first cylinder of the disk.
* Other (non IBM-compatible) PC systems may have different boot sector formats on their disk devices.