Boot Sector - Kinds of Boot Sectors

Kinds of Boot Sectors

Several major kinds of boot sectors could be encountered on IBM PC compatible hard disks, floppy disks and similar storage devices:

• A Master Boot Record (MBR) is the first sector of a data storage device that has been 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 and invoke 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 0x55, 0xAA (called the boot sector signature) at the end of the boot sector (offsets 0x1FE and 0x1FF). 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 to actually load an operating system. It does not indicate the presence of a (or even a particular) file system or operating system, although some old versions of DOS 3 relied on it in their process to detect 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 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 06h 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 0x55, 0xAA on floppies. Since old boot sectors (f.e. 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 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 specifications.

• Non IBM PC compatible systems may have different boot sector formats on their disk devices.