Boot sector

which contains machine code to be loaded into random-access memory (RAM) and then executed by a computer system's built-in firmware (e.g., the BIOS).

The purpose of chain loading first a firmware (e.g., the BIOS), then some code contained in the boot sector, and then, for example, an operating system, is maximal flexibility.

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.

Systems not following the above described design are: On IBM PC compatible machines, the BIOS is ignorant of the distinction between VBRs and MBRs, and of partitioning.

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 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.

To combat this behavior, the system BIOS often includes an option to prevent software from writing to the first sector of any attached hard drives; it could thereby protect the master boot record containing the partition table from being overwritten accidentally, but not the volume boot records in the bootable partitions.

The BIOS option is disabled by default because the message may not be displayed correctly in graphics mode and blocking access to the MBR may cause problems with operating system setup programs or disk access, encryption or partitioning tools like FDISK, which may not have been written to be aware of that possibility, causing them to abort ungracefully and possibly leaving the disk partitioning in an inconsistent state.