The master boot record (MBR), also the partition sector, in IBM PC architecture, is the 512-byte (1/2 kilobyte) boot sector, i.e. the sector on the logical beginning of a hard disk that contains the sequence of commands necessary for booting the operating system(s) (OSes).
The bootstrappingfirmware contained within the ROMBIOS loads and executes the master boot record. The MBR of a drive usually includes the drive's partition table, which the PC uses to load and run the boot record of the partition that is marked with the active flag. This design allows the BIOS to load any OS without knowing exactly where to start inside its partition. Because the MBR is read almost immediately when the computer is started, many computer viruses made in the era before virus scanner software became widespread operated by changing the code within the MBR.
The MBR is the sector at cylinder 0, head 0, sector 1 of a hard disk.
The MBR is first of what could be many partition sectors, each one containing a four entry partition table.
If an active partition is found, that partition's boot record is read into 0000:7c00 and the MBR code jumps to 0000:7c00 with SI pointing to the partition table entry that describes the partition being booted.
The entire MBR is shown, along with the disassembly of the program code and discussion of the new System Indicators, Interrupt 13 extensions and hard drive size limitations.
From the MBR, the maximum number of heads (sides) is 256 (0-255), max cylinders is 1024 (0-1023) and max sectors is 63 (1-63).
FDISK /MBR writes the master boot program to the hard disk without altering the partition table information unless the 2-byte signature at the end of the sector is missing or invalid, in which case it overwrites the partition table entries with all zeros.
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