A fixed disk boot record must be written on the first sector of all fixed disks or logical drives within an extended partition and must contain:

• Code to load and give control to the boot record for one of four possible operating systems.

• A partition table at the specified offset into the boot record. Each table entry is 16 bytes long, and contains the starting and ending cylinder, sector, and head for each of four possible partitions, as well as the number of sectors preceding the partition and the number of sectors occupied by the partition. The "boot indicator" byte is used by the boot record to determine if one of the partitions contains a loadable operating system. FDISK (or equivalent) initialization utilities mark a user-selected partition as "startable" by placing a value of 80h in the corresponding partition's boot indicator (setting all other partition's indicators to 00h at the same time). The presence of the 80h tells the Master Boot Record Program to load the sector whose location is contained in the following 3 bytes. That sector is the actual boot record for the selected operating system, and it is responsible for the remainder of the system's loading process (as it is from diskette). All boot records are loaded at absolute address 0:7C00.

• A Signature to indicate a valid Master Boot Record.