The structure of the MBR is defined by an operating system–independent convention. The first 446 bytes are reserved for program code. They typically hold the boot loader program, in this case, GRUB. The next 64 bytes provide space for a partition table of up to four entries (see Section 1.5.4.1, “Partition Types”). The partition table contains information about the partitioning of the hard disk and the file system type. The operating system needs this table for handling the hard disk. The last two bytes of the MBR must contain a static “magic number” (AA55). An MBR containing a different value is regarded as invalid by the BIOS and all PC operating systems.

Boot sectors are the first sectors of hard disk partitions except for the extended partition, which merely serves as a “container” for other partitions. These boot sectors have 512 bytes of space for code used to boot an operating system installed in the respective partition. This applies to boot sectors of formatted DOS, Windows, and OS/2 partitions, which also contain some important basic data of the file system. In contrast, the boot sectors of Linux partitions are initally empty after setting up a file system. Therefore, a Linux partition is not bootable by itself, even if it contains a kernel and a valid root file system. A boot sector with valid code for booting the system has the same magic number as the MBR in its last two bytes (AA55).