Boot loader

Legacy BIOS

First-stage MBR boot loaders may face peculiar constraints, especially in size; for instance, on the earlier IBM PC and compatibles, a boot sector should typically work with 510 bytes of code (or less) and in only 32 KiB (later relaxed to 64 KiB) of system memory and only use instructions supported by the original 8088/8086 processors. The first stage of PC boot loaders (FSBL, first-stage boot loader) located on fixed disks and removable drives must fit into the first 446 bytes of the master boot record in order to leave room for the default 64-byte partition table with four partition entries and the two-byte boot signature, which the BIOS requires for a proper boot loader — or even less, when additional features like more than four partition entries (up to 16 with 16 bytes each), a disk signature (6 bytes), a disk timestamp (6 bytes), an Advanced Active Partition (18 bytes) or special multi-boot loaders have to be supported as well in some environments.