 The MBR of a hard disk can either define up to four primary partitions, or an extended partition in addition to up to three primary partitions.In November 1987, Compaq Personal Computer DOS 3.31 (a modified OEM version of MS-DOS 3.3 released by Compaq with their machines) introduced what today is simply known as "the FAT16" format, with the expansion of the 16-bit disk sector count to 32 bits in the BPB.Although the on-disk changes were minor, the entire DOS disk driver had to be converted to use 32-bit sector numbers, a task complicated by the fact that it was written in 16-bit assembly language.