The media descriptor byte was used to distinguish between the single-sided and double-sided formats.
An additional format utilizing both sides was supported, with 320KB total capacity. There was no real need to distinguish between various formats, although there was a way to do so through the media descriptor byte in the FAT.ĭOS 1.1 was shipped with updated PCs that had double-sided floppy drives. It’s easy to see why: DOS 1.0 only supported a single storage medium format (a single-sided 5¼” floppy with 40 tracks and 8 sector per track, 160KB total capacity). The basic problem with BPBs is that they were only introduced in DOS 2.0 previous versions of DOS did not use the concept at all. Even when there is a BPB, it’s not necessarily correct. There are DOS disks with no signature in the boot sector and no BPB stored in the boot sector. All DOS programmers also know that every DOS disk contains a boot sector with a 0x55, 0xAA signature in the last two bytes and the BPB stored at offset 3 and starting with an 8-byte OEM identifier. Sadly, it’s not true. All DOS programmers know that DOS storage media (floppies, fixed disks, even RAM drives) have a BPB (BIOS Parameter Block) which describes the basic layout of the storage medium.
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