BIOS PARAMETER TABLE

7/17/2004

The BIOS Parameter Table (BPT) is a small table in the partition boot record of a FAT partition. The BPT is used to identify the geometry used to format the disk and to locate the root directory so that the FAT file system can be accessed. The rest of the boot record is a small program that, either announces that the disk is not bootable, or finds the bootable program (usually IO.SYS with MSDOS and Windows9, but it can have different names); loads it; and executes it.

The partition boot record should not be confused with the Master Boot Record on the first sector of a hard drive. There is only one Master Boot Record and it is not part of any partition. The partition boot record is found at the start of every bootable FAT formatted floppy disk or 'superdisk' (e.g. ZIP disk). A partition boot record is also found at the start of each FAT partition on a hard drive.

The first three bytes of the boot record contain a jump around the BPT. This allows the boot record to be executed starting at a known address (offset 0) while still allowing the size of the BPT to change should there be a reason to change it. The first 8 bytes of the BPT identify the program that formatted the disk. This is followed by sector size, cluster size, number of reserved sectors (sectors not used for fat tables, or data), number to FAT tables, Number of root directory sectors, total sectors, media descriptor byte, sectors per FAT, sectors per track, number of heads, number of hidden sectors,

Unless they are somehow overwritten, most of these parameters are of interest only to programmers writing disk access/copying software. The Media Descriptor byte was important in the early years of MSDOS as it describes the general layout of floppy disks. It was (and may still be) checked by some overaggressive software vendors to make sure that a user was not trying to load software distributed on one medium from a different medium -- e.g. loading 1.2mb 5.25 inch material from a 1.44mb 3.5 inch drive. (Never mind that the usual reason was that the PC didn't have a 5.25 drive installed). Some programs including Windows as late as NT will not accept a disk with a zero Media Descriptor. It is reported that preformatted disks sometimes do not have the Media Descriptor set. Wiping out the media descriptor on a hard drive (it should be F8) will usually cause trouble

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