Format of PC-MOS/386 Block Device Block:
Offset	Size	Description	)
 00h	BYTE	signature byte "H" if allocated from system memory pool
 01h	BYTE	header block ID, "B" = block device block
 02h	WORD	length of block in paragraphs
 04h	WORD	segment address of next header block (0000h if last)
 06h	WORD	segment address of previous header block (0000h if first)
 08h	WORD	pointer to next TCB
 0Ah	WORD	pointer to previous TCB
 0Ch	WORD	pointer to associated TCB (if applicable)
 0Eh	WORD	reserved
---BDB---
 10h	BYTE	logical drive
 11h	BYTE	unit passed to driver
 12h	WORD	sector size
 14h	BYTE	cluster mask
 15h	BYTE	cluster shift count
 16h	WORD	starting sector of first FAT
 18h	BYTE	number of FATs
 19h	WORD	number of root directories
 1Bh	WORD	sector number of first data sector (cluster 0002h)
 1Dh	WORD	number of clusters + 1 (number of highest data cluster)
 1Fh	BYTE	number of sectors in FAT
 20h	WORD	beginning root directory sector number
 22h	DWORD	device driver address
 26h	BYTE	media descriptor byte
 27h  5 BYTEs	???
 2Ch	BYTE	flag: volume > 32MB
 2Dh	BYTE	???
 2Eh	BYTE	number of sectors per cluster
 2Fh	WORD	number of clusters on device
 31h	WORD	number of free clusters (FFFFh = unknown)
 33h	WORD	root directory cluster number
 35h	WORD	pointer to alias/subst string
 37h	WORD	TCB segment address of owner (0000h = none)