DPT/DPB - Drive Parameter Table / Disk Parameter Block

      Offset Size		Description

	00   byte	drive (0 = A, 1 = B)
	01   byte	unit within device, usually equals drive (ramdisk=0)
	02   word	bytes per sector
	04   byte	sectors per cluster minus 1
	05   byte	sectors per cluster (times to shift left or x2)
	06   word	number of sectors before FAT (boot sectors)
	08   byte	number of FAT copies
	09   word	number of root directory entries
	0B   word	number of first data sector
	0D   word	total number of clusters plus 1
	0F   byte	number of sectors used by first FAT

 	The following fields are DOS version dependant

	10   word	number of first sector in root directory
	12   dword	far pointer to current disk device header
	16   byte	media descriptor byte (see MEDIA DESCRIPTOR)
	17   byte	zero if disk accessed, (default=FF, must rebuild DPB)
	18   dword	far pointer to next drive parameter table; offset
			is set to FFFFh if last block in chain
	1C   word	current directory cluster number; 0=root
	1E  64bytes	ASCIIZ current working directory


Fields differing in DOS 3.x

      Offset Size		Description
	1C   word	starting cluster for free space search
	1E  64bytes	number of free clusters; FFFFh = unknown


Fields differing in DOS 4.x

      Offset Size		Description
	11   word	first sector of root directory
	13   dword	far pointer to current disk device header
	17   byte	media descriptor byte (see MEDIA DESCRIPTOR)
	18   byte	zero if disk accessed, (default=FF, must rebuild DPB)
	19   dword	pointer to next drive parameter table; offset
			is set to FFFFh if last block in chain
	1D   word	starting cluster for free space search
	1F   word	number of free clusters, FFFFh = unknown


	- INT 21,32 which is used to read this data resets accessed byte at
	  offset 17h or 18h (depending on DOS version)
	- dword at offset 12h & 13h aren't supported in OS/2 compatability box
	- sector references are DOS logical sectors
	- see	Disk Base Table  BPB  INT 21,1F  INT 21,32