Format of CP/M-86 unknown-drive paramter block:
Offset	Size	Description	)
 00h	BYTE	drive number (00h = first unknown drive, etc.)
 01h	BYTE	deblocking flag (00h = normal write, 01h = directory write)
 02h	WORD	track number
 04h	WORD	sector number
 06h	DWORD	data transfer address
 0Ah	BYTE	verify flag (nonzero to verify writes)
Note:	in CP/M-86 v1.1, this is actually a portion of a BIOS disk data
	  table which starts one byte before the address given to the INT E6
	  handler; that extra byte is the current logical drive number