Format of DBLSPACE activation record:
Offset	Size	Description	)
 00h  2 BYTEs	signature "MD" (4Dh 44h)
 02h	BYTE	4Dh ('M') mount command
 03h	BYTE	error code (set to FFh before calling) (see #02806)
 04h	BYTE	host drive number (0=A:)
 05h	???	DISK_UNIT structure (not documented)