Format of VDISK header block (at beginning of INT 19 handler's segment):
Offset	Size	Description	)
 00h 18 BYTEs	n/a (for VDISK.SYS, the device driver header)
 12h 11 BYTEs	signature string "VDISK	 Vn.m" for VDISK.SYS version n.m
 1Dh 15 BYTEs	n/a
 2Ch  3 BYTEs	linear address of first byte of available extended memory