INT 2F - SRDISK v1.30 - INSTALLATION CHECK
        AX = 7200h
Return: AL = FFh if installed
            ES = segment of device driver header (see below)
Note:   SRDISK is a freeware resizeable RAMdisk by Marko Kohtala

Format of device driver header:
Offset  Size    Description
 00h 10 BYTEs   same as standard device driver header (see INT 21/AH=52h)
 0Ah    BYTE    number of subunits (drives) supported by driver
 0Bh  3 BYTEs   signature "SRD"
 0Eh  4 BYTEs   memory type string (currently only "XMS ")
 12h  4 BYTEs   ASCII driver version string "N.NN"
 16h    BYTE    00h
 17h    BYTE    configuration format version (currently 00h)
 18h    WORD    offset of drive configuration data