INT 16 - PC-Cache v6+ - INSTALLATION CHECK
        AX = FFA5h
        CX = 1111h
Return: CH = 00h if installed
            ES:DI -> internal data (see below)
            CL = cache state
                01h enabled
                02h disabled
SeeAlso: INT 21/AH=2Bh/CX=4358h

Format of internal data:
Offset  Size    Description
-1Ch 20 BYTEs   cached drive list, one byte per drive A: to T:
                each byte is either blank (20h) or drive letter (41h-54h)
 -8     BYTE    ???
 -7     WORD    number of physical transfers (scaled down to 0000h-7FFFh)
 -5     WORD    number of saved transfers (scaled down to 0000h-7FFFh)
 -3   3 BYTEs   ???