INT 13 - SWBIOS - GET EXTENDED CYLINDER COUNT
        AH = FEh
        DL = drive number (80h, 81h)
Return: CF clear
        DX = number of cylinders beyond 1024 on drive
Notes:  standard INT 13/AH=08h will return a cylinder count truncated to 1024
        BIOS without this extension would return count modulo 1024
        SWBIOS is a TSR by Ontrack Computer Systems; Disk Manager also supports
          these calls