Query Free Extended Memory (Function 08h)

        ARGS:   AH = 08h

        RETS:   AX = Size of the largest free extended memory block in K-bytes
                DX = Total amount of free extended memory in K-bytes

        ERRS:   BL = 80h if the function is not implemented
                BL = 81h if a VDISK device is detected
                BL = A0h if all extended memory is allocated

This function returns the size of the largest available extended memory
block in the system.

NOTE: The 64K HMA is not included in the returned value even if it is
not in use.