Query A20 (Function 07h)

        ARGS:   AH = 07h

        RETS:   AX = 0001h if the A20 line is physically enabled,
                     0000h otherwise

        ERRS:   BL = 00h if the function succeeds
                BL = 80h if the function is not implemented
                BL = 81h if a VDISK device is detected

This function checks to see if the A20 line is physically enabled. It
does this in a hardware independent manner by seeing if "memory wrap"
occurs.