AH = 1
        AL = character
        DX = port number
Return: AH = RS-232 status code bits
            0: data ready
            1: overrun error
            2: parity error
            3: framing error
            4: break detected
            5: transmission buffer register empty
            6: transmission shift register empty
            7: time out--if set, other bits invalid
        AL = modem status bits
            0: delta Clear-To-Send
            1: delta Data-Set-Ready
            2: trailing edge of ring detected
            3: change, receive line signal detected
            4: Clear-To-Send
            5: Data-Set-Ready
            6: ring detected
            7: receive line signal detected
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