Device driver function 0Ah      Output Status               C

    Output Status specifies whether any characters are in the device
    output buffer.

    This function is required for character device drivers only.

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    srStatus
    Specifies the status of the completed function. If the output
    buffer has any characters, the driver must set the busy bit (bit
    9), indicating that the device is busy. If the output buffer has
    no characters, the busy bit must be zero. In both cases, the
    driver must set the done bit (bit 8).

    If the function is not successful, the driver must set both the
    error and done bits (bits 15 and 8) and copy an error value to the
    low-order byte.