Device driver function 06h      Input Status                C

    Input Status specifies whether any characters are waiting in the
    device-input buffer.

    This function is required for character device drivers only.

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    If the device has no input buffer, the busy bit must be zero.

    Before attempting to read a character, DOS may wait for a device
    to return a not-busy status.

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    srStatus
    Specifies the status of the completed function. If the device
    input buffer has waiting characters, the busy bit (bit 9) must be
    zero, indicating that DOS need not wait to read a character. If
    the buffer has no characters, the driver must set the busy bit. In
    either case, 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.