The serial port is the simplest way to interface with the Taleä Computer System. It receives data over a serial connection.
Note that the transmit (TX) and receive (RX) ports are labeled here from the CPU’s point of view (if we took the serial controler side, they would be reversed).
╭──────┬──────────┬─────╮
│RX │ halfword │ 0x00│
├──────┼──────────┼─────┤
│TX │ halfword │ 0x02│ <- Notice, only a byte is used
├──────┼──────────┼─────┤
│STATUS│ byte │ 0x04│
├──────┼──────────┼─────┤
│CTRL │ byte │ 0x05│
╰──────┴──────────┴─────╯
Sending a byte to the port is easy, one just needs to write it to the TX port.
An interrupt will be fired when there is data incoming from the serial device at priority level 4. Reading from RX[0] will transfer a byte to the system, whereas reading from RX[1] will return the number of remaining incoming bytes. Notice that subsequent reads from RX[0] will return the bytes of the input in a LIFO/FIFO fashion, until the buffer is exhausted, when it will read 0x00.
Writing 0x00 to the CTRL register will swicht the port to a FIFO for readings. The default (0x01) is LIFO. If the buffer contained data before the swicht took place, no warranties are made about the rearrangement of the data, its loss, or its corruption, and reading from a port in that state can lead to undefined behaviour.