Serial-Protocol

The serial port controller handles two basic modes of use: Transmission (Tx), and
Reception (Rx). It has partially separated drivers for these uses.

The reception driver asserts the "Read" signal, then checks if the serial
port asserts the "Ready" signal. If not, it returns early, passing a zero
value out in the "DRIVER_SERIAL.RxGotData" field. If data is found, the 
serial will already be presenting it on the data lines, and we can store it
in the most significant nibble of RxByte. We then cycle the "Read" signal,
and store the least significant nibble. Then, we clean up the bus, and return.

The transmission driver asserts the "Write" signal, then ignores the "Ready"
signal altogether. The serial port is designed such that it is only ever
not ready to write if the buffer is full - and the buffer holds more data than we know what to do with. We present the most significant nibble of TxByte, then
cycle the "Write" signal and present the least significant nibble. Now finished,
we clean up the bus and return.