* I've recently been receiving messages directly from the CH304 TTL2USB module via UART.dr in the Raspberry Pi 4B
（I'm not sure if that's the right way to do, but after RTFM I have no ideas about what's the means of transmitter holding register.)

> if the FIFOs are not enabled, data is stored in the transmitter holding register (the bottom word of the transmit FIFO).
> if the FIFOs are not enabled, the data byte and status are stored in the receiving holding register (the bottom word of the receive FIFO).