The serial port driver is useful because it will help in the initial debugging of the kernel. Serial port drivers are easy to write on most machines because they involve only initializing the interface, sending a byte, and receiving a byte. It is a good idea to initialize the serial port driver early in the boot process (at the beginning of locore after the start: label) so that diagnostic output can be printed out at boot time. Since the serial port driver is a tty driver, and thus a bit tricky to write from scratch, it is easiest to copy another port's serial port driver and modify the small bits of code that change with new hardware.