serial port

A very simple, low-level hardware channel used to print text out of a machine one character at a time.

The serial port predates modern displays and networking and sends data bit by bit over a simple wire. Because it needs almost no software to work, it is the go-to output for early boot messages and kernel crash logs, when nothing else is trustworthy. A fault handler that manages one line on the serial port is using the most reliable output it has.