5.2 Code: Console output

A write system call on a file descriptor connected to the console eventually arrives at uartputc (kernel/uart.c:87). The device driver maintains an output buffer (uart_tx_buf) so that writing processes do not have to wait for the UART to finish sending; instead, uartputc appends each character to the buffer, calls uartstart to start the device transmitting (if it isn’t already), and returns. The only situation in which uartputc waits is if the buffer is already full.

Each time the UART finishes sending a byte, it generates an interrupt. uartintr calls uartstart, which checks that the device really has finished sending, and hands the device the next buffered output character. Thus if a process writes multiple bytes to the console, typically the first byte will be sent by uartputc’s call to uartstart, and the remaining buffered bytes will be sent by uartstart calls from uartintr as transmit complete interrupts arrive.

A general pattern to note is the decoupling of device activity from process activity via buffering and interrupts. The console driver can process input even when no process is waiting to read it; a subsequent read will see the input. Similarly, processes can send output without having to wait for the device. This decoupling can increase performance by allowing processes to execute concurrently with device I/O, and is particularly important when the device is slow (as with the UART) or needs immediate attention (as with echoing typed characters). This idea is sometimes called I/O concurrency.