Nachos provides three classes of I/O devices with read/write interfaces, of which the simplest is the serial console. The serial console, specified by the SerialConsole class, simulates the behavior of a serial port. It provides byte-wide read and write primitives that never block. The machine’s serial console is returned by Machine.console().
The read operation tests if a byte of data is ready to be returned. If so, it returns the byte immediately, and otherwise it returns -1. When another byte of data is received, a receive interrupt occurs. Only one byte can be queued at a time, so it is not possible for two receive interrupts to occur without an intervening read operation.
The write operation starts transmitting a byte of data and returns immediately. When the transmission is complete and another byte can be sent, a send interrupt occurs. If two writes occur without an intervening send interrupt, the actual data transmitted is undefined (so the kernel should always wait for a send interrupt first).
Note that the receive interrupt handler and send interrupt handler are provided by the kernel, by calling setInterruptHandlers().