This is how RTOSs work on microcontrollers that don't have a supervisor mode. A hardware timer interrupts each process and the interrupt handler switches which

This is how RTOSs work on microcontrollers that don't have a supervisor mode. A hardware timer interrupts each process and the interrupt handler switches which process is active. However, there's nothing stopping one process from disabling interrupts for some critical activity (like bit-banging some serial output) and then forgetting to re-enable them.