660 CHAPTER 16 Real-time operating systems 16.6 EVENT BROKERS An event is a case or a condition that requires the services of the microprocessor to handle it. Internal events are triggered by a running program and may result from software errors, such as a protection violation, or from execution exception, such as a page fault. External events are created by sources outside the execution of the current program and may include I/O events such as a disk request, timer-generated events, or even a message arrival in a message-passing multiprocessor system. The response of a microprocessor to an event may be precise or imprecise, and may be concurrent or sequential. The response to an event is precise if proper handler execution ensures correct program behavior. This is guaranteed if the particular instruction causing the event (the faulting instruction) is identified, and any instructions that need the result of the faulting instruction are not issued until the handler has generated this result. This definition of precise event handling enables us to use multitasking or multithreading to implement concurrent event handling; the event handler and the faulting program run concur- rently in different task or thread contexts. This is in contrast to sequential event handling in which an event interrupts the faulting program, brings it to a sequentially consistent state, runs the handler to completion, and resumes the faulting program. Both concurrent and sequential