Memory system ignores those addresses. Device controller sees the operation and performs accordingly. Devices typically have one or more status registers used for communication. May include done bit and one or more types of error bits. Polling process of periodically checking status bits.
Device sets bits, processor checks.
O Used by almost all systems for at least some devices. O interrupts to tell processor that a device needs attention. Unlike other exceptions, an interrupt is not associated with an executing instruction, and does not prevent instruction completion. Interrupt-Driver Overhead Example Assume same hard drive and processor as before. Overhead for each transfer, including the interrupt, is 500 clock cycles.