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The processor executing a program's instructions. In polled input-output the CPU explicitly checks status; in interrupt-driven input-output it enters an interrupt handler; in DMA it delegates payload movement while retaining setup and completion work.

An interrupt redirects CPU execution to an interrupt handler in response to an event. Here the main case is an asynchronous hardware interrupt: a timer or device requests attention independently of the instruction currently being executed. A device completing a receive operation can announce that its data is ready while the CPU is running another program. Interrupt-driven input-output illustrates this with an explicit timeline. ^[ref]

A synchronous processor exception, such as a page fault caused by a particular memory access, is tied to instruction execution. A system call is an intentional program request for an operating-system kernel service. These routes can share entry machinery, and an instruction named a software interrupt can implement a system call, but this does not make that request an asynchronous device event. For example, the Linux kernel on x86 documents separate entries for external interrupts, divide exceptions, syscall and int 0x80. ^[ref]

The usual device route is a request through an interrupt controller, architecture-specific entry, and dispatch to an interrupt handler. In the Linux kernel, generic handling separates controller operations from flow handling and device driver callbacks. Requests can arrive through a pin or a message, and several devices can share a request source. Routing, priorities and interrupt masking determine which requests can be delivered; notification therefore need not mean immediate execution of the interrupt handler. ^[ref][ref]

The CPU and entry code preserve the state needed to resume interrupted execution, including the instruction pointer. Handling may enter a higher processor privilege level when interrupting ordinary program execution, but an interrupt can also arrive while privileged code is already running. The protected entry route and handler placement depend on the architecture and operating system; a microkernel need not place all device driver work where a monolithic kernel does. An interrupt entry is also distinct from a context switch to another operating-system process. ^[ref][ref]

// Target: computer-science.bigb

In interrupt-driven input-output, a device signals an interrupt when attention is needed, letting the CPU avoid repeatedly checking an idle device. The interrupt handler determines what happened and arranges the required work; notification itself does not move the payload. DMA can supply that separate data-transfer mechanism.

For a toy comparison, let events occur at times $2,5,9$. Ideal interrupt notification happens at those same times, while polled input-output at $0,2,4,6,8,10,12$ detects them at $2,6,10$. The polling delays are $0,1,1$; ideal notification delays are all zero. Events are assumed to remain pending, with a simultaneous event visible to a poll. The chart is a model, not a measurement: it excludes interrupt delivery and dispatch latency, interrupt handler service time, and polling overhead. Its zero-delay row is not a real-hardware guarantee.

The useful idea is to spend attention when something changes. Sparse events can leave long stretches for useful CPU work instead of busy checks. At high event rates, repeated entry and service overhead can consume that saving. Batching notifications can reduce their frequency while delaying some events; switching to polled input-output during active periods is another option. No strategy is universally faster. ^[ref]

The Linux kernel's NAPI mechanism is a concrete combination: an interrupt normally schedules event processing, which polls for work with a receive budget. Further interrupts remain masked during that polling, and are enabled again when polling finishes. NAPI also supports busy polling; the exact processing context and configuration vary. This is a documented Linux kernel design, not a requirement on every operating system. ^[ref]