Overview

This chapter shows how the WdmIo and PHDIo drivers handle interrupts, use Deferred Procedure Calls (DPCs), and catch time-outs. It shows how Read and Write interrupt-driven I/O is started and demonstrates how interrupt handling routines should work.

A driver's interrupt handler must service a hardware device's immediate needs. Any major processing must wait until a driver's DPC routine runs. Eventually, the DPC completes the read or write request and starts the next queued IRP.

The chapter also covers two types of timer. A basic "one-second interval" timer is best used to implement a time-out for interrupt-driven operations. However, custom timers and their associated Custom DPCs can be used for finer grain timing.

Interrupt Handling

First, let me be clear exactly what a hardware interrupt is. Devices that generate interrupts use an electrical signal to indicate that some special condition has occurred. The interrupt system is designed to stop the processor (or one of the processors) in its tracks as soon as possible and start some code to service the interrupt.

When an interrupt occurs, the processor saves a small amount of information on the kernel stack: the processor registers and the instruction pointer before the interrupt. This is just enough to restore the context when the interrupt service routine has completed.