How Programmed I/O Works

This section describes the events that occur during a programmed I/O operation as well as the actions that a driver must take in response to these events.

What Happens During Programmed I/O

In a programmed I/O operation, the CPU transfers each unit of data to or from the device in response to an interrupt. The following sequence of events occurs.

  1. An IRP request (typically an IRP_MJ_READ or IRP_MJ_WRITE) determines that device interaction is required to complete the request. The Dispatch routine queues the IRP for eventual delivery to the driver's Start I/O routine.

  2. The Start I/O routine performs any necessary preprocessing and setup based on the IRP's function code. It then starts the device, typically by writing or ...

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