.. _io_wait:


Waiting for I/O
================

Many types of input / output (I/O) do not occur immediately.  So the 
process must wait in a waiting queue and the device driver needs a strategy
of how to effectively wait for I/O data.

Polling
----------------------------

Polling is not very efficient because the system must continually check the
device for data.

.. note::

    I can not remember where I read this, but it is my understanding that
    only joy--sticks (game ports) use polling in modern operating systems.

.. index:: polling

.. figure:: polling.png
   :align: center

Interrupt Driven
----------------------------

.. figure:: interrupt-driven.png
   :align: center

   It is more efficient to start an I/O peripheral doing a task and let
   it interrupt the system when the I/O operation is finished.

The interrupt driven approach causes drivers to consist of a top and bottom
half.