Add an architecture diagram

And make it obvious everything important happens on the GPU

docs/fov.rst

@@ -40,6 +40,26 @@ Framerate adjustments, as done with :attr:`~PiCamera.framerate_delta` are
 achieved by manipulating the number of "padding" lines added to the end of a
 frame.
 
+At this point, a reader familiar with operating system theory may be
+questioning how a non-real-time operating system like Linux could possibly be
+reading lines from the sensor? After all, to ensure each line is read in
+exactly the same amount of time (to ensure a constant exposure over the whole
+image) would require precision timing, which cannot be guaranteed in a
+non-real-time OS. The answer is quite simply that Linux *doesn't* control the
+sensor.
+
+In fact, none of the camera processing occurs on the CPU at all.  Instead, it
+is done on the Pi's GPU (VideoCore IV) which is running its own real-time OS
+(VCOS). From the Linux side we merely send "messages" to VCOS requesting that
+it do certain things (initialize the camera, set an exposure time, configure a
+JPEG encoder, begin streaming data), and from time to time VCOS sends messages
+back (e.g. here's a frame of JPEG encoded data).
+
+The diagram below roughly illustrates the architecture of the system:
+
+.. image:: images/camera_architecture.*
+    :align: center
+
 The other important factor influencing sensor counts, other than line read-out
 time, is the sensor's gain. Specifically, the gain given the by
 :attr:`~PiCamera.analog_gain` attribute. The corresponding