Using RPMSG to implement a simple calculator on PRU
===================================================

Code
----

.. code:: python

   init_message_channel();

   while : true {
       int option := receive_message();
       int a := receive_message();
       int b := receive_message();

       if : option == 1 {
           send_message(a+b);
       }
       elif : option == 2 {
           send_message(a-b);
       }
       elif : option == 3 {
           send_message(a*b);
       }
       elif : option == 4 {
           if : b != 0 {
               send_message(a/b);
           }
           else {
               send_message(a);
           }
       }
       else
       {
           send_message(a+b);
       }
   }

-  Following code works on PocketBeagle, to use on other boards, please
   change the pins accordingly.

Explanation
------------

``init_message_channel();`` starts the message channel for communication
with ARM <-> PRU cores. Then ``while : true`` loops runs endlessly.

``int option := receive_message();`` receives which operator to be
executed and stores it in option variable. 1 for addition, 2 for
subtractions, 3 for multiplication and 4 for division.
``int a := receive_message();`` receives the value of first operand, and
``int b := receive_message();`` receives the value of second operand.

if-elseif ladder checks if value of option is 1, 2, 3 or 4 and
accordingly sends the value of operation back to ARM core using
``send_message``. While division, it makes sure that divisor is not 0.
If value of option is anything other than 1, 2, 3, 4, then it defaults
to else condition, that is a+b.

This runs endlessly since it is inside a ``while : true`` loop.