\chapter{Message Queue}
\label{chp:messagequeue}

This chapter describes how all of the messaging in the system is handled.

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\section{Message Format}

TBD


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\section{Queue Implementation}

Two pointers are maintained into the Message queue; the head and
tail pointers.  There is also a variable which contains the number
of messages currently in the queue.  These variables are global for
all tasks, and thus the mechanisms for queueing and dequeueing
messages into the system are provided by the kernel.

<<Message RAM>>=
	    ; messages
	msgbase		= (ram + 0x0ca0)
	msgmax		= (msgbase + 0x003f)
@

\subsection{Queueing a Message}

We need a way to continue adding messages onto the queue while
circulating around the ram buffer, so we will have a ram buffer
that is 256 bytes large, so that we can just [[AND]] the offset
with [[0x00FF]] to determine the correct offset into the message
queue.

\begin{enumerate}     
    \item If number of messages is greater than 256, fail.
    \item Store the message at the RAM location that the tail pointer references
    \item Increment the tail pointer
    \item [[AND]] the tail pointer with [[0x00FF]]
    \item Add the tail pointer with the base of the message queue
    \item increment the number of messages
\end{enumerate}


\subsection{Dequeueing a Message}

Similarly, we need a way to pop a message off of the queue, so a  
similar process is used.

\begin{enumerate}
    \item If number of messages is 0, fail
    \item Set the message at the head pointer aside
    \item Increment the head pointer
    \item [[AND]] the head pointer with [[0x00FF]]
    \item Add the head pointer with the base of the message queue    
    \item Decrement the number of messages
    \item Return the message
\end{enumerate}