There are actually two types of CxMessages.  The first, CXM_IEVENT,
corresponds to an input event and travels through the Commodities Exchange
network.  The other type, CXM_COMMAND, carries a command to a commodity.
A CXM_COMMAND normally comes from the controller program and is used to
pass user commands on to a commodity.  A commodity receives these commands
through an Exec message port that the commodity sets up before it calls
CxBroker().  The NewBroker's nb_Port field points to this message port.  A
commodity can tell the difference between the two types of CxMessages by
calling the CxMsgType() function.

    ULONG  CxMsgType( CxMsg *cxm );
    UBYTE *CxMsgData( CxMsg *cxm );
    LONG   CxMsgID  ( CxMsg *cxm );

A CxMessage not only has a type, it can also have a data pointer as well
as an ID associated with it.  The data associated with a CXM_IEVENT
CxMessage is an InputEvent structure.  By using the CxMsgData() function,
a commodity can obtain a pointer to the corresponding InputEvent of a
CXM_IEVENT message.  Commodities Exchange gives an ID of zero to any
CXM_IEVENT CxMessage that it introduces to the Commodities network but
certain CxObjects can assign an ID to them.

For a CXM_COMMAND CxMessages, the data pointer is generally not used but
the ID specifies a command passed to the commodity from the user operating
the controller program.  The CxMsgID() macro extracts the ID from a
CxMessage.

 A Simple Commodity Example 
 Controller Commands 
 Shutting Down the Commodity 

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