The PM Messaging Environment

First consider the non-PM application programming model as shown in the following diagram:

This diagram illustrates the following points:

• Non-PM application threads run in a relatively unconstrained environment (compare this with the following situation).

• The Operating System provides a black-box set of services and interfaces.

• The Hardware is not directly accessible by the application.

For PM message threads, the environment is radically different. The key difference is that application code that runs on a PM message thread is effectively a subroutine of the WinGetMsg API even though WinGetMsg is called by the application. The terminology often used to describe this reversal is Program Inversion. WinGetMsg is said to be inverted with respect to the application's message thread.

We see this illustrated in the following diagram.

Also illustrated by this diagram are the following points:

• PM Message threads act in a co-operative way. They wait for messages, and pass them on to the appropriate application if not for themselves.

• PM Message threads should spend most of their elapsed time waiting for notification of messages - because of their co-operative nature.

• Application code that runs on the message thread should be limited to very short duration processing. We often speak of the tenth-of-a-second rule, which is intended to imply the transient nature of application code processing rather than a precise measure.

• If a message thread communicates with another thread or the operating system, then this should be done either asynchronously or so as not to violate the tenth-of-a-second rule.