Windows Does Not Support Real-Time Programming

The information in this article applies to:

• Microsoft Windows Software Development Kit (SDK) for Windows versions 3.0 and 3.1

In no sense can Microsoft Windows be considered a "real-time" system. It is a message-driven, event-polling system, with nonpreemptive scheduling. The following is additional information:

1. It is possible to write a Windows-based application that sits on some interrupt in order to watch board-level activity. In general, this facility is used by manufacturers of boards to write drivers that watch and respond to interrupts used by the board. It is extremely dangerous to allocate/sit on any interrupt used by Windows itself (keyboard, mouse, etc.).

2. Dedicated systems (those that will not run general-purpose Windows applications) may sit on the timer interrupt, as long as Windows is eventually notified of the timer ticks. Because of the nonpreemptive aspect of the Windows system, it is insensitive to delays in the arrival of timer ticks. Trying to run more than one time-critical application that sits on the timer interrupt is likely to fail.

3. You must arrange for a Windows "library" of routines to provide access to information available from a board. The issue is one of how to divide the work between the following portions:

   a. The driver portion, which watches the interrupt, logs data, and

         notifies clients [through PostMessage()] of the availability of
         data
   

   b. The client portion, which obtains data from the driver portion,

         and processes, displays, and stores data
   

   Drivers can be implemented as Windows libraries, and clients of the device can be implemented as Windows-based applications. The driver portion can be made NEAR real time; the application portion is going to be message driven.