The Windowing System and System Classes

The Windows windowing system is called User. The most significant change to User is the desynchronization of the per-window message queue from the system message queue. This change prevents errant, looping applications that stop processing their messages from blocking the computer system’s entire user interface, thus making other applications unavailable.

With the addition of input queue time thresholds, the system can provide default handling for a looping or an otherwise nonresponsive process. From an end-user perspective, this means the they can do other things while one application is busy. For example, if a word processing program is busy printing a 100 page document, a user can minimize that application and begin working on a spreadsheet. This effectively minimizes the time the user waits with an “hourglass” on their screen.

The desynchronization of the message queue is completely compatible with the Windows Versions 3.0 and 3.1 message model. The message ordering is the same. If WM_xyz came after WM_abc, it still does. This compatibility is entirely necessary because in Windows 32 systems, existing Windows applications run on top of the Windows 32-Bit message system. The messages are simply copied from the 32-bit stack to the 16-bit stack and passed onto the application; therefore message order cannot change.