History
Presentation Manager was codenamed Winthorn and co-developed by Microsoft and IBM's Hursley Lab in the UK in 1987-1988. It was a cross between Microsoft Windows and IBM's mainframe graphical system (GDDM). Like Windows, it was message based and many of the messages were even identical. But there were a number of significant differences as well. Although Presentation Manager was designed to be very similar to the upcoming Windows 2.0 from the user's point of view, and Presentation Manager application structure was nearly identical to Windows application structure, source compatibility with Windows was not an objective. For Microsoft, the development of Presentation Manager was an opportunity to clean up some of the design mistakes of Windows.
One of the most significant differences was the coordinate system. While in Windows the 0,0 coordinate was located in the upper left corner, in PM it was in the lower left corner. Another difference was that all drawing operations went to the Device Context (DC) in Windows. PM also used DCs but there was an added level of abstraction called Presentation Space (PS). OS/2 also had more powerful drawing functions in its Graphics Programming Interface (GPI). Some of the GPI concepts (like viewing transforms) were later incorporated into Windows NT. The OS/2 programming model was thought to be cleaner, since there was no need to explicitly export the window procedure, no WinMain, and no non-standard function prologs and epilogs.
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