Presentation Manager

Presentation Manager (PM) is the graphical user interface (GUI) that IBM and Microsoft introduced in version 1.1 of their operating system OS/2 in late 1988.

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.

One of the most-cited reasons for the split was the divergence of the APIs between Presentation Manager and Windows, which was probably driven by IBM. Initially, Presentation Manager was based on Windows GUI code, and often had developments performed in advance, like the support for proportional fonts (which appeared in Windows only in 1990). One of the divergences regarded the position of coordinate (0,0), which was at the top-left in Windows, but at bottom-left (as in Cartesian coordinates) in Presentation Manager. In practice it became impossible to recompile a GUI program to run on the other system; an automated source code conversion tool was promised at some point. Both companies were hoping that at some point users would migrate to OS/2.

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