Complex sentence: When IBM and Microsoft began collaborating on the design of OS/2 in 1986, the Graphics Presentation Interface (GPI) which was the graphics API in Presentation Manager was based on IBM's GDDM and the Graphics Control Program (GCP).
Split into simpler sentences: When IBM and Microsoft began collaborating on the design of OS/2 in 1986. The Graphics Presentation Interface (GPI), the graphics API in the OS/2 Presentation Manager, was based on IBM's GDDM and the Graphics Control Program (GCP).