About 1985 IBM had a product called TopView that ran a multitasking supervisor on top of DOS and could do real multitasking with the right disk drivers. No protected memory though. TopView was really memory heavy, though -- you gave up at least 160KB just with TopView. Two guys created a TopView clone called Mondrian that ran in just 40K, and ran much faster. Those two guys were Nathan and Cameron Myhrvold. Mondrian was sold to Microsoft and those two guys went with it. Mondrian was a really clever engineering feat.
