Windows (not only the 16-bit 3.x, but going as far as the newest 64-bit ones) uses an event loop for the GUI: your program runs a loop calling the GetMessage fu

Windows (not only the 16-bit 3.x, but going as far as the newest 64-bit ones) uses an event loop for the GUI: your program runs a loop calling the GetMessage function, which blocks until the next window message is available. As long as you don't take too much time processing each window message, you don't have to even think about cooperative task scheduling, since Windows will do it for you while waiting on GetMessage. (In fact, AFAIK even in modern Windows you should avoid taking too much time to process each window message, since blocking the event loop thread makes the UI for your program unresponsive.)