Ha, I did exactly the same in exactly same language but that was the end of 80s. Interactivity was just fine as I've increased frequency of timer interrupt to 4096 times / s and would pass control to standard handler every so often so it would work as standard at about 18 times/s. When not calling old interrupt I would execute threads switch and control logic. Turbo Pascal supported built in assembly so writing all low level stuff was piece of cake. I also used built in assembly to implement graphics.