Did this on my Atari ST 68000 back in the 90s... I did not even heard about the word "preemptive" at the time (guys, I did not even know the Amiga OS did this natively), it was just an idea. Task switching every 10 or 20 HBL or so. I was so glad to have two routines running, each one changing color index 0 register to red and blue so I can see it realtime.