In 1987 I published a preemptive multitasking for 6502[1] (Apple II). If I remember properly (it&#x27;s only been 30 years or so), the stack is fixed at $100, so all you need to do on a context switch is save the stack somewhere else and restore it. That&#x27;s 256 bytes to move around. I used the mouse card as an interrupt source (video refresh) to get a regular source of interrupts.<p>[1] I can&#x27;t find any trace online, except for some weird Word document of what must have been the original README file.