I once implemented a pre-emptive kernel and OS on a very crude hardware, the Hewlett-Packard Graphing Calculator HP48GX, running a Saturn CPU (4Bits with 64bits

I once implemented a pre-emptive kernel and OS on a very crude hardware, the Hewlett-Packard Graphing Calculator HP48GX, running a Saturn CPU (4Bits with 64bits registers, a weird beast). It can still be downloaded here : https://www.hpcalc.org/hp48/apps/shell/ https://www.hpcalc.org/hp48/apps/shell/ You don't need MMU or anything of the sort. The only thing you need is programmable interrupts. My implementation was using a 64Hz timer, that was also synchronized on the LCD screen "vsync" as it had to be serviced for kernel-level double buffering and different modes of simulated multiple grey levels. It is much less difficult than it seems, save registers, save context, load context, load registers, return to the current process. Optimizing the code is just a good knowledge of the CPU constraints and bit of juggling with registers. It was a simpler time, you could benchmark code by counting cycles, instruction by instruction. No cache, no branch predictors, no throttling.