I don&#x27;t know how symbios does it but I don&#x27;t think it&#x27;s that hard to create something pretty close to preemptive multitasking on a z80. You have the vsync interrupt (50 times per second). Add to that some stack manipulation, some bookkeeping and a register store&#x2F;restore procedure and you&#x27;re done. It wouldn&#x27;t be fast though, and it would also limit what a process is allowed to do (disable interrupt, write outside of its own memory region). The z80 provides no mechanism to enforce this, of course. But technically, it&#x27;s a primitive kind of preemptive multitasking.