Expanding Multitasking System -> Multitasking+Threading

[tigerfish]

How would one go about expanding a existing multitasking system into a multithreading system?

[xyzzy]

So you have just single processes executing? In that case, just change it so that threads are what get executed on the CPU rather than processes, and a process only contains threads and resources shared between all of it's threads (e.g. open files, address space).

When you switch to a thread, you switch to the address space of the process containing it, so that you end up with all threads in one process running inside the same address space.