Well, I've been looking at it, and it seems to be driven off of one of the NMI interrupts, and he is doing full page stack copies on context switch. There are 2 tasks set up. The basic idea for multitasking is to preserve the stack and all of the CPU registers for each task, and then copy them down for a context switch. In practice page zero locations, at least some of them, need to be preserved as well, due to the fact that page 0 is used as virtual registers a lot.