2 questions about threads

[Chicken Blood Machine]

Again, if the cpu stays in one thread, you don't need threads, as threads are used when you want other processes to work in parallel, i.e CPU switches between them.
As answer to what you said, here, is that you can't really know/ have control over what the CPU does so your statmet may or may not be true, depends on the system and your application.

That's not quite true. The setting and getting of his boolean variable will be an atomic instruction and hence thead-safe. He may need a mutex though if he has a function that sets/gets the value and then executes further statements that depend on the value not having changed.

[high_flyer]

What I said is true, and has nothing to do with atomic instructions but with the fact that if the CPU is doing a task (not instruction) where it is not needed to share CPU time with other tasks, threads are not needed.

[Chicken Blood Machine]

I was referring to the second part of your statement, not the first.