Lecture Notes Project B								
by Beverly Guangji XU

Develop plan
·	Milestone A(requires most work): Non-preemptive (coroutine with scheduler) 
·	Milestone B (small step from A): Non-preemptive with priorities (uth_join call)
·	Milestone C (medium step from B ): Preemptive with synchronization:
	o	generate spontaneous interrupts that don't interfere each other
	o	the ability to block the interrupts
·	Milestone D (medium step): condition variable

Notes on the source code:
ucontext-basic
·	Don't need to deal with dynamic stack size so far. 64kB is large enough.
·	'child' and 'parent' are typedef structs that allocate memory
·	the concept of 'Context' includes:
	o	Hardware Regs.
		*	Stack Pointer
		*	Program Counter
		*	General purpose Reg.
	o	Misc. (Signal mask)
·	the Control: the parent gets the current execution context by calling 'getcontext'; 
			it then calls 'makecontext(&A,&B, arg) 'to create the new context, and 
			pass the 'arg' along with it; it calls 'swapcontext (&B,&A)' to do the 
			context switch between main thread and child thread (from B to A);after 
			the child context is executed, the context is switched again by calling 
			'swapcontext (&A,&B)'; at the end, it exits and frees the stack.

B-public
·	tickSz is the measured in millisecond. If tickSz is 0, it is non-preemptive; if tickSz is non-zero, it is preemptive.
·	uth_exit call exits a single thread, you can add another call to do 'stack-destroy'.