handler() {
	if (user mode) {
		process_data[pid] = stored registers from kernel stack
	} else {
		handler_stack.push(stored registers from kernel stack)
	}
	
	do; work; for; this; interrupt
	
	if (user mode) {
		resume user mode
		longjmp(process_data[pid])
	} else {
		longjmp(handler_stack.pop())
	}
}