1. The composition of an operating system (OS)
    - Process 
        - A running program
	- Instructions, data, stack (procedure calls)
    - Thread
	- The segment of a process
	- Each thread has its own variable and states
	- Share memory space with other threads
	- Take less time in the creation
    - Kernel space
	- Privilege instructions
    - User space
	- User program, process
    - System call
	- back and forth the kernel space and the user space, e.g. trap
    - Trap
	- User program issues a signal
	- Used to execute a specific functionality in the OS
	- Use return-from-trap instruction to leave kernel space
    - Interrupt
    	- A hardware or software signal
	- Notify the CPU a critical process needs urgent execution
	- interrupt vector (INT 80)
    - Exception
	- Synchronous events generated by the process
	- When should we use exception? illegal instructions, divide-by-0
    - Address space
	- The abstraction of physical memory
	- Code (top)-> instructions, code segment
	    - Why do we put code at the top of the address space?
	- Stack (bottom)-> Keep track of where it is in the function call chain 
	- Heap (top)-> dynamically-allocated, user-managed memory e.g. malloc()
	- Could we change the order of the stack and heap ?
    - Virtual memory
	- Hide the memory access in the physical memory
	- Goals of the virtual memory
	    - transparency
	    - efficiency
		- How to efficiently and flexibly virtualize memory?
	    - protection
		- protect processes from one another (isolation)

	- Memory address translation
	    - base and bound 
	    - memory management unit (MMU) as the part of the processor
	- Paging
	    - solving memory space-management problem
	    - segmentation -> chop things up into variable-sized pieces
		- different-size chunks -> fragmented
	    - paging -> chop up space into fixed-sized pieces (page)
	    - page table
		- per-process data stucture
		- store address translations for each of the virtual pages of the address space
		- Where are page tables stored?
		- the paging is slow, why?
	- Lock
	    - ensure the correctness of the concurrency
	- File system