title: What is process and thread?
body: <p>Yes, I have read many materials related to operating system. And I am still reading. But it seems all of them are describing the process and thread in a "abstract" way, which makes a lot of high level elabration on their behavior and logic orgnization. <strong>I am wondering what are they <em>physically</em>?</strong> In my opinion, they are just some in-memory "data structures" which are maintained and used by the kernel codes to facilitate the execution of program. For example, operating system use <em>some process data structure (PCB)</em> to describe the aspects of the process assigned for a certain program, such as its priority, its address space and so on.  Is this all right?</p>