OS: syscall

1. show the linux architecture for system call;
2. show the steps of modification, each meaning;
3. how to return a value from system call, how to ask a system call to do something for us;
4. how to compile a kernel, load a kernel, use a system call;
5. put up the lab far before the lab, so students can start or have some idea of it before hand;
5. before the lab, run it again to make sure it works.