Lecture 3: Operating System Structures

Question 1

Policy vs Mechanism

Answer 1

• Policy specifies what needs to be done
• e.g. Which virtual memory page should the OS evict? * e.g. What process should the OS run
  next?
• Mechanism specifies how to do it
• e.g. how to save dirty pages, how to update the CPU’s page table to
• reflect the page-out/page-in operations, other bookkeeping
• e.g. how to save the current process’ context, how to restore the new process’ contex

Question 2

reasons for cooperating processes

Answer 2

• Information sharing: Several processes may need to access
  the same data (such as stored in a file.)
• Computation speedup: A task can ofter be run faster if it is
  broken into subtasks and distributed among different
  processes.
• Modularity: It may be easier to organize a complex task into
  separate subtasks, then have different processes or threads
  running each subtask.
• Convenience: An individual user can run several programs
  at the same time, to perform some task.

Question 3

The two Inter process communication mechanisms

Answer 3

Two methods
* Shared memory: shared memory space for two or more process
* Message passing

Question 4

What are signals?

Answer 4

• Very short messages
• No arguments only signal number
• Signals serve two main purposes:
• To make a process aware that a specific event has occurred
• To cause a process to execute a signal handler function included in its
  code