OS - Ch 3,4

Reference : http://pages.cs.wisc.edu/~remzi/OSTEP/cpu-intro.pdf

Process

• Process is a running program.

• Virtualization runs multiple processes by turns, hence providing a virtual environment that contains multiple number of CPUs

  • Context Switch : run & stop …
  • Modern OS uses time sharing mechanism which lowers each processors performance.

• Scheduling: OS policy to decide run which program at which point.

  • Scheduling refers to
    • past info: which programs ran for the last one minute
    • workload info: what kind of programs ran
    • performance measure: system tends to increase communication, workload?

Definition

• Main components of machine states are:
  • Memory: space where instructions and resources are saved/loaded
  • Register:
    • Program Counter: which instruction is being excuted aka instruction pointer
    • Stack pointer: used for managing stack where function varaible and return addresses are saved.

Process API

• Create: OS needs to provide a way to create new process
  • Shell, double-click…
• Destroy: Needs to able to forcibly close processes
• Wait: different kinds of process being waited to take turns (wait interface)
• Miscellaneous Control: Pause | resume …
• Status: Interface that provides process status information such as how long has it been running, current stauts…

In-Depth

• OS steps to run a program
  1. Load program code and static data (initialized variables) into memory (process address space).
     • Early OS loaded both program code and static data before executing a program.
     • Modern OS loads necessary part of it while running a program.
       • Late load requires paging and swapping (later be discussed).
  2. Memory allocation for run-time stack purpose.
     • C uses stack for saving variables, return address…
     • OS resets the stack with given parameters especially by using main() function variables argc and argv vectors.
  3. Memory allocation for heap
     • Heap is for saving dynamically allocated data in C; malloc and free.
     • Heap is used for linked list, hash table, tree. All the different size (dynamic) data structures.
  4. Has three file descriptors - FOR I/O
     • STDIN
     • STDOUT
     • STDERR
  5. Now ready to execute the program
     • Branch to main()

Process Status

• Running
• Ready: Process is ready, but OS is not; executing another process.
• Blocked: Pause the process while waiting for another events; waiting for data

Data Structures

• Register Context: saves register values when the process stopped.
  • Once process resumes, use the saved values.
• Process has states such as initial, and final
  • Final state: Process is completed, but remain in the memory; useful for parent process to check if the task given to its child is successfully completed.