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

Processes

• Process: A running program
• The program itself is “lifeless”: Just code sitting on a disk
• To allow a computer to run more programs than CPUs “concurrently”, we “virtualize” the CPU: Each program “sees” a CPU and treats it as if it is the only program running on that CPU.
• Time sharing
• Context switch: Changing the program running on a CPU.
• How would you explain Mechanism vs. Policy?
  • Mechanism: How to make something happen
  • Policy: The choice of what to do.
• Apply mechanism vs policy to time sharing a CPU.
  • Mechanism is the stuff that needs to happen so that one process can replace another on the CPU without either process knowing it happens.
  • Policy includes things like choosing when to do a context switch.

Process State

• If we want to be able to switch processes, what all do we need to preserve?
  • Memory state (address space)
  • General purpose registers
  • Special purpose registers (Program counter, stack pointer, flags)
  • Open file handles
  • Current working directory
  • Other stuff

Process API

• What operations does an OS need to be able to take on processes?
  • Create
  • Destroy
  • Wait
  • Status
  • Misc others (e.g., suspend)

Process Creation

• Load machine code from disk into memory
• Initialize any static data e.g., global constants.
• Set up the stack and heap
• Set up I/O (stdin, stdout)
• Set PC to first instruction in main

Process States

• Running
• Ready
• Blocked

Data Structures

• OS needs to maintain a list of processes.
• For each process, it needs to track the status, and the “Process State” information listed above.
• Show Figure 4.5 on p8 https://pages.cs.wisc.edu/~remzi/OSTEP/cpu-intro.pdf