OS provides process abstraction and OS data structures.How does OS create a process.

OS provides process abstraction:)

• When you run an exe file, the OS creates a process = a running program
• OS timeshares CPU across multiple processes: virtualizes CPU
• OS has a CPU scheduler that picks one of the many active processes to execute on a CPU.
  – Policy: which process to run
  – Mechanism: how to “context switch” between processes

What constitutes a process:)

• A unique identifier PID)
• Memory image
  – Code and data (static)
  – Stack and heap (dynamic)
• File descriptors
  – Pointers to open files and devices

How does OS create a process?

• Allocates memory and creates memory image
  – Loads code, data from disk exe,
  – Creates runtime stack, heap
• Opens basic files – STD IN, OUT, ERR
• Initializes CPU registers
  – PC points to first instruction

States of a process

Dead: terminated

OS data structures

• OS maintains a data structure (e.g., list) of all active processes
• Information about each process is stored in a process control block (PCB).
  – Process identifier
  – Process state
  – Pointers to other related processes (parent)
  – CPU context of the process (saved when the process is suspended)
  – Pointers to memory locations. – Pointers to open files