The concept of process

• An operating system executes a variety of programs:F Batch system – jobsF Time- shared systems – user programs or tasks
• Textbook uses the terms job and process almost interchangeably.
• Process – a program in execution; process execution must progress in sequential fashion.
  A process includes:
  -program counter
  - stack
  -data section

Process State

• As a process executes, it changes state
  1. new: The process is being created.
  2. running: Instructions are being executed.
  3. waiting: The process is waiting for some event to occur.
  4. ready: The process is waiting to be assigned to a process.
  5. terminated: The process has finished execution
  

Process State Diagram

Process Control Block

• Information associated with each process.
• Process ID
• Process state
• Program counter
• CPU registers
• CPU scheduling information
• Memory-management information
• Accounting information
• I/O status information

Process Control Block

Threads

• One view of a thread is as an independent program counteroperating within a process
• A thread is sometimes called a lightweight process (LWP), butis a smaller execution that a process
• A thread consists of:

-a thread execution state (Running, Ready, etc.)

-a context (program counter, register set)

-an execution stack
-some per-thread static storage for local variables
-access to the memory and resources of its process (shared withall other threads in that process)
-OS resources (open file, signals, etc.)