Operating Systems Process Concepts

Process Concept

• A program being executed is called a process.
• A process is an active entity that represents the basic unit of work in a system.
• A process is comprised of multiple parts:
  • Program code (text section)
  • Current activity (program counter, processor registers)
  • Stack (temporary data, function parameters, return addresses, local variables)
  • Data section (global variables)
  • Heap (dynamically allocated memory)
• A program is a passive entity stored on disk (executable file).
• A program becomes a process when loaded into memory.

Process State

• A diagram of the process state depicts a process moving between states:
  • New: The process is being created.
  • Running: The process is currently using the CPU.
  • Waiting: The process is waiting for an event, such as I/O completion.
  • Ready: The process is waiting to be assigned to the CPU.
  • Terminated: The process has completed execution.

Process Control Block (PCB)

• The PCB (Process Control Block), also known as the task control block, holds information associated with each process.
• The PCB contains:
  • Process state
  • Program counter (location of the next instruction to execute)
  • CPU registers (contents of all process-centric registers)
  • CPU scheduling information (priorities, scheduling queue pointers)
  • Memory management information (memory allocated to the process)
  • Accounting information (CPU usage, elapsed time, time limits)
  • I/O status information (I/O devices allocated, list of open files)

Process Scheduling

• Process scheduling optimizes CPU usage and facilitates time-sharing by quickly switching processes on and off the CPU.
• The process scheduler decides which process runs next.
• Process scheduling is a key component of multiprogramming operating systems.

Context Switch

• A context switch occurs when the CPU transitions from one process to another.
• The system saves the state of the old process and loads the saved state of the new process.
• The context switch involves saving and restoring the contents of the PCB.
• Context switch time is overhead, as the system isn't performing useful work during the switch.
• Faster hardware, like CPUs with multiple sets of registers, can reduce context switch time.

Multiprogramming

• Multiprogramming allows multiple processes to reside in memory simultaneously.
• Two types of multiprogramming:
  • Pre-emption: A process is forcefully removed from the CPU (time-sharing, multitasking).
  • Non-pre-emption: Processes run until completion without interruption.
• The degree of multiprogramming indicates the maximum number of processes that can be in the ready state.

Process Termination

• A process can be terminated due to various reasons:

  • Normal completion of execution
  • Parent process request
  • Resource violation (e.g., attempting to write to a read-only file)
  • I/O failure
  • Parent process termination (cascading termination)
  • Memory shortage

• The wait() system call allows a parent process to wait for a child process to terminate.

Synchronization

• Message passing is a method of inter-process communication (IPC) and can be blocking or non-blocking.
• Blocking message passing is synchronous:
  • Blocking send: The sender process blocks until the message is received.
  • Blocking receive: The receiver process blocks until a message is available.
• Non-blocking message passing is asynchronous:
  • Non-blocking send: The sender sends the message and continues execution.
  • Non-blocking receive: The receiver receives either a valid message or a null message.

Buffering

• Buffering is a technique used in message passing involving a queue of messages attached to a communication link.
• Buffering options include:
  • Zero capacity: No messages can be queued, requiring the sender to block until the receiver processes the message.

Explore the fundamental concepts of processes in operating systems. Understand how a program becomes an active process, the different states of a process, and the components that make up a process. Test your knowledge of this essential topic in system management.