Processes in Operating Systems

A process is a fundamental concept in operating systems. It can be defined in several ways:

• A program in execution.
• An instance of a program running on a computer.
• The entity that can be assigned to and executed on a processor.
• A unit of activity characterized by the execution of a sequence of instructions, a current state, and an associated set of system instructions.

A process is comprised of:

• Program code, which may be shared with other processes.
• A set of data.
• A number of attributes that describe the state of the process.

Process Identity and State Information

For an operating system to manage and identify a process, it must maintain specific attributes. The minimum required information includes:

1. Identifier: A unique ID for the process.
2. State: The current state of the process (e.g., running, waiting).
3. Priority: The priority level relative to other processes.
4. Program counter: The address of the next instruction to be executed.
5. Memory pointers: Pointers to the code, data, and other memory segments used by the process.
6. Context data: Data held in processor registers.
7. I/O status information: Includes outstanding I/O requests, open files, etc.
8. Accounting information: CPU time used, time limits, etc.

This collection of information is stored in a data structure called a Process Control Block (PCB).

Process Control Block (PCB)

The PCB is a critical data structure managed by the operating system.

• It contains all the essential elements and attributes of a process.
• The operating system creates and manages a PCB for every process, which allows it to support the execution of multiple processes.
• The specific structure and format of a PCB are unique to each operating system; they are generally not interchangeable between different OS types.

Processes in Memory

When user processes are loaded into virtual memory, they each have their own private user address space, which contains their programs and data, as well as a user stack. Processes can also utilize a shared address space to communicate or share resources with other processes. Each process is associated with its own Process Control Block, which holds its identification, processor state, and other control information.

Process States

A process transitions through various states during its lifecycle. A common model is the five-state process model. Inter-Process Communications (IPC) may or may not cause a process to block.

Unix Process States

Unix and Unix-like systems define a more detailed set of process states:

State	Description
User Running	Executing in user mode.
Kernel Running	Executing in kernel mode.
Ready to Run, in Memory	Ready to run as soon as the kernel schedules it.
Asleep in Memory	Unable to execute until an event occurs; the process is in main memory (a blocked state).
Ready to Run, Swapped	The process is ready to run, but the swapper must swap it into main memory before the kernel can schedule it to execute.
Sleeping, Swapped	The process is awaiting an event and has been swapped to secondary storage (a blocked state).
Preempted	The process is returning from kernel to user mode, but the kernel preempts it and does a process switch to schedule another process.
Created	The process is newly created and not yet ready to run.
Zombie	The process no longer exists, but it leaves a record for its parent process to collect.

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