Operating System Processes

Process Description and Control

• Processes are a fundamental concept in operating systems, vital for managing and executing programs.

Objectives

• Define the term 'process.'
• Explain the relationship between processes and process control blocks.
• Elaborate on the concept of process states and state transitions.
• Describe the purpose and elements of data structures used by an OS to manage processes.
• Detail the requirements for process control by the OS.

Topics

• What is a process?
• Process states
• Process description
• Process control

Computer Platform and Operating System (OS)

• A computer platform uses hardware resources like monitors, keyboards, and printers.
• Computer applications operate on these resources to perform tasks.
• The OS is developed to provide a secure, consistent interface for applications.

Multitasking

• Resources are made available to multiple applications.
• The processor switches among applications, creating the appearance of simultaneous progression.
• The processor and I/O devices need to be used efficiently.

Definition of Process

• A process is an executing entity triggered when the processor begins to execute program code.
• It has program code, a set of associated data, and an execution context.

Execution Context

• The execution context is a set of elements that uniquely characterize the process during its execution.
• Identifier: A unique identifier distinguishes the process from all other processes.
• State: The current condition of the process, such as running, waiting, or blocked.
• Priority: The relative importance of the process compared to other processes.
• Program Counter: The address of the next instruction to be executed in the program.
• Memory Pointers: Pointers to the program code and data, including any shared memory blocks.
• Context Data: Data present in registers in the processor while the process is executing.
• I/O Status Information: Details about outstanding I/O requests, assigned devices, and files in use.
• Accounting Information: Amount of processor time and clock time used, time limits, and account numbers.

Process Control Block (PCB)

• The PCB contains elements that make it possible to interrupt a running process and resume its execution later as if it hadn't been interrupted.
• The OS creates and manages the PCB.
• The PCB is a key tool for supporting multiple processes.

Process States

• During execution, a process or task is created for that program.
• From the processor's point of view, it executes instructions in a sequence as dictated by the program counter register.
• From the program's point of view, execution involves sequence of instructions within that program.
• A trace is a listing containing the sequence of instructions executed for the process.

Dispatcher

• The dispatcher is a small program that switches the processor from one process to another.

Process Execution

• Memory contains dispatcher, process A, process B, and process C
• Each process runs to completion.

Combined Trace

• The OS allows a process to continue execution for a maximum of six instruction cycles before interruption.
• This prevents one process from monopolizing processor time.

Process Creation

• New Batch Job: A batch job is submitted with a batch job control stream.
• Interactive Logon: A user logs onto the system.
• Created by OS: The OS creates a process to perform a function on behalf of a user program, like printing.
• Process Spawning: A user program dictates the creation of multiple processes.
• Parent Process: The original process that creates other processes.
• Child Process: The processes created by a parent process.

Process Termination

• Completion Indication: There must be a way for a process to indicate its completion.
• Halt or Termination Call: Batch jobs should include a halt instruction or an explicit OS service call for termination.
• User Action: For interactive applications like quitting applications, indicate when the process is completed.

Reasons for Process Termination

• Normal Completion: The process executes an OS service call to indicate it has completed running.
• Time Limit Exceeded: The process has run longer than its allotted time.
• Memory Unavailable: The process requires more memory than the system can provide.
• Bounds Violation: The process tries accessing memory it's not allowed to.
• Protection Error: The process tries to use a resource it can't use.
• Arithmetic Error: A prohibited computation occurs, like division by zero.
• Time Overrun: The process has waited longer than specified for an event.
• I/O Failure: An error occurs during input or output.
• Invalid Instruction: The process tries to execute a nonexistent instruction.
• Privileged Instruction: The process tries to use a reserved instruction.
• Data Misuse: A data piece is wrong or uninitialized.
• Operator/OS Intervention: The operator or OS terminates the process.
• Parent Termination: When the parent terminates, so do the offspring.
• Parent Request: A parent process can terminate its offspring.

Two-State Process Model

• When the OS creates a new process, it enters the "Not Running" state.
• The process's existence is known, and it awaits execution.
• The running process is interrupted periodically, and the dispatcher selects a new process to run.
• Processes move from "Running" to "Not Running," and vice versa.

Process States

• Running: The process is being executed.
• Not-Running: The process exists but is not being executed.

Queuing Diagram

• Processes that are not running are kept in a queue, waiting for their turn to execute.
• There is a single queue in which entry is a pointer to a specific process.
• The queue is a FIFO list, and the processor operates in round-robin fashion.

Limitations of Single Queue

• A single queue is ineffective if processes in the "Not Running" state are blocked or waiting.
• Dispatcher has to scan the queue looking for the process that is not blocked and has been waiting in queue the longest

Multi-State Models

• Split "Not Running" state into two: "Ready" (ready to execute) and "Blocked" (waiting for I/O)
• Now, there are ready, running, blocked states.

Five-State Process Model

• This process model has Ready, Running, and Blocked states plus New and Exit

Five-State Process Model Definitions

• New: Process that hasn't been admitted to executable processes by OS. The process control block might have been created, but not loaded into main memory (RAM).
• Ready: A process that is prepared to execute when given permission.
• Running: The process that is currently being executed. With a single processor, at most one process can be in this state at a time.
• Blocked: Process cannot execute until an event occurs, like I/O operation or waiting for OS resources.
• Exit: A process that has been released from the pool of executable processes. It has either halted or aborted for any reason.

Queues

• Processes waiting due to an event transition into the blocked queue until the event has occurred.
• Then the process may transition into the ready queue once the event completes

Multiprogramming and I/O

• All processes need to be completely in main memory for execution.
• Processors are faster than I/O operations, causing all processes to wait for I/O.
• Even with multi-programming, processors could be idle

Solutions to Processor Idling

• Expanding the size of main memory can allow for more processes
• Swapping can increase space in main memory by moving processes to disk

Swapping

• It involves moving part of or all of a process from main memory to disk.
• If none of the processes in main memory are "Ready", the OS swaps a blocked process out to the disk.

Suspended State

• The process is removed from main memory.
• The process is not immediately available for execution.
• The process may or may not be waiting on an event.
• It was placed in that location via itself, parent process, or the OS to prevent execution.
• Processes are not removed until specifically told so.

Process States - Suspend

• Suspend: Process suspended in states of ready and blocked

Reasons for Suspension

• Swapping: The OS needs to release sufficient main memory.
• Other OS Reasons: The OS suspends a process suspected of causing a problem.
• Interactive User Request: A user may want to suspend a program for debugging.
• Timing: The process is executed periodically and suspended while waiting for the next schedule.
• Parent Process Request: The parent process wants to examine the activities of its descendant.

Process and Resource Allocation

• Process P1 is running and has control of two I/O devices.
• Process P2 is also in main memory but is blocked, waiting for an I/O device allocated to P1.
• Process Pn has been swapped out.

Process Control by OS

• The OS controls events within the computer system, schedules/dispatches/allocates.

OS Management

• In order to manage resources, the OS must collect and store current data.
• The OS system manages its resources through data, structure, and tables

OS must include

• Memory tables containing allocation of main and secondary memory.
• I/O tables containing status of I/O operations.
• File tables containing existence of a file and its location.
• The process location - which includes a program and needs sufficient memory

Process Attribute

• The OS requires a number to manage and control the process
• Unique Identifier
• Sate
• Context data like the accounting information
• The memory location needs good management to track

Process Image

• Collection of program, data, stack, attributes

Typical Element of Process Image

• User data
• OS may allow user level programs to access some portion of address space
• Program
• process
• Stack used store parameters

Process Control Block Grouping

• Three categories for storing process control:
  • I. Identification
  • II. Processor state
  • III: Process control

Process Control

• OS operates in two modes for safety/security purposes:
  • Kernel/system Mode (privileged)
  • User Mode (less privileged)

Two Mode Operation Is Required for OS Protection

• Necessary to protect the OS and other OS tables such as PCB
• The kernel mode gives access to software having all the processer controls

Process Mode Questions and Answers

• To know what the process will execute
  • Bit in PSW (program status word) that indicates the mode of execution
  • Bit changed in response to certain event like a user needing an OS service (system mode) or routine set to OS
  • Service is returned based on the user mode

Process Creation

• The OS has all the data after the new process so it can assign/allocate data

Three-Step Process Switch

• If a process wants to move states (ready, blocked), the switch happens

OS Control Events

• Interrupt - The external instruction results due to an asynchronous external event
• Trap - The process results after following an error/exception condition
• Supervisor Call - It functions through the operating system for an explicit request

Interrupt Processing

• Clock Interrupt
  • It indicates that the process and it's execution has run for the maximum time/slice so its then transferred to the system
• I/O Interrupt
  • It moves it to the first process of the events so it is then waiting as desired
• Memory Fault
  • Needs to have in Virtual Memory, moving to blocked state, then wait for the I/O to complete

Mode Switching and Process

• It may start based on an interrupt but only return the interrupted program without process change
  • The control is based on the state of the program not needing changes

Processor Management

• Process of moving to the stage is done based on the interrupt to know where to set it and then continue the process

Process Environment Changes

• Process block allows movement to another area by update memory and restoring any data

Security Issues

• When issues come up to control the whole process, it is essential to allow admin privileges to control them