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Process Description and control. G.Anuradha (Referred from william stallings and galvin 8 th edition). Contents. Introduction Process states Process description Process control. Introduction. OS must interleave the execution of multiple processes to maximize processor utilization

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process description and control

Process Description and control


(Referred from williamstallings and galvin 8th edition)

  • Introduction
  • Process states
  • Process description
  • Process control
  • OS must interleave the execution of multiple processes to maximize processor utilization
  • Proper allocation of resources and avoidance of deadlocks
  • Interprocess communications
process states
Process states
  • What is it?
  • A two-state Process model
  • Creation and termination of processes
  • Five state model
  • Suspended processes
what is it
What is it?
  • An operating system executes a variety of programs:
    • Batch system – jobs
    • 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
trace of the process
Trace of the Process
  • The behavior of an individual process is shown by listing the sequence of instructions that are executed
  • This list is called a Trace
  • Dispatcher is a small program which switches the processor from one process to another
process execution
Process Execution
  • Consider three processes being executed
  • All are in memory (plus the dispatcher)
  • Lets ignore virtual memory for this.
trace from the processes point of view
Trace from the processes point of view:
  • Each process runs to completion
trace from processors point of view
Trace from Processors point of view





queuing diagram
Queuing Diagram

Etc … processes moved by the dispatcher of the OS to the CPU then back to the queue until the task is competed

review till now
Review till now
  • What is a Trace?
  • What is a Dispatcher?
  • What are the two states of a two state process model?
creation and termination of processes
Creation and Termination of Processes
  • Life of a process is bounded by its creation and termination
  • When a new process is added
    • OS builds data structures
    • Allocates address space in main memory
process creation
Process Creation
  • The OS builds a data structure to manage the process
  • Traditionally, the OS created all processes
    • But it can be useful to let a running process create another
  • This action is called process spawning
    • Parent Process is the original, creating, process
    • Child Process is the new process
process termination
Process Termination
  • There must be some way that a process can indicate completion.
  • This indication may be:
    • A HALT instruction generating an interrupt alert to the OS.
    • A user action (e.g. log off, quitting an application)
    • A fault or error
    • Parent process terminating
five state process model
Five State Process Model
  • Running: process that is currently being executed
  • Ready: process that is prepared to execute when given the opportunity
  • Blocked: process that is waiting for some event to occur
  • New: process that has not yet been loaded into main memory
  • Exit: process that is released because its halted or its aborted
process model contd
Process model contd…
  • In New state a process is defined which happens in 2 stages
    • OS gives a Id
    • Tables needed are allocated and built
  • While in new state the program remains in secondary storage
process model contd1
Process model contd…
  • Termination moves the process to exit state
  • Exit also occurs in two stages
    • The tables and other information associated with job are Temporarily preserved by OS
    • Once the utility programs extracts the needed information the OS no longer maintains any data and the process is deleted from the system.
events that lead to state transition
Events that lead to state transition
  • NullNew: new process is created
  • Newready : Admit – When OS is prepared to take an additional process
  • ReadyRunning: Dispatch When OS has to select a process it selects one from the ready state
  • Running  exit : Release – When process is completed or is aborted
  • RunningReady: Time-out or when the process is preempted
  • Running  Blocked: EventWait– Waiting for I/O or any other system call which is performed by the OS
  • BlockedReady :Event Occurs
  • ReadyExit: In some systems a parent may terminate a child process at any time(not shown in diagram)
  • Blocked exit:
suspended processes
Suspended Processes
  • What is it?
  • Need for Swapping
  • Other uses of suspension
need for swapping
Need for Swapping
  • The I/O takes more time than processor execution
  • Even with multiprogramming with lots of I/O operations the execution speed of processor comes down
  • This can be avoided by expanding the main memory. But
    • Cost of main memory is more
    • Larger memory results in larger processes not more processes
  • Second solution is Swapping
  • Swapping is moving part or all of a process from main memory to disk
  • This happens when none of the processes in main memory are in Ready state
  • In the disk the processes are in a suspend queue – which has existing processes that has been kicked out of main memory
  • After this the OS brings some other process in the suspend queue or some new process
  • With swapping we have an additional state-suspend state
need for suspend states
Need for suspend states
  • When swapping takes place the OS can decide to get the process from the new state or from suspend state. This can include more states
  • Ready: Process in main memory and available for execution
  • Blocked: Process in main memory and awaiting an event
  • Blocked/Suspend: Process in secondary memory and awaiting an event
  • Ready/Suspend: Process in secondary memory but is available for execution
process description
Process Description
  • What is it?
  • OS control Structures
    • Memory tables
    • I/O tables
    • File tables
    • Process tables
  • Process Control Structures
    • Process Location
    • Process Attributes
    • Processor state information
    • Process Control Block
what is process description
What is Process Description?

OS is an entity that manages the use of system resources by processes

What information does the OS need to control processes and manage resources for them?

Current status of each process and resource

How the OS does it?

Maintains tables of each entity it’s managing

process control structures
Process Control Structures
  • For managing and control a process the OS should know
    • Where the process is located
    • Attributes of the process that are necessary for its management (Process ID, Process State, Location in Memory)
process location
Process Location
  • Location of a process image will depend on the memory management scheme
  • Process image is maintained as a contiguous or continuous block of memory
  • This block is maintained in the disk and OS needs to know the location in the disk
  • For OS to manage the process at least a small portion of its image must be maintained in main memory
  • Modern OS uses memory management schemes like paging or segmentation for bringing only a portion of process image in the main memory
  • These details should also be stored in the process table
process control block
Process Control Block
  • Information about a process is stored in a Process control Block(PCB)
  • PCB information is grouped into 3 heads
    • Process Identification
    • Process state information
    • Process Control information
process identification
Process Identification
  • Identifier of this process
  • Identifier of the process that created this process (parent Process)
  • User Identifier
role of pcb
Role of PCB
  • It is the most important data structure in an OS
  • Each PCB contains all of the information about a process that is needed by the OS
  • The blocks are read and/or modified by every module in the OS including
    • Scheduling
    • Resource allocation
    • Interrupt processing
    • Performance monitoring and analysis
role of pcb contd
Role of PCB Contd…
  • Direct access to PCB acts as a threat to its protection
    • A bug in a single routine could damage PCB which could destroy the ability to manage other processes
    • A design change in PCB could affect a number of modules in the OS
  • Solution is to use a handler routine to protect the PCB
process control
Process Control
  • Modes of execution:-
    • User mode-less privileged mode
    • Kernel mode- more privileged mode
  • Reasons for two modes
    • Protect the OS, and OS tables like PCB from interference by user programs
  • In the kernel mode, the software has complete control of the processor and its instructions, registers, and memory
  • Program Status Word (PSW) indicates the mode of execution
steps in process creation
Steps in Process Creation
  • Assign a unique PID to the new process:- entry is made to the primary process table
  • Allocate space for the process:-all elements in the process image like, user address space, user stack are to be set
  • Initialize PCB
    • Process ID portion will contain the IDs
    • Processor State Information portion will be initialized to zero(mostly)
    • Process Control Information is initialized based on standard default values plus attributes requested for this process
  • Set Appropriate linkages
  • Create or expand other data structures
process control block pcb
Process Control Block (PCB)

Information associated with each process

  • Process state
  • Program counter: Address of the next instruction to be executed
  • CPU registers: State information which must be saved when an interrupt occurs.
  • CPU scheduling information: process priority, pointers to scheduling queues, and other scheduling parameters.
  • Memory-management information: base and limit registers, base table and segment table
  • Accounting information: time limits, account and process numbers etc
  • I/O status information: list of I/O devices allocated to the process a list of open files and so on


2.complete control of processor

6.less priviledged mode process creating another

8.List of Instructions