Differ between Process and Thread? Process states and PCB

                                    Process And Process States

Process:

Process is a program which is in execution. A process is the unit of work in a modern time sharing
system.

A system therefore consists of a collection of  processes  operating     system processes   executing  

system    code    and   user    processes    executing   user  code.

A program becomes a  process  when  an  executable  file  is  loaded  in  to   memory.

Two  common techniques  for  loading  executable  files   are double  clicking an icon

representing  the executable file and  entering  the  name  of   the  executable  file   on

the command line .

A process includes :

>program counter and other registers.

>Stack

>work and data space, along with other resource, files etc.

>PCB(process control block )or process descriptor

PCB

>PCB: PCB,to holds information of state of a process,

Proces Control Block Each  process is represented in the operating system by a process   control

block ( PCB )  also  called a  task  control  block

PCB of all processes are maintained in some doubly linked queue.


Process States:

As a process executes , it changes state.

 The state of a process is defined in part by the current activity of that process. A process may be in

one  of the  following   states:

• New. The process is being created.

• Running. Instructions    are  being   executed   (Process is holding CPU).

• Waiting. The process is waiting for some event to occur or complete (such as an I/O completion or

reception of a signal).

• Ready. The process is waiting to be assigned to a processor.

• Terminated. The process has finished execution.

                                         



                                        Process States Diagram

 Process state. The state may be new, ready, running, waiting,  and so on.
PCB(process control block) OR  PD(process descriptor)

contains  information  associated with each process
•Program counter:
The   counter   indicates   the   address   of   the  next    instruction  to  be executed   for   this   process.
 • CPU  registers. The    registers   vary  in  number   and   type  ,   depending  on  the   computer architecture. They   include   accumulators  ,   index   registers  ,  stack   pointers  ,  and   general purpose   registers ,  plus   any   condition  code   information .  Along  with  the  program  counter, this  state  information  must  be  saved  when   an   interrupt   occurs ,  to  allow  the process  to   be   continued   correctly    . 
 • CPU  scheduling   information.This   information   includes   a  process  priority ,  pointers  to scheduling   queues,  and  any   other   scheduling   parameters.
• Memory  management   information. This   information   may  include   such items as  the  value    of   the  base  and  limit  register  and  the  page  tables  , or the  segment   tables, depending   on   the   memory   system   used   by   the   operating   system .

                                                           (Process Control Block)PCB           
                                                           
• Accounting  information. This   information   includes  the   amount   of   CPU   and   real time used,   time   limits , account   numbers,  job  or   process  numbers ,  and  so on. 
• I/O   status   information. This   information   includes   the   list   of   I/O   devices   allocated to   the   process  ,  a   list  of  open   files  ,  and   so   on.

What is  Thread?
Thread: 
Thread is the smallest unit  of  processing    that  can  be performed  in an Operating System(OS).Thread   is    the   light   weight   process . Thread    exists     within  a   process , a process   may  contain    multiple    threads.

Multi-threading:
When    multiple   threads    are   running   concurrently   ,  this   is   known   as   multi-threading.
For Example:
Downloading   a   video  song   while   playing     video song   at   the   same   time. Multi-threading   is   also   used     in   computer  generated   animation.
Widely  used   programming    languages    that   allow  developers   to  work   on   threads   in their   programs   source   code   are   Java  ,  Python   and   .NET.
Differ between  thread and process:

Thread  and   Process  are  two closely   related  terms  in  multi-threading. 

The   main  difference   between   these   terms  is  that  thread  is  a   part  of  a  process, 

 A  process   may  contain one or  more   threads  ,  but   a   thread   cannot   contain   a  process.

Thread  is  a   light   weight   processes:
Threads can generate   other  threads.
Process  exists    independent   on the other hand threads are subset  of process.
Context    switching    between   threads   in   the   same   process  is     faster   than context   switching   between   processes.
Threads  share  their   address  spaces  while  process  have separate address spaces
Treads are used for the small tasks,  where as processes are  used  for the  heavy  weight  tasks.