Process Description and control
G.Anuradha
(Referred from william stallings and
galvin 8th edition)
Contents
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Introduction
Process states
Process description
Process control
Introduction
• OS must interleave the execution of multiple
processes to maximize processor utilization
• Proper allocation of resources and avoidance
of deadlocks
• Interprocess communications
Process states
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What is it?
A two-state Process model
Creation and termination of processes
Five state model
Suspended processes
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
• 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
• 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:
• Each process runs to completion
Trace from Processors
point of view
Timeout
I/O
A Two-State Process Model
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
• What is a Trace?
• What is a Dispatcher?
• What are the two states of a two state process
model?
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
• 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
Reasons for process creation
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
Process Termination
Five State 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…
• 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 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
• 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:
Single blocked queue diagram
Multiple blocked queues
Suspended Processes
• What is it?
• Need for Swapping
• Other uses of suspension
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
• 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 statesuspend state
State transition diagram with single
suspend state
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
• What is it?
• OS control Structures
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Memory tables
I/O tables
File tables
Process tables
• Process Control Structures
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Process Location
Process Attributes
Processor state information
Process Control Block
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
General Structure of OS control tables
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 Image
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
• 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
• Identifier of this process
• Identifier of the process that created this
process (parent Process)
• User Identifier
Processor State Information
Process Control Information
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
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Scheduling
Resource allocation
Interrupt processing
Performance monitoring and analysis
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
• 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
1. Assign a unique PID to the new process:- entry is made to
the primary process table
2. Allocate space for the process:-all elements in the process
image like, user address space, user stack are to be set
3. Initialize PCB
1.
2.
3.
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
4. Set Appropriate linkages
5. Create or expand other data structures
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
Process Control Block (PCB)
CPU Switch From Process to Process
Quiz
Across
2.complete control of processor
6.less priviledged mode
7.one process creating another
8.List of Instructions
Down
1.process waiting for input output operation
3.Program to switch processor from one process to
another process
4.moving part or all process from main memory to disk
5.Program in execution
Quiz answers