Process Basics.ppt

© Oxford University Press 2014. All rights reserved.
Principles of Operating
Systems
Naresh Chauhan

Chapter 5
Fundamentals of Process
Management

Objectives
• Difference between job, program, task and process
• Process environment
• Life cycle of a process with its states and state transitions
• Implementation of Processes with process control block
• Context switching
• Process switching
• Process schedulers
• Various operations on processes

Program and Job
Program and Job used in batch systems.
Program is classical term used for user’s computation. A program is a
piece of code which may be a single line or millions of lines.
a job is the unit of work that a computer operator (or a program
called a job scheduler) gives to the operating system. For example, a
job could be the running of an application program such as a weekly
payroll program.
A job is usually said to be run in batch (rather than interactive) mode.

Job as a sequence of programs/work

Program/Process
• program in execution is called a process
• we write our computer programs in a text file and when we
execute this program, it becomes a process which performs all
the tasks mentioned in the program.
• a process is able to compete for resources
• program is a passive entity
• ‘task’ was used when there was need to have concurrent
execution on a single processor, i.e. more than one programs of
a single user.

Process environment
• When a program is loaded into the memory and it becomes a
process, it can be divided into four sections ─ stack, heap,
text/code and data.
• The following image shows a simplified layout of a process
inside main memory −
Heap

Program/Process
S.
N.
Component & Description
1 Stack
The process Stack contains the temporary data such as method/function
parameters, return address and local variables.
2 Heap
This is dynamically allocated memory to a process during its run time.
3 Text/Code Section
This includes the current activity represented by the value of Program
Counter and the contents of the processor's registers.
4 Data
This section contains the global and static variables.

Difference between program and
process
Program Process
Passive/Static Active/Dynamic
Cannot compete for resources Competes for resources
Has a code section Has a code section, data section,
stack, program counter

Life cycle of a process

Process states and state transitions

Process States
New
This is the state when the process has just been created. It is the initial state in
the process life cycle.
Ready
In the ready state, the process is waiting to be assigned the processor by the
short term scheduler, so it can run. This state is immediately after the new state
for the process.
Ready Suspended
The processes in ready suspended state are in secondary memory. They were
initially in the ready state in main memory but lack of memory forced them to
be suspended and gets placed in the secondary memory.
Running
The process is said to be in running state when the process instructions are
being executed by the processor. This is done once the process is assigned to
the processor using the short-term scheduler.

Process state diagram with
suspended states

Process States
Blocked
The process is in blocked state if it is waiting for some event to occur. This event
may be I/O as the I/O events are executed in the main memory and don't
require the processor. After the event is complete, the process again goes to
ready state.
Blocked Suspended
This is similar to ready suspended. The processes in blocked suspended state
are in secondary memory. They were initially in the blocked state in main
memory waiting for some event but lack of memory forced them to be
suspended and gets placed in the secondary memory. A process may go from
blocked suspended to ready suspended if its work is done.
Terminated
The process is terminated once it finishes its execution. In the terminated state,
the process is removed from main memory and its process control block is also
deleted.

Event types
Event Current State New State OS Actions
A new process is
created.
-- NEW Assigns an ID to the
process and some other
related information.
Process makes a
resource or I/O
request.
RUNNING BLOCKED Schedules the next
process from ready queue
and dispatches it to the
processor.
Resource or I/O is
released.
BLOCKED READY (If the resource
or I/O released is what
the BLOCKED process
requires)
Schedules the next
processor.

Event types
Event Current State New State OS Actions
An interrupt is
generated by
another process or
due to any other
reason.
RUNNING READY Schedules the next
process (interrupting
process if the interrupt
has come from this
process) from ready
queue and dispatches it to
the processor.
Process reaches to
its end of execution
or aborted.
RUNNING TERMINATED Schedules the next
processor.

Process control block
A Process Control Block is a data structure maintained by the Operating
System for every process.
The PCB is identified by an integer process ID (PID).
The following are the fields associated with a PCB:
• PID
• PC
• Registers
• State
• Priority
• Event information
• Pointer to parent process
• Pointer to child process
• Memory related information
• Scheduling related information
• Pointer to address space of the
process

.N. Information & Description
1 Process State
The current state of the process i.e., whether it is ready, running, waiting, or
whatever.
2 Process privileges
This is required to allow/disallow access to system resources.
3 Process ID
Unique identification for each of the process in the operating system.
4 Pointer
A pointer to parent process.
5 Program Counter
Program Counter is a pointer to the address of the next instruction to be
executed for this process.

.N. Information & Description
6 CPU registers
Various CPU registers where process need to be stored for execution for running
state.
7 CPU Scheduling Information
Process priority and other scheduling information which is required to schedule
the process.
8 Memory management information
This includes the information of page table, memory limits, Segment table
depending on memory used by the operating system.
9 Accounting information
This includes the amount of CPU used for process execution, time limits,
execution ID etc.
10 IO status information
This includes a list of I/O devices allocated to the process.

Process image

Implementation of processes

Queues
The various queues used here are implemented as linked lists.
There are mainly following queues:
Ready Queue for storing the processes with state ready.
Blocked queue for storing the processes which needs to wait
for some I/O or resource.
Suspended queue for storing the blocked processes which
have been suspended.
Free process queue for the information of empty space in
memory where a new PCB can be created.

PCB queues in memory

Sequence of activities during process
switching

Schedulers
Long-term scheduler
Short-term scheduler
Medium-term scheduler

Long-term, medium-term and short-term
scheduler

Process operations
Creation
The process identification (process ID) and its priority are assigned.
The memory and other resources required by the process are
allocated to it.
The code is copied in the code area, thereby setting the process
environment.
The operating system looks for a free PCB space in the free-process
queue for the newly created process and initializes all its fields.
An existing process may also create another process. This is known
as process spawning.

Process Hierarchy

Termination
When a process terminates, the memory occupied, open files,
and other resources are taken away from it.
Another reason for the termination of a process may be some
error or exception generated in the execution of the process. It
may be the case that process may
require more memory than allowed.
reference a memory location out of its limits.
Try to access a resource or I/O that is not allowed to use.
attempt an arithmetic operation that is not allowed, e.g. divide by
zero.
find an error while accessing an I/O.
child process termination

Process Basics.ppt

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