2. NASIR
AHMID
PRESTON BS(CS)
UNI
Prof.
1st
Naeem
Semester
Iqbal
1421-212007
3. PROCESS CONCEPT
PROCESS:
A program in execution, process execution must
progress in sequential fashion.
4. A Process includes
A program in execution
An instance of a program running on a computer
The entity that can be assigned to and executed on a
processor
A unit of activity characterized by the execution of a
sequence of instructions a current state and an
associated set of system resources
5. PROCESSES
All the run able software on comp including OS is organized
into a number of sequential processes or just processes for
short processes is just an executing program, including the
current values of the registers and variables. Conceptually each
process has its virtual cpu.
The cpu switches back and forth from process to processes,
but to understand the system much easier to think about a
collection of processes running in pseudo parallel, than to keep
track of how the cpu switches from prog to prog .This back and
forth switching is called multiprogramming.
6. THE PROCESS MODEL
(a) Multiprogramming of four programs.
(b) Conceptual model of four independent, sequential processes.
(c) Only one program active at any instant.
8. PROCESSES CREATION
OS need some way to make sure all the necessary processes
exits.
In general purpose systems. Some way is needed to create
and terminate processes as needed during operation.
Principal events that cause processes creation.
1. System initialization.
2. Executing of a processes creation system call.
3. User request to create new process.
4. Initiation of a batch job.
9. PROCESSES CREATION
When OS is booted, typically several processes are created.
Foreground processes: Processes that interact with
human user and perform work them.
Background processes: Not associated with particular
users, but instead have specific function. E.g. designed to
accept incoming email ,sleeping most of the day but comes
to life when email arrives.
Daemons: Processes that stay in the background to
handle some activity such as email, web pages and so on
called daemons.
10. PROCESSES CREATION
Processes can be created after boot time as well.
Often a running process issue system calls to create one or more new
processes to help it do its job.
Creating new processes is particularly useful when the work to be done
can easily be formulated in terms of several related, but otherwise
independent interacting processes.
In interactive systems, users can start a program by typing a command
or double clicking an icon.
In command based UNIX systems running X windows, the new process
takes the new window. In windows , when a processes started it doesn’t
have a window, but it can create one or more and most do.
11. PROCESS TERMINATION
After a process has been created, it starts running and does
whatever its job is. However, no process lasts forever.
Sooner or later the new process will terminate, usually on
one of the following conditions:
1. Normal exit(voluntary).
2. Error exit(voluntary).
3. Fatal error(involuntary).
4. Killed by another processes(involuntary).
12. PROCESS TERMINATION
Normal exit(voluntary): Terminate because they have done
their work.
Fatal error(involuntary): The process discovers a fatal error e.g.
user types the command cc foo.c to compile the program foo.c and no
file exists the compiler simply exits.
Error exit(voluntary): An error caused by a process often due to
a program e.g. executing an illegal instruction , referencing nonexistent
memory, or divide by zero.
Killed by another processes(involuntary): A process
executes a system call telling the OS to kill some other process. Using
kill system call.
13. PROCESS HIERARCHIES
Parent creates a child process, child processes can create its
own processes.
Forms a hierarchy
UNIX calls this a “process group”.
Windows has no concept of hierarchy.
All processes are created equal.
15. PROCESS STATES
As a process executes, it changes states.
New: The process is being created.
Running: Instructions are being executed.
Waiting: The process is waiting for some event to
occur.
Ready: The process is waiting to be assigned to a
processor.
Terminated: The process has finished execution.
17. IMPLEMENTATION OF PROCESS AND (PCB)
To implement the process model, the OS maintains a table
(an array of structures called the process table with one
entry per process). Some authors call these entries process
control block.
These entry contains information about the:
Process state.
Program counter.
CPU registers.
CPU switching information.
Memory management information.
Accounting information.
I/O status information.
