Operations on Process - Creation, Termination and Concurrent Processes in OS:

 

Operations on Process:

In most of the operating systems, a processor supports at least two modes of execution:

1.    User mode – it is a less privileged mode. All user programs run in this mode.

2.    Kernel mode – it is a more privileged mode. All the OS related tasks are executed in this mode. A kernel mode is also called as a system mode or control mode.

 

The mode of execution is indicated by a bit in the program status word (PSW).

 

1.    Process Creation:

 

An OS creates a new process with the specified or default attributes and an identifier.  A process may create several new sub-processes.

 

Syntax for new process creation: CREATE (ProcessID, attributes), where CREATE is a system call. They have two types - Parent process and Child process.

 

A parent process is a creating process whereas; a child process is created by the parent process. A child process may create another sub processes for which it acts as a parent.  As such, it forms a tree of processes. When OS issues a CREATE system call, it obtains a new PCB from the pool of free memory, fills the fields with parameters (provided and default) and insert the PCB into the ready list. Thus, this makes the specified process eligible for execution.

Commonly required parameters when a process is created include:

·         priority of a process

·         privilege level

·         memory resources

·         access permissions

·         memory protection information, etc.

 

Moreover, a process will require certain resources such as CPU time, memory, files and IO devices to complete its operation.  When a process creates a sub-process, it may obtain its resources either directly from an OS or it uses the resources of parent process.

 

When a parent process creates a new child process, they can execute in one of the following way:

1.    Parent continues to execute concurrently with its child.

2.    Parent waits until some or all of its child processes have terminated.

 

Similarly, with regards to an address space, the two possibilities that can occur:

1.    Child process is a duplicate of the parent process.

2.    Child process has a program loaded into it.

 

Following are the possible reasons for process creation:

1.    New batch job: a batch job is provided to OS.

2.    Interactive logon: user at a terminal logs on to the system.

3.    Created by OS on behalf of a user program.

 

2.    Process Termination :

 

A DELETE system call is used for terminating a process.  A process can terminate itself or it may be terminated by another process using an appropriate system call. When a process is terminated, OS reclaim all the resources that were allocated to it and close all files opened by that process. Further, OS removes PCB and returns it to the free pool. The delete service is a normally invoked as a part of orderly program termination.

 

Possible reasons for process termination are as follows:

1.    A process has completed its normal operation.

2.    When parent terminates, it terminates the child process.

3.    A process has run longer than the specified total time length.

4.    Process requires more memory than the system can provide.

5.    Process tries to access un-allowed memory location.

6.    An attempt to execute invalid instruction.

7.    Privileged instruction- process tries to use an instruction reserved for OS only.

1.    Protection error - process attempts to use a resource that it is not allowed to use.

2.    If a deadlock exists in the system, an OS may terminate a process.

3.    Data misuse - a piece of data is of wrong type or is not initialised.

 

 

Concurrent Processes:

 

The two processes are said to be serial processes, if the execution of one must complete before the execution of another process starts. The two processes are called concurrent, if they are not serial and their execution can overlap in time. 

 

Consider the following figure:

In Fig (a), process P1 starts its execution at time T0; process P2 starts its execution only after process P1 has finished its execution.  Similarly, process P3 starts its execution only after process P2 has finished its execution.  Thus, these processes are said to be serial processes.

 

Now, consider Fig (b), where the execution time of processes P1, P2 and P3 are overlapped. Therefore, they are called as concurrent processes.