- Definition of Process
- Process State
- Process Operations
- Process Control Block
Definition of Process
The notion of process is central to the understanding of operating systems. There are quite a few definitions presented in the literature, but no "perfect" definition has yet appeared.
Definition
The term "process" was first used by the designers of the MULTICS in 1960's. Since then, the term process, used somewhat interchangeably with 'task' or 'job'. The process has been given many definitions for instance
- A program in Execution.
- An asynchronous activity.
- The 'animated sprit' of a procedure in execution.
- The entity to which processors are assigned.
- The 'dispatchable' unit.
and many more definitions have given. As we can see from above that there is no universally agreed upon definition, but the definition "Program in Execution" seem to be most frequently used. And this is a concept are will use in the present study of operating systems.
Now that we agreed upon the definition of process, the question is what is the relation between process and program. It is same beast with different name or when this beast is sleeping (not executing) it is called program and when it is executing becomes process. Well, to be very precise. Process is not the same as program. In the following discussion we point out some of the difference between process and program. As we have mentioned earlier.
Process is not the same as program. A process is more than a program code. A process is an 'active' entity as oppose to program which consider to be a 'passive' entity. As we all know that a program is an algorithm expressed in some suitable notation, (e.g., programming language). Being a passive, a program is only a part of process. Process, on the other hand, includes:
- Current value of Program Counter (PC)
- Contents of the processors registers
- Value of the variables
- The process stack (SP) which typically contains temporary data such as subroutine parameter, return address, and temporary variables.
- A data section that contains global variables.
A process is the unit of work in a system.
In Process model, all software on the computer is organized into a number of sequential processes. A process includes PC, registers, and variables. Conceptually, each process has its own virtual CPU. In reality, the CPU switches back and forth among processes. (The rapid switching back and forth is called multiprogramming).
Process State
The process state consist of everything necessary to resume the process execution if it is somehow put aside temporarily. The process state consists of at least following:
- Code for the program.
- Program's static data.
- Program's dynamic data.
- Program's procedure call stack.
- Contents of general purpose registers.
- Contents of program counter (PC)
- Contents of program status word (PSW).
- Operating Systems resource in use.
- New State: The process being created.
- Running State: A process is said to be running if it has the CPU, that is, process actually using the CPU at that particular instant.
- Blocked (or waiting) State: A process is said to be blocked if it is waiting for some event to happen such that as an I/O completion before it can proceed. Note that a process is unable to run until some external event happens.
- Ready State: A process is said to be ready if it use a CPU if one were available. A ready state process is runable but temporarily stopped running to let another process run.
- Terminated state: The process has finished execution.
Process Operations
Process Creation
In general-purpose systems, some way is needed to create processes as needed during operation. There are four principal events led to processes creation.
- System initialization.
- Execution of a process Creation System calls by a running process.
- A user request to create a new process.
- Initialization of a batch job.
A process may create a new process by some create process such as 'fork'. It choose to does so, creating process is called parent process and the created one is called the child processes. Only one parent is needed to create a child process. Note that unlike plants and animals that use sexual representation, a process has only one parent. This creation of process (processes) yields a hierarchical structure of processes like one in the figure. Notice that each child has only one parent but each parent may have many children. After the fork, the two processes, the parent and the child, have the same memory image, the same environment strings and the same open files. After a process is created, both the parent and child have their own distinct address space. If either process changes a word in its address space, the change is not visible to the other process.
<Figure 3.2 pp.55 From Dietel>
Following are some reasons for creation of a process
- User logs on.
- User starts a program.
- Operating systems creates process to provide service, e.g., to manage printer.
- Some program starts another process, e.g., Netscape calls xv to display a picture.
Process Termination
A process terminates when it finishes executing its last statement. Its resources are returned to the system, it is purged from any system lists or tables, and its process control block (PCB) is erased i.e., the PCB's memory space is returned to a free memory pool. The new process terminates the existing process, usually due to following reasons:
- Normal Exist Most processes terminates because they have done their job. This call is exist in UNIX.
- Error Exist When process discovers a fatal error. For example, a user tries to compile a program that does not exist.
- Fatal Error An error caused by process due to a bug in program for example, executing an illegal instruction, referring non-existing memory or dividing by zero.
- Killed by another Process A process executes a system call telling the Operating Systems to terminate some other process. In UNIX, this call is kill. In some systems when a process kills all processes it created are killed as well (UNIX does not work this way).
Process States
A process goes through a series of discrete process states.
- New State The process being created.
- Terminated State The process has finished execution.
- Blocked (waiting) State When a process blocks, it does so because logically it cannot continue, typically because it is waiting for input that is not yet available. Formally, a process is said to be blocked if it is waiting for some event to happen (such as an I/O completion) before it can proceed. In this state a process is unable to run until some external event happens.
- Running State A process is said t be running if it currently has the CPU, that is, actually using the CPU at that particular instant.
- Ready State A process is said to be ready if it use a CPU if one were available. It is runable but temporarily stopped to let another process run.
Process State Transitions
Following are six(6) possible transitions among above mentioned five (5) statesFIGURE
- Transition 1 occurs when process discovers that it cannot continue. If running process initiates an I/O operation before its allotted time expires, the running process voluntarily relinquishes the CPU.
This state transition is:
Block (process-name): Running → Block.
- Transition 2 occurs when the scheduler decides that the running process has run long enough and it is time to let another process have CPU time.
This state transition is:
Time-Run-Out (process-name): Running → Ready.
- Transition 3 occurs when all other processes have had their share and it is time for the first process to run again
This state transition is:
Dispatch (process-name): Ready → Running.
- Transition 4 occurs when the external event for which a process was waiting (such as arrival of input) happens.
This state transition is:
Wakeup (process-name): Blocked → Ready.
- Transition 5 occurs when the process is created.
This state transition is:
Admitted (process-name): New → Ready.
- Transition 6 occurs when the process has finished execution.
This state transition is:
Exit (process-name): Running → Terminated.
Process Control Block
A process in an operating system is represented by a data structure known as a process control block (PCB) or process descriptor. The PCB contains important information about the specific process including
- The current state of the process i.e., whether it is ready, running, waiting, or whatever.
- Unique identification of the process in order to track "which is which" information.
- A pointer to parent process.
- Similarly, a pointer to child process (if it exists).
- The priority of process (a part of CPU scheduling information).
- Pointers to locate memory of processes.
- A register save area.
- The processor it is running on.
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