5 State Process Model in Operating System

Pre-requisites:

• States of a Process in Operating Systems
• Two-State Process Model in Operating System

A process is a program under execution that consists of a number of elements including, program code and a set of data. To execute a program, a process has to be created for that program. Here the process may or may not run but if it is in a condition of running then that has to be maintained by the OS for appropriate progress of the process to be gained.

Need for Five-State Process Model

In five- state model the states have been split into two non-running states: ready and blocked, and along with this, two more states are added for the purpose: New and Exit/Terminate. These two states are used because, in the previous models, the main memory is capable to store all programs but that is not true because the programs are now very large, and loading those processes in the main memory is very tough/ or even not possible and also if there’s a requirement for using the previous resources that are released by the process, then that is not possible here.

The five states that are being used in this process model are:

1. Running: It means a process that is currently being executed. Assuming that there is only a single processor in the below execution process, so there will be at most one processor at a time that can be running in the state.
2. Ready: It means a process that is prepared to execute when given the opportunity by the OS.
3. Blocked/Waiting: It means that a process cannot continue executing until some event occurs like for example, the completion of an input-output operation.
4. New: It means a new process that has been created but has not yet been admitted by the OS for its execution. A new process is not loaded into the main memory, but its process control block (PCB) has been created.
5. Exit/Terminate: A process or job that has been released by the OS, either because it is completed or is aborted for some issue.

Execution of Process in Two-state Model

This model consists of five states i.e, running, ready, blocked, new, and exit. The model works when any new job/process occurs in the queue, it is first admitted in the queue after that it goes in the ready state. Now in the Ready state, the process goes in the running state. In the running state, a process has two conditions i.e., either the process goes to the event wait or the process gets a time-out. 

If the process has timed out, then the process again goes to the ready state as the process has not completed its execution. If a process has an event wait condition then the process goes to the blocked state and after that to the ready state. If both conditions are true, then the process goes to running state after dispatching after which the process gets released and at last it is terminated. 

Five State Process Model

Possible State Transitions

There can be various events that lead to a state transition for a process. The possible state transitions are given below:

1. Null -> New: A new process is created for the execution of a process.
2. New -> Ready: The system will move the process from new to ready state and now it is ready for execution. Here a system may set a limit so that multiple processes can’t occur otherwise there may be a performance issue.
3. Ready -> Running: The OS now selects a process for a run and the system chooses only one process in a ready state for execution.
4. Running -> Exit: The system terminates a process if the process indicates that is now completed or if it has been aborted.
5. Running -> Ready: The reason for which this transition occurs is that when the running process has reached its maximum running time for uninterrupted execution. An example of this can be a process running in the background that performs some maintenance or other functions periodically.
6. Running -> Blocked: A process is put in the blocked state if it requests for something it is waiting. Like, a process may request some resources that might not be available at the time or it may be waiting for an I/O operation or waiting for some other process to finish before the process can continue.
7. Blocked -> Ready: A process moves from blocked state to the ready state when the event for which it has been waiting.
8. Ready -> Exit: This transition can exist only in some cases because, in some systems, a parent may terminate a child’s process at any time.

Benefits:

1. The New and Exit state are very useful constructs for managing the process.
2. It is an efficient way of the previous two-state process model.

Drawbacks:

1. If a process gets terminated or exit from the OS, its data is not preserved by the OS.
2. There can be a performance issue as in a situation where each process goes to a blocked state the CPU stays idle until at least one process leaves the waiting state.