Dispatcher in Operating System and its Functions

The Dispatcher in an Operating System (OS) is a vital component of the CPU scheduling system. Once the short-term scheduler selects the next process to execute, the dispatcher transfers control of the CPU to that process. This transition is not just a simple switch but it involves several complex tasks like:

• Performing a context switch
• Moving the CPU from kernel mode to user mode
• Jumping to the correct execution point in the user program

In short, the dispatcher ensures that the selected process can resume or begin execution correctly and safely.

Dispatcher provides control of the CPU to the process picked by the short-term scheduler. When the scheduler finishes its task of selecting a process, then the dispatcher takes that process to the destination state. 

What is Dispatcher in Operating System?

A Dispatcher in Operating System (OS) is a system module responsible for transferring control of the CPU to the process selected by the short-term scheduler. It performs tasks such as:

• Context switching (saving and loading process states)
• Switching to user mode (ensuring secure user execution)
• Jumping to the correct program counter (resuming execution)
• Changing process state from ready to running
• Updating accounting and memory management info

For example, in FIFO scheduling, once the scheduler selects the next process (e.g., P1, P2, etc.), the dispatcher removes it from the ready queue and assigns it to the CPU for execution.

Diagram: Flow from Scheduler → Dispatcher → CPU

Here’s a visual explanation of how the dispatcher works in process scheduling:

+-----------------------+         +-----------------------+         +-----------------------+
|  Short-term Scheduler | ---->   |       Dispatcher      | ---->   |          CPU          |
|  (selects process)    |         | (switch, load, assign)|         | (executes the process)|
+-----------------------+         +-----------------------+         +-----------------------+

                     |                                 ↑
                     |     Ready Queue (P1, P2, ...)   |
                     +---------------------------------+

Explanation:

• Scheduler picks the next process (e.g., P2).
• Dispatcher prepares and assigns that process to the CPU.
• CPU begins execution of the selected process.

Example of Dispatcher in OS (FIFO Scheduling)

Let’s take an easy example to understand the role of a dispatcher:

Suppose there are five processes: P1, P2, P3, P4, and P5, with respective arrival times T1, T2, T3, T4, and T5.

The system uses FIFO (First In, First Out) scheduling:

1. P1 arrives first, so the scheduler picks it.
2. The dispatcher removes P1 from the ready queue and assigns it to the CPU.
3. After P1 finishes, the scheduler selects P2.
4. The dispatcher again moves P2 from ready to running state.
5. This cycle continues for P3, P4, and P5.

💡 This example clearly shows how the scheduler chooses the process, and the dispatcher executes the transition from ready state to running state.

This is how a dispatcher works. The scheduler provides the dispatcher with an ordered list of processes that the dispatcher gives to the CPU. The dispatcher does many tasks such as memory mapping, context switching and setting up user registers.

Functions of Dispatcher

1. Context Switching

The dispatcher saves the state (context) of the currently executing process and loads the saved state of the next process to run. This context includes CPU registers, program counters, and memory management information.

2. Switching to User Mode

Processes run in user mode, where they have restricted access to system resources. The dispatcher switches the CPU to user mode before handing control over to the process to ensure safe execution of user programs.

3. Jumping to the Correct Location

The dispatcher moves the program counter to the position where the process was last interrupted. This ensures the process resumes execution from the correct state, not starting over from the beginning.

4. Managing Process State

The dispatcher changes the state of the process from ‘ready’ to ‘running’ and updates various scheduling and management tables used by the operating system.

5. Prioritizing CPU Allocation

In systems with priority-based scheduling, the dispatcher may consider the priority of processes, ensuring that high-priority processes receive CPU time before lower-priority ones.

6. Accounting

The dispatcher updates system and process accounting information. This includes tracking CPU usage, waiting time, and overall system load, which can be used for performance tuning and ensuring fairness.

7. Memory Management Adjustments

In some cases, switching processes may require changes to memory management settings, such as refreshing the memory map to reflect the memory allocation of the incoming process.

Summary: Role of Dispatcher in CPU Scheduling

Conclusion

The dispatcher in OS plays a crucial role in process execution and CPU scheduling. Without it, the transition between processes would be chaotic and inefficient. From context switching to mode changing and memory management, the dispatcher ensures that every process gets fair and efficient access to the CPU.

Understanding the functions of dispatcher helps you grasp how operating systems manage multitasking, process transitions, and system stability.