Context Switching in OS

The process of context switching in OS involves the storage of the context/state of a given process in a way that it can be reloaded whenever required, and its execution can be then resumed from the very same point as earlier. It is basically a feature of the multitasking OS, and it allows the sharing of just a single CPU by multiple processes.

What is Context Switching in OS?

Context switching in OS is the process of storing the state of a running process or thread, so that it can be restored and resume execution at a later point, and then loading the context or state of another process or thread and run it.

Context switching is a fundamental concept in operating systems, and is necessary for multitasking and efficient resource management. In a multitasking operating system, multiple processes or threads can be running concurrently. The operating system uses context switching to switch between these processes or threads, so that each one can have a fair share of the CPU's time.

Context switching in OS involves saving and restoring the following information:

• The contents of the CPU's registers, which store the current state of the process or thread.
• The process or thread's memory map, which maps the process or thread's virtual memory addresses to physical memory addresses.
• The process or thread's stack, which stores the function call stack and other information needed to resume execution.

Context switching can be a costly operation, as it requires a significant amount of overhead. However, it is essential for multitasking and efficient resource management.

Why Do We Need Context Switching in OS?

Context switching is necessary in operating systems for the following reasons:

• Multitasking: Context switching allows multiple processes to run concurrently on a single CPU. This is essential for modern operating systems, which need to support a wide range of applications, such as web browsers, email clients, and video games.
• Efficient resource management: Context switching allows the operating system to efficiently manage the CPU and other resources. For example, if one process is blocked waiting for I/O, the operating system can switch to another process that can run while waiting for the I/O to complete.
• Fairness: Context switching ensures that each process gets a fair share of the CPU's time. Without context switching, a single process could monopolize the CPU and prevent other processes from running.

In addition to these reasons, context switching is also used in a number of other operating system features, such as:

• System calls: When a process makes a system call, the operating system needs to switch to kernel mode to handle the request. This requires saving the state of the user-mode process and loading the state of the kernel process.
• Interrupts: When an interrupt occurs, the operating system needs to switch to kernel mode to handle the interrupt. This also requires saving the state of the current process and loading the state of the kernel process.
• Scheduling: When the operating system schedules a new process to run, it needs to switch to the state of that process. This is also known as a context switch.

Context Changes as a Trigger

Context changes, or context switches, are frequently triggered events in operating systems. These switches occur when the operating system needs to switch from executing one process or task to another. Several events or conditions can trigger context changes:

1. Time Quantum Expiry: In time-sharing or round-robin scheduling, each process is allocated a fixed time quantum (a slice of CPU time). When a process's time quantum expires, the operating system initiates a context switch to allow the next process in the queue to execute.
2. Higher Priority Process: When a higher-priority process becomes ready to run, the operating system may preempt the currently running process and switch to the higher-priority one.
3. I/O Operations: A process may request an I/O operation, such as reading from a disk or receiving data from a network. Since I/O operations are often slow, the CPU can be more productive by executing another task while waiting for the I/O to complete. Once the I/O operation is finished, a context switch occurs to resume the original process.
4. Interrupts: Hardware and software interrupts, such as timer interrupts, keyboard interrupts, and network interrupts, can trigger context switches.
5. Process Termination: When a process completes its execution or is terminated for any reason, a context switch is triggered to release the CPU and resources associated with that process. The operating system then selects the next process to run.

These trigger events are managed by the operating system's scheduler and are crucial for efficient multitasking, process management, and ensuring fairness and responsiveness in a system.

Process Control Block in Context Switching in OS

The Process Control Block (PCB) plays a crucial role in context switching in OS. The PCB is a data structure that contains all of the information that the operating system needs to manage a process.

When the operating system needs to switch contexts, it saves the state of the current process in its PCB and then loads the state of the new process from its PCB. This allows the operating system to quickly and efficiently switch between processes without losing any data.

Here is an example of how the PCB is used in context switching:

1. Process A is running and has the CPU's attention.
2. Process B interrupts Process A with a system call.
3. The operating system saves the state of Process A in its PCB, including the contents of its registers, memory map, and stack.
4. The operating system loads the state of Process B from its PCB, including the contents of its registers, memory map, and stack.
5. Process B starts executing.
6. When Process B is finished executing, the operating system saves its state in its PCB and loads the state of Process A from its PCB.
7. Process A resumes executing from where it left off.

State Diagram of Context Switching

Steps Of Context Switching in OS

The steps involved in context switching in OS are as follows:

1. Save the state of the current process: This includes saving the contents of the CPU registers, the memory map, and the stack.
2. Load the state of the new process: This includes loading the contents of the CPU registers, the memory map, and the stack.
3. Update the process scheduler: The process scheduler needs to be updated to reflect the new state of the system.
4. Switch to the new process. This involves transferring control to the new process's instruction pointer.