In an operating system scheduler, what is context switching between processes or threads?

Introduction / Context:
Multitasking operating systems allow many processes or threads to share the CPU. At any given instant, the CPU runs only one thread on a core, but over time it appears that many tasks progress in parallel. This illusion is created by context switching, which is a core responsibility of the scheduler and dispatcher in the operating system.

Given Data / Assumptions:

Multiple processes or threads are competing for CPU time.
The operating system uses a scheduling algorithm to decide which one should run next.
The state of a process or thread includes registers, program counter and other context information.
The question asks what context switching means in this setting.

Concept / Approach:
A context switch occurs when the operating system stops executing one process or thread and starts executing another. To do this safely, it must save the current state of the outgoing process in its process control block and load the saved state of the incoming process. This state includes the program counter, CPU registers and information about memory mappings. After the switch, the new process continues execution as if it had never been paused.

Step-by-Step Solution:
Step 1: When a running process uses up its time slice or is blocked by an event such as I O, the scheduler selects another ready process. Step 2: Before switching, the kernel saves the current CPU register values and program counter of the outgoing process into its control block. Step 3: The kernel then loads the saved register values and program counter of the incoming process from its control block into the CPU. Step 4: After this loading, the CPU resumes execution at the point where the new process last left off. Step 5: This save and restore operation is called a context switch and is fundamental to preemptive multitasking.

Verification / Alternative check:
Operating system texts consistently describe context switching in terms of saving and restoring process or thread context around scheduling decisions. Compiler design, permanent relocation to disk or user interface settings are described elsewhere and are not linked to the term context switch in this technical sense.

Why Other Options Are Wrong:
Converting source code to machine code is compilation, not context switching.
Moving programs permanently to secondary storage is about loading and storage management, not about switching between running processes.
Changing screen resolution is a user interface operation and is unrelated to CPU scheduling.

Common Pitfalls:
A frequent misunderstanding is to underestimate the cost of context switching. It consumes CPU time and can affect cache performance, so excessive context switching can reduce throughput. Another pitfall is to think that context switching happens only between processes; many systems also switch between threads inside the same process.

Final Answer:
Context switching is the operating system action of saving the execution state of the current process or thread and restoring the state of another, so that the CPU can switch execution from one to the other.