Differentiate between process switch and mode switch in OS

A process is defined as a program in execution and an entity that represents the basic unit of work to be implemented in the system. Understanding the difference between process switch and mode switch is crucial for comprehending how operating systems manage CPU resources and maintain system security.

Process Switch

A process switch (also called context switch) occurs when the processor switches from one thread/process to another thread or process. It involves saving the contents of CPU registers and instruction pointer for the current process.

For the new task, the registers and instruction pointer are loaded into the processor, then execution of the new process may start or resume. The old program will not execute further, but its state is saved in memory so the kernel can resume it later when ready.

Features of Process Switch

• Affects system performance due to overhead

• Increases load on CPU processor

• Requires saving and restoring complete process context

• Enables multitasking by time-sharing the CPU

• Triggered by scheduler or interrupts

Mode Switch

A mode switch occurs when the CPU changes privilege levels between user mode and kernel mode. The kernel operates at a higher privilege level than standard user tasks to maintain system security and resource control.

During a mode switch, the currently executing process does NOT change. The processor uses different modes to protect the OS from misbehaving programs and control access to system resources like RAM and I/O devices.

Steps for Mode Switch

• Program execution starts in user mode with limited privileges

• When system resources are needed, a mode switch to kernel mode occurs

• Mode switch happens through system calls or hardware interrupts

• Kernel functions execute with full system access

• After completion, system returns to user mode

Note − A general protection fault occurs when a user application attempts unauthorized actions, such as accessing restricted memory areas.

Comparison

Conclusion

Process switch changes the executing process entirely with high overhead, while mode switch only changes privilege levels within the same process. A mode switch is often required before a process switch can occur, as only the kernel has the authority to perform context switching between processes.