Definition
Mode Switch
A mode switch is the atomic hardware transition of the CPU privilege level between User Mode and Kernel Mode. It is the fundamental mechanism that allows the operating system to regain control of the processor.
Triggers
A mode switch occurs when the CPU encounters one of the following:
- Supervisor Call: An explicit request from a user program for an OS service.
- Trap: An exception triggered by the current instruction (e.g., division by zero or an invalid memory access).
- Interrupt: A signal from a source external to the current process, such as a hardware timer or I/O device.
Mode Switch vs. Process Switch
A mode switch is a prerequisite for a process switch, but not every mode switch results in a process switch.
- Mode Switch: A lightweight operation that involves saving only enough state (e.g., Program Counter and Registers) to handle the kernel-level event. The CPU remains within the context of the same process.
- Process Switch: A heavyweight operation that occurs if the OS decides to switch execution to a different process. This requires updating the PCB, switching address spaces, and potentially flushing the TLB.
Performance
Mode switching is significantly faster than process switching because it does not involve changing the memory management context or affecting the processor’s caches as drastically.