Definition
Mode Switch
A mode switch is the atomic hardware transition of the CPU privilege level between User Mode and Kernel Mode. It allows the OS to regain control of the processor.
Triggers
Supervisor Call
Explicit request from a user program for an OS service.
Trap
Exception triggered by current instruction (e.g., division by zero, invalid memory access).
Interrupt
Signal from external source (hardware timer, I/O device).
Mode Switch vs. Process Switch
Mode Switch
Lightweight operation saving only enough state (PC, registers) to handle the kernel event. CPU remains in the same process context.
Process Switch
Heavyweight operation requiring PCB update, address space switch, and potential TLB flush. Requires a mode switch as prerequisite.
Performance
Mode switching is significantly faster than process switching as it does not change memory management context or affect caches.