 Solution

A system call or other synchronous trap from user code enters the privileged handler, saving the prior execution status; entry may also mask interrupts depending on the architecture. A device or timer interrupt similarly saves the interrupted context, selects the privileged interrupt handler and commonly adjusts the interrupt mask to control nesting. A return-from-trap or return-from-interrupt instruction restores the saved status, ordinarily returning to user state with interrupts enabled.

These are three distinct transition situations. In addition, the operating-system kernel can explicitly enable or disable interrupts around a critical section using privileged instructions. The exact entry mask is architecture-dependent; entering supervisor state does not logically require interrupts to remain disabled throughout the handler.