Complex sentence: The operating system executes at the highest level of privilege, and allows applications to request services via system calls, which are often executed via interrupts; an interrupt automatically puts the CPU into the privileged mode, hence elevating the privilege level to the required level.
Split into simpler sentences: The operating system executes at the highest level of privilege, and allows applications to request services via system calls, which are often executed via interrupts. An interrupt automatically puts the CPU into the privileged mode, hence elevating the privilege level to the required level.