Raw device access must be protected because allowing an application to directly control devices would bypass operating-system protection.
The trap table must also be configured only by trusted kernel code.
Otherwise, a user program could redirect traps or system calls to an arbitrary location.
The return-from-trap instruction changes the privilege level and restores protected processor state, so it is also privileged.
Ordinary arithmetic and accesses to valid locations in the process’s own user address space do not require kernel mode.
Answer : A; B; D
