Need protection from user programs that:
Hardware: Bit in CPU to indicate mode and check on monitor mode for privileged instr.s
e.g., I/O instructions are privileged, so a user program must request I/O through the OS
Remember that only one program (in a single CPU system) can be executing at a time so when the OS turns control over to a user program it has "lost control."
Modifications to the CPU timer are privileged