Privileged vs Non-Privileged Instructions [7M]

Context: Modern OS must protect itself, memory and devices from user programs. CPU provides dual-mode operation: Supervisor/Kernel mode (privileged) and User mode (non-privileged). Mode indicated by PSW bit (e.g. x86 CPL / PE flag, ARM M bits). Some instructions allowed only in supervisor mode.

1. Privileged Instructions:

Define: Executable only when CPU in supervisor mode ($mode=0$). If attempted in user mode, triggers privileged instruction trap / General Protection Fault.

• Examples:

  • I/O instructions: IN, OUT, INS, OUTS (x86), HLT, CLI, STI (halt, disable/enable interrupts)
  • Control registers: MOV CR0, EAX, LIDT, LGDT, WRMSR, TLB flush (INVLPG)
  • Memory management: LMSW, MOV to CR3 (page table base)
  • Interrupt & DMA control: IRET, enable/disable DMA
  • HALT, RESET processor

• Purpose: Directly manipulates shared hardware resources. Only OS kernel should do that.

2. Non-Privileged (Unprivileged / User) Instructions:

Executable in any mode. Ordinary computation not affecting system state.

• Examples: MOV, ADD, SUB, MUL, CMP, JMP, CALL, PUSH, POP (within user stack), floating point arithmetic.
• Some sensitive but non-privileged: e.g. POPF may not allow changing interrupt flag if in user mode (x86 masks it).

3. Why distinction? Protection

• Prevents user program from: corrupting other processes' memory (via MMU), stealing devices, masking interrupts, halting system, changing scheduling.
• System calls: User wants I/O -> executes trap / INT 0x80 / SYSCALL which switches to kernel mode, OS validates parameters then executes privileged operation on behalf.
• Virtualization: Hypervisor uses privileged instructions to emulate hardware for guest.

4. Mechanism:

Hardware checks mode bit before decoding. E.g. pseudo:

if (instruction.isPrivileged && PSW.mode == USER) -> exception(13)

x86 has 4 rings (0=kernel most privileged, 3=user). I/O privilege level (IOPL) in EFLAGS controls IN/OUT access. ARM has SVC (supervisor) vs USR.

Comparison Table:

Aspect	Privileged	Non-privileged
Mode	Kernel only	User & Kernel
Function	System control	Computation
Trap if violated	Yes - #GP fault	No
Example	HLT, CLI, IN, LGDT	ADD, MOV, AND

Without this separation, multi-programming and OS isolation would be impossible.