Intel Privileged and Sensitive Instructions (114473)

SUMMARY

The Intel architecture defines "privileged" instructions and "sensitive" instructions. The privileged instructions may only be executed when the Current Privilege Level is zero (CPL = 0). Attempting to execute a privileged instruction when CPL != 0 will generate a general protection (GP) exception. Windows traps GP exceptions caused by executing privileged instructions and usually generates an application error.

The sensitive instructions (also called IOPL-sensitive) may only be executed when CPL <= IOPL (I/O Privilege Level). Attempting to execute a sensitive instruction when CPL > IOPL will generate a GP exception. This should usually not cause a fatal error. The Windows Virtual Machine Manager (VMM) traps GP exceptions caused by executing sensitive instructions and (depending on the instruction) either simulates the instruction's behavior in the VM in which the instruction was executed, or dispaches it to a virtual device driver, which simulates the instruction's behavior.

The Virtual Machine Manager (VMM), which is in WIN386 along with the core Virtual Device Drivers (VxDs), runs at CPL = 0. By Intel hardware definition, Virtual 8086 (V86) mode always runs at CPL = 3. In enhanced mode, the machine can be either in IOPL = 0 or IOPL = 3.

MORE INFORMATION

The privileged instructions include:

    CLTS - Clear Task-Switched Flag    LMSW    - Load Machine Status
    HLT  - Halt Processor              LTR     - Load Task Register
    LGDT - Load GDT Register           MOV CRn - Move Control Register
    LIDT - Load IDT Register           MOV DRn - Move Debug Register
    LLDT - Load LDT Register           MOV TRn - Move Test Register

The sensitive instructions in protected mode include:

    IN  - Input           OUTS - Output String
    INS - Input String    CLI  - Clear Interrupt-Enable Flag (IF)
    OUT - Output          STI  - Set IF

The instruction POPF is both IOPL and CPL sensitive. IOPL will only be changed by POPF when executed at CPL = 0. The interrupt flag (IF) will only be changed by POPF if executed with CPL <= IOPL. If POPF does not change IOPL or the interrupt flag due to lack of privilege, no exception is generated.

The sensitive instructions V86 mode include:

    CLI   - Clear IF      POPF  - Pop Flags
    STI   - Set IF        INT n - Software Interrupt
    PUSHF - Push Flags    IRET  - Interrupt Return

The CPL = 3 by definition in V86 mode. If IOPL < 3, attempting to execute V86 mode sensitive instructions will generate a GP exception. The Windows VMM will trap the GP exception and emulate the instruction in the VM in which it was originally executed.

I/O instructions are special in that they are sensitive not only to IOPL, but also to the IO permission bitmap in the Task State Segment (TSS). If CPL <= IOPL, then the I/O operation will proceed without checking the I/O permission bitmap. If CPL > IOPL, or if in V86 mode, then the I/O permission bitmap is consulted. If the bits corresponding to the I/O port are clear, the I/O operation proceeds, otherwise, a GP exception is generated. In response to this GP exception, the VMM gets control and dispatches control to the VxD that traps the port.

REFERENCES

For additional information, please see the following article(s) in the Microsoft Knowledge Base:

100947 Port Trapping in Windows 3.0/3.1

112298 DOCERR: Port I/O with inp() and outp() Fails on Windows NT

Also see the Intel "i486(TM) Microprocessor Programmer's Reference Manual" and the section "The Effect of Priviledge Levels" in the article "The Tao of Interrupts" on the Microsoft Developer Network (MSDN) Development Library CD.