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Q114473: Intel Privileged and Sensitive Instructions

Article: Q114473
Product(s): Microsoft Windows Software Development Kit
Version(s): 3.1
Operating System(s): 
Keyword(s): kb16bitonly
Last Modified: 01-FEB-2002

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The information in this article applies to:

- Microsoft Windows Software Development Kit (SDK) 3.1 
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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:

  Q100947 Port Trapping in Windows 3.0/3.1

  Q112298 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.

Additional query words: 3.10

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Keywords          : kb16bitonly 
Technology        : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK310
Version           : :3.1

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