Q100947: Port Trapping in Windows 3.0/3.1

Article: Q100947
Product(s): Microsoft Windows Software Development Kit
Version(s): WINDOWS:3.0,3.1
Operating System(s): 
Keyword(s): kbfile kbsample kb16bitonly kbOSWin310 kbOSWin300
Last Modified: 09-DEC-1999

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The information in this article applies to:

- Microsoft Windows Software Development Kit (SDK) versions 3.0, 3.1 
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SUMMARY
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In enhanced mode Windows, the I/O permission bitmap (IOPM) is used by the
virtual machine manager (VMM) to determine whether I/O is permitted at a given
I/O port. The IOPM is clear when the VMM is initialized, which means that any
I/O port can be accessed. VxDs can trap specific I/O ports to prevent direct
access. Therefore, unless a VxD traps a port, direct I/O access is permitted. In
standard mode, the IOPM is not used, so direct I/O access is always permitted.
This does not apply to Windows NT.

The sample IOPORT.ZIP lists ports trapped by the standard VxDs included in
enhanced mode Windows (IOPMPORT.XLS and IOPMPORT.TXT), and an MS-DOS program
IOPM.EXE, which lists all currently trapped ports. Also, the DT (dump TSS)
command in WDEB386 lists all currently trapped ports.

MORE INFORMATION
================

The following files are available for download from the Microsoft Download
Center:

Ioport.exe

For additional information about how to download Microsoft Support files, click
the article number below to view the article in the Microsoft Knowledge Base:

  Q119591 How to Obtain Microsoft Support Files from Online Services

Microsoft used the most current virus detection software available on the date of
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The IOPM is contained in the task state segment (TSS). The I/O privilege level
(IOPL) and the I/O permission bitmap determine whether a task is allowed to
perform I/O. The IOPL determines the minimum privilege level required to perform
I/O without checking the permission bitmap. For example, if IOPL = 1, then a
procedure must have a code privilege level (CPL) of 1 or 0 (zero) to perform
unrestricted I/O. However, if the CPL of the current task is greater than the
IOPL, or the processor is operating in virtual 8086 mode, then the I/O
permission bitmap is checked to determine whether I/O is permitted at the
requested port.

In enhanced mode Windows, CPL is always greater than IOPL, so the IOPM is always
checked to determine whether direct I/O access is permitted. In standard mode,
CPL equals IOPL, so the IOPM is never checked.

Each bit in the bitmap corresponds to an I/O port byte address. For example, the
bit for port address 60 (decimal) corresponds to the 60th bit, or the 4th bit in
the 8th byte of the bitmap. In enhanced mode Windows, the bitmap is always 8192
bytes, and the bitmap is initialized to allow I/O at any port.

When a VxD is initialized, it can call Install_IO_Handler to trap specific I/O
ports. Install_IO_Handler sets the corresponding bits in the IOPM. When a
trapped port is accessed, the VxD I/O handler will take over and perform the
appropriate action. Windows maintains a single TSS for all VMs. When a VM switch
occurs, the TSS IOPM is updated to reflect the local IOPM of the incoming VM.
Port trapping for a specific port may be disabled locally or globally from a VxD
by calling Disable_Local_Trapping or Disable_Global_Trapping.

Additional query words: softlib IOPORT.ZIP kbfile

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Keywords          : kbfile kbsample kb16bitonly kbOSWin310 kbOSWin300 
Technology        : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK300 kbWinSDK310
Version           : WINDOWS:3.0,3.1

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