Registrations are now open. Join us today!
There is still a lot of work to do on the wiki yet! More information about editing can be found here.
Already have an account?

Microsoft KB Archive/100947

From BetaArchive Wiki

Port Trapping in Windows 3.0/3.1


The information in this article applies to:

  • Microsoft Windows Software Development Kit (SDK) versions 3.0, 3.1


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.


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


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 posting to scan this file for viruses. Once posted, the file is housed on secure servers that prevent any unauthorized changes to the file.

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

Keywords : kbfile kbsample kb16bitonly kbOSWin310 kbOSWin300
Issue type :
Technology : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK300 kbWinSDK310

Last Reviewed: December 9, 1999
© 2001 Microsoft Corporation. All rights reserved. Terms of Use.