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Q68250: VFINTD.exe Capturing Global Interrupts in Enhanced Mode

Article: Q68250
Product(s): Microsoft Windows Device Driver Kit
Version(s): 3.0,3.1
Operating System(s): 
Keyword(s): kbfile kbsample kb16bitonly kbDDK kbOSWin300
Last Modified: 16-DEC-1999

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The information in this article applies to:

- Microsoft Windows Device Development Kit (DDK) for Windows, versions 3.0, 3.1 
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SUMMARY
=======

VFINTD.exe is a sample application that is used later in this article to
demonstrate how to virtualize a particular IRQ. There are two pieces of code to
the VFINTD sample: the VxD and a sample MS-DOS application. All the files that
are named VFINTD are part of the VxD. The MS-DOS application is called TSTFINT.
The only purpose of the MS-DOS application is to capture floppy disk interrupts.
The sample application does not release the focus capture, but the VxD provides
this functionality.

A hardware interrupt in the 386 enhanced mode of the Microsoft Windows graphical
environment is classified as either "global" or "owned." A global interrupt is
typically reflected into whichever virtual machine (VM) is currently running,
while an owned interrupt is reflected into the VM that owns that interrupt. For
example, the floppy disk drive interrupt is typically made global, because any
VM should be able to use the floppy disk drive and receive interrupts from it.
On the other hand, an application that exclusively handles communications with a
nonstandard device would become confused if interrupts from the device were
routed to some other VM. For this reason, these interrupts should be "owned."

In some circumstances, it may be advantageous to "capture" a global interrupt
temporarily in one VM, effectively making the interrupt "owned." The text below
discusses VFINTD.386, a sample virtual device (VxD) that does just that. With
this VxD installed in a Windows system running in 386 enhanced mode, an
application can issue the appropriate function calls to reflect all interrupts
from the floppy disk drive into the application's VM, regardless of which VM is
currently running.

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

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

  Vfintd.exe
  (http://download.microsoft.com/download/winddk/enhmode/1/W31/EN-US/Vfintd.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
posting to scan this file for viruses. Once posted, the file is housed on secure
servers that prevent any unauthorized changes to the file.

Background
----------

Standard hardware for an MS-DOS compatible computer includes a programmable
interrupt controller (PIC). One register in the PIC is the interrupt mask
register (IMR), which defines which hardware interrupts are currently enabled
and which are disabled (or masked). The MS-DOS utility DEBUG can display the
contents of the IMR. To demonstrate this, run DEBUG from the MS-DOS prompt. Then
enter the following command to display the value of port 21 on your machine:

  " i21 " (without the quotation marks)

Port 21 is the IMR for the hardware interrupts IRQ0 through IRQ7. The format of
the IMR is that each bit (0-7) that is set corresponds to a disabled interrupt
request (IRQ). Therefore, a typical IMR value might be 98h, which indicates that
IRQ3, IRQ4, and IRQ7 are all masked, and that those interrupts are disabled:

     Hexadecimal 98 = Binary 10011000 = IRQ3, IRQ4, and IRQ7 off

NOTE: A PC/AT compatible system has two hardware PICs, and thus, two IMRs, that
correspond to the fifteen possible IRQs. (The second PIC is cascaded to IRQ2,
which consumes one IRQ.) While this point is included here for completeness, is
not necessary to understand the overall picture.

When an MS-DOS device driver or terminate-and-stay-resident program (TSR)
installs an interrupt handler for a particular IRQ, the device driver or TSR
hooks the interrupt vector and unmasks the corresponding bit in the IMR. When
Windows is running, MS-DOS drivers and TSRs are normally shared by all virtual
machines. Thus, if an interrupt occurs while an arbitrary VM is running, Windows
is not required to perform a virtual machine task switch to access the interrupt
handler, because the interrupt handler resides in the address space of each
virtual machine.

Alternatively, if an interrupt handler for a previously masked IRQ is installed
within a virtual machine, and an interrupt occurs while another virtual machine
is executing, Windows must make a task switch so that the local interrupt
handler is accessible and executable.

Windows Default Behavior
------------------------

The VPICD VxD virtualizes the PICs when Windows is running in 386 enhanced mode.
When hardware is virtualized, each virtual machine has access to its own
"virtual" PIC. This allows transparently sharing hardware among all virtual
machines.

Unless some other mechanism processes an IRQ, the VPICD handles interrupts
according to the following default behavior:

When Windows boots into 386 enhanced mode, VPICD examines the value of the IMRs
in the system to determine which interrupts are masked. If an interrupt is not
masked (is enabled), Windows must assume that a valid interrupt handler is
already installed for this IRQ. These IRQs are then flagged as "global," so that
Windows can avoid the overhead of a task switch to service them. The remaining
IRQs, which are masked (disabled), are flagged as "local" or "owned." An
interrupt for one of these IRQs will be reflected only into each virtual machine
that unmasks the IRQ.

Changing the Default Behavior
-----------------------------

It is possible to change the default behavior of interrupt handling (described
above) by installing a VxD that virtualizes an IRQ with the VxD call
VPICD_Virtualize_IRQ. This call replaces the VPICD default callback routines
with routines supplied in the VxD. For example, the virtual communication device
(VCD) virtualizes the communication interrupt so that it can control interrupts
from communication devices regardless of the state of the IMR at Windows load
time.

The VFINTD sample VxD also uses VPICD_Virtualize_IRQ to allow an application to
"capture" the interrupts on IRQ6 (the floppy disk device), which is normally
global. For example, a hard disk backup program that directly accesses the
floppy disk controller hardware to improve performance would "lose" interrupts
to other virtual machines unless it changed the global nature of IRQ6. VFINTD
can be used in this case to capture the "focus" of the floppy disk device and
the interrupts it generates.

Using VFINTD
------------

There are two pieces of code to the VFINTD.exe sample: the VxD and a sample
MS-DOS application. All the files that are named VFINTD.exe are part of the VxD.
The MS-DOS application is called TSTFINT. The only purpose of the MS-DOS
application is to capture floppy disk interrupts. The sample application does
not release the focus capture, but the VxD provides this functionality.

To install the VxD in your machine, perform the following steps:

1. Exit Windows.

2. Modify the [386enh] section of your SYSTEM.INI file to include the following
  line:

  device=VFINTD.386

3. Copy the VFINTD.386 file into the directory where the Windows system files
  are stored. By default, this is the C:\WINDOWS\SYSTEM directory.

4. Start Windows.

With the VxD installed, you can run the TSTFINT application in an MS-DOS window
to capture interrupts from the floppy drive. TSTFINT displays the version number
of the VxD to indicate that the application has successfully communicated with
the VxD.

This sample has been provided to demonstrate how to virtualize a particular IRQ.
It is a good place to start when developing another VxD to perform a similar
function.

Additional query words:

======================================================================
Keywords          : kbfile kbsample kb16bitonly kbDDK kbOSWin300 
Technology        : kbAudDeveloper kbWin3xSearch kbWinDDKSearch kbWinDDK300 kbWinDDK310
Version           : :3.0,3.1
Issue type        : kbinfo

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