I/O Device Serialization and Virtualization (68975)

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Background

Usually, when an application performs an I/O operation, it calls functions in a high-level applications programming interface (API) to perform the task. For example, an application developed for Windows can call the WriteComm function to send characters to a communications port. Eventually, however, that transmission is accomplished through a combination of IN and OUT instructions in Intel assembly language that communicate with the desired hardware device.

In enhanced mode, Windows uses features of the Intel 386 processor that enable multiple MS-DOS applications, as well as the Windows graphical environment, to run concurrently. During normal system operation, the processor rapidly switches its context between these applications, creating the impression that all applications are running simultaneously. This process creates the need to arbitrate access to hardware devices. Returning to the communications example, if the system would allow an MS-DOS-based communications package to write characters to a COM port that was currently being used by the Windows Terminal application, the characters from the two applications would be intermixed, and the communications data would be corrupted.

Protection Mechanisms

The 386 processor has various features that enable enhanced mode Windows to provide a protected environment for each application. This environment is called a virtual machine (VM). Because the Windows graphical environment and each DOS application runs in its own VM, the processor checks each instruction that it processes to enforce the protection mechanism. This hardware checking enables the system to take control when an application attempts an operation that would normally interfere with the other running tasks.

For example, one protection mechanism that allows Windows to maintain control over I/O devices is known as the I/O permission bitmap, a data area created by the operating system that is referenced directly by the processor. When an application issues an IN or OUT instruction to a specific I/O port, the processor uses the port address as an index into the bitmap first. Then, depending on the value of the bit at the corresponding offset, it will either allow the operation to proceed or interrupt the operation and start the general-protection fault interrupt handler. This provides the operating system with the tool it needs to trap I/O instructions on a port-by-port basis.

Serialization

Now that you have the background information you need, let's take a closer look at the communications example to see how Windows handles device contention. If you try to concurrently use the Windows Terminal application and an MS-DOS communications program on the same COM port under Windows 3.0, Windows display a dialog box warning you of the device contention, and asking which application should gain ownership of the device. This process is known as device serialization. One VM is allowed unlimited access to a device for a certain period of time and all other applications that want to use the device must wait until it is available.

The processor's I/O permission bitmap makes serialization possible. For the first application that uses the port, Windows sets the appropriate bit in the permission bitmap for its VM to 0 (zero) to allow I/O operations. However, Windows sets that same bit in the bitmaps of all other VMs to 1 (one), signaling that I/O is "trapped." Therefore, when a second application attempts to read or write to the COM port, the instruction causes a general-protection fault, Windows receives control, and displays the dialog box.

This concludes a very low-level analysis of device contention. Fortunately, it is not necessary to dig around in the I/O permission bitmap to make this all work. Instead, Windows provides functions that allow an application to manipulate the protection mechanisms for a particular piece of hardware and for a particular VM. However, to use these functions, and in general, to provide this type of functionality in enhanced mode Windows, a virtual device (VxD) is required.

The VCD

The virtual communications device (VCD) has actually been the main subject of this article to this point. The VCD is a VxD provided with Windows that, among other things, keeps track of which VM currently owns a particular COM port, and displays a contention dialog box when a second VM tries to use a port concurrently. Because the source of the VCD is available in the Microsoft Windows Device Development Kit (DDK), it is possible to study the instructions that the VCD uses to perform these functions and to use them as a base for writing a new VxD that supports another device.

Unfortunately, the VCD is not the best example for a beginning VxD programmer because it is quite large and uses multiple code source files. It also contains a considerable amount of complex code that has been added to improve performance.

VDIALOG

Because the VCD is so complex, a small VxD sample demonstrates how to implement device serialization instead. VDIALOG is a VxD that, when installed on a Windows enhanced mode system, controls access to a nonexistent device. The companion programs WINACC and DOSACC are application programs that attempt to use the fictitious device. When both applications are run concurrently, VDIALOG tracks the device "owner" and displays a dialog box when contention occurs. VDIALOG is one of samples provided with version 3.1 of the Windows DDK.

Note that the mechanism works correctly regardless of whether actual hardware exists at the protected port address. The protection mechanisms themselves do not depend on what is connected to the system.

One Step Further

Sometimes, device serialization is not enough. For example, imagine how unusable the system would be if a dialog box would be displayed each time an MS-DOS application tried to write to the screen. For this reason, the virtual display device (VDD) uses a technique called device virtualization. Essentially, this means that the system informs an application that the I/O operations the application issues succeed, when in fact the real (physical) device may have been affected only indirectly.

Using the display device to illustrate this concept, one of the more advanced features of Windows in enhanced mode is the ability to display multiple MS-DOS applications simultaneously on the Windows desktop. Previous versions of Windows had attempted, often unsuccessfully, to perform this feat by trapping screen writes at the system BIOS function level. The problem with this approach is that there is no protection at the device level; it has always been possible for some application to bypass the BIOS and write directly to the device. Windows enhanced mode fully exploits features of the 386 processor to prevent so-called "bad applications" from ruining the display context.

More Protection Mechanisms

Video adapters not only have hardware registers, as COM ports do, but normally also have a memory mapped I/O buffer. Therefore, when an MS-DOS application, for example, writes to the screen, the text is moved out to the video buffer, usually located at some address between 640K and 1 MB, and the text then automatically appears on the screen. Obviously, if the multiple VMs that can run under enhanced mode Windows could all write physically to that buffer, and the adapter was using that single buffer to display text on the screen, the screen would display a jumble of characters and the multiple VM facility would be useless.

Windows is able to avoid that problem. To do so, Windows uses the page translation facility of the 386 processor. Where the I/O permission bitmap is a mechanism to protect I/O ports, page translation is a mechanism to protect memory. Page translation works as follows: as a program executes, it constantly makes read and write accesses to memory. For example, a typical MOV instruction forms a memory address from the operand of the instruction, often using an offset within the instruction, some combination of general registers, and a segment register. The values are combined to eventually form an address, a number that is sent to the memory unit of the computer to identify the desired memory location. When paging is enabled on the 386 processor, as it normally is when enhanced mode Windows is running, an extra step is inserted before the address is sent to the memory unit. The processor uses the address as an index into page tables that are set up by the system. The value found in the tables is the one sent to the memory unit.

The VDD

Page mapping is the tool used by the VDD to virtualize these I/O buffers. When an MS-DOS application writes to the screen (that is, when the application thinks it writes to the screen), Windows reroutes the actual text to an entirely separate area in memory which is managed by the VDD.

When Windows updates the physical screen--for example, a screen that has a number of windowed MS-DOS applications--Windows sends requests to the VDD for the contents of each "virtual video buffer" and paints the text from each of these buffers into an MS-DOS window. The fact that this entire process is transparent to each of the MS-DOS applications is the essence of virtualization.

VITD

As you might expect, the VDD is quite complicated. In fact, it is much more complicated than the VCD, and, as such, the beginning VxD programmer would undoubtedly have a difficult time understanding how it does what it does. Therefore, the virtual interval timer device (VITD) has been written to demonstrate some of the principles of device virtualization. VITD is somewhat larger than the VDIALOG sample, but it is tiny in comparison with the VDD. However, despite its small size and relative simplicity, it provides full virtualization of a simulated hardware device. VITD is one of the samples provided with version 3.1 of the Windows DDK.

To understand what VITD does, imagine a totally new hardware device in a PC called an interval timer. This new device would have nothing to do with the real hardware timer that is used to keep time on each PC. Instead, the way this timer would work is that an application would program in an interval and start the timer. The timer would count down, return a single interrupt to the application, and stop. This works differently than the standard timer on the PC, which provides interrupts repeatedly at some programmed rate. Another difference between this imaginary timer and the real one is that the count specified is the number of milliseconds for the interval, instead of some number of "clock ticks." If all this sounds like a potentially useful device, which it would be, then it certainly is a shame that it is not part of the standard PC hardware.

The interval timer is not part of the PC hardware unless the VITD VxD is installed on an enhanced mode Windows system. The VITD provides a simulation of the imaginary interval timer for each virtual machine. Therefore, an application developed for MS-DOS or Windows can program the interval timer (using IN and OUT instructions) just as if it were a real hardware device, and it will produce an interrupt in that virtual machine after the desired interval.

It is important to note that the VITD simulates hardware that is not there; however, the same principles can be applied to providing virtualization of real hardware devices. Such a VxD would work similarly to VITD, but would also work with the real hardware and "massage" data going to or coming from the virtual state of the device in each VM. Again, a perfect example is the VDD, which must maintain and read from its virtual buffers so that the data can be painted on the screen.

Conclusion

The VxD API under enhanced mode Windows contains the functions necessary serialize and virtualize I/O devices. It is through this API that programmers can protect, change the behavior of, or even completely simulate, I/O hardware.

The VDIALOG and VITD VxDs are small, straightforward samples that are a good place to begin to learn about this aspect of Windows programming. VDIALOG and VITD are provided with the version 3.1 of the Windows DDK.