Virtual Device Drivers (VxDs)

VxDs were once the backbone of device communication in older versions of Windows, but they still play a role in specific virtualized environments and legacy systems.

You would be surprised at how many embedded devices run Old Windows or even DOS…

This article mainly is for interest in how these worked in the old days :)

What Are Virtual Device Drivers (VxDs)?

VxDs are special types of drivers that operate in a virtualized environment, enabling multiple applications to share access to system hardware without direct control.

Unlike traditional hardware drivers, VxDs don’t always interact with physical hardware—instead, they create a virtualized version of it.

These drivers were widely used in Windows 3.x, 95, and 98 before the transition to the Windows Driver Model (WDM).

How Virtual Device Drivers Work

VxDs act as intermediaries between applications and hardware.

Instead of allowing direct access to hardware components, they provide a controlled interface that multiple applications can use.

Application Requests Access – An application tries to communicate with hardware.
VxD Intercepts Request – Instead of directly accessing hardware, the request goes through the VxD.
VxD Manages Resources – The VxD determines whether to fulfill the request itself or pass it to the real hardware.
Virtualized Hardware Interaction – If needed, the VxD simulates hardware responses for the application.

Why Were VxDs Important?

Back in the golden age of Windows 95, VxDs were crucial for managing:

Virtualized Memory Access – Preventing applications from directly interfering with system memory.
Hardware Emulation – Allowing multiple applications to share devices like sound cards and network adapters.
System Stability – Reducing direct hardware access meant fewer crashes caused by rogue applications.
Performance Optimization – By handling requests more efficiently, VxDs improved multitasking.

Writing a Virtual Device Driver

Even though modern Windows versions use WDM instead of VxDs, writing a virtual device driver can still be useful for specialized applications in virtual machines and embedded systems.

Tools You’ll Need:

Windows Driver Kit (WDK) – The modern toolset for writing drivers, including virtualized ones.
Visual Studio – The preferred environment for writing and compiling drivers.
Emulated Windows Environments – If you’re working with legacy software, virtual machines running Windows 95/98 can help test VxDs.

A Simple Virtual Device Driver Example

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#include <windows.h>
#include <vmm.h>

BOOL VXDInit()
{
    _asm { int 20h }
    return TRUE;
}

void VXDExit()
{
    _asm { int 20h }
}

A Basic “Hello World” VxD

The following code demonstrates a minimal VxD driver that prints “Hello, World” when loaded.

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; Hello World VxD
.386P
include vmm.inc  ; Include Virtual Machine Manager definitions

Begin_VxD HELLO_WORLD

VxD_DATA_SEG
    Msg db "Hello from VxD!", 0
VxD_DATA_ENDS

VxD_CODE_SEG

VxD_ENTRY MyVxDEntry
    mov esi, offset Msg   ; Load message address
    call Debug_Printf     ; Print to debug console
    clc                   ; Clear carry flag (success)
    ret                   ; Return to OS
VxD_ENTRY_END MyVxDEntry

End_VxD HELLO_WORLD
VxD_CODE_ENDS
end

Explanation:

VxD_ENTRY MyVxDEntry – This is the entry point of the driver.
Debug_Printf – Prints a message to the Windows debug console.
clc / ret – Indicates successful execution and returns control.

This is an oversimplified VxD structure for older Windows versions.

Installing and Using Virtual Device Drivers

Although Windows no longer supports traditional VxDs, virtualization platforms and legacy systems still make use of similar concepts.

So if you have an old internet enabled toaster.. Running Embedded Windows 95… then you can install your test driver on that…

Installing VxDs on Older Systems:

Modify SYSTEM.INI – Add a line under [386Enh] to include your VxD.
Use VM Additions – In a virtual machine, you may need to install legacy driver support.
Manually Load VxDs – Some applications require manually copying .vxd files to the Windows directory.

Modern Virtualized Drivers:

For modern Windows versions, User-Mode Virtual Drivers (UMDF) or Kernel-Mode Virtual Drivers provide similar functionality without using legacy VxDs.

When to Use Virtual Device Drivers Today

While traditional VxDs are largely obsolete, their concepts live on in various forms:

Virtualization Platforms – Hyper-V, VMware, and VirtualBox use similar mechanisms to manage guest OS interactions.
Emulated Hardware – Some legacy applications require virtual device drivers to run properly.
Gaming and DOS Emulation – Older games and software that need direct hardware access sometimes rely on virtualized drivers.
Security Sandboxing – Certain security applications use virtualized drivers to isolate programs from the system kernel.

The Downsides of Virtual Device Drivers

1. Compatibility Issues

Since VxDs were mostly used in older Windows versions, modern Windows doesn’t natively support them anymore. Applications relying on VxDs often require virtualization or emulation.

2. Performance Bottlenecks

Virtualized drivers can introduce latency, making them unsuitable for real-time applications.

3. Security Risks

Because VxDs operate at a deep system level, they can be exploited if improperly designed, especially in unprotected virtual environments.

Conclusion

Virtual Device Drivers (VxDs) were an essential part of Windows computing in the 90s, helping applications share hardware resources without stepping on each other’s toes.

While modern Windows no longer supports them, the principles behind VxDs still apply in virtualization, security, and embedded systems.