VxD

VxD

VxD, or Virtual Device Driver, is a type of device driver used in Microsoft Windows operating systems, particularly in the Windows 3.x and Windows 9x series. VxDs operate at the kernel level and are responsible for managing hardware devices and providing an interface between the hardware and the operating system.

Overview

VxDs are designed to run in the virtual 8086 mode of the Intel 386 and later processors, allowing them to manage hardware resources and provide services to both the operating system and applications. They are loaded into memory at system startup and remain resident, handling interrupts and other low-level operations.

VxDs are written in assembly language or C and are compiled into a binary format with the .VXD file extension. They are capable of directly accessing hardware and can perform tasks such as:

• Managing input/output operations
• Handling interrupts
• Providing virtual memory management
• Facilitating communication between hardware and software

History

The concept of VxDs was introduced with Windows/386, an early version of Windows that provided multitasking capabilities on top of MS-DOS. With the release of Windows 3.0 and subsequent versions, VxDs became a crucial component of the Windows architecture, enabling the operating system to support a wide range of hardware devices.

In Windows 95, VxDs continued to play a significant role, although the introduction of the Windows Driver Model (WDM) in Windows 98 and later versions began to phase out VxDs in favor of more modern driver architectures.

Technical Details

VxDs operate in ring 0, the most privileged level of the x86 processor architecture, allowing them to execute any instruction and access any memory location. This level of access is necessary for their role in managing hardware resources and providing critical system services.

VxDs communicate with the operating system and applications through a set of predefined interfaces and APIs. They can intercept and handle interrupts, perform direct memory access (DMA), and manage hardware registers.

Advantages and Disadvantages

Advantages

• Performance: VxDs can provide high-performance access to hardware due to their low-level operation and direct access to system resources.
• Flexibility: They can be used to implement complex device management and control logic that is not possible with higher-level drivers.

Disadvantages

• Stability: Because VxDs operate at the kernel level, a bug or error in a VxD can lead to system crashes or instability.
• Compatibility: VxDs are specific to the Windows 9x architecture and are not compatible with newer versions of Windows, which use different driver models.

Transition to Modern Driver Models

With the advent of Windows 2000 and Windows XP, Microsoft introduced the Windows Driver Model (WDM), which provided a more robust and flexible framework for developing device drivers. WDM drivers are designed to be compatible with both Windows 98 and later versions of Windows, facilitating a smoother transition for hardware manufacturers and developers.

The introduction of the Windows Driver Foundation (WDF) further modernized driver development, providing a more structured and reliable environment for creating drivers for Windows Vista and later versions.

Also see

• Device driver
• Windows Driver Model
• Windows Driver Foundation
• Kernel mode
• Virtual 8086 mode