In computing, the Windows Driver Model (WDM) also known at one point as the Win32 Driver Model is a framework for device drivers that was introduced with Windows 98 and Windows 2000 to replace VxD, which was used on older versions of Windows such as Windows 95 and Windows 3.1, as well as the Windows NT Driver Model. Overview WDM drivers are layered in a stack and communicate with each other via I/O request packets (IRPs). The Microsoft Windows Driver Model unified driver models for the Windows 9x and Windows NT product lines by standardizing requirements and reducing the amount of code that needed to be written. WDM drivers will not run on operating systems earlier than Windows 98 or Windows 2000, such as Windows 95 (before the OSR2 update that sideloads the WDM model), Windows NT 4.0 and Windows 3.1. By conforming to WDM, drivers can be binary compatible and source-compatible across Windows 98, Windows 98 Second Edition, Windows Me, Windows 2000, Windows XP, Windows Server 2003 and Windows Vista (for backwards compatibility) on x86-based computers. WDM drivers are designed to be forward-compatible so that a WDM driver can run on a version of Windows newer than what the driver was initially written for, but doing that would mean that the driver cannot take advantage of any new features introduced with the new version. WDM is generally not backward-compatible, that is, a WDM driver is not guaranteed to run on any older version of Windows. For example, Windows XP can use a driver written for Windows 2000 but will not make use of any of the new WDM features that were introduced in Windows XP. However, a driver written for Windows XP may or may not load on Windows 2000. WDM exists in the intermediary layer of Windows 2000 kernel-mode drivers and was introduced to increase the functionality and ease of writing drivers for Windows. Although WDM was mainly designed to be binary and source compatible between Windows 98 and Windows 2000, this may not always be desired and so specific drivers can be developed for either operating system. Device kernel-mode drivers With the Windows Drivers Model (WDM) for devices Microsoft implements an approach to kernel mode drivers that is unique to Windows operating systems. WDM implements a layered architecture for device drivers, and every device of a computer is served by a stack of drivers. However, every driver