Editing VxD

{{Short description|Device driver model used in Microsoft Windows/386}}
{{Moreref|date=September 2014}}
{{Use dmy dates|date=July 2019|cs1-dates=y}}
'''VxD''' is the [[device driver]] model used in [[Windows/386|Microsoft Windows/386]] 2.x, the 386 enhanced mode of [[Windows 3.1x|Windows 3.x]], [[Windows 9x]], and to some extent also by the [[Novell DOS 7]], [[OpenDOS 7.01]], and [[DR-DOS 7.02]] (and higher) multitasker (<code>TASKMGR</code>).<ref name="Paul_1997_NWDOSTIP"/> VxDs have access to the [[memory]] of the [[Kernel (operating system)|kernel]] and all running [[Process_(computing)|processes]], as well as raw access to the hardware. Starting with [[Windows 98]], [[Windows Driver Model]] was the recommended driver model to write drivers for, with the VxD driver model still being supported for [[backward compatibility]], until [[Windows Me]].

== Name and design ==
The name "VxD" is an abbreviation for "virtual xxx driver", where "xxx" is some class of hardware device. It derives from the fact that most drivers had filenames of the form <code>vxxxd.386</code> in [[Windows 3.1x|Windows 3.x]]. Some examples are vjoyd.386 (joystick) and vmm.386 ([[Memory management|memory manager]]). VxDs under Windows 3.x usually have the [[filename extension]] <code>.386</code>, while those under Windows 9x have <code>.vxd</code>. Windows 9x VxDs are in [[Linear Executable]] format; its code can be 16-bit and 32-bit hybrid, or pure 32-bit.<ref>{{Cite web |title=Writing Windows Virtual Device Drivers |url=https://www-user.tu-chemnitz.de/~heha/vxd/vxd.htm |access-date=2025-05-02 |website=www-user.tu-chemnitz.de}}</ref> VxDs written for Windows 3.x can be used under Windows 9x but not vice versa.

== History ==
Prior to the advent of Windows, [[DOS]] applications would either communicate directly with the various pieces of hardware (responding to [[interrupt]]s, reading and writing [[device memory]] etc.) or go through a DOS [[device driver]]. As DOS was not multitasking, each application would have exclusive and complete control over the hardware while running. Though [[Microsoft Windows|Windows]] applications don't often communicate directly with hardware, it was the only way for Windows drivers; and still is in the [[real mode|real]] and standard modes of Windows 3.x.

[[Windows/386]] and onward allowed multiple DOS applications to execute concurrently by executing each within its own [[virtual DOS machine|virtual machine]]. To share physical resources among these virtual machines, Microsoft introduced virtual device drivers. These drivers solved issues relating to conflicting usage of physical resources by intercepting calls to the hardware. Instead of a [[Input/output#Port-mapped I/O|machine port]] representing an actual device, it would represent a "virtual" device, which could be managed by the operating system.

== Obsolescence ==
Although Windows 98 introduced the [[Windows Driver Model]] (WDM), VxD device drivers can be used under Windows 98 and Windows Me. Using VxD drivers instead of WDM drivers in Windows 9x may result in advanced [[Advanced Configuration and Power Interface|ACPI]] states like [[Hibernation (computing)|hibernation]] being unavailable.

VxDs are not usable in [[Windows NT]] or its descendants. Windows NT-based operating systems from [[Windows NT 3.1|3.1]] to [[Windows NT 4.0|4.0]] must use drivers written specifically for them. These drivers are otherwise known as the [[Windows NT#Major features|Windows NT Driver Model]]. Starting with [[Windows 2000]], Windows NT-based operating systems have adopted the Windows Driver Model from Windows 98.

VxDs should not be confused with the similarly named [[NTVDM]]-specific 'VDDs' (Virtual Device Drivers), which provide a method of emulating direct I/O under a Windows NT "[[cmd.exe|DOS Box]]".  NTVDM VDDs run as regular, [[32-bit]], [[user-mode]] [[dynamic-link library|DLLs]], and must rely on the [[Win32 API]] (or another WDM driver) to emulate the desired I/O on behalf of the [[16-bit]] program.

== See also ==
* [[VXD (CONFIG.SYS directive)]]
* [[Windows Driver Model]] (WDM)
* [[Architecture of Windows 9x]]

== References ==
{{reflist|refs=
<ref name="Paul_1997_NWDOSTIP">{{cite book |title=NWDOS-TIPs &mdash; Tips & Tricks rund um Novell DOS 7, mit Blick auf undokumentierte Details, Bugs und Workarounds |series=MPDOSTIP, Release 157 |author-first=Matthias R. |author-last=Paul |date=1997-07-30 |edition=3rd|language=de |url=http://www.antonis.de/dos/dos-tuts/mpdostip/html/nwdostip.htm |access-date=2014-09-06 |url-status=live |archive-url=https://web.archive.org/web/20170910194752/http://www.antonis.de/dos/dos-tuts/mpdostip/html/nwdostip.htm |archive-date=2017-09-10}} (NB. NWDOSTIP.TXT is a comprehensive work on [[Novell DOS 7]] and [[OpenDOS 7.01]], including the description of many undocumented features and internals. It is part of the author's yet larger MPDOSTIP.ZIP collection maintained up to 2001 and distributed on many sites at the time. The provided link points to a HTML-converted older version of the NWDOSTIP.TXT file.)</ref>
}}

== Further reading ==
* {{cite book |title=Unauthorized Windows 95 - Developer's Resource Kit |author-last=Schulman |author-first=Andrew |date=November 1994 |publisher=[[John Wiley & Sons, Inc.]] |edition=1st|location=Foster City, California, USA |isbn=1-56884-305-4 |url=https://archive.org/details/unauthorizedwind00schu }}
* {{cite book |author-first1=Andrew |author-last1=Schulman |author-first2=Ralf D. |author-last2=Brown |author-link2=Ralf D. Brown |author-first3=David |author-last3=Maxey |author-first4=Raymond J. |author-last4=Michels |author-first5=Jim |author-last5=Kyle |title=Undocumented DOS: A programmer's guide to reserved MS-DOS functions and data structures - expanded to include MS-DOS 6, Novell DOS and Windows 3.1 |publisher=[[Addison Wesley]] |edition=2nd|date=1994 |orig-year=November 1993<!-- first printing --> |isbn=0-201-63287-X |location=Reading, Massachusetts |url=https://archive.org/details/undocumenteddosp00andr_0 }} (xviii+856+vi pages, 3.5"-floppy) Errata: [https://web.archive.org/web/20190417215556/http://www.cs.cmu.edu/afs/cs/user/ralf/pub/books/UndocumentedDOS/errata.ud2][https://web.archive.org/web/20190417212906/https://www.pcjs.org/pubs/pc/programming/Undocumented_DOS/#errata-2nd-edition]
* {{cite book |title=Programmer's Guide to Microsoft Windows 95: Key Topics on Programming for Windows from the Microsoft Windows Development Team |series=Technical Reference |author=<!-- staff writers --> |publisher=[[Microsoft Press]] |edition=1st|date=1995-07-01 |isbn=1-55615-834-3 <!--|ISBN=978-1-55615-834-6 --> |location=Redmond, Washington, USA |url=https://books.google.com/books?id=MLZQAAAAMAAJ |access-date=2016-05-26}}
* {{cite book |title=Systems Programming for Windows 95 |author-first=Walter |author-last=Oney |publisher=[[Microsoft Press]] |edition=1st|date=1995 |isbn=1-55615-949-8 |location=Redmond, Washington, USA |url=https://archive.org/details/systemsprogrammi00oney }}
* {{cite book |title=Windows 95 System Programming Secrets |author-last=Pietrek |author-first=Matt |author-link=Matt Pietrek |date=November 1995 |publisher=[[John Wiley & Sons, Inc.]] |series=The Secrets Series |edition=1st|isbn=1-56884-318-6 |url=https://books.google.com/books?id=E9Q7KAAACAAJ |access-date=2016-05-26}}
* {{cite book |title=Writing Windows VxDs and Device Drivers - Programming Secrets for Virtual Device Drivers |author-first=Karen |author-last=Hazzah |publisher=[[Miller Freeman, Inc.]] |location=Lawrence, Kansas, USA |edition=2nd printing, 2nd |date=1997 |isbn=0-87930-438-3 }}

{{Microsoft APIs}}

[[Category:Microsoft application programming interfaces]]
[[Category:Device drivers]]
[[Category:Windows components]]