An undocumented feature is an unintended or undocumented hardware operation, for example an

undocumented instruction An illegal opcode, also called an unimplemented operation, unintended opcode or undocumented instruction, is an opcode, instruction to a CPU that is not mentioned in any official documentation released by the CPU's designer or manufacturer, wh ...

, or

software feature In software, the term feature has several definitions. The Institute of Electrical and Electronics Engineers defines the term ''feature'' in IEEE 829 as " distinguishing characteristic of a software item (e.g., performance, portability, or functio ...

found in computer hardware and

software Software is a set of computer programs and associated software documentation, documentation and data (computing), data. This is in contrast to Computer hardware, hardware, from which the system is built and which actually performs the work. ...

that is considered beneficial or useful. Sometimes the documentation is omitted through oversight, but undocumented features are sometimes not intended for use by

end users In product development, an end user (sometimes end-user) is a person who ultimately uses or is intended to ultimately use a product. The end user stands in contrast to users who support or maintain the product, such as sysops, system administrato ...

, but left available for use by the vendor for software support and development. Also, some unintended operation of hardware or software that ends up being of utility to users is simply a bug, flaw or quirk. Since the suppliers of the software usually consider the

software documentation Software documentation is written text or illustration that accompanies computer software or is embedded in the source code. The documentation either explains how the software operates or how to use it, and may mean different things to people in ...

to constitute a contract for the behavior of the software, undocumented features are generally left unsupported and may be removed or changed at will and without notice to the users. Some user-reported defects are viewed by software developers as working as expected, leading to the catchphrase "it's not a bug, it's a feature" (INABIAF) and its variations.

Hardware

Undocumented instructions, known as illegal opcodes, on the

MOS Technology 6510 300px, Image of the internals of a Commodore 64 showing the 6510 CPU (40-pin DIP, lower left). The chip on the right is the 6581 SID. The production week/year (WWYY) of each chip is given below its name. The MOS Technology 6510 is an 8-bit mic ...

of the Commodore 64 and

MOS Technology 6502 The MOS Technology 6502 (typically pronounced "sixty-five-oh-two" or "six-five-oh-two") William Mensch and the moderator both pronounce the 6502 microprocessor as ''"sixty-five-oh-two"''. is an 8-bit microprocessor that was designed by a small te ...

of the Apple II computers are sometimes utilized. Similarly programmers (notably game and

demo Demo, usually short for demonstration, may refer to: Music and film *Demo (music), a song typically recorded for reference rather than release * ''Demo'' (Behind Crimson Eyes), a 2004 recording by the band Behind Crimson Eyes * ''Demo'' (Deafhea ...

) on the

Commodore Amiga Amiga is a family of personal computers introduced by Commodore in 1985. The original model is one of a number of mid-1980s computers with 16- or 32-bit processors, 256 KB or more of RAM, mouse-based GUIs, and significantly improved graphi ...

take advantage of the unintended operation of its coprocessors to produce new effects or optimizations. In 2019 researchers discovered that a manufacturer debugging mode, known as VISA, had an undocumented feature on

Intel Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the developers of the x86 seri ...

Platform Controller Hubs, known as chipsets and which are included on most Intel-based motherboards, which made the mode accessible with a normal motherboard. Since the chipset has

direct memory access Direct memory access (DMA) is a feature of computer systems and allows certain hardware subsystems to access main system memory independently of the central processing unit (CPU). Without DMA, when the CPU is using programmed input/output, it is ...

this is problematic, mostly for security reasons.

Software

Undocumented features (for example, the ability to change the

switch In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...

character in

MS-DOS MS-DOS ( ; acronym for Microsoft Disk Operating System, also known as Microsoft DOS) is an operating system for x86-based personal computers mostly developed by Microsoft. Collectively, MS-DOS, its rebranding as IBM PC DOS, and a few ope ...

, usually to a

hyphen The hyphen is a punctuation mark used to join words and to separate syllables of a single word. The use of hyphens is called hyphenation. ''Son-in-law'' is an example of a hyphenated word. The hyphen is sometimes confused with dashes ( figure ...

) can be included for compatibility purposes (in this case with

Unix Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, an ...

utilities) or for future-expansion reasons. However; if the software provider changes their software strategy to better align with the business, the absence of documentation makes it easier to justify the feature's removal. New versions of software might omit mention of old (possibly superseded) features in documentation but keep them implemented for users who've grown accustomed to them. In some cases, software bugs are referred to by developers either jokingly or conveniently as undocumented features. This usage may have been popularised in some of Microsoft's responses to bug reports for its first

Word for Windows Microsoft Word is a word processor, word processing software developed by Microsoft. It was first released on October 25, 1983, under the name ''Multi-Tool Word'' for Xenix systems. Subsequent versions were later written for several other pla ...

product, but doesn't originate there. The oldest surviving reference on

Usenet Usenet () is a worldwide distributed discussion system available on computers. It was developed from the general-purpose Unix-to-Unix Copy (UUCP) dial-up network architecture. Tom Truscott and Jim Ellis conceived the idea in 1979, and it wa ...

dates to 5 March 1984.First Use on Usenet
Google Groups Between 1969 and 1972, Sandy Mathes, a systems programmer for

PDP-8 The PDP-8 is a 12-bit minicomputer that was produced by Digital Equipment Corporation (DEC). It was the first commercially successful minicomputer, with over 50,000 units being sold over the model's lifetime. Its basic design follows the pioneer ...

software at

Digital Equipment Corporation Digital Equipment Corporation (DEC ), using the trademark Digital, was a major American company in the computer industry from the 1960s to the 1990s. The company was co-founded by Ken Olsen and Harlan Anderson in 1957. Olsen was president un ...

(DEC) in Maynard, MA, used the terms "bug" and "feature" in her reporting of test results to distinguish between undocumented actions of delivered software products that were ''unacceptable'' and ''tolerable'', respectively. This usage may have been perpetuated.Memories of Sandy Mathes, now Sandra Lee Harris,

MIT The Massachusetts Institute of Technology (MIT) is a private land-grant research university in Cambridge, Massachusetts. Established in 1861, MIT has played a key role in the development of modern technology and science, and is one of the m ...

Class of 1969, as of August 2010. Undocumented features themselves have become a major feature of computer games. Developers often include various

cheats Cheating generally describes various actions designed to subvert rules in order to obtain unfair advantages. This includes acts of bribery, cronyism and nepotism in any situation where individuals are given preference using inappropriate crit ...

and other special features ("

easter eggs Easter eggs, also called Paschal eggs, are eggs that are decorated for the Christian feast of Easter, which celebrates the resurrection of Jesus. As such, Easter eggs are common during the season of Eastertide (Easter season). The oldest tra ...

") that are not explained in the packaged material, but have become part of the "buzz" about the game on the

Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a '' network of networks'' that consists of private, pub ...

and among gamers. The undocumented features of foreign games are often elements that were not localized from their native language.

Closed source Proprietary software is software that is deemed within the free and open-source software to be non-free because its creator, publisher, or other rightsholder or rightsholder partner exercises a legal monopoly afforded by modern copyright and inte ...

APIs can also have undocumented functions that are not generally known. These are sometimes used to gain a commercial advantage over third-party software by providing additional information or better performance to the application provider.

References

{{DEFAULTSORT:Undocumented Feature Software anomalies Technical communication

Hardware

Software

See also

References