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The chess engine decides what moves to make, but typically does not interact directly with the user. Most chess engines do not have their own graphical user interface (GUI) but are rather console applications that communicate with a GUI such as XBoard, WinBoard or Gnome Chess via a standard protocol. This allows the user to play against multiple engines without learning a new user interface for each, and allows different engines to play against each other.
The command-line interface of GNU Chess became the initial de facto standard, called the Chess Engine Communication Protocol and was first supported by XBoard. When XBoard was ported to the Windows operating system as WinBoard, this protocol was popularly renamed to 'WinBoard Protocol'. The WinBoard Protocol was itself upgraded and the two versions of the protocols are referred to as: 'WinBoard Protocol 1' (original version) and 'WinBoard Protocol 2' (newer version). There is another protocol, the Universal Chess Interface. Some engines support both major protocols, and each protocol has its supporters. The Winboard Protocol is more popular but many chess engine developers feel that the Universal Chess Interface is easier to implement. Some interfaces support both protocols, while others, such as WinBoard, support only one and depend on subsidiary interpreters to translate.
Chess engines increase in playing strength each year. This is partly due to the increase in processing power that enables calculations to be made to ever greater depths in a given time. In addition, programming techniques have improved, enabling the engines to be more selective in the lines that they analyze and to acquire a better positional understanding. According to one survey,, the top engines have been increasing in strength by an average of 67 Elo per year since 1986.
A chess engine often uses a vast previously computed opening "book" to increase its playing strength for the first several moves up to possibly 20 moves or more in deeply analyzed lines.
Some chess engines maintain a database of chess positions, along with previously computed evaluations and best moves, in effect, a kind of "dictionary" of recurring chess positions. Since these positions are pre-computed, the engine merely plays one of the indicated moves in the database, thereby saving compute time, resulting in stronger more rapid play.
Some chess engines use endgame tablebases to increase their playing strength during the endgame. An endgame tablebase is a database of all possible endgame positions with small groups of material. Each position is conclusively determined as a win, loss, or draw for the player whose turn it is to move, and the number of moves to the end with best play by both sides. Endgame tablebases in all cases identify the absolute best move in all positions included (identifying the move that wins fastest against perfect defense, or the move that loses slowest against optimal opposition). Such tablebases are available for all positions containing three to six pieces (counting the kings) and for some seven-piece combinations. When the maneuvering in an ending to achieve an irreversible improvement takes more moves than the horizon of calculation of a chess engine, an engine is not guaranteed to find the best move without the use of an endgame tablebase, and in many cases can fall foul of the fifty-move rule as a result.
Many engines use permanent brain (speculative analysis while the opponent is thinking) as a method to increase their strength.
The results of computer tournaments give one view of the relative strengths of chess engines. However, tournaments do not play a statistically significant number of games for accurate strength determination. In fact, the number of games that need to be played between fairly evenly matched engines, in order to achieve significance, runs into the thousands and is, therefore, impractical within the framework of a tournament. Most tournaments also allow any types of hardware, so only engine/hardware combinations are being compared.
Historically, commercial programs have been the strongest engines. If an amateur engine wins a tournament or otherwise performs well (for example, Zappa in 2005), then it is quickly commercialized. Titles gained in these tournaments garner much prestige for the winning programs, and are thus used for marketing purposes.
- Dutch open computer chess championship
- Internet Computer Chess Tournament (CCT)
- International Paderborn Computer Chess Championship
- North American Computer Chess Championship
- Top Chess Engine Championship (TCEC)
- World Computer Chess Championship
Chess engine rating lists aim to provide statistically significant measures of relative engine strength. These lists play multiple games between engines on standard hardware platforms, so that processor differences are factored out. Some also standardize the opening books, in an attempt to measure the strength differences of the engines only. These lists not only provide a ranking, but also margins of error on the given ratings. Also rating lists typically play games continuously, publishing many updates per year, compared to tournaments which only take place annually.
There are a number of factors that vary among the chess engine rating lists:
- Time control. Longer time controls, such as 40 moves in 120 minutes, are better suited for determining tournament play strength, but also make testing more time-consuming.
- Hardware used. Faster hardware with more memory leads to stronger play.
- 64-bit (vs. 32-bit) hardware and operating systems favor bitboard-based programs
- Multiprocessor vs. single processor hardware.
- Ponder settings (speculative analysis while the opponent is thinking) aka Permanent Brain.
- Transposition table sizes.
- Opening book settings.
These differences affect the results, and make direct comparisons between rating lists difficult.
|Rating list||Time control
|Top three engines||Rating|
|2005||February 8, 2017||1791||667,877||Stockfish 8 x64
Houdini 5.0 x64
Komodo 10.3 x64
|2006||February 8, 2017||181||978,292||Stockfish 8.0 x64
Houdini 5.0 x64
Komodo 10.3 x64
|2006||February 8, 2017||184||468,770||Stockfish 8
- Note that the listings in the above table only count the best entry for a given engine.
These ratings, although calculated by using the Elo system (or similar rating methods), have no direct relation to FIDE Elo ratings or to other chess federation ratings of human players. Except for some man versus machine games which the SSDF had organized many years ago (which were far from today's level), there is no calibration between any of these rating lists and player pools. Hence, the results which matter are the ranks and the differences between the ratings, not the absolute level of the numbers. Also, each list calibrates their Elo via a different method. Therefore, no Elo comparisons can be made between the lists. Nevertheless, in view of recent man versus machine matches, it is generally undisputed that top computer chess engines should be rated at least in the range of top human performances, and probably significantly higher.
Missing from many rating lists are IPPOLIT and its derivatives (e.g. Fire). Although very strong and open source, there are allegations from commercial software interests that they were derived from disassembled binary of Rybka. Due to the controversy, all these engines have been blacklisted from many tournaments and rating lists. Rybka in turn was accused of being based on Fruit, and in June 2011, the ICGA formally claimed Rybka was derived from Fruit and Crafty and banned Rybka from the International Computer Games Association World Computer Chess Championship, and revoked its previous victories (2007, 2008, 2009, and 2010) The ICGA was criticized for this decision by Dr. Søren Riis, a longstanding supporter of the Rybka program. Rybka is still included on several society ranking lists.
Engines can be tested by measuring their performance on specific positions. Typical is the use of test suites, where for each given position there is one best move to find. These positions can be geared towards positional, tactical or endgame play. The Nolot test suite, for instance, focuses on deep sacrifices. Then there are the BT2450 and BT2630 test suites by Hubert Bednorz and Fred Toennissen. These suites measure the tactical capability of the engine and have been used at least by REBEL. There is also a general test suite called Brilliancy by Dana Turnmire. The suite has been compiled mostly from How to Reassess Your Chess Workbook.
Strategic Test Suite (STS) by Swaminathan and Dann Corbit, tests chess engine's strategical strength.
The most difficult modern test suite is Nightmare II by Walter Eigenmann (2016). This collection contains 30 extremely complicated puzzles, compiled especially for chess engines.
These chess playing systems include custom hardware or run on supercomputers.
- Bebe, a strong bit-slice processor in the 1980s
- Chess (Northwestern University)
- Cray Blitz
- Deep Blue
- Deep Thought
- Hydra, predecessor was called Brutus
- Jonny, won the 2015 WCCC running on a "btrzx3" linux cluster of the University of Bayreuth using 2,400 AMD x86-64 2.8 GHz cores in total.
Commercial dedicated computers
In the 1980s and early 1990s, there was a competitive market for strong dedicated chess computers. Many form-factors were sold, from handheld peg-board computers to wooden auto-sensory boards with state-of-the-art processors. This market changed in the mid-90s when the economical embedded processors in dedicated chess computers could no longer compete with the fast processors in personal computers. Nowadays, most dedicated units sold are of beginner and intermediate strength.
- Chess Challenger, a line of chess computers sold by Fidelity Electronics from 1977 to 1992. These models won the first four World Microcomputer Chess Championships.
- ChessMachine, an ARM-based dedicated computer, which could run two engines:
- Excalibur Electronics sells a line of beginner strength units. Excalibur was started in 1992 by the son of the founder of Fidelity Electronics.
- Mephisto, a line of chess computers sold by Hegener & Glaser. The units programmed by Richard Lang won six consecutive World Microcomputer Chess Championships. They bought out Fidelity in 1989.
- Novag sold a line of tactically strong computers, including the Constellation, Sapphire, and Star Diamond brands.
- Phoenix Chess Systems makes limited edition units based around StrongARM and XScale processors running modern engines and emulating classic engines.
- Saitek sells mid-range units of intermediate strength. They bought out Hegener & Glaser and its Mephisto brand in 1994.
These chess programs run on obsolete hardware.
- 1K ZX Chess
- Colossus Chess
- Mac Hack
- Microchess was the first commercial game for a personal computer, developed first for the KIM-1 and later Commodore PET, Apple II, TRS-80 and others. Bobby Fischer played against MicroChess.
- Socrates II
There are hundreds of free and/or open source chess engines which conform to one of the above communication protocols. The top 50 strongest, freely available engines, according to the CCRL 40/40 rating list, are listed here.
- Also available: 40 moves in 4 minutes
- Also available: 40 moves in 4 minutes, 40 moves in 120 minutes
- blog.chess.com Creating a chess engine from scratch (Part 1: Basics), Link date 28 June 2012
- "mizarchessengine.com". Retrieved 25 September 2016.
- "CCRL 40/40 - Index". November 5, 2016. Retrieved November 6, 2016.
- "CEGT 40/20". Chess Engines Grand Tournament. February 7, 2016. Retrieved February 9, 2016.
- "IPON". IPON. November 16, 2016. Retrieved February 3, 2016.
- Chess engine controversy at chessvibes.com, retrieved 28/May/2010
- Rybka disqualified and banned from World Computer Chess Championships | ChessVibes
- Riis, Dr. Søren (January 2, 2012). "A Gross Miscarriage of Justice in Computer Chess (part one)". Chessbase News. Retrieved 19 February 2012.
- Nolot test suite
- BT2450 test suite
- Rosenboom, Manfred. "Rebel Century FAQ: 3. Using Rebel". Retrieved 25 September 2016.
- Brilliancy suite TalkChess forum
-  Strategic Test Suite
-  Nightmare II
- Sousa, Ismenio. "Fidelity Chess Challenger 1 - World's First Chess Computer". Retrieved 25 September 2016.
- "Microchess". Retrieved 25 September 2016.
- Elo ratings taken from "CCRL 40/40, full list". Computer Chess Rating List. January 29, 2012. Retrieved February 9, 2012.
- Chess Engine's Polyglot Opening Book for WinBoard GUI - A general (learning) purpose Chess Engine's Polyglot Opening Book for WinBoard GUI.