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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 glChess 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.
Interface protocol 
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 interface support both protocols whereas others, such as WinBoard, support only one and depend on subsidiary interpreters to translate.
Increasing strength 
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 analyse and to acquire a better positional understanding.
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 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. To some extent, this is a self-fulfilling prophecy; 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
- 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. All listed engines are 64-bit.
|Rating list||Time control
|Top three engines||Rating|
|2005||November 10, 2012||1241||422,452||Houdini 3 x64 4CPU
Critter 1.6a x64 4CPU
Rybka 4 x64 4CPU
|2006||March 24, 2013||1112||656,607||Houdini 3 x64 4CPU
Critter 1.6 x64 4CPU
Equinox 1.70 x64 4CPU
|2006||October 17, 2012||110||213,250||Houdini 3.0 STD x64 1CPU
Komodo 5 x64 1CPU
Critter 1.4a x64 1CPU
|2009||January 17, 2012||67||161,160||Houdini 2.0c x64 1CPU
Critter 1.4 x64 1CPU
Rybka 4 Exp. 42 x64 1CPU
|1984||March 23, 2013||311||123,376||Deep Rybka 4 x64 2GB Q6600 2.4 GHz
Deep Hiarcs 14 2GB Q6600 2.4 GHz
Naum 4.2 MP x64 2GB Q6600 2.4 GHz
|2001||September 9, 2012||226
|100,749||Rybka 4 x64 2CPU
Thinker 5.5.4A1 x64 2CPU
Stockfish 2.2.2 x64 2CPU
- 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.
Test suites 
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.
Dedicated hardware 
These chess playing systems include custom hardware or run on supercomputers. All are historical; chess supercomputers have not competed in computer tournaments since Hydra played in 2006.
- Bebe, a strong bit-slice processor in the 1980s
- Chess (Northwestern University)
- Cray Blitz
- Deep Blue
- Deep Thought
- Hydra, predecessor was called Brutus
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 sells 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.
See also 
Notes and references 
- blog.chess.com Creating a chess engine from scratch (Part 1: Basics), Link date 28 June 2012
- "CCRL 40/40 - Complete list". November 10, 2012. Retrieved November 19, 2012.
- Also available: 40 moves in 4 minutes
- "CEGT 40/20". Chess Engines Grand Tournament. March 24, 2013. Retrieved March 30, 2013.
- Also available: 40 moves in 4 minutes, 40 moves in 120 minutes
- "IPON". IPON. August 29, 2012. Retrieved August 29, 2012.
- "SWCR". SWCR. January 17, 2012. Retrieved May 21, 2012.
- "The SSDF Rating List". Swedish Chess Computer Association. March 23, 2013. Retrieved March 30, 2013.
- "BayesianElo Ratinglist of WBEC Ridderkerk". September 9, 2012. Retrieved March 30, 2013.
- Chess engine controversy at chessvibes.com, retrieved 28/May/2010
- Riis, Dr. Søren (02.01.2012). "A Gross Miscarriage of Justice in Computer Chess (part one)". Chessbase News. Retrieved 19 February 2012.
- Nolot test suite
- BT2450 test suite
- Brilliancy suite TalkChess forum
-  Strategic Test Suite
- 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.