, Feng-hsiung Hsu
IBM T.J. Watson Research Center, Yorktown Heights, NY 10598, USA
Sandbridge Technologies, 1 N. Lexington Avenue, White Plains, NY 10601, USA
Compaq Computer Corporation, Western Research Laboratory, 250 University Avenue,
Deep Blue is the chess machine that defeated then-reigning World Chess Champion Garry
Kasparov in a six-game match in 1997. There were a number of factors that contributed to this
• a single-chip chess search engine,
• a massively parallel system with multiple levels of parallelism,
• a strong emphasis on search extensions,
• a complex evaluation function, and
• effective use of a Grandmaster game database.
This paper describes the Deep Blue system, and gives some of the rationale that went into the
design decisions behind Deep Blue.
2001 Elsevier Science B.V. All rights reserved.
This paper describes the Deep Blue
computer chess system, developed at IBM
Research during the mid-1990s. Deep Blue is the culmination of a multi-year effort to
build a world-class chess machine. There was a series of machines that led up to Deep
Blue, which we describe below. In fact there are two distinct versions of Deep Blue, one
which lost to Garry Kasparov in 1996 and the one which defeated him in 1997. This paper
will refer primarily to the 1997 version, and comparisons with the 1996 version, which we
Corresponding author. The ordering of the authors is alphabetic.
Feng-hsiung.Hsu@compaq.com (F.-h. Hsu).
0004-3702/01/$ – see front matter
will call Deep Blue I, will be made where appropriate. For clarity, we will sometimes refer
to the 1997 version as Deep Blue II. A brief summary of the chess machines described
here can be found in Appendix B. A fuller history of the Deep Blue project can be found
1.1. ChipTest and Deep Thought
Earlier efforts in building a chess machine, ChipTest and Deep Thought, took place
at Carnegie Mellon University in the 1980s. In 1988 Deep Thought was the ﬁrst chess
machine to beat a Grandmaster in tournament play. These systems used a single-chip chess
move generator  to achieve search speeds in the neighborhood of 500,000 positions
per second (ChipTest) to 700,000 positions per second (Deep Thought). Deep Thought is
described in detail in [16,17].
1.2. Deep Thought 2
In 1989–90, part of the Deep Thought team (Anantharaman, Campbell, Hsu) moved to
the IBM T.J. Watson Research Center to continue the effort to build a world-class chess
machine. In late 1990, Joe Hoane replaced Thomas Anantharaman in the group. Deep
Thought 2, aka Deep Blue prototype, was the ﬁrst result of this effort. Although the primary
purpose of the system was as an intermediate stepping stone to Deep Blue, Deep Thought
2 played in a number of public events from 1991 through 1995.
Although Deep Thought 2 used the same move-generator chip as Deep Thought, it had
1. Medium-scale multiprocessing. Deep Thought 2 initially had 24 chess engines,
although over time that number decreased as processors failed and were not replaced.
This compares with Deep Thought, which usually used two processors (although
there were four- and six-processor versions that made a few appearances).
2. Enhanced evaluation hardware. The Deep Thought 2 evaluation hardware used larger
RAMs and was able to include a few additional features in the evaluation function.
Nonetheless, the evaluation function was relatively simple. For example, the Deep
Thought 2 hardware was not able to recognize “bishops of opposite color”, a feature
that chess players know greatly increases the chance for a drawn endgame. In order to
address this (and other similar problems), the Deep Thought 2 system implemented a
software “band-aid” mechanism. Unfortunately this slowed down the overall system
speed and created numerous search anomalies at the hardware/software boundary. In
spite of these drawbacks, the missing evaluation function features were sufﬁciently
important to use this evaluation patch.
3. Improved search software. The search software was rewritten entirely for Deep
Thought 2, and was designed to deal better with the parallel search, as well as
implement a number of new search extension ideas. This code would later form the
initial basis for the Deep Blue search software.
4. Extended book . The extended book (see Section 8.2) allowed Deep Thought 2 to
make reasonable opening moves even in the absence of an opening book. This feature
was also inherited by Deep Blue.
The major competitive successes of Deep Thought 2 included victories in the 1991 and
team in 1993.
Deep Blue I was based on a single-chip chess search engine, designed over a period of
three years. The ﬁrst chips were received in September of 1995. A number of problems
were found with these chips, and a revised version was received in January of 1996.
Deep Blue I ran on a 36-node IBM RS/6000
computer, and used 216 chess chips.
The chips each searched about 1.6–2 million chess positions per second. Overall search
speed was 50–100 million chess positions per second.
The full 36-node Deep Blue I played only six games under tournament conditions, all
in the February 1996 match with Garry Kasparov. This match was won by Kasparov by
a fairly decisive 4–2 score, although the match was tied at 2–2 after the ﬁrst four games.
Three additional tournament-condition matches were played in preparation for the 1996
Kasparov match, all using a single-node version of Deep Blue with 24 chess chips. This
system, aka Deep Blue Jr., beat Grandmaster Ilya Gurevich 1.5–0.5, drew Grandmaster
Patrick Wolff 1–1, and lost to Grandmaster Joel Benjamin 0–2.
1.4. Deep Blue II
After the 1996 match with Kasparov, it was clear that there were a number of deﬁciencies
in the play of Deep Blue I. A series of changes were made in preparation for the rematch
which took place in May of 1997. First, a new, signiﬁcantly enhanced, chess chip was
designed. The new chess chip had a completely redesigned evaluation function, going
from around 6400 features to over 8000. A number of the new features were in response to
speciﬁc problems observed in the 1996 Kasparov games, as well as in test games against
Grandmaster Joel Benjamin. The new chip also added hardware repetition detection, a
number of specialized move generation modes (e.g., generate all moves that attack the
opponent’s pieces: see Section 3.1), and some efﬁciency improvements that increased the
per chip search speed to 2–2.5 million positions per second. The second major change was
to more than double the number of chess chips in the system, and use the newer generation
of SP computer to support the higher processing demands thereby created. A third change
was the development of a set of software tools to aid in debugging and match preparation,
e.g., evaluation tuning and visualization tools. Finally, we concluded that the searching
ability of Deep Blue was acceptable, and we spent the vast majority of our time between
the two matches designing, testing, and tuning the new evaluation function.
Deep Blue defeated Garry Kasparov in the 1997 match by a score of 3.5–2.5. For
this victory, the Deep Blue team was awarded the Fredkin prize for defeating the human
world champion in a regulation match. There were two additional matches played by Deep
Blue Jr. in preparation for the Kasparov match. The two matches, against Grandmasters
This last match went a long way to convincing us that Joel Benjamin would be an excellent Grandmaster