Bitboard

A bitboard represents a set of chessboard squares with a 64-bit number. A bit is a unit that can hold only 0 or 1. Because a chessboard has 64 squares, one bit can be assigned to every square.

Set of one piece class

White-pawn mask

8

7

6

5

4

3

2

a1

b

c

d

e

f

g

h

The active bits correspond to the White pawns on c3, e3, a2, and h2.

One bit per square

Fixed local convention: indicators are read by ranks from a8 through h8, then a7 through h7, continuing down to a1 and h1. A 1 marks membership in the set. Displayed set, White pawns: 00000000/00000000/00000000/00000000/00000000/00101000/10000001/00000000. The side to move and the other FEN data are not part of this mask.

1. The active bits correspond to the White pawns on c3, e3, a2, and h2.

A value of 1 does not always mean that a piece is present. It means that the square belongs to the set described by that bitboard. A bitboard might mark all White pawns, every square attacked by a knight, all occupied squares, or any other useful group.

Set of attacked squares

Knight-attack mask

8

7

6

5

4

3

2

a1

b

c

d

e

f

g

h

The eight geometric destinations of the knight on d4 form the set even though they are empty.

Ones without pieces

The same rank-order convention is retained. Displayed set, squares attacked by the White knight on d4: 00000000/00000000/00101000/01000100/00000000/01000100/00101000/00000000. The ones describe a geometric property even when no piece occupies those squares.

1. The eight geometric destinations of the knight on d4 form the set even though they are empty.

A chess engine usually maintains several bitboards at once. Bitwise operations, which combine the zeros and ones directly, can intersect sets, unite them, remove squares, or shift patterns with very few processor instructions. This supports fast move and attack generation.

Combining masks

Attack and occupancy intersection

8

7

6

5

4

3

2

a1

b

c

d

e

f

g

h

d5 and h1 belong to both A and B: the rook on d1 attacks them and a Black piece occupies them.

Result of A ∩ B

Using the declared rank-order convention, A is the set of squares attacked by the White rook on d1 and B is the set of squares occupied by Black pieces. The displayed set is the intersection of A and B: 00000000/00000000/00000000/00010000/00000000/00000000/00000000/00000001. The active bits correspond to d5 and h1. The Black king on h8 belongs to B but not to A.

1. d5 and h1 belong to both A and B: the rook on d1 attacks them and a Black piece occupies them.
2. The king on h8 belongs to the Black-piece set but lies outside the set attacked by the rook.

What one bitboard does not store by itself

• Every piece type, unless other bitboards provide that separation.
• Which side is to move.
• Castling rights, meaning whether the special king-and-rook move is still allowed, and en passant availability, meaning whether a pawn can make that special capture.

A complete position combines those facts with several sets. A bitboard is also not a search method such as alpha-beta pruning; it is a representation of information.