bitboard

A bitboard is a technique used in board game programming where the state of a board is encoded into binary words. In chess, a standard bitboard uses 64 bits; each bit corresponds to a square on an 8x8 board, typically mapped from a1 to h8. Engines store multiple bitboards, one for each piece type and color (for example, white pawns, black knights), and also occupancy bitboards such as all white pieces, all black pieces, and all pieces overall. A square is marked in a bitboard if that square is occupied by the corresponding piece.

Operations on bitboards rely on bitwise operators. Bitwise AND, OR, XOR and shifts manipulate bitboards to perform move generation and evaluation. For example, shifting a pawn bitboard can represent forward moves, while masks enforce move legality. Attack sets for knights and kings are derived from fixed shift patterns; sliding pieces use occupancy masks and, in optimized forms, magic bitboards to compute ray attacks efficiently.

Advantages of bitboards include the ability to process many squares in parallel within a single CPU word, leading to fast move generation, evaluation, and pruning. They are memory-efficient and align well with modern 64-bit processors, enabling highly optimized engines.

Extensions and variants include using multiple 64-bit bitboards for boards larger than 64 squares, and employing techniques such as magic bitboards and precomputed lookup tables to accelerate sliding moves. Bitboards are also used in other games and hybrid representations, but remain most prominent in high-performance chess engines.