EPD operation

EPD means Extended Position Description. An EPD line begins with four fields describing the pieces, side to move, castling rights, which show whether each side still retains the special move involving its king and a rook, and the en passant target square. It can then include EPD operations that attach instructions or additional information to that position.

Each operation contains an opcode, a short name identifying its meaning, followed by zero or more operands, the operation's values. It always ends with a semicolon. In id "test-001";, id is the opcode and the quoted string is the operand. Multiple operations are separated by one space.

Standardized opcodes cover tasks such as identifying the position, proposing best moves, marking moves to avoid, storing a predicted variation, or adding comments. Software that does not recognize an opcode should ignore that operation rather than reject the entire line. This extensibility lets the same basic position description serve test suites, analysis, and communication between programs.