Hello,

In Sargon I listing, there is :

;************************************************* ***********
; BOARD -- Board Array. Used to hold the current position
; of the board during play. The board itself
; looks like:
; FFFFFFFFFFFFFFFFFFFF
; FFFFFFFFFFFFFFFFFFFF
; FF0402030506030204FF
; FF0101010101010101FF
; FF0000000000000000FF
; FF0000000000000000FF
; FF0000000000000060FF
; FF0000000000000000FF
; FF8181818181818181FF
; FF8482838586838284FF
; FFFFFFFFFFFFFFFFFFFF
; FFFFFFFFFFFFFFFFFFFF
; The values of FF form the border of the
; board, and are used to indicate when a piece
; moves off the board. The individual bits of
; the other bytes in the board array are as
; follows:
; Bit 7 -- Color of the piece
; 1 -- Black
; 0 -- White
; Bit 6 -- Not used
; Bit 5 -- Not used
; Bit 4 --Castle flag for Kings only
; Bit 3 -- Piece has moved flag
; Bits 2-0 Piece type
; 1 -- Pawn
; 2 -- Knight
; 3 -- Bishop
; 4 -- Rook
; 5 -- Queen
; 6 -- King
; 7 -- Not used
; 0 -- Empty Square
;************************************************* ********

And another flag to reset this flag when unmove the ...first move ;-)
;************************************************* *********
; MOVE LIST SECTION
;
; MLIST -- A 2048 byte storage area for generated moves.
; This area must be large enough to hold all
; the moves for a single leg of the move tree.
;
; MLEND -- The address of the last available location
; in the move list.
;
; MLPTR -- The Move List is a linked list of individual
; moves each of which is 6 bytes in length. The
; move list pointer(MLPTR) is the link field
; within a move.
;
; MLFRP -- The field in the move entry which gives the
; board position from which the piece is moving.
;
; MLTOP -- The field in the move entry which gives the
; board position to which the piece is moving.
;
; MLFLG -- A field in the move entry which contains flag
; information. The meaning of each bit is as
; follows:
; Bit 7 -- The color of any captured piece
; 0 -- White
; 1 -- Black
; Bit 6 -- Double move flag (set for castling and
; en passant pawn captures)
; Bit 5 -- Pawn Promotion flag; set when pawn
; promotes.
; Bit 4 -- When set, this flag indicates that
; this is the first move for the
; piece on the move.
; Bit 3 -- This flag is set is there is a piece
; captured, and that piece has moved at
; least once.
; Bits 2-0 Describe the captured piece. A
; zero value indicates no capture.
;
; MLVAL -- The field in the move entry which contains the
; score assigned to the move.
;
;************************************************* *********


Cordially.