2 bits per square
00 empty
01 white pawn
10 black pawn
11 other piece
=128 bits
4 bit index of white king on board (one of the others)
4 bit index of black king
for each "other piece" on board that is not a king: 1 bit color + 2 bit type
1 bit for current player
2 bits for each king if they moved (optional, only exists if king is at thair initial square. Otherwise it is moved)
1 bit for each pawn if they can be captured en passant (optional, these bits only exists for their pawns if there is a pawn at 4th row for white or 5th row for black)
4 bit index of black king
for each "other piece" on board that is not a king: 1 bit color + 2 bit type
1 bit for current player
2 bits for each king if they moved (optional, only exists if king is at thair initial square. Otherwise it is moved)
1 bit for each pawn if they can be captured en passant (optional, these bits only exists for their pawns if there is a pawn at 4th row for white or 5th row for black)
(And i will ignore the draw rule)