[Smalltalker-80]

Why not do it simpler? : Create an array with 16 elements, one element per piece, black + white. Every array element is 7 bits wide, 1 bit for captured or not, and 6 bits for the square number the piece is on (8 x 8). Then you need 16 * 7 = 112 bits = 14 bytes. (And the captured-bit can even be compressed further as a 65th square, but that makes it more calculation intensive to extract a position)

[Scarblac]

Each side has 16 pieces, so you need 32 elements.

[Smalltalker-80]

Ah how silly of me, that woud make it 28 bytes. (I had the nagging feeling I was missing something :-) And promotions are also not covered by this...

[veidelis]

+ 3 bits for piece type?

[addaon]

You only need the piece type for pawns (that can be upgraded), and a bit on the king to track if castling is possible; otherwise a single bit for on-board/captured is sufficient, since the types of the other pieces are implicit in the array index. (You can shave single bits in a few places -- if the state represents a game in progress the king-captured bit isn't needed; natural bishops only need 5 bits for position on board, etc. This doesn't really add up though.)

On the other hand, there are 32 pieces (max) on a chess board, not 16, so grandparent is off by a factor of more than two.