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The first two bytes would contain the board status as bits.

[11eKQkqm] [rrrrffff]

where in the first byte the 2 high bits are set and each additional bit in the first byte would be set if a condition exists:

   e = en passant is possible (this bit is redundant because if en passant is possible the second byte <> $F0 (a "$" precedes hexadecimal digits).
  KQ = castling available for White Kingside or Queenside
  kq = castling available for Black Kingside or Queenside
   m = cleared if White's move (w), or set if Black's move (b)

  If en passant is possible the second byte indicates the square where en passant is possible.  It has it's upper nibble (4 bits) set to the rank (always 3, 6, or $F) and it's lower nibble set to the file #(1 - 8).
  If en passant is not possible this byte will equal $F0.

Now the piece information follows.  Moving across the board from the starting position for the Black Queen's Rook down to the starting position for the White King's Rook (moving across files first, then by rank) you would store the position of each piece.

This would be done with a single byte for each remaining piece.  The type of piece would be represented in the upper 4 bits (values 1 - 6 for white pieces {P,N,B,R,Q,K}, 10 - 16 for black pieces {p,n,b,r,q,k}).  The lower 4 bits would represent how many open spaces on the board until the next piece appears.  This would allow a range of 0 - 14 spaces to be represented.  If more than 14 are needed (and this would only be needed at the most 3 times on any given board), a value of 15 would be used and the byte that follows would instead contain the total amount of spacing needed, (1 - 61).
  To represent space before the first piece it would be coded with a piece type of zero.

So for example:
   3 spaces before the first piece would be represented by     $03 
   a black king followed by 60 spaces would be represented by  $AF $3C
   a white night followed by no spaces would be represented by $20.


The byte for the last piece always has the spacing information set to zero.



To show the whole board representation of a starting position with bytes within brackets (where piece types are represented by letters instead of numerals):

[%110KQkqw] [$F0]
[r0][n0][b0][q0][k0][b0][n0][r0][p0][p0][p0][p0][p0][p0][p0][p15][32][P0][P0][P0][P0][P0][P0][P0][P0][R0][N0][B0][Q0][K0][B0][N0][R0]

for a total of 35 bytes.  The representation would range between 6 - 37 bytes.  If you wanted to keep track of move counts you can use 4 more bytes to cover that range nicely and still be at or below  41 bytes (all status bytes would come first so that everything that remains would follow the piece placement format.

Here's a game in progress:
[1100000b] [$F0]
[r1][b2][r3][p1][n0][q1][b2][n0][p1][p1][k0][p0][p10][P0][N4][P2][B5][Q1][B0][P0][P0][P2][R0][R2][K0]

and the same game in hexadecimal bytes:
[C1][F0]
[D1][C2][D3][A1][B0][E1][C2][B0][A1][A1][F0][A0][AA][10][24][12][35][51][30][10][10][12][40][42][60]


and one involving en passant:
[11eKQkqb] [$34]
[r0][n0][b0][q0][k0][b0][n0][r0][p0][p0][p2][p0][p0][p3][p15][15][P0][p3][P0][P8][P1][P0][P0][P0][P0][R0][N0][B0][Q0][K0][B0][N0][R0]

and in hexadecimal bytes:
[FF][34]
[D0][B0][C0][E0][F0][C0][B0][D0][A0][A0][A2][A0][A0][A3][AE][E][10][A3][10][18][11][10][10][10][10][40][20][30][50][60][30][20][10]

With the way the values were chosen here each byte will have a non-zero value.  This means they can be read into and manipulated in strings (as buffers) if desired.