Character Memory format of Famicom roms spritesheets.
The chr file format contains a series of bits equivalent to pixels for each tile in the spreadsheet of a rom. A chr tile is 8x8 pixels, the data for each tile is made up of 128 bits, where the first 64 bits are the first color, the next 64 bits the second color, and where the colors overlap result in the third color, for a total of 4 colors including the background.
0000 1e3f 3f3f 3f1e 78fc fcfc fc78 0000
For a simple editor and ANSI C implementation, see Nasu.