FAT_ENT_OFFSET_SECTOR
ra = FAT_ENT_OFFSET_SECTOR(sb, chain);
end = FAT_ENT_OFFSET_SECTOR(sb, chain + len - 1);
sec = FAT_ENT_OFFSET_SECTOR(sb, chain);
sec = FAT_ENT_OFFSET_SECTOR(sb, loc);
sec = FAT_ENT_OFFSET_SECTOR(sb, loc);