This is an idea I had for a custom mapper that would raise the number of background tiles from 256 to 1024. The idea is that you divide each 16x16 block into 4

This is an idea I had for a custom mapper that would raise the number of background tiles from 256 to 1024. The idea is that you divide each 16x16 block into 4 different graphics banks. Top-Left tile gets its own back, Top-Right tile gets its own bank, Bottom-Left tile gets its own bank, Bottom-Right tile gets its own bank. Wouldn't require very much hardware at all, just 32K of SRAM for the CHR-RAM, and a few discrete logic chips. NES repeats this sequence of reads: * Read Nametable byte * Read Attribute Table byte * Read background tile graphics low byte * Read background tile graphics high byte Then when it's time for sprites: * Read garbage Nametable byte * Read garbage Nametable byte * Read sprite Tile graphics (sprite) low byte * Read sprite Tile graphics (sprite) high byte - The critical part of this pattern is that it's always an Attribute table byte before background tile graphics, and always a Nametable byte before sprite tile graphics. Nametables are at 2000-23BF and repeating every 400 bytes later (appears four times). Attribute tables are at 23C0-23FF and repeating every 400 bytes later (appears four times) Distinguishing the two requires not much more than ANDing a bunch of address lines together. So now, when you see a Nametable read: - Save the low bit of column and low bit of row, use those to make a value 0-3. - Bankswitch CHR-RAM to bank 4 for sprites Then when you see an Attribute table read: - Bankswitch CHR-RAM to your value 0-3 for the background tile read