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

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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