NES-AOROM

Cartridge Board

What Is It?

A discrete logic Nintendo-made board.
Max 256K of PRG ROM, a 74161, and an 8K VRAM.
Note: this board has bus conflicts.

The Tech:

The LS161 acts as a 4 bit latch. It latches D0,D1,D2, and D4. Writing anywhere in the 8000h- FFFFh range will set this latch using the above data lines.

7  bit  0
---- ----
xxxM xBBB

The B bits select a 32K PRG ROM page at 8000-FFFFh.

The M bit is sorta odd.  It selects which nametable is to be displayed.
0 selects the first nametable (2000-23FFh)
1 selects the second nametable (2400-27FFh)

Note: Since the M bit controls the mirroring line, the cartridge does not have
an H or V mirroring and instead uses "1 screen" mirroring where all 4 quadrants
display the same nametable. (The nametable is chosen via the M bit).