How does PPU mirroring work?
Well, there is mirroring across the PPU, in terms of NAME tables. There is only enough physical RAM in the PPU to store 2 different name tables at one time; however, there is still the address space for 4 different name tables. Thus, two of the name tables in the PPU are copies of the other two. Which table is a copy of which is determined by the mirroring selection lines in an NES cartridge. On games with no memory mappers, the mirroring is fixed, depending on which of two contact points a blob of solder is sitting. However, most memory mappers also provide for PPU mirroring switching on-the-fly. Some game cartridges also have extra RAM on-board, which provides for a total of 4 individual name tables to be accessed. There is also, of course, the capability for single-screen mirroring, which means that there is only 1 active name table in the PPU, and the other 3 are copies of it. The two major kinds of name table mirroring are horizontal and vertical. With horizontal mirroring, the tw
