Mirroring

There are two types of mirroring that appear commonly within the scope of NES emulation.

Nametable Mirroring

Nametable mirroring affects what is shown past the right and bottom edges of the current nametable. When mirroring is enabled for a particular axis (horizontal and/or vertical), the coordinates simply wrap around on the current nametable. A background "mirrored" in this way is repeated, not flipped. When mirroring is disabled, a second nametable is used. There are four common combinations of mirroring:

Horizontal

A vertical arrangement of the nametables results in horizontal mirroring, which makes a 32x60 tilemap.

This is most commonly used for games which only scroll vertically or in all directions.

Doing any horizontal scrolling using horizontal mirroring is hard to do smoothly because the data on the right of the screen is immediately show on the left due to mirroring. Clever use of hardware left-side screen clipping will hide all name table glitches, but because the attribute tables have a resolution of 2x2 tiles, there will always be attribute glitches on the left and/or the right side of the screen. The best possible way to hide it is to have 4 pixels with potentially wrong attributes on both sides, but most commercial games did worse than that having usually 8 or even more glitchy pixels, so that is why so many NES games have color glitches on the border of the screen.

Some televisions overscan up to 8 pixels on both left and right border, but most doesn't. Perfectionist programmers could use solid black sprites on the right border to hide attribute glitches and make the screen look symmetrical and hide absolutely all attribute glitches, as in the game Alfred Chicken, but very few games do this because it reduces the number of sprites per scanline to 7 and wastes a lot of OAM space (roughly 1/4 in 8x16 pixel sprite mode).

To configure a cartridge board for horizontal mirroring, connect PPU A11 to CIRAM A10. On cartridge boards made by Nintendo, this is selected by shorting the "V" solder pad (for "vertical arrangement"). Note using inverse logic may make more sense, "Leaving the 'H' jumper open gives horizontal mirroring" on Nintendo boards.

Vertical

A horizontal arrangement of the nametables results in vertical mirroring, which makes a 64x30 tilemap.

This is most commonly used for games which only scroll horizontally. Games that scroll vertically (by any amount and without status bar) and that never scroll horizontally by more than one screen would use this mirroring (e.g. Lode Runner, Bomberman, Fire Emblem, Crystal Mines), so that they don't have to load anything when scrolling horizontally.

Of course it is also used for games which scroll in both directions without a status bar. Because data that is on the top/bottom of the screen will immediately show up on the other side, a clever use of NTSC overscan can make it glitch-less multidirectional scrolling, but glitches will appear on PAL televisions (and NTSC televisions with a overscan range which is a little off). The best possible way to hide glitches is to make 4 pixels with wrong tiles and 4 additional pixels with wrong color on both sides, but most commercial games did much worse than this, that's why they look so bad if overscan is disabled.

Perfectionist programmers could use raster split to hide glitches (and possibly also provide more blanking time to update VRAM) as in the games Jurassic Park and M.C. Kids, but it was rarely done because it complicates the code a lot for little benefits.

To configure a cartridge board for vertical mirroring, connect PPU A10 to CIRAM A10. On cartridge boards made by Nintendo, this is selected by shorting the "H" solder pad (for "horizontal arrangement"). Note using inverse logic may make more sense, "Leaving the 'V' jumper open gives vertical mirroring" on Nintendo boards.

Single-Screen

Single-screen mirroring is only available with certain mappers, such as the AxROM, SxROM, and TLSROM boards, resulting in two 32x30 tilemaps.

Its main advantage is that it allows using a status bar at the top or bottom of the screen while also allowing the playfield to extend equally in any direction - this can be done by storing the status bar in one nametable, rendering the playfield in the other nametable, and switching mirroring (and scrolling parameters) at the appropriate screen location during rendering.

There is also a lot of other things that can be drastically simplified when using 1-screen mirroring: The formulas used to calculate PPU address of data to be updated to the screen are also significantly simpler, and if the status bar have a variable size or is scrolling, all this would be a headache without 1-screen mirroring.

When this mirroring is used to scroll horizontally, similar glitches and scrolling problems that those of horizontal mirroring will happen. However, as long as there is a status bar, no glitches will happen vertically since the data that falls off the bottom (or the top) of the screen will come in the area that is "hidden" by the status bar, regardless of overscan factors.

There are several different ways to configure a cartridge board for single-screen mirroring. Some boards have single screen mirroring which is mapper controlled like the MMC1 and AxROM. For AxROM connect the output of a register (e.g. 74HC161) to CIRAM A10 (AxROM latches PRG D4). A simpler way to create one screen mirroring that's fixed (non-changable by software) is to simply connect CIRAM A10 to Gnd or Vcc. The board that implements iNES Mapper 218 wires CIRAM A10 to PPU A10, A11, A12, or A13, so that the game can store tiles in CIRAM without having any CHR ROM or CHR RAM in the cartridge.

4-Screen

With additional RAM and/or PPU address mapping present on the cartridge, 4 unique nametables can be addressed through the PPU bus, creating a 64x60 tilemap, allowing for more flexible screen layouts. Very few games made use of this type of mirroring.

Example games using 4-screen RAM nametables:

• Rad Racer II
• Gauntlet
• Napoleon Senki
• Rocman X (Sachen)

Example games using 4-screen with ROM nametables:

• Final Lap
• King of Kings

Mappers capable of 4-screen mirroring:

• MMC3 allows 4-screen RAM in some configurations (Rad Racer II, Rocman X).
• iNES 206 implements a subset of MMC3 features, and can use 4-screen RAM (Gauntlet).
• iNES 77 maps RAM across the PPU memory space, combining with internal VRAM to provide 4 RAM nametables, and a combination of CHR-RAM and ROM for pattern tables (Napoleon Senki).
• Vs. System had twice as much VRAM as the NES, giving a permanent 4-screen setup. This is most visible as iNES 99, but several other mappers were also used on this hardware.
• VRC6 allows 1k CHR-ROM pages to be arbitrarily mapped into the 4 nametable screens.
• Namco 163 allows 1k CHR-ROM pages to be arbitrarily mapped into the 4 nametable screens (Final Lap, King of Kings).
• The JY Company mapper allows 1k CHR-ROM pages to be arbitrarily mapped into the 4 nametable screens.
• MMC5 can use 1k of extra RAM to create a 3rd nametable, while procedurally generating a blank data page for a 4th, allowing (just barely) 4 different screens to be mapped at once.
• The iNES format can specify 4-screen mirroring in the header, allowing 4-screen RAM nametables to be applied to any mappers that don't structurally conflict with this (if supported by the emulator).

There are several ways to implement extra nametable RAM on a cartridge board:

• Add an extra 2 KB of RAM on the board. Decoder logic enables CIRAM only for $2000-$27FF and the cartridge RAM $2800-$2FFF.
• Add a 6264 8 KB RAM on the board. CIRAM /CE is pulled high, and the cartridge RAM is enabled at $2000-$3FFF. The PPU itself never uses $3000-$3FFF during rendering, but 8 KB RAMs are easier to find than 4 KB RAMs.
• Add a larger RAM on the board and map it to the entire PPU address space. This allows 8 KB of pattern tables at $0000-$1FFF, 4 KB of nametables at $2000-$2FFF, and almost 4 KB of extra memory at $3000-$3EFF.

Other

• 

  Diagonal

• 

  L-shaped

• 

  3-screen vertical

• 

  3-screen horizontal

• 

  3-screen diagonal

Other uncommon types of mirroring are available in other boards, such as TxSROM variations of the MMC3, extended techniques available to the MMC5, arbitrary VRAM mirroring arrangements by the Namco 163, or ROM mirroring arrangements using mappers that allow ROM nametables.

Diagonal mirroring (CIRAM A10 = PA11 XOR PA10) would facilitate changes in scrolling direction without having to flip between Horizontal and Vertical mirroring.

L-shaped mirroring (CIRAM A10 = PA11 OR PA10), seen in mapper 243 and the Sachen 8259 family, allows scrolling in four directions as long as scrolling changes directions only at screen boundaries.

Unusual cases:

• Castlevania 3 uses the third nametable RAM available on the MMC5
• Laser Invasion uses the third nametable RAM available on the MMC5
• The Sunsoft-4 mapper used by After Burner allows ROM nametables, but only in 2-Screen mirroring configurations.
• Mighty Morphin Power Rangers III, IV (JY Company) uses ROM nametables in a 3-Screen horizontal configuration, though the lower screen is never shown.

Mirroring chart

This table helps to choose which mirroring to use, it is of course not an ultimate chart or anything, just a guide if someone is writing a game and don't know which mirroring to use.

Scrolling Type	With Status Bar	With Status Bar and Parallax Scrolling Effect	With Parallax Scrolling Effect	Without Status Bar and Parallax Scrolling Effect	Game example	Comment
1-screen fixed Only	-	Vertical, Horizontal2	Vertical, Horizontal2	-	Donkey Kong
Horizontal Only	Vertical	Vertical, Horizontal2	Vertical, Horizontal2	Vertical	Super Mario Bros.
Vertical Only	Vertical, Horizontal, Single Screen, 3-screen V/H3	Vertical, Horizontal2, Single Screen2, 3-screen V/D3	Horizontal2, Vertical1, Diagonal, 3-screen V/D3, 4-screen4	Horizontal	Gun.Smoke, Recca	Horizontal mirroring with a status bar works best with as status bar will have to change address when scrolling vertically. If you are using a parallax scrolling effect on the entire screen with a status bar, such as the Recca is best to use a Single Screen, Horizontal, or the Vertical mirroring with a scanline counter IRQ.
Horizontal Only, field limited horizontally	Vertical	Vertical	Vertical	Vertical	Lode Runner	No data has to be loaded at all when scrolling horizontally, but the area is limited to two screens. This screen type, some of parallax scrolling effect does not work.
Vertical Only, field limited vertically	Horizontal	Horizontal	Horizontal	Horizontal	Wrecking Crew	No data has to be loaded at all when scrolling vertically, but the area is limited to two screens (less the status bar if present).
Horizontal/Vertical aligned screens	Vertical, Horizontal2, Diagonal, 3-screen V/D3, 4-screen4	Vertical, Horizontal2, Single Screen2, 3-screen V/D3, 4-screen4	Vertical, Horizontal2, Diagonal, 3-screen V/D3, 4-screen4	Alternate H/V, L-shaped, Diagonal, 3-screen D3, 4-screen4	Metroid, Mega Man series, Air Fortress, Guardian Legend	For use if scrolling direction changes only on a screen-based pattern, as in Metroid and Mega Man 3, 4, 5, 6. Horizontal mirroring with a status bar works best with as status bar will have to change address when scrolling vertically (Also work as Diagonal mirroring) or playfield will have to be written twice to memory. (Panic Restaurant) If you are using a parallax scrolling effect on the entire screen with a status bar, such as the Recca is best to use a Single Screen, Horizontal, or the Vertical mirroring with a scanline counter IRQ.
Horizontal/Vertical/Bidirectional aligned screens	Vertical <-> Single Screen2, Horizontal2, Vertical, 4-screen4	Vertical <-> Single Screen2, Single Screen2, Horizontal2, Vertical, 4-screen4	Alternate V1/H2, Diagonal, 4-screen4	Alternate V1/H2, Diagonal, 4-screen4	Akumajou Special, Rockman 4 Minus Infinity	Horizontal or 4-screen mirroring with a status bar works best with as status bar will have to change address when scrolling vertically or playfield will have to be written twice to memory. (Teenage Mutant Ninja Turtles III: The Manhattan Project) Vertical mirroring Only with a status bar works best with a scanline counter IRQ. (Teenage Mutant Ninja Turtles II: The Arcade Game) If you are using a parallax scrolling effect on the entire screen with a status bar, such as the Recca is best to use a Single Screen, Horizontal, or the Vertical mirroring with a scanline counter IRQ.
Bidirectional, field limited vertically	Horizontal2	Horizontal2	Horizontal2	Horizontal2	Super Mario Bros. 3	No data has to be loaded at all when scrolling vertically, but the area is limited to two screens (less the status bar if present).
Bidirectional, field limited horizontally	Vertical	Vertical	Vertical1	Vertical1	Fire Emblem	No data has to be loaded at all when scrolling horizontally, but the area is limited to two screens. Vertical mirroring with a status bar works best with a scanline counter IRQ. This screen type, some of parallax scrolling effect does not work.
Bidirectional, field limited 4-screen	4-screen4	4-screen4	4-screen4	4-screen4	Gauntlet	No data has to be loaded at all when scrolling 4-screen, but the area is limited to four screens (less the status bar if present).
Free bidirectional	Single Screen2, Horizontal2, Vertical, 4-screen4	Single Screen2, Horizontal2, Vertical, 4-screen4	Vertical1, Horizontal2, Diagonal, 4-screen4	Vertical1, Horizontal2, Diagonal, 4-screen4	Kirby's Adventure	If using horizontal or 4-screen mirroring with a status bar there are 2 possibilities: The status bar will have to change address when scrolling vertically (Double Dragon series, Conquest of Crystal Palace, Gradius II) The playfield will have to be written twice to memory (Kirby's Adventure, Little Nemo: The Dream Master), which only works if the status's bar size remains constant. Vertical mirroring with a status bar works best with a scanline counter IRQ. (Jungle Book, Mickey's Safari in Letterland)
Depth screen	Vertical, Horizontal, 4-screen4	Vertical, Horizontal, 4-screen4	Vertical, Horizontal, 4-screen4	Vertical, Horizontal, 4-screen4	Mach Rider, Rad Racer	Bidirectional Pseudo 3D depth perspective works best with a CHR bank switching ROM nametables. (After Burner)

• ¹: Vertical glitches on PAL screens and on NTSC that doesn't overscan.
• ²: Horizontal glitches will be unavoidable when scrolling (see above).
• ³: 3-screen mirroring, you can use only specific mapper of MMC5.
• ⁴: 4-screen mirroring, and can be used only when you added an additional RAM to the cartridge board.

Memory Mirroring

Memory mirroring refers to the appearance of memory or I/O registers at more than one range of addresses, with the same byte being accessible at more than one address. This occurs when the full address isn't completely decoded, that is, when a chip ignores one or more address lines. Because completely decoding an address usually takes a lot more pins on a chip, incomplete decoding is used to reduce the hardware required; if the mirror occupies otherwise unused address space, it poses no problems.

Within the NES, many things are mirrored:

• System memory at $0000-$07FF is mirrored at $0800-$0FFF, $1000-$17FF, and $1800-$1FFF - attempting to access memory at, for example, $0173 is the same as accessing memory at $0973, $1173, or $1973.
• PPU I/O registers at $2000-$2007 are mirrored at $2008-$200F, $2010-$2017, $2018-$201F, and so forth, all the way up to $3FF8-$3FFF.
• The single registers on most simple mappers are mirrored throughout $8000-$FFFF.
• Registers on many common ASIC mappers (such as the MMC1 and MMC3) are also mirrored, in groups, throughout $8000-$FFFF.
• Nametable mirroring, as described above, happens due to memory mirroring within PPU $2000-$2FFF (hence its name). However, in this case the memory mirroring is intentional and necessary.
• In NROM-128, the 16k PRG ROM is mirrored into both $8000-$BFFF and $C000-$FFFF.
• In most mappers, banks past the end of PRG or CHR ROM show up as mirrors of earlier banks. For example, UNROM PRG banks 8-15 are duplicates of banks 0-7 respectively.

See also

• PPU scrolling