Mirroring

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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 pariticular axis (horizontal and/or vertical), the coordinates simply wrap around on the current nametable. When mirroring is disabled, a second nametable is used. There are four common combinations of mirroring:

Horizontal

A A
B B

A vertical arrangement of the nametables results in horizontal mirroring. This is most commonly used for games which only scroll vertically. It can also be used by games which scroll both horizontally (by any amount), vertically (by slightly less than one screen), and make use of a status bar at the top or bottom of the screen (e.g. Super Mario Bros. 3). Finally, many games that scrolls to both directions at the same time (by any amount) without the use of a status bar prefer use this mirroring. 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 resulution of 2x2 tiles, there will always be attribute glitches on the left and/or the right side of the screen, so that is why so many NES games have color glitches on the border of the screen. Some NTSC televisions may overscan up to 8 pixels on both left and right border, but most doesn't. Perfectionists programmers could use solid black sprites on the right border to hide attribute glitches and make the screen look symetrical, however very few games does this due to the fact it reduce the number of sprites per scanline to 7, and wastes a lot of OAM space.

On cartridge boards made by Nintendo, this is selected by shorting the "V" solder pad (for "vertical arrangement").

Vertical

A B
A B

A horizontal arrangement of the nametables results in vertical mirroring. This is most commonly used for games which only scroll horizontally. Games that scroll vertically (by any amount and without status bar) and that does never scroll horizontally by more than one screen would use this mirroring (e.g. as Fire Emblem), so that they don't have to load anything when scrolling horizontally. It is also used for games which scroll in both directions (by any amount and without status bar), but only one direction at the exact same time (e.g. Dragon Warrior). Doing so will look nice on most NTSC televisions (thank to overscan), but glitches will commonly appear on PAL television on the top and the bottom of the screen. Because the data on the top of the screen becomes immediately on the bottom due to mirroring, it is then hard on PAL monitors to scroll the screen smoothly vertically using horizontal mirroring.

On cartridge boards made by Nintendo, this is selected by shorting the "H" solder pad (for "horizontal arrangement").

Single-Screen

A A
A A
B B
B B

Single-screen mirroring is only available with certain mappers, such as the AxROM, SxROM, and TLSROM boards. 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. 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 if the monitor is NTSC or PAL.

4-screen VRAM

A B
C D

With an additional 2 KB of RAM present on the cartridge, 4 unique nametables can be addressed, allowing for more flexible screen layouts.

Very few games use this method, though.

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 wich mirroring to use. A ! is there to say horizontal glitches will be unavoidable when scrolling (see above). A * indicates vertical glitches on PAL screens and on NTSC that doesn't overscan.

Scolling Type With Status Bar Without Status Bar Comment
Horizontal Only Vertical Vertical
Vertical Only Vertical/Single screen Horizontal
Horizontal/Vertical separate screens Vertical Alternate H/V For use if scrolling direction changes only on a screen-based pattern, as in Metroid.
Bidirectional, field limited vertically Horizontal ! Horizontal ! 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 * No data has to be loaded at all when scrolling horizontally, but the area is limited to two screens.
Bidirectional (slow) Horizontal ! Vertical * If present, status bar will have to change address when scrolling vertically.
Bidirectional (fast) Single screen ! Horizontal !

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. Because completely decoding an address often takes slightly more hardware, 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 16 KiB 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.