The .NES file format (file name suffix .nes) is the de facto standard for distribution of NES binary programs, with use even in licensed emulators such as commercialized PocketNES and Wii Virtual Console. It is often called the iNES format, as it was created by Marat Fayzullin for an emulator called iNES. The format was later extended with NES 2.0 to fix many of its shortcomings.
iNES was an early NES emulator developed by Marat Fayzullin. Its most lasting contribution to the NES scene was its popularization of the iNES ROM file format and mapper numbering system.
This file format is commonly referred to as the iNES file format/iNES header format. The file extension is .nes, so it is sometimes referred to as the .nes file format, and files in it as .nes files. Now that the NES 2.0 file format exists, which uses the same .nes extension, a .nes file/the .nes file format could mean the iNES file format or NES 2.0 format, so the full format names should be used where the differences in the formats are relevant, like specifications or format support.
An iNES file consists of the following sections, in order:
Some ROM-Images additionally contain a 128-byte (or sometimes 127-byte) title at the end of the file.
The format of the header is as follows:
76543210 |||||||| |||||||+- Mirroring: 0: horizontal (vertical arrangement) (CIRAM A10 = PPU A11) ||||||| 1: vertical (horizontal arrangement) (CIRAM A10 = PPU A10) ||||||+-- 1: Cartridge contains battery-backed PRG RAM ($6000-7FFF) or other persistent memory |||||+--- 1: 512-byte trainer at $7000-$71FF (stored before PRG data) ||||+---- 1: Ignore mirroring control or above mirroring bit; instead provide four-screen VRAM ++++----- Lower nybble of mapper number
In the iNES format, cartridge boards are divided into classes called "mappers" based on similar board hardware and behavior, and each mapper has an 8-bit number (or 12-bit in NES 2.0). The low 4-bits of this mapper are given here in bits 4-7 of this field.
The presence of persistent saved memory is given by bit 1. This usually takes the form of battery-backed PRG-RAM at $6000, but there are some mapper-specific exceptions:
See: Nametable Mirroring
For mappers with hard-wired mirroring, connecting CIRAM A10 to PPU A10 or A11 for a vertical or horizontal arrangement is specified by bit 0.
Some mappers have a 4-screen variation of the board, which is specified with bit 3:
A few mappers override normal usage of the nametable mirroring bits:
Ambiguity:
The trainer usually contains mapper register translation and CHR-RAM caching code for
It is not used on unmodified dumps of original ROM cartridges.
76543210 |||||||| |||||||+- VS Unisystem ||||||+-- PlayChoice-10 (8KB of Hint Screen data stored after CHR data) ||||++--- If equal to 2, flags 8-15 are in NES 2.0 format ++++----- Upper nybble of mapper number
The PlayChoice-10 bit is not part of the official specification, and most emulators simply ignore the extra 8KB of data. PlayChoice games are designed to look good with the 2C03 RGB PPU, which handles color emphasis differently from a standard NES PPU.
Vs. games have a coin slot and different palettes. The detection of which palette a particular game uses is left unspecified.
NES 2.0 is a more recent extension to the format that allows more flexibility in ROM and RAM size, among other things.
76543210 |||||||| ++++++++- PRG RAM size
Size of PRG RAM in 8 KB units (Value 0 infers 8 KB for compatibility; see PRG RAM circuit)
This was a later extension to the iNES format and not widely used. NES 2.0 is recommended for specifying PRG RAM size instead.
76543210 |||||||| |||||||+- TV system (0: NTSC; 1: PAL) +++++++-- Reserved, set to zero
Though in the official specification, very few emulators honor this bit as virtually no ROM images in circulation make use of it.
76543210 || || || ++- TV system (0: NTSC; 2: PAL; 1/3: dual compatible) |+----- PRG RAM ($6000-$7FFF) (0: present; 1: not present) +------ 0: Board has no bus conflicts; 1: Board has bus conflicts
This byte is not part of the official specification, and relatively few emulators honor it.
The PRG RAM Size value (stored in byte 8) was recently added to the official specification; as such, virtually no ROM images in circulation make use of it.
Older versions of the iNES emulator ignored bytes 7-15, and several ROM management tools wrote messages in there. Commonly, these will be filled with "DiskDude!", which results in 64 being added to the mapper number.
A general rule of thumb: if the last 4 bytes are not all zero, and the header is not marked for NES 2.0 format, an emulator should either mask off the upper 4 bits of the mapper number or simply refuse to load the ROM.
Over the years, the header of the .NES file format has changed as new features became needed. There are three discernable generations:
| Thing | Archaic iNES | iNES | NES 2.0 |
|---|---|---|---|
| Byte 7 | Unused | Mapper high nibble, Vs. | Mapper high nibble, NES 2.0 signature, PlayChoice, Vs. |
| Byte 8 | Unused | Total PRG RAM size (linear) | Mapper highest nibble, mapper variant |
| Byte 9 | Unused | TV system | Upper bits of ROM size |
| Byte 10 | Unused | Unused | PRG RAM size (logarithmic; battery and non-battery) |
| Byte 11 | Unused | Unused | VRAM size (logarithmic; battery and non-battery) |
| Byte 12 | Unused | Unused | TV system |
| Byte 13 | Unused | Unused | Vs. PPU variant |
| Byte 14 | Unused | Unused | Miscellaneous ROMs |
| Byte 15 | Unused | Unused | Default expansion device |
| Mappers supported | 0-15 | 0-255 | 0-4095 |
Recommended detection procedure:
Categories: File formats