.NES File Format – Header

File layout

Some tools add a title of about 127 or 128 bytes at the very end of the file. Nothing in the header says so; a reader notices it only because the file is longer than the parts above.

Header

Flags 6

The trainer usually holds code that translates mapper register writes and caches CHR RAM. It was used by early RAM cartridges that could not imitate mapper chips, and by the DOS emulator NESticle. Unmodified dumps of cartridges do not include one.

Flags 7

Flags 8

The whole byte is the PRG RAM size in 8 KiB units, where 0 means 8 KiB (for files written before the field existed). It was added late and almost no files use it.

Flags 9

Part of the official header, but very few files set it, so few emulators honour it.

Flags 10 (unofficial)

Not part of the official header, and relatively few emulators read it.

Mapper number

The mapper number names the cartridge board, chiefly the chip that switches banks of ROM and RAM. It is (flags 7 AND 0xF0) OR (flags 6 >> 4), 0 to 255. Archaic headers had only the low four bits (0 to 15). The numbering was made popular by the iNES emulator; the NESdev Wiki documents each assigned number on its own page. The table below is the NESdev Wiki’s list of commonly met mappers:

Other numbers up to 255 are listed, without names, in the NESdev Wiki’s mapper grid; a full named register of them is not given here. Numbers above 255 exist only in NES 2.0 headers.

Header generations and DiskDude!

Because early versions of iNES ignored bytes 7 to 15, tools used to write their own text there. The commonest is DiskDude! from byte 7, whose D (0x44) read as flags 7 adds 64 to the mapper number. The NESdev rule: if the last four bytes are not all zero and the header is not marked as NES 2.0, an emulator should ignore the upper four mapper bits or refuse the file.

The generations: archaic iNES (Marat Fayzullin’s first header, also used by NESticle, mapper 0–15 only); iNES 0.7, which added the upper mapper nibble in byte 7, supported by emulators from about 2000; and iNES as described here, with bytes 8 and 9.

Worked example

4E 45 53 1A   "NES" + 0x1A
02            PRG ROM: 2 × 16 KiB = 32 KiB
01            CHR ROM: 1 × 8 KiB
01            flags 6: bit 0 set (vertical mirroring), no battery,
              no trainer, mapper low nibble 0
00            flags 7: not Vs., not PlayChoice, iNES, mapper high nibble 0
00 00 00      flags 8, 9, 10: unused (8 KiB PRG RAM implied, NTSC)
00 00 00 00 00   padding

For a battery-saving MMC1 game with 256 KiB of PRG ROM and CHR RAM, bytes 4 to 7 would be 10 00 12 00: 16 × 16 KiB of PRG, no CHR ROM, and flags 6 = 0x12, that is mapper low nibble 1 and the battery bit.

Sources

• iNES – NESdev Wiki: file layout, every header byte and flag bit, DiskDude!, header generations and the detection procedure
• List of mappers – NESdev Wiki: the commonly met mappers in the table
• Mapper – NESdev Wiki: the grid of all assigned mapper numbers

The worked examples are made up for this page.