.NES

Introduction

iNES is the original header for .nes files, named after the iNES emulator that introduced it in the 1990s. It stores a Nintendo Entertainment System cartridge’s program ROM and graphics ROM behind a 16-byte header that tells the emulator how the cartridge was wired, above all which mapper chip it used to switch banks of ROM in and out. Most of the NES ROM images in circulation use it.

Structure

The file starts with the 16-byte header. The first four bytes are 4E 45 53 1A (NES and an MS-DOS end-of-file character). Byte 4 gives the size of the program ROM in 16 KB units and byte 5 the size of the character (graphics) ROM in 8 KB units, where 0 means the cartridge uses RAM for graphics. Bytes 6 and 7 hold flags: the screen mirroring, whether there is battery-backed save RAM, whether a 512-byte trainer follows the header, and the mapper number split across the two high nibbles. Byte 8 can give the size of work RAM and byte 9 the TV system, but few files fill them in, and the rest of the header is zero.

Telling it apart

The file starts with 4E 45 53 1A and bits 2 and 3 of byte 7 are 00. When they are 10 in binary, the file is NES 2.0.

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