.NES

Introduction

NES 2.0 is a backwards-compatible extension of the iNES header, made by the NES emulation community. The original header cannot describe many cartridges exactly: it has only 256 mapper numbers, no way to tell apart boards that share a mapper number, and no reliable field for RAM sizes or the console region. NES 2.0 uses the header’s spare bytes to record all of these, so an emulator does not have to guess them from a database.

Structure

Bytes 0 to 7 are laid out as in iNES: the magic 4E 45 53 1A, the ROM sizes, and the flags with the low eight bits of the mapper number. Bits 0 and 1 of byte 7 give the console type, such as a plain NES or a Vs. System arcade board, and bits 2 and 3 are 10 in binary to mark the header as NES 2.0. The other eight bytes become:

A RAM size given as a shift count n means 64 bytes shifted left by n; 0 means none.

Telling it apart

The file starts with 4E 45 53 1A and bits 2 and 3 of byte 7 are 10 in binary (byte 7 AND 0C is 08).

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