Nintendo header

About 33% of licensed Nintendo NES games have an header present at their last bank, at addresses $ffe0-$fff9, right before the interrupt vectors.

The info in them is frequently incomplete and/or inaccurate. When info on a particular filed is not present, a byte $00 or $ff is used for padding

• $ffe0-$ffef : Name of the game, in ASCII, zero terminated (sometimes the name is abbreviated or replaced by Nintendo's code).
• $fff0-$fff1 : PRG checksum (either the last 16k bank, or the whole PRG-ROM). Clamped sum of all bytes, excluding the two checksum bytes.
• $fff2-$fff3 : CHR checksum.
• $fff4 : High nybble = prg size (0=64kb, 2=32kb, 3=128kb, 4=256kb 5=512kb).
• $fff4 Low nybble = chr size (0 or 8 = 8kb of RAM, 1=16kb, 2=32kb 3=128kb, 4=256kb).
• $fff5 : Mirroring. ($04 = mapper controlled, $02 = hardwired to vertical, $81 or $82 = hardwired to horizontal.
• $fff6 : Version ($01 by default, incrementing on revisions)
• $fff7 : Unknown.
• $fff8 : Maker's code, the same used for the FDS, GB, GBC and SNES headers. $01 = Nintendo, $08 = Capcom, etc.
• $fff9 : Unknown.