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 Famicom Box reads this header to determine the title of the inserted game.

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. (sometimes the name is abbreviated or replaced by Nintendo's code). ASCII codes 20h-5Ah allowed. Should be right-justified.
• $FFF0-$FFF1 : PRG checksum (either the last 16k bank, or the whole PRG-ROM). Sum of all bytes in the relevant area, excluding the two checksum bytes. Big-endian.
• $FFF2-$FFF3 : CHR checksum. 0x00 if RAM. Big-endian.
• $FFF4 : Data sizes. Values: 0 = 8 KiB or 64 KiB based on board type, 1 = 16 KiB, 2 = 32 KiB, 3 = 128 KiB, 4 = 256 KiB, 5 = 512 KiB
  • D7-D4: PRG size
  • D3: 0 = CHR ROM, 1 = CHR RAM
  • D2-D0: CHR size
• $FFF5 : Board Type.
  • D7: 0 = Horizontal nametable arrangement, 1 = Vertical arrangement (inverse of iNES mirroring bit)
  • D6-D0: 0 = NROM, 1 = CNROM, 2 = UNROM/AOROM, 3 = GNROM, 4 = MMC (any). Determines which area to checksum.
• $FFF6 : Title encoding : 0 = No title entered, 1 = ASCII, 2 = Another encoding
• $FFF7 : Valid Title Length - 1. 0 if no title entered.
• $FFF8 : Maker's code, the same used for the FDS, GB, GBC and SNES headers : 1 = Nintendo, 2-254 = everyone else. 255 must be reserved.
• $FFF9 : Header Validation Byte. 8-bit checksum of $FFF2-$FFF9 should = 0

Calculating the checksum

Everynes claims that the Famicom Box system software applies these rules:

NROM and CNROM

Sum of $E000-$FFFF, $C000-$FFFF, or $8000-$FFFF. The system software detects the size.

UNROM

Sum of $8000-$BFFF in banks 0-7. Unsure how this interacts with $FFF0-$FFF1 exclusion in the case of UOROM, BxROM, or AxROM.

GNROM

Each 32 KiB bank has its own header with a checksum of that bank.

MMC

Sum of $C000-$FFFF, as if NROM-128. No bank switching is attempted.

Caveats for particular mappers

MMC1

Sum of $C000-$FFFF at power on, as if NROM-128. No bank switching is attempted. Any 16 KiB bank that can be switched into $C000-$FFFF needs its own header with a checksum of that bank.

MMC3, most VRCs, FME-7

Sum of $C000-$FFFF at power on, as if NROM-128. These mappers have a fixed bank at $E000-$FFFF and an unpredictable switchable bank at $C000-$DFFF. In order to get the checksum to match regardless of state, the sum of bytes in all 8 KiB banks other than the last must be identical. We seek evidence of games on these mappers having been released for Famicom Box at all.