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

Reverse engineering of the Famicom Box's system software revealed that the checksum covers these areas:

NROM and CNROM

Sum of $8000-FFFF

UNROM, UOROM, and AOROM

Sum of $8000-BFFF for each of banks 0-7

GNROM

Sum of $8000-FFFF for the current bank (0-3)

MMC1, MMC2, and MMC3

Sum of $C000-FFFF