Game Genie

The Game Genie is a enhancement cart for the NES designed by Camerica and distributed by Galoob and Camerica. It functions as a pass-thru, with a 72-pin cartridge connector connecting it to the NES, and a 72-pin cartridge slot for a game to be inserted into. When plugged in between a game and the NES and turned on, it provides a simple interface to enter up to three cheat codes, which then modify the behavior of the game.

The Game Genie is not assigned a mapper number.

Technical

The Game Genie works by intercepting CPU reads and replacing the game cart's response with its own response. It can intercept any three addresses in CPU $8000…$FFFF and respond with a single replacement for each. To make the tool more compatible with bank-switching, each of the three codes has an optional compare value which can be used to only replace the byte if the original byte matches the compare, hopefully limiting the cheat to functioning on the desired bank.

When first booted, the Game Genie presents its own 4-KiB PRG ROM and a series of simple gates masquerading as a CHR ROM. The included PRG ROM runs code to show a simple code entry user interface. When the user presses Start, the cheat values are written to memory-mapped registers, and then another register is written which switches the Game Genie into game mode, where the attached game cart's CHR and PRG is passed through, save whatever code replacements were defined. The Game Genie remains in game mode until power-cycled, and will respond to no further writes.

Registers

Master Control ($8000)

7  bit  0
---- ----
.DDD CCCE
 ||| ||||
 ||| |||+- Write 1 to switch into game mode
 ||| +++-- Compare enable for each of the three codes
 +++------ Disable each of the three codes

Bit 1 and 4 correspond to the code at $8001…$8004.

Game Genie writes first a value with bit 0 set and then writes 0x00 to this register. Because after the first write, the GameGenie logic switches into game mode, any further writes to range $8000-$ffff will cause the slave cartridge /ROMSEL to become low for that write cycle. As a result, the second write will be seen and interpreted by the hardware inside slave game cartridge. The reason for this write is unknown, maybe it initializes the bank select register for MMC3 games?

Address High ($8001, $8005, $8009)

7  bit  0
---- ----
.AAA AAAA
 ||| ||||
 +++-++++- Bits 8:14 of address for this cheat (Bit 15 fixed to 1)

Address Low ($8002, $8006, $800A)

7  bit  0
---- ----
AAAA AAAA
|||| ||||
++++-++++- Bits 0:7 of address for this cheat

Compare ($8003, $8007, $800B)

7  bit  0
---- ----
CCCC CCCC
|||| ||||
++++-++++- Compare value for this cheat (write 0 if unused?)

Replace ($8004, $8008, $800C)

7  bit  0
---- ----
RRRR RRRR
|||| ||||
++++-++++- Replacement value for this cheat

Unknown ($FFF0, $FFF1)

The Game Genie rom writes 0 to $FFF0, $FFF1, $FFF0 in that sequence.

Pattern Tables

When game mode is inactive, the Game Genie generates PPU pattern tables through PPU $0000…$1FFF by the following method:

• When PPU A2 = 1:
  • PPU A4 → PPU D4…D7
  • PPU A5 → PPU D0…D3
• When PPU A2 = 0:
  • PPU A6 → PPU D4…D7
  • PPU A7 → PPU D0…D3

References

• nesgg.txt – NES Game Genie Code Format DOC v0.71 by Benzene of Digital Emutations, 1997-07-10
• Patent US5112051A – Interfacing device for a computer games system

External links

• NES Game Genie ROM disassembly by Kevin Selwyn (GitHub page)
• NES Game Genie ROM disassembly by qalle