Both consoles and games were targeted by unauthorised parties, many of whom ended up producing their own replicas . Bootleg hardware was made using a combination of off-the-shelf and reverse-engineered chips, aiming to offer a cheaper alternative to SNK's hardware.
Thus, on the one hand, you've got SNK trying to fend off bootleggers. On the other, game studios were enforcing that their code only ran on a genuine console and cartridge.
Combating fake consoles
Neither SNK nor game studios favoured running on bootleg hardware (whether it was a Neo Geo clone or a bootleg cartridge). Hence, many games bundled routines that fiddled with very particular areas of the motherboard (e.g., status flags, the real-time clock, the Watchdog, etc.) in an attempt to spot any defects in clone boards . Whenever any of these checks failed, the game would display a warning message and refuse to proceed.
Fighting fake cartridges
Considering cartridges housed an exceptional number of integrated circuits, games implemented creative measures to combat piracy.
Starting with the common techniques, the System ROM contains a portion of data called Security Code that is compared side by side with the cartridge's P ROM during boot . The bootloader will proceed only if both areas match.
Additionally, games like Fatal Fury 2 also interact with their companion 'PRO-CT0' chip (a multiplexer for the CHA Board) to ensure the program is residing in an official cartridge . Otherwise, it assumes it has been cloned, triggering many anti-piracy unpleasantries (such as making the opponent invincible).
Later on, newer entries also stored encrypted data in the C ROM, designed to be seamlessly decrypted by the 'NEO-CMC' (found on the CHA Board) . This chip contained a set of unusual XOR functions to deobfuscate the graphics data before it reached the VDC . The NEO-CMC also acts as a multiplexer for tile data within the C ROMs, allowing manufacturers to omit the inclusion of S ROM chips altogether.
For more extreme measures, games like 'Metal Slug X' bundled specialised ICs in their PROG board, which complicated matters further . This is reminiscent of Nintendo's CIC system, which required constant game-chip communication (albeit significantly distinct in terms of goals and architecture).
All in all, this shows that the anti-piracy battle was just another cat-and-mouse game. However, in this case, SNK was trying to steer both home and arcade owners away from cheaper knock-offs.
Post-market utilities
Once the lifecycle of this console had reached an end, a new phase emerged: Homebrew (running homemade software and/or hardware). At the time of writing (June 2024), the most common practices are hardware modifications, firmware replacement and flashcarts.
Thanks to its enthusiastic community, there are many online resources available that teach how to correct and/or modify MVS and AES hardware. These include converting MVS models to work outside cabinets (the so-called consolisation), while others focus on addressing deficiencies in particular models. For instance, the AES model I own had its video encoder bypassed, improving the quality of the RGB output.
On the firmware side, there have been developments like the 'UNIVERSE BIOS'/UniBIOS. This is the equivalent of a custom firmware made to replace the original System ROM. Behind the scenes, the UniBIOS is made of an MVS System ROM modified to provide the following additional abilities :
- Switch between 'MVS' and 'AES' mode. This is very useful for AES systems, as games have a hardcoded number of coins/lives, and switching to MVS mode enables to increase this amount arbitrarily.
- Change the region. Some games are censored depending on the region they run on.
- Activate cheats.
Finally, to run arbitrary code in this console, stores are selling third-party flashcarts which can reprogram themselves to replicate any past CHA and PROG boards. Due to their complexity, they are not exactly affordable, but that is another product Neo Geo owners have available nowadays.