Namco Gorgon ("System 22.5") for MiSTer FPGA — Rapid River

1 Game • 3 Versions • 1 FPGA Core

We're excited to bring another unique piece of Namco arcade history to MiSTer FPGA: Rapid River (1997), running on Namco's Gorgon hardware!

Often referred to as "System 22.5," Gorgon represents a fascinating transition between Namco's Super System 22 and System 23 arcade platforms.

This hybrid architecture combines the MIPS R4650 CPU, H8 I/O processor, and C352 sound hardware associated with System 23, alongside a Super System 22-style 3D rendering pipeline and C404 sprite hardware.

Rather than extending the existing System 23 implementation, we've developed XNGORGON as a dedicated FPGA core, bringing Rapid River to MiSTer with its unique hardware characteristics intact.

And yes, you can finally paddle your way through the rapids. or just the turbo assist.

Enjoy a preview below of Rapid River launching through a preview of MiSTerSpin, a new front-end for MiSTer inspired by Hyperspin.

What's Supported

XNGORGON — Namco Gorgon (1997)

Rapid River

• US — RD3 Ver. C (Primary Version)

• World Version

• Prototype Version

The primary US release is located in Arcade/XelaNotPu/, using its full MAME game title.

Additional versions are available under:

XelaNotPu/alternatives/_Rapid River/

This includes dedicated 480i and 240p variants of all three versions, providing additional options for enthusiasts using original 15 kHz CRT displays.

Release Highlights

Authentic Paddle Controls

Rapid River's original arcade cabinet used a unique two-axis raft paddle controller. We've adapted that experience for conventional MiSTer controls while preserving the original game's paddle mechanics.

• Up / Down: Paddle strokes

• Left / Right: Blade direction

• Authentic stroke detection: Paddle movements must travel sufficiently beyond the center position to register, just like the original cabinet.

We've also introduced an Auto Paddle button. Hold it to paddle forward at the maximum rate accepted by the game while using the D-pad or analog stick to steer.

Complete Sprite Layer Implementation

The Gorgon's C404 sprite hardware is now implemented, restoring important visual elements layered over the 3D environment.

These include:

• Name-entry letter wheel

• Environmental signs

• Smoke effects

• Water spray and other visual overlays

• Sprite elements missing from certain screens in MAME

These additions help deliver a more complete representation of the original arcade presentation.

Service Menu & DIP Switch Support

The game's original diagnostic and configuration features are accessible directly from MiSTer.

• Navigate the test menu using D-pad Up / Down

• Select options using Service Enter

• Configure all eight DIP switches through the MiSTer OSD

• DIP switch labels follow MAME's documented naming

CRT-Friendly Video Options

Support for both 480i and 240p output, with dedicated variants for users running original CRT hardware.

• 480i: Weave and Bob deinterlacing options for HDMI

• 240p: Blend and Drop line modes

• CRT Adjust: Included for additional display tuning

Pause & Supporter Features

• Pause support with a dedicated supporter screen

• Optional Pause when OSD is open setting (default: Off)

• CRT Adjust support

DB9 / DB15 controller support is planned for a future update.

Persistent Game Saves

NVRAM support allows important game data to survive between sessions, including:

• Game settings

• High scores and records

• Calibration data

NVRAM is maintained independently for each game version.

Thank You for Your Support!

Every new core brings another piece of arcade history to life, and Gorgon is a particularly interesting milestone in Namco's hardware evolution.

From the Super System 22 rendering architecture to the next-generation MIPS hardware that would power System 23, Rapid River offers a unique glimpse into a transitional era of Namco arcade engineering.

Thank you to everyone supporting the continued development of these FPGA cores.

Another Namco arcade platform joins the MiSTer family.

Grab a paddle. The rapids are calling!

Legal Notices & Disclaimers

Independent, Unofficial Project

XNGORGON is an independent, community-developed FPGA implementation created for research, technical education, hardware preservation, and compatibility with historically significant arcade software.

This project is not affiliated with, sponsored, endorsed, authorized, or approved by Bandai Namco Entertainment, Bandai Namco Holdings, Namco, or any of their subsidiaries, affiliates, or successors.

References to original arcade systems, manufacturers, games, and hardware are provided solely for historical identification, technical documentation, and compatibility purposes.

Trademarks & Intellectual Property

Namco, Bandai Namco, Rapid River, and related names, logos, game titles, and trademarks are the property of their respective owners.

MiSTer and MiSTer FPGA are referenced solely to identify the intended hardware platform and its associated ecosystem.

All third-party trademarks, copyrights, and other intellectual property rights remain with their respective owners. Their mention does not imply any association with or endorsement of this project.

No ownership of third-party trademarks, game software, artwork, audio, or other proprietary assets is claimed.

ROMs & Copyrighted Game Content

This release does not include, distribute, or provide access to commercial game ROMs, BIOS images, copyrighted game assets, or other proprietary game software.

The FPGA core is a hardware implementation and does not grant any license or authorization to reproduce, download, distribute, or use copyrighted game software.

Users are solely responsible for obtaining and using game software in compliance with applicable copyright laws and any required permissions or licenses.

Ownership of an original arcade board or game does not necessarily authorize downloading or copying ROM images. Applicable laws and exceptions vary by jurisdiction.

The developers do not endorse or facilitate unauthorized distribution of copyrighted software.

Beta Software & Warranty Disclaimer

THIS SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, TO THE EXTENT PERMITTED BY APPLICABLE LAW.

The software is experimental and may contain bugs, incomplete features, compatibility limitations, or other defects.

No guarantee is made regarding performance, accuracy, compatibility, uninterrupted operation, or fitness for a particular purpose.

To the extent permitted by applicable law and the governing licenses, the developers and contributors disclaim liability for damages arising from the use, modification, or distribution of this software, including loss of data, device malfunction, or other consequential damages.

Nothing in this notice limits rights or obligations that cannot legally be excluded.

Supporter Contributions

Supporter contributions help fund ongoing development, research, testing, and maintenance of independent FPGA projects.

Contributions do not purchase or confer ownership of any commercial game software, ROMs, trademarks, or other third-party intellectual property.

All third-party names and trademarks are acknowledged as the property of their respective owners.

Source Code: Github