Upgrades for Dendy: Part 2/2

Overclocking

When discussing personal computer upgrades, the topic usually involves installing a faster processor or increasing its clock frequency. Enthusiasts haven't overlooked old gaming consoles either.

In the Dendy, you can't simply replace the processor with a faster equivalent. Even in the original version, it's a specialized chip containing a 6502 core, sound synthesizer, and I/O ports on a single die. In Chinese clones, it's a system-on-chip that also includes the video system and RAM.

Nevertheless, mild overclocking is possible by replacing the quartz resonator. In the NTSC version, it runs at 21.4773 MHz. All other frequencies are derived by dividing this one: the processor clock of 1.79 MHz and the video controller frequencies.

Simply swapping the resonator breaks the video signal. Proper overclocking requires installing an additional clock generator for the video controller, allowing it to operate at its original frequency, while replacing the main resonator overclocks the processor by approximately 20–30 percent.

With such overclocking, some games that suffer from slowdowns see improvements, but the effect is modest. The game architecture is such that overclocking simply causes the processor to spend more time waiting for the next frame. A side effect is that sound pitch rises, since the synthesizer shares the same crystal with the processor.

In the late 1980s, developers considered solving the performance issue by installing an additional processor in the cartridge. In 1990, Color Dreams planned to place a Z80 processor in the cartridge of the game Hellraiser (Super Cartridge) along with 64 kilobytes of RAM. The Z80 would perform software rendering into a video buffer that the video controller would see as external video memory. This would have lifted the restrictions on sprite count and even enabled 3D rendering. A prototype was built, but the code was never written. The idea proved viable for the Super Nintendo.

Sprite flickering is often attributed to insufficient processing power, but it's actually an artifact of the video controller, which cannot display more than eight sprites per scanline. Developers intentionally added flickering manually so that extra sprites would at least be visible some of the time.

Memory

The console has less RAM installed than its address space allows. Most likely, the original plan was to install the maximum amount, but a cost-reduction compromise was needed.

For processor RAM, the address range from $0000 to $1FFF (8 kilobytes) is allocated, but only 2 kilobytes are actually installed, mirrored four times. You can restore the full amount of memory, and most games will work unchanged. However, there won't be any improvements: old games don't know about the additional memory.

The video controller has a 16-kilobyte address space, but only 2 kilobytes reside inside the console. These contain descriptions of two screen pages. Another 8 kilobytes of video memory are placed on the cartridge board.

The video controller can use an additional 2 kilobytes to implement four screen pages. This enables seamless scrolling without artifacts in all eight directions. With only two kilobytes, there are limitations and artifacts. In the game Gauntlet, this missing video RAM was installed in the cartridge.

Some mappers implement additional video memory. MMC5 adds a kilobyte of special-purpose memory that's invisible to the video controller directly but affects graphics through clever address switching. This memory is used for increased color resolution and storing extended tile numbers, increasing the number of simultaneously displayed background elements.

There's a "hole" in the address space from $4018 to $8000 (16 kilobytes). Cartridges place their memory there. Typically, the area from $6000 to $8000 is used for additional RAM of 2–8 kilobytes, often battery-backed for saving game state. With bank switching, up to 64 kilobytes can be added.

The simplest mapper switching a 16-kilobyte window with an 8-bit register can address up to 4 megabytes of cartridge memory.

Enthusiasts implemented a variant with a 12-bit register, allowing up to 64 megabytes to be addressed. This made possible a music album called "A Winner Is You" with realistic sound and digitized music videos in the "You Are Error" project.

Chinese Developments

Improvements to the Famicom and NES architecture were pursued not only by Nintendo but also by Chinese engineers, who created an upgraded version from an alternate reality that few people ever saw.

The Taiwanese company V.R. Technology, starting in 2001, developed chipsets for building Famicom/NES clones, introducing OneBus technology. They created a series of enhanced VTxxx chipsets (from VT01 to VT369).

VT01 was designed for portable Famicom clones with TFT screens. The video system was reworked for direct screen control. There were no other differences from standard clones.

VT02 substantially expanded capabilities. It added a built-in MMC3-like mapper with the OneBus system. Built-in raster interrupt generation appeared, along with simultaneous PAL and NTSC support, an RS-232 interface, per-tile and per-sprite graphics bank selection, and DMA for transferring data to graphics RAM. A second sound synthesizer was added (doubling polyphony) as well as an 8-bit DAC with DMA.

VT03 substantially expanded graphical capabilities. It became possible to use 16-color sprites and tiles, a 16-pixel-wide sprite mode appeared, and an extended 12-bit palette in HSL format was introduced. VT09 was a cost-reduced version optimizing RAM usage. The VT16x and VT18x versions included a TFT screen controller.

VT32 added a hardware multiplier and divider, changed the palette to standard R4G4B4, and added playback of two ADPCM compressed audio channels. The second synthesizer was removed.

VT369 — the last chip in the series — added processor overclocking to 5.37 MHz (triple speed), a 256-color video mode, a high-resolution 512x480 pixel mode, up to 16 wide sprites per scanline, and a 15-bit R5G5B5 palette.

V.R. Technology created English documentation and a fork of the NNNesterJ emulator with VT03 support. The chips appeared in rare clones, including the Generation NEX (2005).

In the late 1990s, the company UMC created the UM6578 chip with 16-color graphics, 10 kilobytes of on-chip video RAM, a built-in mapper with eight 2-kilobyte windows, a programmable timer, an 8-bit DAC, and support for peripheral devices (keyboard, mouse) with interrupt handling.

Hobbyists and Enthusiasts

While engineers on different continents were developing console capabilities, enthusiasts were implementing unusual projects.

The Famicom's video controller (RP2C02) has a simple interface and can be connected to other devices. In prehistoric times, a domestic enthusiast known as HardWareMan connected the controller to a Mikrosha computer based on the KR580VM80 microprocessor (Intel 8080). Modern projects repeat this with Arduino, connecting the NES PPU.

In general, the PPU can be controlled by any processor, creating amazing hybrids. The problem is the availability of the RP2C02: the original Nintendo chip wasn't intended for free sale, and the Chinese equivalent UA6538 stopped being used in the early 1990s with the transition to high integration. However, chips salvaged from defective consoles occasionally appear for sale.

In 1995, Sergey Veremeyenko, a well-known ZX Spectrum developer, created a "Video Processor for ZX-Spectrum" — a hybrid to which an entire Dendy was connected. The development was described in detail in the magazine "ZX-Review," but few attempted to replicate it. The system was theoretically functional.

The development consisted of a cartridge ("interface board") and a video output merging scheme. The board contained 2 kilobytes of ROM with a loader, up to 32 kilobytes of RAM for code and graphics, and an 8-bit bidirectional port. On power-up, the driver code would launch, receiving data from the Spectrum and operating as a normal cartridge.

The video merging scheme combined two images: the Dendy output was displayed only in character cells with the Spectrum's blink attribute set. This required serious modification of the Spectrum's circuitry, synchronizing it with the Dendy's PAL video signal. Sync pulses were extracted from the video signal to synchronize the Spectrum.

The two images were output in different formats: RGB from the Spectrum and composite video from the Dendy. This required simultaneous connection to a television and support for a "window" signal to switch between sources on a pixel-by-pixel basis.

A significant drawback was the complexity of creating new software. It required assembly skills for the Spectrum and deep knowledge of Dendy architecture and 6502 — practically impossible requirements for 1995. Without new software, the system could only reproduce simple early "mapperless" games loaded from floppy disks. Such games were inferior to most late-era Spectrum titles. For these reasons, the development remained a thing of legend.

The last hobbyist development from the early 2000s was CopyNES. This was a system for interfacing the Dendy (specifically, the original NES) with a personal computer.

The interface was achieved by installing an additional board in place of the original 2A03 processor. The processor itself was desoldered and installed on the additional board. The board also contained RAM, a loader ROM, and a parallel port interface. A later USB CopyNES variant also existed. The design allowed any manipulation of the processor's addresses, substituting different memory areas with loadable code, intercepting reset and interrupt vectors.

Essentially, CopyNES was an in-circuit debugger. You could halt the processor, read or modify memory, and execute code step by step. You could read data from any cartridge, analyze any mapper, load and execute your own code for debugging on real hardware. This wasn't a consumer device: it was aimed at the needs of developers and hackers, not ordinary users.

Network Technologies

In our time, consoles without internet are hard to imagine. Network technologies also touched Dendy-like consoles.

In 1988, the Japanese market saw an official device from Nintendo — the Family Computer Network System. It was designed to connect to a special service via dial-up over telephone lines.

Unlike later developments for Sega Genesis and Super Nintendo, this early implementation had an unclear purpose. Essentially, it was a teletext equivalent: check the weather, stock quotes, place sports bets, read jokes. Games were somehow not included, though downloadable content was planned. The service lasted until 1991.

Technically, the device resembled the Famicom Disk System without the disk drive, but with a modem section. It came with a special controller-joystick with a numeric keypad and additional buttons.

In modern times, enthusiasts have achieved more. The emergence of cheap IoT microcontrollers with modern network interfaces (TCP/IP, WiFi) has enabled the creation of NES devices with internet connectivity. These include the NESnet universal cartridge project for network applications (early development stage) and the game Super Tilt Bro., which implements one-on-one networked fighting via WiFi on a standard NES console.

Super Tilt Bro. had a successful Kickstarter in 2023, raising 89,000 euros. Both projects use the ESP8266 module for network communications.

"The Cherry on Top"

The apex of placing additional hardware in an NES cartridge is the Doom port. Someone actually ran real Doom on an 8-bit NES. But there's a catch.

The computational power of the 6502 processor (1.79 MHz) isn't enough for a Doom-like game, although Wolf-like projects exist. The described port (2019) runs on a Raspberry Pi 3A+.

This powerful single-board computer runs a full, uncut Doom port. It converts the output image into the console's format and transmits it via USB to an additional FX2LP microcontroller. The latter receives data over USB and emulates a regular video RAM chip for the console. The console itself runs synchronization code, polls the joystick, passes button press information to the RPi, and plays specially created 8-bit music.

Although it's a trick and a cheat (the game runs on modern hardware), interfacing this hardware with a 1983 console is an interesting technical challenge and achievement. Whether it's an upgrade or the Ship of Theseus is an open question.

Modern Reproductions

Let's wrap up with "modern reproductions" — contemporary clones created by enthusiasts who truly love this system.

There are kits for assembling a full replica: a new PCB and vintage through-hole components. Options exist for installation in a custom case or an existing one from classic models like the original Famicom or Dendy Junior. The result is a quality console assembled by your own hands, identical to the genuine article, running games from original cartridges. They differ from old Chinese clones in material quality, and in the refinement of power supply and output signal amplification circuits.

There are FPGA replicas that don't use original chips. Usually, these aren't standalone systems but entire multi-system platforms supporting NES as one of many consoles. While they offer no advantages over software emulators when running ROM images instead of real cartridges, they differ in their approach to emulation.

Some FPGA solutions enjoy great popularity. MiSTer, built on the Terasic DE10-Nano development board, supports a whole range of second- through fifth-generation consoles.

There are FPGA systems that implement exclusively the NES, supporting real cartridges and peripherals. Although the hardware essence is the same, emulation is implemented closer to real hardware with corresponding timings. FPGA opens new possibilities: HDMI output, quick saves, and much more.

RetroUSB AVS — the first among such consoles, developed by the creator of the first NES Flash cartridge, PowerPak. It differs from software emulators in its compatibility with nearly all real hardware (except the light gun) and HDMI output at 720p with no lag. The system has a unique plastic housing and two slots for NES and Famicom cartridges. No service functions are provided.

Analogue Nt Mini offers similar advantages, built on FPGA: two slots, HDMI output at 1080p, support for real cartridges and peripherals. Correct light gun operation is promised.

Analogue Nt — the king of reproductions using authentic hardware. Built on a new PCB with original components salvaged from defective consoles: real 2A03 (processor) and 2C02 (NTSC video controller) chips. The regional protection chip is excluded. Two cartridge slots for Famicom and NES. RGB output, component, S-Video, and composite video outputs. Optional HDMI support via an additional board.

Housed in a gorgeous CNC-milled aluminum enclosure. The only drawback — the price: $500 at launch (2015). Such devices, aimed at hardcore fans, are always expensive.

Conclusion

And once again it all comes down to the classic meme about the bread loaf: with a few simple tools and skilled hands you can make quite a lot of things — but why? The answers vary. "Just because it's interesting" is not the worst answer.