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Anyone with at least one foot in the Tandy Color Computer (CoCo) world these days will have already been hearing about something called the Game Master Cartridge (GMC). What is this device? Another whiz-bang flash-in-the-pan hardware announcement with little real utility? Or is this a good and useful product that everyone should acquire for their CoCo collection? Well, as the designer of the GMC I will say that it is perhaps neither…or maybe it is both?

To start, the GMC is not really a consumer product at all. The GMC is more of a technology, one designed to solve a particular set of problems. For most folks, the only way to acquire GMC technology will be to buy a game cartridge with the technology built into it.

In particular, the GMC is designed to enable better (i.e. more entertaining) cartridge games on the CoCo with only a modest increase in price versus a “normal” game cartridge. For many collectors, cartridge games offer a tangible quality that makes them prized parts of a retro computing collection. When the cartridges are built around GMC technology, they can truly offer a game experience that would be unavailable otherwise.

So what is the big deal? Cartridges have existed for the entire life of the CoCo. How is the GMC technology different? Well, normal cartridges for the CoCo 1 and CoCo 2 are limited in storage capacity to about 16K. The GMC extends that through the time-honored technique of bank switching, expanding available storage to nearly 64K on the cartridge. Combined with the ability of the CoCo 1 and CoCo 2 to run at faster clock speeds while executing programs on a cartridge, and this feature alone is enough to justify claims of enabling bigger and (hopefully) better CoCo 1 and CoCo 2 games with more graphics than ever before and at the fastest speeds possible. Even on the CoCo 3, no other cartridge solution currently in production offers any more storage than the GMC.

But…what really makes the GMC special is the inclusion of an on-board TI SN76489 Digital Complex Sound Generator. Used in the BBC Micro, the Sega Master System, the ColecoVision, the Tandy 1000, and a number of other vintage systems, this little chip punches above its weight. With this hardware on-board, the GMC offers continuously playing music and sound effects for games on any version of the CoCo. Easily programmed and at worst only requiring software service during vertical sync periods, this heroic little chip can add to a game enjoyable music in 3-part harmony (plus a noise/percussion channel) without requiring complex cycle counting or tricky interrupt handling. This little gem can add value to nearly any game while requiring very little time or effort from the game’s creator.

In my mind, perhaps the best part of the GMC is it’s affordability. Depending on packaging, there will always be real, upfront costs associated with producing game cartridges. Plastic cases, elctronic parts, PCBs, boxes, manuals, etc all take their toll. Fortunately, the bank-switching bit of the GMC only requires a little 74xx-series logic and a bigger ROM chip. Moreover, the simple little SN76489 only needs one more 74xx-series chip and a few miscellaneous electronic parts to do its job in the GMC. Yes, the design is “period correct” (a.k.a. “old skool”) — no CPLDs or FPGAs here, and no surface mount parts! In the end, even at current eBay prices the GMC technology only adds ~$3 to the cost of building a cartridge. That price is well worth it for the extra value provided!

If you are thinking of writing a CoCo game, by now you must be wondering how to take advantage of the Game Master Cartridge technology for your own creations. For most, the simplest way to start is by using MAME. There, emulation of the GMC is already fully implemented thanks to Tim Lindner. Those that feel the need to test on real hardware can reach-out to me (John Linville <linville@tuxdriver.com>) to inquire about the availability of developer kits. Further, anyone that completes a GMC-based game and is ready to ship cartridges to the CoCo community can likewise contact me for production details.

The CoCo may not be well-known as a game platform. But I assure you that under the control of the right software package, the CoCo can provide hours of wholesome video game entertainment. Beyond that, writing 8-bit game software for the CoCo is a joy for those of us that love the machine. Anyone that goes to such effort will want to provide the best gameplay experience possible to those that play their creation. Producing high quality and entertaining music and sound effects for a game on the CoCo can be difficult, even on the CoCo 3. Now, the GMC technology for the CoCo puts enjoyable music production within the reach of any competent programmer without requiring a background in music.

Cartridge games based on Game Master Cartridge technology will be available at CoCoFEST! in Lombard, IL on April 21-22, 2018. Come out and get them all!

http://www.glensideccc.com/cocofest/