when Radio Shack released the third and final edition of Color Computer Tandy Color Computer 3It involves a routine ASIC Chip. This chip is called GIME (Graphics Interrupt Memory Enhancement), mimics most of the features of Motorola 6847 Video display generator chip used in earlier models. It also added new features such as high-resolution graphics with more colors and a memory management unit. This allowed the CoCo 3 to be backwards compatible while allowing 640×225 resolution graphics and the ability to access up to 512K or RAM.
The functions of GIME are well documented Tandy Service Manual for Color Computer 3 Sold by Radio Shack.
GIME could only display 16 colors (from 64 palettes) on one screen at a time, but clever programmers quickly figured out how to display all 64 hardware at once. Radio Shack knew this trick and offered a diagnostic program that did just that.
Even smarter programs started doing things like:
Although the demo scene in Cocoa was very small, it was there.
Even 256 colors?
During the CoCo 3’s production days, some commercial programs were sold that claimed to offer 256 colors. These colors can only be viewed on a composite NTSC monitor, so those using an RGB CM-8 (or similar) monitor or PAL versions of the CoCo 3 cannot participate.
In 2010, the Cocoa programmer John Linville Made a full motion video player using this 256 color mode:
But… was there more?
Quest for “other” 256 color mode
Several features were planned for the CoCo 3 that never made it to production. Early PCB prototypes had a built-in floppy disk controller. The CoCo 3s used a more standard DB9 port for the monitor rather than the odd header connector on the bottom of the product.
New hardware capabilities were implemented through a series of logic chips. These chips are then integrated into a custom GIME chip for manufacturing.
In fact, an early specification document submitted to Microware Systems Corp. specifically mentioned a 256 color mode. (Microware was the company chosen by Radio Shack to make extensions to Microsoft Extended Basic to support the new hardware.)
More information on GIME…
Although GIME is well understood with excellent documentation, nobody really knows how it works. Programmers of various emulators and FPGA projects have to develop their own Clean room implementation GIME uses only technical specifications and any behaviors discovered by the programmers.
CoCo 3 demos like the one above often don’t work initially with these reverse-engineered GIME implementations. Developers find programs that don’t work and figure out how to modify their virtual GIME until it behaves like real hardware. This means that, to date, many GIME-bashing demos look different depending on which emulation/recreation you’re running.
His goal was to sacrifice some GIME chips (including the original buggy 1986 version and later the “standard” 1987 version) and replace them. This process to destroy Cocoa’s original video chip is already used to confirm what the embedded font characters are. The end result for his project is believed to be enough knowledge to perform a full clone of GIME via FPGA. (The GIME-X project has already done a cleanroom FPGA plug-in version of GIME and added more features, but no one has done a 100% recreation yet, so no information to make it possible.)