Here is a summary of what I know about the Mitsubishi/Renesas 7790 series MCUs
16-bit MCU, which can be switched in software to operate in 8-bit mode
84-pin PLCC package
103 instructions
Mask ROM or one time programmable EPROM.
A cousin of the 68HC11 architecture, but with many differences.
Used in various Mitsubishi Electric Automotive ECUs, as well as multiple JECS (Nissan, Subaru) ECUs.
Only sold to Japanese manufacturers. Possibly only MEA and JECS.
The only datasheet I've been able to find was posted by Calum Johnson of Calumsult. This was a 6-page partial datasheet for a 7790 MCU. He also has a pinout only for a 7791. Documentation is pretty much non-existant, and must be derived from other Mitsu/Renesas 7700 series MCUs. There are a lot of hardware differences, so this is not easy. Fortunately, the 7700 series software manual IS available. This has been invaluable in my dissection of this MCU.
My experience is with the 7793 and 7798 as used in the Mitsubishi 3000GT and Dodge Stealth. Specifically:
M37793E4TJ and M37793M4TJ. 2K RAM, 32K ROM. E in suffix = EPROM version and M = Mask ROM.
M37798E7TJ and M37798M7TJ. Unknown RAM (2K?) and 48K ROM.
The only way to enable reprogramming is by installing something like a Techtom daughtercard.
Unfortunately, this disables multiple functions on the MCU, as the pins are reassigned to communications with the external EPROMs. Those functions must be recreated via a device such as an FPGA. This is no trivial task.
I owe a LOT to Calum Johnson. His notes and explanations have provided invaluable help in this project.