Konami made great games for the MSX computer platform. Their cartridges have become expensive collectibles instead of cheap games. The author's goal was to make a functional cartridge to play Konami's Knightmare II: The Maze of Galious.
For years, the author wanted to play The Maze of Galious on MSX. They looked at the used market but the price was too high. The obstacle was understanding the mapping scheme. After talking to bsittler and getting some Konami-style cases and a sample EEPROM, the author started working on the project.
A cartridge mapper is needed when cartridges get bigger than 64k. The MSX standard has a slot system that allows for page-switching. The Konami MegaROM mapper divides a 1Mbit ROM into eight 16k segments and each MSX cartridge slot can point to one of these segments.
The author initially struggled with implementing the mapper. They realized that A14 wasn't needed to distinguish between paged and unpaged access. After some debugging, they found that the DIP-switch was wired backward and added pull-down resistors. Finally, the MSX booted into Salamander and Galious with the correct adjustments.
The author also got a custom sticker made for the cartridge and fixed some issues with the PCB. They played a short clip of Galious using a custom controller. The project was a lot of fun and the cartridge KiCad files and Gerbers are available on GitHub.
The MSX standard has some rules for slot mapping and page-switching. There are also some limitations and caveats, such as the fixed mapping of the $4000 to $7fff slot on early Konami mappers. Overall, it was a challenging but rewarding project.