« on: June 04, 2019, 10:21:30 pm »
Hi, you first need to understand the Genesis memory map. The Genesis maps the Rom on the first 4MB of memory. This is the best of the worlds, because the Rom addresses starts on 0x0. So, if your Rom has only 1MB, for example, it's because the developers saved some money using only one 1MB chip, instead of a more expensive 4MB chip.
So answering your question, you only need to increase the file size to 4MB to expand. But like the other guys said, you need to be aware of where the game maps the Sram. It's an implementation detail chosen by the developers, but affects the PCB, because the game's hardware needs to know when to access the data chip or the Sram in the cartridge. Most games maps the Sram to 20:0000-ffff. The PS4 also maps the save to 20:0000-ffff, so the developers made some kind of mapper in the cartridge using the !TIME pin because the Rom has 3MB and also uses the 20:0000-ffff for Rom data. But I think to do a translation you don't need to worry about that.
Adding a little info, the Lunar Expand was made because on the Snes the Rom is mapped in several parts of the memory map, so there are several places to put the Rom data, and depending on the Rom size some parts of the Rom will be mapped in different regions. So expanding is not so straightforward like the Genesis, that only maps on 00-3f:0000-ffff.
Another info, actually the Genesis maps the first 10MB to Rom data, and not only the first 4MB. You can use the 40-7f banks if you want, but no one used this region back on the day because this region is used by the Sega CD. So you can make an 8MB cartridge and it will work without a mapper, as long as the Sega CD is not connected. For emulators you can use this region, as long as the emulator implements this behaviour. Most emulators only cares about licensed games, so most emulators don't support this.
The last 2MB, banks 80-9f, you can also use for Rom, but seems that in this region the processes demands a manual manipulation of the DTACK pin, so is a little more work.