The SMS has 3 types of memories: - 2kB of RAM, directly accessible from the CPU - 8kB of video RAM, used by the VDP (read/write through IO ports) - the game ROM, from 32kB to 512kB
We can fit the "internal" memories in block ram but the ROM has to be stored outside, in the SRAM of the Papilio Plus. We have to fill it with the ROM image before boot.
Two ways to do that: the easy way, and the cool way.
The easy way is to fill the SRAM trough the USB connection. A patched version of papilio-prog allows that.
The cool way is a custom boot rom, stored in the FPGA, that reads data from an SD card. After some struggle with the timings of the various SD vendors, it seems we've reached something that works fine.