 Well, RAM is nearly always (can&#39;t really say always, because some programmers are gay) used as decompression buffer for the compressed mappings inside the ROM. And usually, they&#39;re mappings like 256x256 or 128x128 (i.e. level art mappings), which aren&#39;t directly copied to VDP, but just the parts of it etc. You have to build smaller chunks, then larger ones, too. 