The ROM list is src/memory.cpp:87-100. A missing ROM is filled with 0xFF and the emulator starts, so only N88.ROM, N88SUB.ROM and N88EXT0.ROM, without which nothing boots, are required. The directory is QUASI88_ROM_DIR, else <home>/rom (src/file-op.cpp:444-521). The CMake ROMDIR setting only appears in the help text.
