-The return value read from un-mapped space: change from current "0" to "1", to represent that in real hardware circuit(TTL, or MOS with pull-up resistor);
-For the final compiled "mame.exe": instead of linking everything to a huge single file, use something like dynamic linking technology, let each lib file be loaded in run-time.
-For multi-cpu(dsp) target machine, run each cpu/dsp emulation in a seperate thread.