Currently, MAME seems to return "0" when accessing un-mapped spaces. However, in a real electronic circuit, the result is usually the opposite. It usually returns "1":
-For the widely used TTL standard, because it has a pull-up resistor in the input, so always returns "1" when not connected;
-For MOS circuit, when input is floating/high-z, even if without a pull-up resistor present(rarely), the return is usually random, not always zero. In this case, a return of "1" will not be worse than 0.
While the above condition doesn't affect "Normal" cases, some programs might not coded very formally, so might behavior differently and even weired, and hard to debug.
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Return value from un-mapped spaces in a real hardware circuit
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