Sorry, let me try that again 
What I mean is, even if the soldered RAM is not responding, will the IIsi try to assume it's still there? Like for instance are the SIMM sockets hardwired to only map from addresses 1 MB and higher? Or does the MMU transparently handle all of that? (I honestly don't know anything about how the RAM works in these computers...this question is mainly for my curiosity)
What I mean is, even if the soldered RAM is not responding, will the IIsi try to assume it's still there? Like for instance are the SIMM sockets hardwired to only map from addresses 1 MB and higher? Or does the MMU transparently handle all of that? (I honestly don't know anything about how the RAM works in these computers...this question is mainly for my curiosity)