[xfer]

Loading a program into memory is part of runtime(it happens before your program starts executing; so how is that useful to model that?); Why would calling a function be a side-effect?

Not sure what exactly you mean by writing a register; if you directly mutate a location, it is considered side-effect. Otherwise any register rewrites that happen are just implementation details and if you have a different architecture than von-neumann than it could also be implemented without writing to a register.