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No you are not wrong and your understanding is basically correct. So you'll need to debug some more to determine what/why your dual port RAM is being corrupted/affected by NIOS software size. Probably the easiest thing to change (and possibly get different crash symptom) would be to relocate the dual port RAM elsewhere in the address map, not near to where your on-chip memory is located.
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Thank you ted. Another possibility is to allocate the dual port ram memory region as a global array of uint8_t for example. So that this region can't be overwritten randomly for some reason. By the way it seems that it's not possible to allocate some memory given the starting address and the vector size.
thank you !