Addressing unmapped memory space results in unpredictable behavior

Thanks for the ideas.

I'm using Quartus II 13.1 (latest version to support our Cyclone III FPGA).

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Have you tested and proven that something is getting written somewhere that you don't expect?

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The actual error I'm targeting occurs only once per month or so, so it's very hard to find and I couldn't prove that it's actually caused by a write to unmapped memory. But what I proved is, that when NIOS writes to unmapped memory deliberately, the write gets assigned to another custom memory mapped component which as a result causes the error I'm looking for.

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I created several instances and set the addressing so that they collectively covered most of the 32 bit address space that wasn't used by other components.

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I also thought about filling the unused memory space, just to cover all the addresses, but I haven't tried it yet.

I have two masters in my system that can write to the component in question (NIOS and an external DSP). It would help a lot, if I knew which (if at all) is causing the problem...