using SignalTap for internal RAM

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But it's a "simple dual-port RAM". It's not a"true dual-port RAM".

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Simple dual-port is also dual-port. It uses both address/data ports of the RAM block, so there's none left for the ISMCE.

Of course, if one port is read only and you don't need write access through ISMCE, it would be still possible by doubling the RAM. I didn't yet come across a Quartus option to use ISMCE this way, but it should work manually.