How to initialize content of a RAM without using Megawizard

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Unfortunately, this proprietary mechanism won't work in other tools or, importantly, during simulation.

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It works fine in ModelSim simulation (in Quartus simulation anyway), when using *.hex rather than *.mif initialization files. But ModelSim has special requirements for the *.hex file location.

In any cases, where the initialization file content can be generated by a well-defined (not necessarily simple) algorithm, e.g. a function lookup-table, you have the option to calculate it at compile time by VHDL code, which is the most portable construct to my opinion.