in-system memory read in VHDL

There's no functional difference between instantiating lpm_rom and using the Quartus MegaWizard, that creates a altsyncram instance internally. Both are referencing identical low-level libraries and generate most likely identical but surely similar code. Personally, I prefer to use the MegaWizard as a template generator and include the generated code directly, allowing parameter modifications without running the MegaWizard.

Both ROM variants allow to specify a *.hex instead of a *.mif initialisation file. Also Quartus can translate between both formats. If you intend ModelSim simulation, you should use *.hex, cause ModelSim doesn't understand *.mif.

I'm still wondering about your previous requirement of using the ROM inside a procedure or process. B.T.W.: The syntax error shown in the posting is a missing semicolon, but other errors are present, too.