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the only differences are the introduction of 2 clocks and the rden_a and rden_b port
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"The only difference" actually matters. Your code requests synthesis of a RAM output register, that holds the previous value if rd_en is inactive. But there's no equivalent register avaible in the hardware. Please notice that the clocked read action you are associating with rd_en is actually "absorbed" by the register level at the RAM input, so it's not available at the RAM output.
I don't clearly see, what's the purpose of the rd_en action in your design. The RAM itself doesn't require it.
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