If an invalid address is specified on the ADDR bus, Xs may appear on the DOUT bus during read operations, and during set and reset operations in the simulation. In the device, you might get unpredictable data on the DOUT.
The Xs on the output might continue to appear until the invalid address is overwritten internally with a new address. When output registers are used, this could take a couple of clock cycles.
To avoid this issue, specify only the valid addresses. The valid address are 0 to the depth of the memory chosen during the core generation.