DDR3 Memory
The Nexys Video board comes with 512~MB DDR3 memory.
The Xilinx Memory Interface Generator (MIG) 7 is used for interfacing. Docs: https://support.xilinx.com/s/article/46226?language=en_US
To speed up RTL simulation, the MIG is by default not included in RTL simulations. The MIG is always used in synthesis and therefore always present in netlist simulations.
0x80000000 / 0x90000000 are strangely mirrored. This seems like a MIG / memory problem. This means that currently, only 256 MB DDR memory are accessible.