Physical RAM Disk
Here, talking about physical RAM disk, not the software version.
There are three types interface for physical RAM disk, one is PCIe interface, another SATA, or the external drive.
There are two controllers on such board, one is to simulate device, second one is to address the RAM array.
Following web page shows the design of such board
The way we made an external PCIe RAM disk based on the DDR memory Cache
Full overview of the new DDR RAM disk
If a stick of RAM is given a constant supply of power, could it be used as a permanent storage?