1. What is dual channel mode in DDR4? 2. How to enable dual channel mode in Arria 10 GX board?

Dual slot interface is referring to 2 separate DIMM slots (the table on page 227 of the guide you linked). The user guide is saying that with a single instance of the EMIF IP, you can control up to 4 ranks, meaning you can have up to 4 CS signals. A DIMM in a single slot may have 1, 2, or 4 ranks, whereas with dual slots, you could have 2 DIMMs, each with 2 ranks (2 CS signals for each DIMM), both controlled by a single instance of the IP.

Your questions:

1) Yes, you can have more than one memory controller IP in the top level. You can have as many as can fit within the EMIF I/O resources of your selected device. And some resources can be shared between multiple interfaces to allow for more interfaces.

2) As mentioned above, it depends on the number of ranks for each DIMM. If the DIMMs are only single or dual rank, you can get away with only a single instance of the EMIF IP. If you require in total more than 4 CS signals, you need an additional EMIF instance.

3) As mentioned, a single dual-rank DIMM is a single DIMM in a slot that requires two CS signals. Read "DIMM Options" on page 226 and the notes at the bottom of table 261 in the guide you linked.

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