All 900 Series oscilloscopes have fully installed FPGA RAM.
But what's it used for?
Nobody knows...
Maybe that memory was originally
meant to be dedicated to the digital channels' data, working in parallel with the standard RAM for the analog channels? I.e. the idea was to have twice the memory bandwidth in the DHO900 series vs. the 800 models, to support the additional digital channels without sacrificing analog sampling rate? And for whatever reason that did not work out and Rigol had to fall back, last minute, onto a solution with shared memory and compromised sampling rates?
That's speculation, of course. But it would be an explanation for the awkward sampling rate compromise, and also for the fact that Rigol forgot to mention it in the datasheet.
If it should be true, a hacked DHO800 might work as an MSO just as well (or not so well) as the DHO900. And on the other hand, maybe Rigol are still hoping to improve the DHO900 sampling rates, by tweaking the FPGA code and enabling that extra RAM? Or to come out with an upgraded "DHO900 deluxe" in the future?