I'm planning to put together a new computer using one of the new Ryzen processors.

I heard that 3600 MHz memory would provide the best performance for it, because
using faster memory would require something to be slowed down to two memory
cycles instead of one. Then I learned some more details: the Infinity Fabric
interconnect within the CPU runs at half the memory speed, to a maximum of 1800
MHz. So the faster memory even makes the chip work faster internally.

However, when looking for a motherboard, some were billed as supporting memory
up to 2666 MHz, and others as supporting memory up to 3200 MHz. But when I
looked at the *detailed* specs for the latter boards, they _did_ support
DDR4-3600 memory... as long as you were overclocking.

What?

If the memory's rated speed is 3600, how is running it at 3600 overclocking? Is
this some multiplier thing, the chip can't get its Infinity Fabric up to 1800 if
it stays at its rated clock speed?

Or is this just a garble, with the truth not as bad as it sounds: to use the
memory at its full rated speed, you will just have to go into the same BIOS
screen that people use if they're overclocking, but you don't have to actually
overclock anything.

Then I found that by buying two 8 GB sticks for my build, I had dodged another
bullet!

Had I gotten 32 GB of RAM on two 16 GB sticks instead, one web site told me I'd
never be able to use it at its rated speed - and I ran a good chance of the
computer not booting at all!

However, further searching turned up that this was an issue with the first-gen
Ryzen, and it got fixed by a BIOS update. Basically, Ryzens didn't like dual-
ranked memory, and 16 GB memory sticks are almost always dual-ranked, and 8 GB
memory sticks now are almost always single-ranked.

Still, apparently there is some risk in dual-ranked memory with Ryzen. I did
note that some memory was advertised as compatible with Ryzen, or as compatible
with Intel, so I chose, from what was available, something that didn't say it
was only for Intel, and which seemed to be on one site's list of memory that did
work with Ryzen.

In any case, it certainly seems like getting memory for a Ryzen system is a
complicated process.

John Savard