[fluoridation]

>now the majority of desktops with DDR5 have 4 channels, not 2 channels

Source? I just looked up two random X870E boards from Gigabyte and both are dual channel.

>To avoid ambiguities, one should always write the width of the memory interface.

They're incomparable quantities. More channels support more parallel operations, while a wider bus at a constant frequency supports higher throughput.

The bus width is not even that useful of a metric. It's more useful to talk about bits per second, which is the product of bus width and frequency.

[sliken]

Sadly motherboards, tech journalist, and many other places confuse the difference between a dimm and channel. The trick is the DDR4 generation they were the same, 64 bits wide. However a standard DDR5 dimm is not 1x64 bit, it's actually 2x32 bit. Thus 2 DDR5 dimms = 4 channels.

For some workloads the extra channels help, despite having the same bandwidth. This is one of the reasons that it's possible for a DDR5 system to be slightly faster than a DDR4 system, even if the memory runs at the same speed.

[fluoridation]

>However a standard DDR5 dimm is not 1x64 bit, it's actually 2x32 bit. Thus 2 DDR5 dimms = 4 channels.

Uh, surely that depends on how the motherboard is wired. Just because each DIMM has half the pins on one channel and the other half on another, doesn't mean 2 DIMM = 4 channels. It could just be that the top pins over all the DIMMs are on one channel and the bottom ones are on another.

[sliken]

I think there's a standard wiring for the dimm and some parts are shared. Each normal ddr5 dimm has 2 sub channels that are 32 bits each, and the new specification for the HUDIMM which will only enable 1 sub channel and only have half the bandwidth.

I don't think you can wire up DDR5 dimms willy nilly as if they were 2 separate 32 bit dimms.

[fluoridation]

Well, I don't know what to tell you. I'm not a computer engineer, but I assume Gigabyte has at least a few of those, and they're labeling the X870E boards with 4 DIMMS as "dual channel". I feel like if they were actually quad channel they'd jump at the chance to put a bigger number, so I'm compelled to trust the specs.

[sliken]

In computer manufacture speak dual channel = 2 x 64 bit = 128 bits wide.

So with 2 dimms or 4 you still get 128 bit wide memory. With DDR4 that means 2 channels x 64 bit each. With DDR5 that means 4 channels x 32 bit each.

Keep in mind that memory controller is in the CPU, which is where the DDR4/5 memory controller is. The motherboards job is to connect the right pins on the DIMMs to the right pins on the CPU socket. The days of a off chip memory controller/north bridge are long gone.

So if you look at an AM5 CPU it clearly states:

   * Memory Type: DDR5-only (no DDR4 compatibility).

   * Channels: 2 Channel (Dual-Channel).

   * Memory Width: 2x32-bit sub-channels (128-bit total for 2 sticks).

[fluoridation]

Why are you quoting something that contradicts you? It clearly states it's a dual channel memory architecture with 32-bit subchannels. The fact the two words are used means they mean different things.

>In computer manufacture speak dual channel = 2 x 64 bit = 128 bits wide.

Yes, because AMD64 has 64-bit words. You can't satisfy a 64-bit load or store with just 32 bits (unless you take twice as long, of course). That you get 4 32-bit subchannels doesn't mean you can execute 4 simultaneous independent 32-bit memory operations. A 64-bit channel capable of a full operation still needs to be assembled out of multiple 32-bit subchannels. If you install a single stick you don't get any parallelism with your memory operations; i.e. the system runs in single channel mode, the single stick fulfilling only a single request at a time.