Is it Intel Core i5-7640X support just 2 ram sticks? I have 4 x 4Gb ram but just 2 of them is visible. 8 memory slots on my mobo but just 2 is working. Situation changes when installing another cpu. i7-9800x all is fine with this cpu. What is wrong ?

[AlHill]

Tell us about the motherboard -

• What model board and manufacturer,
• does it have the latest bios,
• what memory it supports,
• what is the speed of the memory,
• is the 7640x supported,
• etc?

Also, here are the two processors compared.

https://ark.intel.com/content/www/us/en/ark/compare.html?productIds=189122,121500

Doc

[AUzie]

I replaced 3 motherboards for my child the first was asrock X299M Extreme4 second asrock X299 Extreme4 and the last one Gigabyte X299 UD4 Pro. I was thinking the problem is with asrock motherboards because these memory sticks was working fine in my main pc ROG RAMPAGE VI EXTREME OMEGA so i go for gigabyte and again the same problem. The last thing i done just instal my main cpu 9800x to that gigabyte mobo and all 4 rams start working fine. In 7640x cpu memory specifications says Max # of Memory Channels 2 Is it means just 2 sticks can be used?

[AlHill]

You did not answer my questions.

Doc

[AUzie]

What model board and manufacturer - GIGABYTE X299 UD4 Pro

does it have the latest bios - not sure but asrock had 

what memory it supports - Support DDR4 up to 4400(O.C.)

what is the speed of the memory -3200MHz HyperX HX432C16PB3K4/16

is the 7640x supported - Yes

[AlHill]

Drop the speed of the memory to 2666. Make sure the board has the latest bios. I am not concerned with the asrock board.

Also, you may want to try different, lower speed, memory.

Doc

[AUzie]

I will update my bios tomorrow and will see how it goes. I will tell you if that helps. Thank you

[AUzie]

Thanks again today I done bios update and finally all works fine :)

[AlHill]

glad you have it working.

Doc

[Lewis-H]

Most i5 motherboards support 4 sticks of DDR4, but in a dual-Channel configuration. This means that the CPU reads from 2 memory sticks at a time. In a quad channel setup, the CPU will read from 4 sticks of memory at a time. Each stick provides 64 bits of data to the CPU. So, you can see that a quad-channel CPU will require more pins than a dual channel. This is why CookieMania66 pointed out that this is not a feature of most desktop CPUs. What is your application ? Most games are GPU bound, not CPU bound, so most of us spend money on our GPUs.

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Lewis