Motherboard has 4 DIMM slots but only supports dual channel?

Solution
Yes, 4 modules will work in a dual channel system.

Each DRAM module (stick) has a 64 bit wide buss to the memory controller. A single channel system can only read/write 64 bits at a time.

Dual channel makes the buss appear to be 128 bits wide by accessing two modules simultaneously, meaning it can read/write 128 bits at a time. So a dual channel system with four modules appears to have two 128 bit wide paths between the modules and memory controller.

A quad channel system with four modules installed would appear to the memory controller to have a single 256 bit wide path to the memory controller.

atomicWAR

Glorious
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The extra two slots just allow for more ram to be address even though it is still only dual channel. The problem with populating all dimms with Ryzen is the fastest ram speed you can get is 2400mhz (actually even intel has issues with ram speed when you populate 4 slots on a dual channel chip or 8 slots on a quad channel chip). If you only populate 2 dimms on Ryzen you can get over 3000mhz out of your ram.
 

The Gigabyte B250M-DS3H (rev. 1.0) is an Intel motherboard. I never ran into issues running 4 modules in my Intel motherboards (I buy 4 module kits).
 
Yes, 4 modules will work in a dual channel system.

Each DRAM module (stick) has a 64 bit wide buss to the memory controller. A single channel system can only read/write 64 bits at a time.

Dual channel makes the buss appear to be 128 bits wide by accessing two modules simultaneously, meaning it can read/write 128 bits at a time. So a dual channel system with four modules appears to have two 128 bit wide paths between the modules and memory controller.

A quad channel system with four modules installed would appear to the memory controller to have a single 256 bit wide path to the memory controller.
 
Solution

maxalge

Champion
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dual channel mode gives a performance boost to pairs of ram


it has nothing to do with how many ram slots you have

you can buy 4 sticks of ram and they will all work in dual channel mode because they are paired


a single stick will work in single channel mode, with no benefits from dual channel


if you had three ram sticks, the pair get dual channel boost, with the single not
 

atomicWAR

Glorious
Ambassador
Oops my bad for some reason I saw b370...and it is b250. Regardless the max frequency of your board for ram is 2400mhz which is supported by intel with all four dimms as it is stock ram frequency of that gen chips (much like 2400mhz is with Ryzen). It is in cases where you have OCd ram that the issue I mentioned applies but in your use case that argument is out the window becuase of motherboard restrictions. So there is no real reason to not populate all four dimms on your board if you want to other then heat and power consumption (which is higher with 4 dimms compared to 2).
 

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