Why are we putting RAM sticks into 2nd and 4th slot first?

[Zaporro]

Hello,

Since long time already, we are told, when assembling PC and where motherboard has 4 RAM slots, that we should insert first pair of RAM into 2nd and 4th RAM slot first (or other two assigned RAM slots) and only after that, if we get 2nd pair we should put it into 1st and 3rd slot.

Why is that? And please, no "because manual says so" answers. I know that most consumer grade motherboards and chipsets support dual channel so because of way in which controller is connected to RAM slots there might be preference for 2nd and 4th slot over 1st and 3rd slot
but
aren't all slots made to be used?

I've seen lot of issues where people would put only 2 RAM sticks into 1st and 3rd slots and their systems would not boot up but swapping them to 2nd and 4th slot would solve the issue. Why is that happening? And whatever is happening, is this the same reason why manufacturers of motherboard in their QVL list will say "X ram model will work in X slot but not in Y"?

 

[scout_03]

it is the way motheboard maker made wiring on them so it detect those 2 slot as first channel for ram and made there bios to use it this way .

 

[delaro]

A1 "1"and B1 "3" are normally reserved for Single Channel of the same type

A2 "2" and B2 "4" are typically reserved for Dual Channel of the same type

Current boards don't really seem to give a * where you put dual channel but single channel still is preferred on A1 and B1.If you need to know more its time to start reading on the difference between Single and Dual Channel.

https://www.gamersnexus.net/guides/1349-ram-how-dual-channel-works-vs-single-channel?showall=1

 

[USAFRet]

Hello,

Since long time already, we are told, when assembling PC and where motherboard has 4 RAM slots, that we should insert first pair of RAM into 2nd and 4th RAM slot first (or other two assigned RAM slots) and only after that, if we get 2nd pair we should put it into 1st and 3rd slot.

Why is that? And please, no "because manual says so" answers. I know that most consumer grade motherboards and chipsets support dual channel so because of way in which controller is connected to RAM slots there might be preference for 2nd and 4th slot over 1st and 3rd slot
but
aren't all slots made to be used?

I've seen lot of issues where people would put only 2 RAM sticks into 1st and 3rd slots and their systems would not boot up but swapping them to 2nd and 4th slot would solve the issue. Why is that happening? And whatever is happening, is this the same reason why manufacturers of motherboard in their QVL list will say "X ram model will work in X slot but not in Y"?

1. Because that's the way the slots are wired. 1 & 3 on one channel, 2 & 4 on the other channel.

2. Why 2 & 4? To prevent any possible interference with the large CPU air cooler. Sometimes those will overhang slot 1.

 

[InvalidError]

From the electrical engineering point of view, the DRAM dies provide adjustable bus termination and putting the only active DIMM for each channel on each channel's last slot puts them at the end of the bus where those bus terminations will be most effective at mitigating signal reflections, which should give you the likely best chance at trouble-free operation - the 5mm unterminated bus stub from an unpopulated DIMM slot before the bus termination is less disruptive than a 10-15mm stub at the end of the bus from the extra PCB trace distance and connector.

Support for multiple DIMMs per channel almost got scrapped from the DDR4 spec due to such signal integrity concerns.

 

[faslanetech]

It's pretty silly, I mean I know why they do it as people have explained above but they easily could have just flipped it to make it 1 and 2 instead of A2 to and B2 also just so it reads left to right one two three and four it's kind of silly but for whatever reason they wired it up that way. And it's not because the CPU Cooler overhangs LOL most CPU and heatsink are much taller than the ram sitting on your board so it's not going to overlap and if it does it's not going to be close enough to actually damage the ram if it's properly cooled that's just an old wives tale, you have to literally be baking that RAM for it to be an issue from the CPU being close to a one and with the popper CPU cooler a1 isn't going to get hot whatsoever anyway.

 

[drea.drechsler]

1. Because that's the way the slots are wired. 1 & 3 on one channel, 2 & 4 on the other channel.
...

If that's numbering the dimm slots sequentially going away from the CPU I'm not sure it's correct if you want a dimm in each channel for dual channel operation. So correctly, wouldn't it be 1 & 2 on one channel and 3 & 4 on the other?

So you install one dimm in 2, one in 4 for 2-dimm operation, adding one in 1 and the last in 3 for 4 dimm operation.

 

[drea.drechsler]

From the electrical engineering point of view, the DRAM dies provide adjustable bus termination and putting the only active DIMM for each channel on each channel's last slot puts them at the end of the bus where those bus terminations will be most effective at mitigating signal reflections, which should give you the likely best chance at trouble-free operation - the 5mm unterminated bus stub from an unpopulated DIMM slot before the bus termination is less disruptive than a 10-15mm stub at the end of the bus from the extra PCB trace distance and connector.

Support for multiple DIMMs per channel almost got scrapped from the DDR4 spec due to such signal integrity concerns.

That should have been obvious to an old RF engineer, but I just wasn't thinking that way at first.

This also suggests two-socket motherboard configurations should have much better potential since they completely lack unterminated stubs to the unpopulated sockets. Has this been shown true in practice?

 

[faslanetech]

You're correct. That's how most mobos are today. They call it A1,A2,B1, B2, so if two dimms used you want to occupy A2 and B2 OR if going left to right (thinking in sequential numbering) slots 2 and 4. That's how my Gigabyte board was and ASRock Z390 board is currently.

 

[faslanetech]

I just filled each of the four with an 8GB DDR4 300 so now I no longer have to care ;-)