Computer won't start with RAM in certain slots

Looking at the manual for your motherboard from MSI's website, it says the following:

Always insert memory modules in the DIMMA2 slot first.

So no, it's not just a recommendation. It's a requirement, although the wording on the diagram in the manual is misleading. You must always install the first module in DIMMA2, but it's up to you how to install after that first one.

You certainly want to follow the manual's recommendation when it comes to which slots to use, as installing the memory incorrectly could limit memory throughput significantly (you want to be able to leverage dual-channel memory whenever possible).

The slots on your board are labeled DIMMA1, DIMMA2, DIMMB1, DIMMB2. DIMMs A2 and B2 should be filled first to leverage dual-channel memory. If you put one in A2 and another in A1 or B1, the machine might boot, but it will not run in dual-channel mode. Follow this diagram for installing memory (also from the manual):

Sam Forbis

Posted 2020-02-26T19:02:09.513

Reputation: 1 198

Nice bit of research! if you want to see complicated RAM slot mapping, see the old 2008 Mac Pro - this from OWC - https://eshop.macsales.com/tech_center/manuals/Memory/macpro.pdf Not exactly intuitive ;-))

– Tetsujin – 2020-02-26T19:43:43.090

2@Tetsujin Leave it to Apple to complicate something like that even more than it already is! You'd think that with as far as we've come in terms of technology, something like this would be a lot simpler... – Sam Forbis – 2020-02-26T19:51:00.140

2@SamForbis - It actually is simple. – Ramhound – 2020-02-26T21:40:29.393

hmm... but why? – Hp93 – 2020-02-27T07:45:32.480

10RAM installation has had instructions / configurations like this for decades, going back to chip locations and SIMM modules. – Mick – 2020-02-27T08:21:59.927

1This isn't specific to this board either. I got a Tomahawk X470 at home with exactly those pictures and the exact same instructions in it. I think it's been fairly standard for a long time now. – Mast – 2020-02-27T08:42:14.937

@Tetsujin I have that MacPro, and I did upgrade its memory. :) It is still my backup and P4 server. – Almo – 2020-02-27T15:24:51.037

1@SamForbis: The difficulty is that the code to figure out which modules are installed and configure things appropriately needs to have some RAM available when it starts up, before it has figured out what's installed. – supercat – 2020-02-27T17:19:57.247

I realize now it was a bit silly for me to say what I said in my first comment, I do understand everything that everyone's said and I know how CPUs have direct connections to the RAM so it's not so simple to just dynamically change which DIMMs go to which channels. I guess what it comes down to is technological progress only really happens when there's a reason to, and for something as trivial as putting your DIMMs in the right slots, there hasn't been a reason for them to simplify that process. – Sam Forbis – 2020-02-27T17:26:21.960

2It is impossible to change which memory channel a DIMM socket is connected to. One memory "channel" is really just one set of pins on the CPU socket that can be connected to one or more memory modules. On the motherboard, all modules on the same "channel" are simply wired in parallel. The number of memory channels is limited by the number of pins on the CPU socket. Multiple modules on one memory channel is a simple method for providing more memory capacity without significantly increasing the cost. – alex.forencich – 2020-02-27T20:49:07.707

8As for why they want to to install DIMMA2 first as opposed to DIMMA1, this is for electrical reasons. A circuit board trace with nothing at the end of it is a "stub" and it creates reflections. After all, electrical signals are carried by electromagnetic waves, and they can bounce off of stuff just like light can bounce off of a window. Therefore, you have to populate the memory module at the end of the trace first to minimize these reflections and ensure proper operation. – alex.forencich – 2020-02-27T20:51:35.887

1@SamForbis that OWC document is exactly what you would expect for a system with four memory channels. – alex.forencich – 2020-02-27T21:06:40.100

@Tetsujin the new Mac Pro has similar requirements, and a diagram on the memory DIMM covers to show how to install it. https://support.apple.com/en-gb/HT210103

– Tim – 2020-02-27T21:48:19.310

@SamForbis of all the things to blame apple for, over-complicating things like that isn’t really their ball game! – Tim – 2020-02-27T21:49:12.423

5tl;dr RTFM. Seriously, people. – Ian Kemp – 2020-02-28T07:57:03.007

@alex.forencich It wouldn't be impossible to change which channel a DIMM is connected to. You would need high-bandwidth switches similar to those used on PCIe lanes. The issue is they would add latency, and latency is critical in memory. Plus cost, and there's no good reason. – SomeoneSomewhereSupportsMonica – 2020-02-28T20:31:23.437

6You've still options to test… 1&2, 3&4 would potentially confirm an issue with slot 3. Check, if it boots, that you can see all the RAM in both cases. – Tetsujin – 2020-02-26T19:14:06.883

6It's also good to test individual slots alone, in case the specific slot is having trouble. I.E. put RAM only in Slot 3 and try it. I suspect all of the slots will work alone, due to SamForbis' answer below. Knowing Slot 3 is trouble will inform you as to whether to try upgrading to all 4 slots in the future. – Christopher Hostage – 2020-02-26T19:36:47.223

Always read the manual for things like this if you want the optimal configuration for your motherboard. – Thorbjørn Ravn Andersen – 2020-02-29T13:26:55.237

Computer won't start with RAM in certain slots

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