My 2017 5K iMac memory slot experiences

carbontune

macrumors member
Original poster
Sep 11, 2018
89
17
United Kingdom
Adding memory was one of the most frustrating experiences as a new iMac owner. I bought the 8GB RAM 2017 5K iMac so it has two DIMMs installed in slots 1 and 3 (slots 2 and 4 were empty). I had a 32GB memory kit from Crucial. I wanted 40GB but up until the early hours of Saturday morning, I didn't think that I could achieve this. Things finally fell into place after a few hours of trying various configurations out.

In terms of slot numbering, I'm assuming slot 1 is the lowest slot in the memory bay and slot 4 is the highest slot.

Slot 4 - empty
Slot 3 - 4GB Apple DIMM
Slot 2 - empty
Slot 1 - 4GB Apple DIMM

My first issue was a total lack of Apple-verified information available on the Internet into which slots to use for expanding the memory. As things would pan out, leaving the Apple installed 4GB DIMMs in slots 1 and 3 was best. It took a couple of calls across two days with Apple to get towards the right path, who ended up giving me slightly incorrect information.

I used Geekbench 4 to get some system and memory benchmarks as I tried various configurations.

These combinations did not work for me (iMac would not boot)

40GB
Slot 4 - 4GB Apple DIMM
Slot 3 - 16GB Crucial DIMM
Slot 2 - 4GB Apple DIMM
Slot 1 - 16GB Crucial DIMM

40GB
Slot 4 - 4GB Apple DIMM
Slot 3 - 4GB Apple DIMM
Slot 2 - 16GB Crucial DIMM
Slot 1 - 16GB Crucial DIMM

These combinations worked but memory performance was poor

40GB
Slot 4 - 16GB Crucial DIMM
Slot 3 - 16GB Crucial DIMM
Slot 2 - 4GB Apple DIMM
Slot 1 - 4GB Apple DIMM





These combinations worked at maximum performance

8GB
Slot 4 - empty
Slot 3 - 4GB Apple DIMM
Slot 2 - empty
Slot 1 - 4GB Apple DIMM

32GB
Slot 4 - empty
Slot 3 - 16GB Crucial DIMM
Slot 2 - empty
Slot 1 - 16GB Crucial DIMM

40GB
Slot 4 - 16GB Crucial DIMM
Slot 3 - 4GB Apple DIMM
Slot 2 - 16GB Crucial DIMM
Slot 1 - 4GB Apple DIMM



 

wardie

macrumors 6502
Aug 18, 2008
409
133
It’s because it is dual channel memory and works best in pairs in the slots you have documented. I’m sure there’s a few posts elsewhere on the forum and other sites about how to set it up. I just put by extra 2x8GB into the 2 spare slots, rightly assuming Apple stock memory is set up properly as a pair. Thanks for the data proof points, there was some speculation of whether you’d notice the difference or not.
 

carbontune

macrumors member
Original poster
Sep 11, 2018
89
17
United Kingdom
What threw me was why it wouldn't boot with with some combinations. It seems to require the Apple-installed DIMMs in their default slots. Whilst matched DIMM pairs in adjacent slots would work, all the DIMMs run in single channel mode so the overall performance of the iMac is a little slower.
 

jerwin

macrumors 68030
Jun 13, 2015
2,775
4,606
post the relevant section of the system information. Quickest way to get to it is to hold down the option key and open the apple menu.

here's mine. Yes, it's very nearly vintage.

Screen Shot 84.png
 

carbontune

macrumors member
Original poster
Sep 11, 2018
89
17
United Kingdom
Hi jerwin, here's the memory details in System Information:



One observation: notice how the order displayed in this report is different from yours. I'm wondering whether this leads to some confusion on the Internet. If you look on the right-hand side of your report under 'Size', then you'll see 4 / 4 / 8 / 8. If you look under the same column on mine, it shows 4 / 16 / 4 / 16. However, in terms of Bank and DIMM numbers, the layouts are the same.
 

carbontune

macrumors member
Original poster
Sep 11, 2018
89
17
United Kingdom
I don't get it. If you were not going to replace the original modules why didn't you just leave them where Apple installed them?
Because Apple told me to put the 16GB DIMMs into the slots occupied by the Apple 4GB DIMMs and to move the 4GB DIMMs. Sadly, my iMac wouldn't boot in that configuration.
 

mreg376

macrumors 65816
Mar 23, 2008
1,134
328
Brooklyn, NY
Because Apple told me to put the 16GB DIMMs into the slots occupied by the Apple 4GB DIMMs and to move the 4GB DIMMs. Sadly, my iMac wouldn't boot in that configuration.
But why even ask Apple? Did you ever hear of any problems by anyone adding RAM to Macs with accessible RAM slots when they simply installed the RAM into the open slots? I haven't. You KNOW that the machine was working fine with the original modules in their original slots. If you are going to assume anything, you would assume that the modules were installed there by the people who built the machine for a reason, no? Why look for trouble and ignore the obvious?
 

bbnck

macrumors 6502a
Mar 19, 2009
567
311
Because Apple told me to put the 16GB DIMMs into the slots occupied by the Apple 4GB DIMMs and to move the 4GB DIMMs. Sadly, my iMac wouldn't boot in that configuration.
Well now you know not to blindly listen to the advice of an AppleCare representative. They've asked you to swap around the DIMM modules for no good reason. It's rare to get through to anyone that's knowledgeable enough to answer anything more than basic questions correctly. In the future just ask your question here first before resorting to asking someone at AppleCare.

Nonetheless, appreciate the data points you posted. Interesting to see the performance difference between the various working configurations.
 
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