imac 2019 memory upgrade

[seryms]

Hi - Thank you for your response in advance. I bought a 2019 imac with 8GB of memory and bought 16GBx2 crucial RAM. I would like to understand where I place the new ones. One thing I read suggested it is OK to slot the new ones in the empty slots. Another one said that I need to remove the old 4GB ones, put the new 16GB ones in where the old ones were, and then put the 4GB in the slots that were originally empty. Would you please let me know where I can look to get this right? Does Apple give guidance for this? Thanks.

 

[theluggage]

Does Apple give guidance for this? Thanks.

(Caveat: I'm assuming this hasn't changed since the 2017 and earlier iMacs).

You're right - there's a lot of contradictory, just plain wrong or probably correct but garbled and ambiguous info out there.

Basic rule: it should work in any order, but to ensure that 'dual channel' mode is active - which makes a measurable difference to performance - you want a pair of matching DIMMs on each channel which - on an iMac - translates to having matching modules in alternate slots - i.e. you want the new 16GB DIMMS to go in between the exiting 4GB ones, giving 4GB 16GB 4GB 16GB or 16GB 4GB 16GB 4GB. Either of the methods you describe will achieve this.

E.g. in my Mac with 24GB of RAM, About This Mac -> System Report -> Memory shows:

BANK 0/DIMM0: Size: 4 GB

BANK 0/DIMM1: Size: 8 GB

BANK 1/DIMM0: Size: 4 GB

BANK 1/DIMM1: Size: 8 GB


See:
https://forums.macrumors.com/threads/ram-on-2017-imac-5k.2106351/#post-25826705

...where another user actually did some speed tests & booted Windows to verify dual-channel mode and I tracked down an ancient PDF that confirmed what the "Bank0/1" and "DIMM0/1" notation meant in terms of channels..