calling all 2017 iMac owners. Need your help on RAM

Maker of stock RAM

  • Hynix Korea

    Votes: 5 33.3%
  • Micron, China

    Votes: 10 66.7%

  • Total voters
    15

HiFiGuy528

macrumors 68000
Original poster
Jul 24, 2008
1,850
39
I hope you guys can tell me if the factory RAM from Apple in your iMac for 4GB x 2 (8GB) or 8GB x 2 (16GB), etc. are all the sticks same brand? by SK Hynix Korea or Micron China?
 

theluggage

macrumors 601
Jul 29, 2011
4,539
3,337
I hope you guys can tell me if the factory RAM from Apple in your iMac for 4GB x 2 (8GB) or 8GB x 2 (16GB), etc. are all the sticks same brand? by SK Hynix Korea or Micron China?
I have 2x8GB from Micron based on the Manufacturer ID of 0x802C.
2 x 4Gb with the manufacturer ID 0x802C - same as the 2x8GB Micron (Crucial) sticks I added myself.

On the subject of RAM, any views on the best way to mix two different sizes of RAM? Mine came with one Apple stick in slot 0 of each bank (i.e. the 1st and 3rd slots) so I stuck the new 2 new 8GB sticks in the empty slots, giving me:

Bank 0/DIMM 0: 4GB
Bank 0/DIMM 1: 8GB
Bank 1/DIMM 0: 4GB
Bank 1/DIMM 1: 8GB

It works fine, but I suspect the optimum is to have two matching sticks in each bank - which is what I would have done if I'd thought to check the system report to see how the slots were numbered before shutting down and opening the RAM hatch... Everything I'd read on the interwebs led me to expect to find the Apple modules in the top two slots...

Anybody know the "correct" way (specifically for the 2017 iMacs) and/or whether the potential performance boost is worth the bother of re-arranging things (I suspect not)?
 

HiFiGuy528

macrumors 68000
Original poster
Jul 24, 2008
1,850
39
Thank you for the help so far everyone. My eBay RAM arrived today, but my iMac is not here yet. :( I hope my iMac comes with Micron RAM so it matches my eBay purchase. Can someone verify that I bought the correct set of RAM sticks?

My plan:

Bank 0/DIMM 0: 4GB (stock)
Bank 0/DIMM 1: 4GB (eBay)
Bank 1/DIMM 0: 4GB (stock)
Bank 1/DIMM 1: 4GB (eBay)

total: 16GB RAM.

FullSizeRender 3.jpg
 

SaSaSushi

macrumors 601
Aug 8, 2007
4,100
493
Takamatsu, Japan
2 x 4Gb with the manufacturer ID 0x802C - same as the 2x8GB Micron (Crucial) sticks I added myself.

On the subject of RAM, any views on the best way to mix two different sizes of RAM? Mine came with one Apple stick in slot 0 of each bank (i.e. the 1st and 3rd slots) so I stuck the new 2 new 8GB sticks in the empty slots, giving me:

Bank 0/DIMM 0: 4GB
Bank 0/DIMM 1: 8GB
Bank 1/DIMM 0: 4GB
Bank 1/DIMM 1: 8GB

It works fine, but I suspect the optimum is to have two matching sticks in each bank - which is what I would have done if I'd thought to check the system report to see how the slots were numbered before shutting down and opening the RAM hatch... Everything I'd read on the interwebs led me to expect to find the Apple modules in the top two slots...

Anybody know the "correct" way (specifically for the 2017 iMacs) and/or whether the potential performance boost is worth the bother of re-arranging things (I suspect not)?
The correct way is to put matching DIMMs in DIMM0 and DIMM1 exactly as Apple ships it (and as you have it installed already). It will run with mismatched RAM in DIMM0 and DIMM1 but not in dual channel and at a considerable loss in performance.

Always put matching RAM in alternating slots in the machine for best performance.
 
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theluggage

macrumors 601
Jul 29, 2011
4,539
3,337
Your configuration is correct. My factory DIMMS are in BANK 0/DIMM0 and BANK 1/DIMM0.
The correct way is to put matching DIMMs in DIMM0 and DIMM1 exactly as Apple ships it (and as you have it installed already).
Thanks (I hope!). Not doubting you, but are there any sources for this? The amount of contradictory - or just plain ambiguous (along the lines of "The memory modules are installed into matching banks" - uh, right...) - information out there on the tubes beggars belief. Even the wikipedia article seems to contradict itself.

So I take it that, basically, "Bank 0" == "Channel 0" etc. (as opposed to "DIMM 0" == "Channel 0") so that matching DIMMs should be inserted with one in each bank but with the same DIMM numbers, so they can be accessed in parallel...?
 

SaSaSushi

macrumors 601
Aug 8, 2007
4,100
493
Takamatsu, Japan
Thanks (I hope!). Not doubting you, but are there any sources for this? The amount of contradictory - or just plain ambiguous (along the lines of "The memory modules are installed into matching banks" - uh, right...) - information out there on the tubes beggars belief. Even the wikipedia article seems to contradict itself.
I don't blame you for your skepticism because I've seen the conflicting reports myself. The proof for me is that in addition to Apple shipping their own RAM in that config, with mismatched RAM in DIMM0 and DIMM1 on this machine I get Geekbench multi-core scores of around 16000 and with matching sets 20000+. Also, in BootCamp Windows with matching RAM CPU-Z shows dual channel mode active but not with the RAM mismatched.

So I take it that, basically, "Bank 0" == "Channel 0" etc. (as opposed to "DIMM 0" == "Channel 0") so that matching DIMMs should be inserted with one in each bank but with the same DIMM numbers, so they can be accessed in parallel...?
Correct, Bank0 and Bank1 should have matching RAM for each DIMM channel (DIMM0 and DIMM1).

When viewing the RAM configuration in About This Mac > Memory, the channels are: DIMM0 on the top and DIMM1 on the bottom.

 
Last edited:

J.Gallardo

macrumors 6502
Apr 4, 2017
299
74
Spain
I don't blame you for your skepticism because I've seen the conflicting reports myself. The proof for me is that in addition to Apple shipping their own RAM in that config, with mismatched RAM in DIMM0 and DIMM1 on this machine I get Geekbench multi-core scores of around 16000 and with matching sets 20000+. Also, in BootCamp Windows with matching RAM CPU-Z shows dual channel mode active but not with the RAM mismatched.



Correct, Bank0 and Bank1 should have matching RAM for each DIMM channel (DIMM0 and DIMM1).

When viewing the RAM configuration in About This Mac > Memory, the channels are: DIMM0 on the top and DIMM1 on the bottom.

This is my plan for my new iMac as it arrives.
But, just curiosity: does it make a difference to install the big modules in DIMM0 & change small ones to DIMM1?
(Maybe an accessing time response... I don’t know how access to memory works; no tech savy, sorry)
Thank u in advance.
 
Last edited:

SaSaSushi

macrumors 601
Aug 8, 2007
4,100
493
Takamatsu, Japan
This is my plan for my new iMac as it arrives.
But, just curiosity: does it make a difference to intall the big modules in DIMM0 & change small ones to DIMM1?
(Maybe an accessing time response... I don’t know how access to memory works; no tech savy, sorry)
Thank u in advance.
I tried both configurations and it made absolutely no difference at all in my testing. In case you're running modules with different CL the iMac will run all installed memory at the speed of the slowest part no matter where is inserted.
 

kdoug

macrumors 6502a
Jun 2, 2010
969
129
Iowa City, IA USA
I tried both configurations and it made absolutely no difference at all in my testing. In case you're running modules with different CL the iMac will run all installed memory at the speed of the slowest part no matter where is inserted.
SaSaSushi, my machine also just came in today also and I'm going to be installing the 32GB this evening. Just so I understand, you should leave the 8GB in their current slots and put the 2 sticks in the open slots? Also my understanding is they're staggered is this correct? Thanks!
 

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SaSaSushi

macrumors 601
Aug 8, 2007
4,100
493
Takamatsu, Japan
SaSaSushi, my machine also just came in today also and I'm going to be installing the 32GB this evening. Just so I understand, you should leave the 8GB in their current slots and put the 2 sticks in the open slots? Also my understanding is they're staggered is this correct? Thanks!
Yes, the Apple RAM is already in proper dual channel configuration so you just add your new set into the empty slots.

Be sure to firmly seat the new modules and also be aware that on the very first power up with the new RAM installed you are going to have a black screen for about 30-60 seconds while the machine initializes itself for the new configuration.
 
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Clinbio

macrumors newbie
Sep 24, 2017
2
2
Can I add just one , e.g a Crucial CT16G4SFD824A model, to the new 2017 model 27inch iMac? So it goes from 8 to 24 Gb? If that's possible, what slot occupation would be the best ? Currently the two 4Gb are at slot 1 and 3. Have the single 16 Gb module at slot 1 and move one 4Gb to slot 2 (and leaving the other at slot 3?)?
I don't want to fill up the last empty slot already but might do that some time in further future.
 
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