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I recently purchased 4 GB of RAM (2 x 2 GB modules) for a Mac Pro (8-core, early 2008), that runs 3 or 4 OS (Win, Linux and OS X) in out dev environment.

NB: I used the base OS X install to verify memory.

I initially installed the 2 new modules in Riser A/DIMM 2 and Riser B/DIMM 2, then new memory did not show up, total system memory 2 GB.

Then, I swapped the original 2 x 1 GB modules with the 2 GB modules, giving:

Riser A/DIMM 1: 2 GB
Riser B/DIMM 1: 2 GB
Riser A/DIMM 2: 1 GB
Riser B/DIMM 2: 1 GB

This gave me, 4 GB RAM in total.

So, I moved the 1 GB modules to DIMM 3, like this:

Riser A/DIMM 1: 2 GB
Riser B/DIMM 1: 2 GB
Riser A/DIMM 3: 1 GB
Riser B/DIMM 3: 1 GB

Bingo, 6 GB RAM.

Are the DIMM 2 sockets bad? Or am I missing something?


Below is the current System Profiler output, showing 6 GB of RAM:

Memory Slots:

  ECC:  Enabled

DIMM Riser B/DIMM 1:

  Size: 2 GB
  Type: DDR2 FB-DIMM
  Speed:    800 MHz
  Status:   OK
  Manufacturer: 0x802C
  Part Number:  0x3138484632353637324A4438304545314434
  Serial Number:    0xE04920F9

DIMM Riser B/DIMM 2:

  Size: Empty
  Type: Empty
  Speed:    Empty
  Status:   Empty
  Manufacturer: Empty
  Part Number:  Empty
  Serial Number:    Empty

DIMM Riser A/DIMM 1:

  Size: 2 GB
  Type: DDR2 FB-DIMM
  Speed:    800 MHz
  Status:   OK
  Manufacturer: 0x802C
  Part Number:  0x3138484632353637324A4438304545314434
  Serial Number:    0xE04921EE

DIMM Riser A/DIMM 2:

  Size: Empty
  Type: Empty
  Speed:    Empty
  Status:   Empty
  Manufacturer: Empty
  Part Number:  Empty
  Serial Number:    Empty

DIMM Riser B/DIMM 3:

  Size: 1 GB
  Type: DDR2 FB-DIMM
  Speed:    800 MHz
  Status:   OK
  Manufacturer: 0x80AD
  Part Number:  0x48594D5035313241373243503844332D5335
  Serial Number:    0x05049101

DIMM Riser B/DIMM 4:

  Size: Empty
  Type: Empty
  Speed:    Empty
  Status:   Empty
  Manufacturer: Empty
  Part Number:  Empty
  Serial Number:    Empty

DIMM Riser A/DIMM 3:

  Size: 1 GB
  Type: DDR2 FB-DIMM
  Speed:    800 MHz
  Status:   OK
  Manufacturer: 0x80AD
  Part Number:  0x48594D5035313241373243503844332D5335
  Serial Number:    0x05049201

DIMM Riser A/DIMM 4:

  Size: Empty
  Type: Empty
  Speed:    Empty
  Status:   Empty
  Manufacturer: Empty
  Part Number:  Empty
  Serial Number:    Empty
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2 Answers

Here's the Mac Pro (early 2009) manual on installing RAM. It's more than just installing in pairs, so read carefully.

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NOTE: This link is for Early 2009 MacPros - I haven't down voted but you might want to edit for clarity. –  rjstelling May 15 '09 at 14:08
    
Good point, thanks. –  ceejayoz May 15 '09 at 14:16
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You need to follow the memory layout suggested on the side door, and the instructions in the manual that shipped with your Mac Pro. The memory layouts have changed significantly from the older 2007 model Mac Pros.

edit: link to the memory replacement manual for early 2008 Mac Pros

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I thought the only layout was "install in pairs", and I did that? –  rjstelling May 15 '09 at 12:24
2  
Yes, but the definition of a pair changed between the models. This caught me out the first time I tried to add ram to a 2008 Mac Pro –  Dave Cheney May 15 '09 at 13:14
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