Understanding Intel Mac RAM

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This article or section is based on a forum post written by CanadaRAM.

This guide addresses some common questions about RAM for all Apple's Intel-based Macs.


Intel iMacs, MacBooks, MacBook Pros and Mac minis

  • All Intel Core Macs support dual channel memory access if matching modules are installed. The customary estimate is that this gives a 6% - 8% real world performance benefit. The modules do not have to be the same brand. That means it is quite possible but not 100% guaranteed, that adding a 3rd party SODIMM to an Apple supplied SODIMM of the same size will make a matched pair.
  • All Intel Core Macs will run with mismatched RAM, and will run in dual-channel asymmetric mode. Core 2 Duo Macs can run a 3 GB configuration (1 GB + 2 GB), which on average can be faster than a 2 GB matched pair if you are multitasking or using heavy programs like Photoshop, digital audio and digital video production that can benefit from the extra 1 GB. In general, more RAM trumps matched RAM. However, RAM is (mid-2009) currently cheap, and systems that are limited to 3 GB of RAM will accept 4 GB of RAM (2x2GB SODIMMs). For more information, see Matched RAM on Intel Macs.
  • Most Older Core Macs use the same DDR2-667 (PC2-5300) 200 pin SODIMMs. Therefore, choosing MacBook Pro RAM for the pre-April 2008 iMacs, MacBooks and Minis is correct.
  • Starting with the Late 2008 MacBook (Unibody only) and MacBook Pro Most New Core Macs (nVidia chipset) use the same DDR3-1066 (PC3-8500) 204 pin SODIMMs. Again, as with previous generations, choosing RAM for one model will work in others. The exceptions here are the Late 2008 White MacBook which retains DDR2-667 (PC2-5300) 200 pin SODIMMs (although there are reports of success with DDR2-800 (PC2-6400) RAM running at full speed in these machines) and the Mid 2009 White MacBook which officially supports DDR2-800 (PC2-6400) 200 pin SODIMMs.
  • The 667 MHz memory bus machines do not benefit from 800 MHz SODIMMs. This is a hardware limitation of the memory controller on the motherboard which is hardwired at 667 MHz (even though the Santa Rosa Front Side Bus is 800 MHz, FSB is not the same as the memory bus). There have been reports of 800 MHz SODIMMs failing in Penryn MacBooks and MacBook Pros.
  • The April 2008 iMac requires DDR2-800 MHz (PC2-6400) SODIMMs. It uses a 1066 MHz Frontside bus, and 800 MHz memory bus. There are reports that it will run with some 667 MHz SODIMMs, but performance will be compromised.
  • The March 2009 iMac requires DDR3-1066 Mhz (PC3-8500) 204-pin SODIMMs.
  • iMacs starting with the Late-2009 21.5" and 27" models have 4 memory sockets, so they can be expanded without removing the factory pair of SO-DIMMs. This is a first for recent consumer Macs. The i5 and i7 versions support DDR3-1333 (PC3-10660), due to a memory controller embedded inside the CPU. (This does not include the mobile i-series CPUs which are included in the Mid 2010 MacBook Pro).
  • The maximum RAM is different for different logic boards, this is explained in the following table:
Intel Chip TypeRAM Limit RAM Type iMacMac MiniMacBook MacBook Pro
CoreDuo (Yonah)& CoreSolo2 GBDDR2-667Rev A (Jan '06 to Sep '06)Rev A and B (Feb '06 to Aug '07)Rev A (May '06 to Nov '06)Rev A and B (Jan '06 to Oct '06)
Core2Duo (Merom)3 GBDDR2-667Rev B (Sep '06 to Aug '07)Rev C (Aug '07 and later)Rev B and C (Nov '06 to Nov '07)Rev C (Nov '06 to Jun '07)
Core2Duo (Merom w/ Santa Rosa chipset) 4 GBDDR2-667Rev C (Aug '07 and later)None as yetNone as yetNone as yet
Core2Duo (Merom w/ Santa Rosa chipset) 6 GBDDR2-667None as yetNone as yetRev D and E (Nov '07 and later)Rev D and E (Jun '07 and later)
Core2Duo (Penryn) 6 GBDDR2-667None as yetNone as yet(Jan '08 and later)(Jan '08 and later)
Core2Duo 1066 FSB (800 MHz RAM) 4 GBDDR2-800April 2008 iMac8,1None as yetNone as yetNone as yet
Core2Duo (nVidia chipset) 4 GBDDR3-1066March 2009 (edu model)March 2009None as yetNone as yet
Core2Duo (nVidia chipset) 6 GBDDR3-1066None as yetNone as yetOctober 2008October 2008 (15" screen only)
Core2Duo (nVidia chipset) 8 GBDDR3-1066March 2009 (other than edu model)Mid 2010 UnibodyNone as yetEarly 2009 (15" screen other than 2.4GHz, 17" screen)
Core2Duo (Wolfdale) 16 GBDDR3-1066October 2009 iMac10,1None as yetNone as yetNone as yet
i7 and i5 (Arrandale) 8 GBDDR3-1066None as yetNone as yetNone as yetMid 2010 (15" and 17")
i7 and i5 (Lynnfield) 16 GBDDR3-1333October 2009 iMac11,1None as yetNone as yetNone as yet
i7 and i5 (Sandy Bridge) 16 GBDDR3-1333; accepts DDR3-1600May 2011 iMac12,1None as yetNone as yetEarly 2011 (All Units)

  • All compatible DDR2-667 RAM will perform the same in an Intel Core2Duo Mac. Corsair, OWC and a few other companies have introduced some DDR2-667 RAM with a CAS Latency of 4, which is faster than the standard CL5. In real-world testing at Barefeats.com, the Corsair and OWC CL4 RAM averaged about 1% faster than CL5 RAM from Corsair, Kingston, OWC and DMS.
  • Macs with a 3 GB limit will accept 4 GB of RAM, however 700 MB will not be available to the OS due to hardware limitations and OSX will report 3.0 GB. The advantage of this is that two matched 2GB RAM SODIMMs will work in dual channel symmetric mode if this is available, whereas a 2GB and 1GB SODIMM will not.
  • 4 GB SODIMMs will not work in most Macs. 4 GB SODIMMs are relatively expensive. The Macs in the chart above listing 6 GB maximums will take one 4 GB plus one 2 GB, but two 4 GB modules crash the machine as soon as the OS tried to access more than 4096 MB of memory. The exceptions are the Early 2009 iMacs (not education model) and the Early 2009 MacBook Pro (except 2.4GHz) which can each take two 4 GB SODIMMs for a maximum of 8GB and, to some extent, the Late 2007 MacBook and later and the Mid 2007 MacBook Pro and later which can take one 4GB SODIMM along with one 2GB SODIMM for a maximum of 6GB.
  • Any hardware limitations will not be altered by a future OS or firmware update. There is no "32-bit" RAM limitation in OSX - Panther on a G5 tower can address 16 GB. It is not Mac OS-dependent.

Mac Pro and xServe

  • Mac Pro and Intel Xserve RAM is different, these computers both use FB-DIMMs (Fully Buffered DIMMs) and you cannot use conventional DDR-2 667 MHz or 800 MHz DIMM memory.
  1. The Mac Pro has 2 memory "riser" boards, Riser A and Riser B, each with 4 memory slots (1,2,3,4)
  2. The RAM must be installed in matching pairs, in this order: Riser A slot 1&2, Riser B slot 1&2, Riser A slot 3&4, Riser B slot 3&4.
  3. There is a small speed benefit to using 4 matching modules, organized in corresponding banks - for example, a 1 Gb module each in A1 A2 B1 B2 - this enables 256-bit access across the 4 modules. However, do not buy smaller RAM modules solely to preserve 256 bit access.
  4. There is a speed penalty (longer latency) to using slots 3 and 4 of the risers, so your largest RAM modules should be concentrated on slots 1 and 2 of each Riser.
  5. Apple recommends MacPro RAM that uses a large finned aluminum heatsink on both sides of the FB-DIMM. Generic FB-DIMMs such as the Kingston KVR- series do not have these. However the Intel Xserves don't have space for the large heatsinks, and they use flat heat-spreaders.

Mac Pro Xeon (Aug. 2006 - Dec. 2007)

The original 4 Core and 8 Core Mac Pros use 667 MHz FB-DIMMs. There is no point to installing faster FB-DIMMs

Mac Pro Penryn (Jan. 2008)

The 4 Core and 8 Core Penryn Mac Pros use 800 MHz FB-DIMMs, do not substitute the 667 MHz Mac Pro FB-DIMMs.

Mac Pro Nehalem (March 2009 and August 2010)

The Nehalem Mac Pros use DDR3-1066 (PC3-8500) ECC DIMMs (not FB-DIMMs), no large heatsinks, with temperature sensors. There are 4 memory sockets (2009 Quad-core 2.66GHz, 2.93GHz, 3.33 GHz and 2010 2.8GHz and 3.2GHz) or 8 memory sockets (2009 8-core 2.26, 2.66GHz, 2.93GHz and 2010 8-core 2.4GHz). They can implement dual channel or triple channel memory access modes. Fastest performance is using triple-channel mode, with three identical modules in sockets 1 through 3 and leaving socket 4 empty.

Mac Pro Westmere (August 2010)

The Westmere 6-core, 8-core and 12-core Mac Pros use DDR3-1333 (PC3-10660) ECC DIMMs, no large heatsinks, with temperature sensors. Single processor (6 core 3.33 GHz) machines have 4 sockets, Dual Processor (8 and 12 core 2.66GHz and 2.93GHz) machines have 8 sockets.

The 2010 lineup actually has a mixture of Nehalem and Westmere motherboards, so you need to verify your individual machine.

Buying RAM for Intel Macs

Main Article Buying RAM

The most-recommended USA sellers on MacRumors are Data Memory Systems, OWC (MacSales) and Crucial (use the link at the top of the forum pages to help support MacRumors). Another popular memory vendor is Ramjet. The Kingston KTA-MBxxx/xG series are good too, Kingston do not guarantee Mac compatibility with their ValueRAM series. Big-box discounters Newegg, Frys are popular for bargain-seekers, but usually offer no guarantee of compatibility or support on their generic RAM, so the buyer is responsible for choosing and testing. Keep that in mind, but don't let it scare you away from good deals. Since Intel macs use pretty standard hardware, memory compatibility is not as big of an issue as it used to be. The chances of memory not working in your Mac are about the same as on a comparable PC.

Look for a reputable seller with good shipping and return policies (no shipping or restocking charges on DOA RAM, for example), and good customer service. On all Macs except the Mac Pro, avoid RAM with heatspreaders; they will have issues physically fitting in iMacs, Mac minis and MacBooks.

Internationally, look for a seller in your country or economic zone. Some US sellers will ship internationally, but the buyer will be responsible for taxes, duties and customs brokerage. Insurance and shipping will be higher. International buyers are almost always responsible for shipping and border costs both ways for warranty replacements, so factor these costs into your decision.

Installing RAM for Intel Macs

Main Article Installing RAM

Installing third party RAM is fine with Apple and does not void a warranty. It is generally fairly simple to install new RAM in all Intel Macs. Instructions for do-it-yourself installation are on the Apple Support site and in the owner's manual.

Windows RAM usage under Boot Camp

32-bit Windows is limited to using less than 4 GB of RAM -- typically 3.0 - 3.5 GB is available to the OS, how much less depends on the hardware configuration. But only 2 GB is available to a given application, and the Windows user interface will have a maximum of 3 GB available (RAM between 3 GB and the hardware limitation may be used by drivers and OS processes)

On a Mac Pro, currently 32-bit Windows in Boot Camp is limited to 2 GB RAM total, regardless how much physical RAM is in the Mac. This is a combination of the EFI / Firmware and Windows limitations. Some Penryn MacBook Pros can allow Windows to see 3 GB total.

The Windows switches /3GB and /PAE will not make a difference to these limits in XP, Vista or Win 7; they affect the allocation of memory to applications within Windows only.