Understanding Intel Mac RAM
From Mac Guides
- This article is based on a forum post written by CanadaRAM.
This guide addresses some common questions about RAM for all Apple's Intel-based Macs.
Contents |
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 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. For more information, see Matched RAM on Intel Macs.
- Most 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.
- 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 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 maximum RAM is different for different logic boards, this is explained in the following table:
| Intel Chip Type | RAM Limit | iMac | Mac Mini | MacBook | MacBook Pro |
|---|---|---|---|---|---|
| Core Duo (Yonah) | 2 GB | Rev 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) |
| Core 2 Duo (Merom) | 3.3 GB | Rev 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) |
| Core 2 Duo (Merom w/ Santa Rosa chipset) | 4 GB | Rev C (Aug '07 and later) | None as yet | Rev D and E (Nov '07 and later) | Rev D and E (Jun '07 and later) |
| Core 2 Duo (Penryn) | 4 GB | None as yet | None as yet | (Jan '08 and later) | (Jan '08 and later) |
| Core 2 Duo 1066 FSB (800 MHz RAM) | 4 GB | April 2008 | None as yet | None as yet | None as yet |
- 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 3.3GB limit have in reality a limit of 3GB available to the OS as otherwise 700 MB RAM is wasted.
- 4 GB SODIMMs (if you could find any) will not work in any Mac to date.
- 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.
- The Mac Pro has 2 memory "riser" boards, Riser A and Riser B, each with 4 memory slots (1,2,3,4)
- 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.
- 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.
- 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.
- 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.
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). The Kingston KTA-MB667/XX series are good too, however 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; they affect the allocation of memory to applications within Windows only.
