The Ultimate Guide to Mac RAM

Mac|Life
by Florence Ion

Learn everything you need to know about how RAM works inside your Mac — and how to give your mac a warp-speed boost by installing better, faster memory.

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A dozen or so years ago, RAM was relatively pricey and you couldn’t simply buy it online. But as computers started getting faster and performing more intensive operations, cheap, readily available memory became a necessity. Conveniently, that also makes it one of the best, easiest upgrades you can make to your machine. After all, as your Mac’s hardware becomes dated, it may not run as fast as when you first took it out of the box. A little boost in memory could give it that new lease on life you’ve been hoping for — and save you from having to purchase a whole new system just yet. We’ll walk you through how RAM works inside your Mac and show you how to tweak it so it makes your applications run almost twice as fast. We’ll even show you how to test your Mac to find out if an upgrade is really necessary. It’s time to flip the switch and take your Mac up to light speed!

How Does RAM Work?

These days, RAM is so easy and cheap to purchase that upgrading it has become a basic, essential part of computer tinkering. The more memory your Mac has available, the quicker your computer can access data and applications, making it easy for you to switch between application windows and do things like apply a ton of Photoshop filters, then instantly launch a game in Steam.

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But exactly how does it work? The best analogy we’ve come across compares RAM to storage bins (credit to Jon Stokes of Ars Technica for conceiving it). In this model, your operating system is constantly storing a heap of small bits of data and sorting them into bins. Each bin of data is assigned an address, and every time an application needs to access it, your computer’s processor goes in search of the appropriate bin based on its assigned address. Following this model, it’s actually better to have extra memory because it keeps the system from having to utilize the hard drive as additional bins to store data. The more data that RAM can store in its bins, the faster your computer’s operating system can perform.

Currently, computers use synchronous dynamic random access memory, or SDRAM. It has a double data rate (DDR), and that means it gets more than twice the bandwidth of the original type of SDRAM. So, going back to our analogy, SDRAM splits each bin into two but assigns them only one address, making it easier for the RAM to read through data two at a time without having to add extra bins. This keeps memory modules small in size and efficient in performance. Maybe some day, we’ll have RAM reading memory at three times the rate, essentially converting every duplex bin into a triplex.

Let’s Get Technical

Confused by all the jargon used to describe RAM? This quick primer will get you up to speed.

DDR2 vs. DDR3

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The key notch in DDR2 is 7cm from the edge, while the notch in DDR3 is about 5.5cm — this prevents you from mingling incompatible RAM.

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“DDR” stands for “double data rate,” and both DDR2 and DDR3 are commonly used in today’s Macs. Since it’s the older technology, DDR2 runs at lower clock speeds, transfers less data per clock tick, and uses more power than DDR3. DDR2 commonly runs at only 800MHz, while DDR3 can run at higher clock speeds (the standard is 1,333MHz) and thus offers higher bandwidth than DDR2. The two are not compatible.

DIMM

An acronym for “dual inline memory module,” a DIMM is the standard form of memory stick used in most computers today; it replaced single inline memory modules. Among other details, these modules vary by the number of pins (the electrical contacts that look like copper teeth) and the position of the notch that guides them into the motherboard.

SO-DIMM

Laptop memory sticks are called SO-DIMMs (“SO” stands for “small outline”) because they’re so much smaller. Along with MacBooks, iMacs and Mac minis use SO-DIMMs to fit their svelte form factors.

FB-DIMM

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FB-DIMM doesn’t look much different than SDRAM, but trust us — it’s different.

Today’s Mac Pros use DDR3 DIMMs, but the original Intel Mac Pros used Fully Buffered (FB) DIMMs. These RAM sticks featured additional memory controllers that handled the communications between the RAM and memory controller. FB-DIMMs are now obsolete, but they allowed the Mac Pros of that era to run far more RAM than an iMac could. They aren’t compatible with DDR3 memory modules.

Memory Bandwidth

Think of memory bandwidth as how much data a RAM module can process per clock cycle. RAM modules are often labeled by this bandwidth. A DDR3 module running at a clock speed of 1,066MHz, for example, may also be referred to as a “PC3-8500 module,” which indicates that it’s a DDR3 module with a theoretical bandwidth of 8,500MB/s.

Memory Bus Speed

This term refers to the clock speed of the connection between the memory controller and the memory itself. It’s often referred to as “PCnnn”, where “nnn” is any sort of three- or four-digit number. Generally, the higher the bus speed on the motherboard, the faster the processor will access the RAM.

Channels

Memory bandwidth can be upped beyond that of a single module by running multiple modules in multiple channels. If one is good, two is better. MacBooks and MacBook Pros use dual-channel memory, which means that they require two or more memory modules for optimum performance. Current Mac Pros can go up to three channels per processor because of the Intel Nehalem (Xeon series) architecture, meaning that they can get data transfer rates of up to 25.6GB/s when all three channels are engaged.

Virtual Memory

Modern operating systems can use space on your hard drive or SSD to simulate RAM. The operating system will manage what gets moved into the virtual memory so performance is not impacted too significantly. Adding RAM is preferred because virtual memory is dramatically slower than RAM.

RAM Pairs

You can get the best memory bandwidth by using memory in matched pairs. For example, if you wanted to upgrade 2GB of RAM in your MacBook to 4GB, the most bandwidth would come by replacing the two original 1GB sticks with a pair of 2GB sticks.

32-bit vs. 64-bit

Think of your Mac’s ability to address memory as a phone book in a small town. The 32-bit phone book only has enough entries to list the addresses of 4,000 people. The larger 64-bit phone book used in the city has enough entries to store the unique addresses of 144,000 people. The 32-bit Tiger OS is that small-town directory and can only use 4GB of RAM (even if you have 8GB installed), while Snow Leopard and Lion can use an astonishing — and purely theoretical! — 16 billion GB of RAM. In reality, the most RAM we’ve seen to date is a dual-processor Mac Pro with 64GB.

Latency

The delay when the memory controller tells the memory module to access a particular piece of data. Generally, the lower the latency, the better the performance and response in applications.

ECC

Error Correction Code. Mac Pros all use ECC RAM that’s capable of correcting multi-bit errors. MacBooks and Mac Book Pros use non-ECC RAM.

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