Stop the Noiz

Introduction to Computer Memory

Frank Fox - 2011.02.23 - Tip Jar

A while back, I wrote about the ins and outs of hard drives. The mate to the hard drive is RAM (a.k.a. system memory or just plain memory). Where the hard drive is the bookshelf full of facts, RAM is memory for the brains, the processor.

RAM is one of the parts on the computer that has suffered the most from Product Confusion Fatigue (PCF). Do you need SDRAM or DRAM. What's the difference between DDR, DDR2, and DDR3? Does your computer take PC133 or PC3200 memory?

It's all a royal mess, because capacity, speed, and technology are changing too fast to get used to.

The bad news is that for older Macs, you still have to sort out between having SIMMs or DIMMs and all the other choices (Low End Mac has that information in its computer profiles).

The good news is that today's Macs are a different story. Everything is now some flavor of DDR, which is a standard established by JEDEC (Joint Electron Device Engineering Council) for SDRAM style memory.

Just because everything is now one type doesn't mean that it has gotten simple. Manufacturer and retailers throw around acronyms, trying to sound more impressive than they are. We'll start by sorting out the useful information about these acronyms, like what is the difference between DDR and SDRAM?

DDR (double data rate) is how the information on RAM is sent over to the processors. The nice thing about DDR is that it is faster (note the word double in the name). You could use DDR with any type of RAM, but JEDEC picked SDRAM for its standard.

SDRAM (synchronized dynamic random access memory) is memory that is in sync with the computer's bus. Buses on computers are not like a passenger bus. A computer bus is used for the communication line between the processor and the other parts, like RAM. Because it is in sync with the bus clock, information can be sent on a predictable schedule. This way there's not a lot of waiting for things to be ready.

Two other similar terms that is thrown around are DIMM and SO-DIMM. This isn't the kind of memory, but the type of package and connector the memory uses. DIMM is more for desktop computers. It is bulkier, but easy to install. SO-DIMM is about half the size, so it is used mostly in laptops. Apple also uses SO-DIMM in the Mac mini to make it smaller.

Every so often they add more pins to the DIMM connector. Most of the pins are used to pick out the right address for the memory that needs to be accessed. As more memory is packed onto each stick of RAM, more pins are needed. It's like when a city grows and more phone numbers are needed to keep up with all the growth.

In addition to picking the right address, the pins also are used to send the chunks of data. As the size of the chunks grow, the number of pins needed grows. Using separate pins for sending versus receiving data also adds to the number of pins needed. In finding the right kind, you just match the number of pins listed in your owners manual.

The 2006 through 2008 Mac Pro used FB-DIMM, a.k.a. FBD (fully buffered DIMM). This is a special style of RAM that had the possibility of fast transfer rates and more memory slots. Everything comes with trade-offs, and Apple had no choice in using this style of RAM, since it was required for the Xeon processor.

One other quirk brought on by the Mac Pro was the need to pick between single- and dual-rank memory (see PDF). I found the definition of rank on the JEDEC website.

A DIMM is organized as one or two physical sets of memory, called ranks. Note that single rank or dual rank is different from single-sided or double-sided, e.g. a single rank DIMM build from x4 DRAM devices is actually double-sided.

Dual rank memory provides an advantage for the Mac Pro. It can read and write at the same time from multiple processors. This gives a performance boost for the same amount of installed memory.

A special feature of RAM used for workstations and servers is ECC, error-correcting code. This adds a parity bit to show whether or not any of the data stored in memory was changed by accident. Because it costs a little more, it has slowly been phased out of most consumer computers. It may not happen very often that the ECC finds mistakes, but it gives a little extra insurance that the data stored in memory is correct.

Different from FB-DIMM is registered or buffered memory. This style of memory has the information stored up and then transferred over. Yes, it increases reliability, but at the price of speed. Used with ECC, you have the most reliable memory, but it's suited more for servers or mission critical computers. It is not well suited for consumers, especially gamers who judge their computers by speed alone.

Combining several of these features, such as the faster transfer rate (DDR), free of errors (ECC), and synchronized RAM (SDRAM) gives a powerful combination to make RAM that can keep up with today's fast, multiple processor computers.

All of Apple's current computers uses at least two processor cores. Apple has supported specification for RAM that have 2 or 3 of these features. This generally makes the RAM that is in a Mac slightly better than average, but often more expensive.

To those who don't see value in the higher specification, it makes Mac an easy target to criticize over price. One thing is true, third party vendors can provide Apple certified RAM and save you over buying it directly from Apple.

These acronyms only get us through the physical aspects of RAM. There are still another half dozen specifications to cover different aspects of speed, but we'll save that for another article.

When buying RAM for your Mac, I strongly recommend that you shop at a website that gives recommendations specifically for each Mac model. LEM

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