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.
