A computer buyer’s guide: RAM

Today we’re dealing with how to choose RAM for your system. Whether you’re building your computer from scratch or upgrading our desktop or laptop, there are a few things you should know before you attempt to upgrade your machine. RAM is an acronym for Random Access Memory and is a component of a computer that feeds the CPU with the data it needs to access to perform its tasks. Accessing RAM is much faster than reading data from mass storage, which would make the machine crawl because the hard drive wouldn’t be able to provide the CPU with the data it needs to operate at a reasonable speed.

RAM generations explained

Computer memory has been standardised in the late nineties and early nineties in the DDR SDRAM. For the purpose of this article, you don’t need to know what the whole acronym stands for and all the technicalities associated with it. Suffice it to say, RAM has been improved since the early 2000s in four generations. The most recent of these is known as DDR4 and is used on newer motherboards for the latest Intel processors. AMD does not currently support DDR4, but its upcoming Zen architecture will. However, DDR3 is still good enough for most users: the technology website LinusTechTips actually compared DDR3 and DDR4 and found no meaningful difference in performance between the two.

Are the generations intercompatible?

The inexperienced user might ask “what’s the big deal about the two generations? Can’t I just install DDR4 modules in my computer from 2012?” The answer, unfortunately, is no. The three generations are electrically incompatible, so in order to prevent the installation of a, say, DDR4 module in a motherboard that supports DDR3, RAM manufacturers decided to make them physically incompatible as well, so that they cannot installed by mistake.

Right now we’re in the middle of the transition between DDR3 and DDR4, with current Intel processors supporting both generations and motherboard manufacturers offering products sporting either DDR3 or DDR4 RAM slots (also known as DIMM slots). To solve the issue of having to manufacture motherboards with either one slot type or the other, Intel came up with something called UniDIMM, a slot that can accept both generations. So far, however, there are no motherboards to feature such slots, and I honestly doubt there ever will.

How do I know what generation does my motherboard support?

RAM modules have a notch placed slightly off center that ensures that they’re seated properly in the slot (they fit only in one way) and makes the various generations physically incompatible with each other. Never try to fit a module inside an incompatible socket: this will damage both the motherboard and the module itself.

The image below shows a DDR3 and a DDR4 module. Notice how the notch is in a different place.

Physical comparison between a DDR3 and a DDR4 module. Notice the different placement of the notch. (Click to enlarge)
Physical comparison between a DDR3 and a DDR4 module. Notice the different placement of the notch. (Click to enlarge)

There is also another way to tell the two apart, but it’s not exactly a failsafe way.

What specifications should I look for in my memory?

One important metric for RAM is its transfer rate, which determines how often the CPU can transfer information to it in a second.

Good DDR3 RAM has a transfer rate of 1600 MHz, although some models claim they can operate at 1866, 2100 or even 2400 MHz. This is only a half-truth, though. In reality, most (if not all) motherboards run their DIMM slots at 1600 MHz by default, with higher transfer rates becoming available only through overclocking, a practice that voids your motherboard’s warranty and doesn’t yield significant performance improvements.

DDR4 modules are usually sold with speeds of either 2133 MHz or 2400. 2100 MHz models are a bit more common and slightly less expensive, while not being much slower than their 2400 MHz counterparts. If you’re in the market for DDR4 RAM, 2100 MHz models are the definite bang for the buck.

Another important specification is the CAS latency, which defines how long the RAM takes to access a particular segment of memory and is expressed in milliseconds. For either generation, the lower the latency, the better.

What form factors are there?

There are only two form factors to look out for and they’re easy to tell apart. Commonly, desktop computers use DIMM modules, which measure 133.35 mm in length.

A SODIMM DDR3 RAM module, commonly used on laptops.
A SODIMM DDR3 RAM module, commonly used on laptops.

Laptops, some mini-ITX motherboards and Intel NUCs take SODIMM modules, which are easily distinguishable from full-length DIMM modules due to their shorter length (68 mm).

How much RAM do I really need?

What we’ve discussed so far is all well and good, but you’re probably here just to have a definitive answer as to how much RAM does your computer need. Asking that question on any computer enthusiast forum will probably yield twenty different answers, each reflecting that particular user’s preferences and biases. More often than not, the story goes along the lines “get as much RAM as your motherboard can possibly take”. Anyone can see how that’s simply a useless suggestion, because most users don’t need ludicrous amounts of memory for their system. Moreover, memory is expensive and unused memory is wasted memory.

In order to give a really useful answer, we should consider the types of applications we intend to run on our computer. In a run-of-the-mill small office computer, this will probably amount to a web browser, an office suite, possibly an email client and a professional application specific to that particular freelancer’s or business’, like a photo editing application like Photoshop, a video editor like Premiere, and so on and so forth.

You should also be aware of how the operating system handles memory. At boot, modern operating systems copy the information they need to work into memory. This not only includes stuff needed by the operating system itself, like drivers or services, but also the programs that the user wants to execute automatically every time she turns on the computer. Skype is a very common example. After this, the operating system will locate the most-often used files and load those too into memory to use as cache. This is known as prefetching and, although it makes for longer boot times, makes the system more responsive after the whole process is completed, because there’s no need to read commonly-accessed information from disk (which is an order of magnitude slower than RAM).

After this whole process is completed, the memory is almost filled up. In Windows’ Task Manager, used memory is represented in dark purple, cache in light purple, and free memory in white. When deciding how much RAM you need for your next computer, a good starting point is your old one. Open the Task Manager with Ctrl + Shift + Esc, then go to the Performance tab. Alternatively, right-click the taskbar and select the appropriate option. Now, ask yourself the following two questions.

How much RAM is currently in use? Is this more or less than half of the memory currently installed on your system?

In case you’re currently using more than half of your memory, a good rule of thumb suggests that you should install more RAM on your next computer.

How big is your paged pool?

RAM usage in a clean install of Windows 10.
RAM usage in a clean install of Windows 10.

The paged pool is information that should reside on the RAM but for any reason (commonly because the system has run out of RAM) is being paged, i.e. momentarily placed, on your hard drive. As we suggested earlier, information paged to the hard drive is much slower to access than that already loaded in RAM. If your paged pool (or paging file, in Windows Vista and 7) is too big, it’s time to upgrade your RAM on your old system, or get more memory for your new one.

About Andrea Luciano Damico 110 Articles
Andrea Luciano Damico is a freelance translator from Italy. Among his interests are linguistics, technology, video games, and generally being a chill guy. He runs Let's Translate.it and Tech4Freelancers.net.