Wednesday, August 15, 2007

What to Remember about Memory

It seems like it's always a good idea to add memory (RAM) to your computer. Trouble is, memory comes in so many forms, it's hard to remember them all. And any memory you add has to be compatible with your processor and the memory you already have.

For desktop computers memory looks like the picture on the right. There are a few different form factors (sizes and shapes), and sometimes the modules are covered with a heat sink, but the picture gives you the general idea.


Memory is shorthand for random access memory or RAM. It can be confusing, because your hard drive is also a form of memory, as is your USB drive, memory stick, SDD, etc. Generally the term memory is used for the RAM in your computer, but context is key to being sure.

RAM stores the data your processor is using at the moment. It's relatively small, but fast. That's important, because anytime your processor is waiting for the data it needs, your computing is stalled.

In the overall data storage scheme, RAM falls between cache and the hard drive.

Your hard drive stores all of your software and most of your data (documents, music, etc.). Hard drives are big, but they're relatively slow so they're not good for holding data your processor needs instantly.

Cache is a small amount of memory built right into your computer's processor. It keeps things moving by putting the most urgently needed data right at your processor's fingertips.

RAM holds the data that the processor has called up from the hard drive, but hasn't been loaded into cache. Like the cache, it's volatile, meaning it loses all the data when you turn off your computer. Like the hard drive it can hold a fair amount of data. (OK, the "like the cache" sentence is dead on. The "like the hard drive" sentence is a bit of a stretch, but it kept some symmetry to the paragraph. You may have a couple of gigabytes of RAM, but probably more than a hundred gigabytes of hard disk capacity).

Memory modules almost always slide into slots on your computer's motherboard. It's simple and obvious on most desktop motherboards. On laptops it can be anywhere from simple to a freaking nightmare. I once used a computer than had its memory soldered onto the motherboard and no room for any additional modules. I like to think it was a one-time mistake that no computer maker has ever repeated.

In this article, by the way, we're talking about computer memory. That's important because memory with similar names and shapes shows up in printers and other devices and it can have some important differences.

Here's a short field guide to memory:

Form Factor is IMMportant
If you want to add memory to your computer, the first complication you'll encounter is the type of module -- form factor in industry jargon. To make identification easier(?) the various typ0es all have similar names, like DIMM, RIMM, SIMM and so on. Actually, they all sound alike because they're all memory modules (MM) and the chips on them are inline (I), so it's the first word (letter) that distinguishes them.

DIMM stands for Dual Inline Memory Module. It's the most common memory around these days. DIMMs are the next step after 9you guessed it) SIMMs, or single inline memory modules. Why the change? Computer processors went from 32-bit data paths to 64-bit data paths. That's twice the capacity to access data and DIMMs let your 64-bit processor get all of the data from a single chip. A 64-bit processor connected to SIMMs would have to read two separate memory module at the same time.

DIMMS have 168 pines running along the bottom, separated into two groups of 84. There's a notch in between the two sets. The chips are mounted above the pins and they're separated by a gap on the board. DIMMs can have two groups of 4 chips or one group of 5 and oen group of 4, for a reason we'll get to in a moment.

Small-outline dual inline memory modules are -- you see this coming -- smaller than regular DIMMs. These show up in laptops where they help keep the overall package small. The price of that extra smallness is, we'll, price:they cost more. Be careful, though, because your laptop might use an even smaller module.

Smaller yet, and even more expensive.

If you need to add SIMMs to your computer step back and take a look at the big picture. I'll be blunt: you're computer's getting up there in years and it might not be worth the investment.

But I know how it is with the old workhorse that saw you through those pre-XP Windows days and never gave up no matter how many blue screens of death it had to endure. And besides, SIMMs are dirt cheap, even free if you don't mind digging through a dumpster.

Your SIMMs have 72 pins running along the bottom (OK, it could be 30 but if it is, you need to consult with a Hospice).

Rambus inline memory modules are notable because they don't fit the profile, literally. They were developed by a company called Rambus and they have a proprietary design. My impression is that they don't come in computers anymore, they're used in other electronic devices that need memory.

Some Tech Terms
  • Parity, non-parity and ECC (error checking and correction) all refer to ways of checking for errors in a stream of data, or not. Parity and ECC add a bit (literally) of data to the stream to detect errors. For the parity process it means adding a little more memory (on some DIMMs it means a total of 9 chips -- one group of 5 nd another of 4). The ECC process just slows the memory down a tiny bit, not enough to wipe out the extra efficiency of eliminating errors. Non-parity memory doesn't check for errors.
  • Buffereing is a way to make the exchange of data with the processor more efficient.

So What's in Your PC?
OK, now you know what kinds of memory are out there. But what's in there (your computer, I mean)? Well, you might remember seeing the time of RAM on the box. You could take apart your computer and look at the modules (which is the right thing to do before you invest in any more modules)?

But if you want to find out right now in kind of a cool way, you can download
some free software that will examine your computer and report back. It's called CPU-Z. You don't have to install CPU-Z, just download it, unzip it and double-click on cpuz.exe. A little windows will open up. If you click on the "Memory" tab at the top, you'll see what kind of RAM you've got. (If that last sentence sounds a little in-your-face, it's because it is. RAM matters).

It won't take but a second for you to see that there's an opportunity to waste some time here. CPU-Z can tell you a lot about your system -- even more if you read the readme file that comes with it.

But before you wander off, the tab next to Memory is important too. It's labeled "SPD." Remember that your motherboard has slots for memory -- probably 4 of them. Since memory modules come in different configurations, e.g. 512 MB, 1 GB, etc., you have to figure out if you have any open slots. The SPD tab will tell you how many slots you have and what's in each of them, if anything.

If your computer currently as 2 GB of memory, it could be a 1 GB module in slot 1, a 1 GB module in slot 2 and then 2 empty slots.

You could add a GB by slipping a new module into the third slot. But if your 2GB comes from 4 512 MB modules taking up all 4 slots, it's more complicated. Most online memory stores have help in figuring out the best configuration. Just Google "memory."

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