Understanding Memory Specs

With today’s modern PCs, the memory or RAM options are vast and to a lot of people confusing.  I’m hoping with this to explain RAM in such a way to make it easier for a person getting a memory upgrade to be better informed about their product choices.  This is going to assume that this is for a desktop personal computer for a Small Office / Home Office.

First things first you have to know what form factor your desktop pc is compatible with.  You have currently 4 standards of memory that are in use:  DDR, DDR2, DDR3, DDR4 (DDR stands for Double Data Rate, it superseded SDR or Single Data Rate).  With DDR being the oldest and DDR4 being the newest standard.  Pretty much any desktop PC system/motherboard is setup to accept one of these types of RAM.  It is important to know since these are not interchangeable.  Should you be in any doubt as to what kind of memory it is (buying it used) here’s a quick little chart to help determine the types. Again these are for desktop PC

DDR	184-Pins
DDR2	240-Pins
DDR3	240-Pins
DDR4	288-Pins

The other factor that makes them not compatible between them is there is notch between the pins that physically makes them unable to be inserted into the wrong type memory slot.

Now that we’ve determined the type of desktop memory, we need to look at the speed.  This is where we can mix and match a little bit.  However be aware that all the memory will operate at the speed of the slowest memory module in use or the speed the system/motherboard operates.  So memory speed can be displayed in a couple different ways.

The first is just in megahertz.  You can see on the product the following hertz rates (DDR is listed here): 100,133,150,166,183,200,216,240  The faster the better and the more premium the price point you will pay for that memory.

You will also see them displayed like the following (again DDR is used here): PC1600, PC2100, PC2400, PC2700, PC3000, PC3200, PC3500, PC3700.  This actually tells you the transfer rate.  So PC1600 can run at 1600 MB/Sec.  Generally again the higher the number the better, and again the higher the pricepoint for the higher the transfer rate.

A little trick to determine the megahertz of the memory and then vice versa would be to multiply or divide by 16.  It’s not exact but it gets you close if you are mixing memory (not advised) and the memory is each labeled a different way.

Now we know our Type, our Speed, now we need to figure out why one memory module of the same capacity is more expensive than the other, (Brand name cost excluded).  And that’s a spec in memory known as Latency.   It’s the delay the actual chips that make up your memory take before acting on the operation.  It is measured in clock cycles.   So even though the memory is the same type, speed, the chips used change the cost and efficiency of the module.  Typically Lower numbers are better, and again the lower latency values, the more premium the pricepoint will be to get that memory.  Typically the the latency values are displayed in this format.  However for the sake of thoroughness

7-7-7-8(-1)

I will give a quick breakdown of what the numbers are for.  This will be a high level explanation just know the lower the values the better performance you can expect from the memory module.

• The first number is your CAS Latency, which as lot of people feel is the most important value.  This is the number of clock cycles from the time an instruction is sent to the module and information is returned.  In this example it would be 7 clock cycles.

• The second number is the RAS to CAS Delay.  This is the delay between Row and the Column where the instruction is trying to get.

• The third number is the RAS Precharge. this is the delay between disabling and accessing the next line of data.

• The fourth is the Active to Precharge Delay.  This is the delay time from access initiations to that memory.

• There is sometimes a 5th number (which is why it is in parenthesis above) This is the delay from the time the module is able to receive a command after it has been activated.  This Value is a 1 or a 2.

Then the final piece that adds to your price point doesn’t have anything to do with the module itself per se.  Its the brand.  A lot of times to a point, the brand you choose will determine the quality of build and the level or QA that goes into the chips.  I will not go into brands, because to a certain degree it is preference and experience that leans a potential buyer of a product to one over the other, and that’s not what this entry is about.