Types of PC Memory

Before the year 2000, all SDRAMs can only perform under the single data rate (SDR) interface. But as computers grew more powerful, so did the demand for a faster memory. This is when the double data rate (DDR) technology entered the scene. The DDR interface nearly doubles the processing speed of SDR while significantly reducing operational temperature and power consumption which made it the current standard in the industry. Its conception, in turn, gave birth to the three most common types of PC memory: DDR, DDR2 and DDR3.

DDR

The DDR interface is the first step in the evolution and was quite promising during its heyday. If your computer has been around for less than a decade, then most probably, it has a DDR-SDRAM. It comes in 184-pin DIMM format (see form factors) and has a clock rate of 100-200 MHz. Compared to the original SDR-SDRAM, the DDR consumes less electricity at 2.5 volts while delivering faster transfer rates of up to 400 MTps. For as long as they have the same speed, you can attach more than one DDR inside your CPU. 

DDR2

Just a few years after it first came out, the DDR proved insufficient, and DDR2 technology knocked it off from the pedestal. Sporting a 240-pin DIMM format, it promised to give PC owners back then more (200-533MHz clock rate, 1066MTps) for less (1.8 volts). You can even use multiple DDR2 SDRAMs with different speeds, however, the faster one will adapt to match the slower SDRAMÕs speed.

DDR3

Though soon to be dethroned by DDR4, the DDR3 is the current SDRAM chipset standard. It also comes in the 240-pin DIMM format like the DDR2, however, you may not insert it into a DDR2 socket as its slots are slightly different.

It clocks at 400-1066MHz, has a maximum transfer rate of 2133MTps and is even more energy efficient, chugging only 1.5 volts.

Form factor

With so many manufacturers of computer parts all over the world, it is important for them to agree on the dimension, shape and other details of the products they produce, The standard created to refer to these external features is called form factor. With the form factor, it is possible to assemble a PC from parts with different brands. Remember, though, that you can only use a certain SDRAM form factor if your motherboard chipset supports it. Usually, the way to distinguish one form factor from another is the number of pins. The most common form factor today is the DIMM (dual in-line memory module). DDRs in DIMM format have 168 pins, DDR2 has 200 or 240 pins, while DDR3 has 240 pins, as well. Even if two SDRAM types have the same number, as in the case of the 240-pin DDR2 and DDR3, they can't be switched to fit each other's sockets because their "voltage keys" (indents separating the pins) are positioned differently.

With the DDR2's voltage key is near the center while DDR3's nearer to the left. The small outline DIMMs or SO-DIMMs and MicroDIMMs are just the smaller versions. Being more compact, they're more apt for use in devices that use less power such as laptops, sub-notebooks, network printers and routers. The fully buffered DIMM (FB-DIMMs), on the other hand, can be considered the elite version of the DIMM format. The most obvious difference that you'd see is that FB-DIMMS have lower pin count. But what really sets it apart from the mainstream SDRAM form factors is that it has an advanced memory buffer (AMB) that sends out signals among the controller, memory devices, and other modules without compromising its integrity and speed. It's also equipped with a cyclic redundancy check (CRC) to lessen transmission errors, making it the ideal SDRAM format for servers, critical networking equipment and high-end desktop computers.

Memory Speed

SDRAMs have coded monickers that may be enough to turn you off, specially if you don't know how to read them. To understand these codes, you must first know that SDRAMs have a "friendly" name and a corresponding industry name, both of which describes the memory's specs. The friendly name starts with the SDRAM type, like "DDR-", "DDR2-", or "DDR3-" followed by the numerical value of its data rate or bandwidth per second. The industry name, on the other hand, starts out with "PC-", "PC2-", or "PC3-" (which corresponds to DDR, DDR2, and DDR3, respectively), followed by the peak transfer rate which is usually eight times the bandwidth per second. So for example, a DDR3 has the friendly name "DDR3-800", its industry name will then be "PC3-6400".

There are instances however, that SDRAM manufacturers round down the peak transfer rate in the industry name to make it look nicer. For example, DDR3-1066 has a peak transfer rate of 8533MBps, yet its industry name is just PC3-8500. But as mentioned before, faster isn't always better when it comes to PC memory. In simple terms, the clock speed can be described as how fast the memory can perform a task. However, there's also CAS latency (Column Access Strobe) which is how long it takes for the memory to go from one task to another. This means that sometimes, even a high-speed SDRAM with a high CAS latency can be trumped by a slower SDRAM with a lower CAS latency. Current DDR3s have relatively higher latencies (7-9) compared to DDR2s (4-6) but this is because the latter has had time to mature.

Memory Kits

While using one SDRAM may be sufficient for most applications, memory-intensive programs such as games will require more than one. This is where adapting a dual- or even triple-channel architecture becomes handy. You may compare it to having two or three roads leading to the same location instead of just one. More roads will mean less traffic congestion and therefore, faster travel. However, some using the multiple-channels will you require you to buy SDRAMs with identical specs, otherwise, the faster one will have to slow down to match the other SDRAM's speed - that is, if your motherboard will allow the use of mismatched SDRAMs, at all.