In planning for our monthly upcoming series on the Best Memory for the Money, we decided to take a look only at three kinds of PC memory, and to skip the SODIMMs (small outline dual in-line memory modules) so commonly used in notebook PCs nowadays. These will probably make their first appearance later, but for this first iteration, we plan to cover only three types of RAM used in desktop PCs:
- DDR or Dual Data Rate memory is by no means the oldest type of RAM available for sale nowadays, but its heyday has already come and gone, and it's the oldest type of RAM that we will cover. You must actively seek out older motherboards and processors to put a new system together that uses this kind of RAM, though plenty of existing systems use DDR RAM. DDR gets its moniker from its ability to access two individual memory locations (data bits) during each clock cycle (one on the leading edge of the cycle on the incoming transition, the other on the trailing edge of the cycle on the outgoing transition). DDR operates at 2.5 volts (more when overclocked, usually) and is the heaviest power consumer of the three types of memory covered here. Compare Prices on PC Memory
- DDR2 or Double Data Rate 2 memory is the most common type of memory used in current, modern PCs. It's neither the oldest nor the newest type of RAM around, but it appears in most ready-made PCs and notebooks in today's marketplace (while the format differs, DDR2's speed and capabilities in laptops are the same as those for conventional desktop DDR2 flavors). DDR2 generally runs faster than DDR, which is why it offers faster data rates than its older counterpart (the slowest DDR2 is as fast as the fastest DDR). DDR2 operates at 1.8 volts and, like DDR, usually at higher voltages when overclocked. DDR2 falls in the middle of the power consumption range for the three types of memory covered here.
- DDR3 or Double Data Rate 3 memory is the putatively faster new kid on the memory block. Once again, DDR3 ups the speed ante over DDR2, so that its slowest speed (which is generally not commercially available) is as fast as DDR2's fastest speed (800 megatransfers per second). DDR3 is the least power hungry of the three types covered here, while operation is at 1.5 volts and more when it is overclocked.
Memory Specifications Tell Only Part Of The RAM Story
JEDEC is a standards body for the global semiconductor industry. Originally the acronym stood for Joint Electron Device Engineering Council, but now it is decoded as the JEDEC Solid State Technology Association.
From the standpoint of PC memory devices, JEDEC sets the basic specifications to which all RAM must adhere at a minimum. As you look at the values in Table 1 and compare them to information that appears in Tables 2 through 4, however, you'll see that overclocking parts push beyond the JEDEC limits, to the point where there is usually overlap between neighboring types of memory, as with DDR-400 or PC-3200, and DDR2-400 and PC2-3200.
|Header Cell - Column 0||DDR||DDR2||DDR3|
|Voltages||2.5v +/- 0.1v||1.8V +/- 0.1V||1.5V +/- 0.075V|
|Termination||Limited||Limited||All DQ signals|
|Topology||TSOP||TSOP or Fly-by||Fly-by|
|Driver Control||None||OCD Calibration||Self Calibration with ZQ|
|Thermal Sensor||No||No||Yes (Optional)|
Table 1: JEDEC Memory Specification Values
Notes: Fly-by is also sometimes called FBGA (fine-pitch ball grid array); TSOP stands for "thin small-outline package;" OCD stands for off-chip driver circuitry; and ZQ refers to a connection with a built-in, extremely precise 1% 240? RZQ resistor against which the chip calibrates itself.