DDR (which stands for Double Data Rate according to Abbreviationfinder) memory is the standard that replaced traditional SDR SDRAM memories (better known as “SDRAM memories” or even “DIMM memories”), and is very well received by the market, especially in the personal computer segment .But, what made it so well accepted?What are the characteristics that differentiate it from the previous standard?What are its advantages?The answers to these and other related questions are provided below.
Emergence of DDR memories
At the time when Intel’s Pentium III processor was one of the main products of its kind on the market, the standard FSB (Front Side Bus) rate – essentially, the speed at which the processor communicates with RAM – was 133 MHz, equivalent to 1,064 MB per second.However, it is known that, as a rule, the motherboard chipset does not use the FSB frequency to communicate with the memory, but the speed of the latter.At that time, the standard for memory speed was also 133 MHz (the well-known PC-133 SDRAM memories), which also provided a transfer rate of 1,064 MB per second.It is possible to notice, with this, that there was a certain “balance” in the communication speeds between the components of the computer.
However, with the launch of chips such as Pentium 4, from Intel, and Athlon, from AMD, this “balance” ceased to exist, as the FSB of processors started to have more speed, while the memories continued in the PC-standard 133, maintaining the frequency at 133 MHz. In these conditions, this means that the computer as a whole cannot take advantage of all the processing resources.
For Pentium 4 users, there was even an alternative: using Rambus-type memories (or RDRAM).This type was faster than PC-133 memories, but it had some disadvantages: it only worked with Intel processors, it had a very high price and the motherboards that supported Rambus memories were also very expensive.
In the same period, DDR memories were already a reality, but Intel tried to popularize Rambus memories, which made it “ignore” the existence of the first ones.AMD, in turn, needed an efficient alternative that could work fully with its new processors.The company ended up betting on DDR memories and, from then on, this guy started to become popular, especially since Intel, soon after, had to adhere to the idea.
But the mere appearance of DDR memories was not an immediate solution to the speed problems between memories and FSB.Only with the launch of Dual-Channel DDR memories did the solution become effectively effective.The subject that will be discussed later.
Functioning of DDR memories
DDR memories are very similar to SDR SDRAM memories.The latter work synchronously with the processor, avoiding the delay problems existing in previous technologies.The great advantage of DDR technology, however, lies in its ability to perform twice as many operations per clock cycle.Thus, while an SDR SDRAM PC-100 memory works at 100 MHz, for example, a DDR module with the same frequency makes it correspond to twice, that is, 200 MHz.
But, how is this possible?In memories, data is stored in spaces called cells.These are organized in a kind of matrix, that is, they are oriented in a scheme that resembles rows and columns.The crossing of a line with a column forms what is known asa memory address.
Normally, in read and write operations, it is only possible to access one line at a time.But DDR memories have a “trick”: they access two different positions, but both on the same line.That is why this technology is able to double operations per cycle, one at the beginning of the cycle and one at the end.
Because of this characteristic, DDR memories now have a different standard of nomenclature.In the SDR SDRAM modules, expressions such as PC-100 and PC-133 are found, where the number indicates the frequency.Thus, a PC-133 comb informs that the device works at 133 MHz. In DDR memories, this also occurs, but considering the characteristic of duplication per cycle.Thus, a DDR-200 module, for example, actually works at a rate of 100 MHz. But, in the alternative nomenclature, such as PC-1600, for example, the amount of megabytes transferred per second is what is considered.Observe the table:
|PC-100 SDRAM||800 MB / s|
|PC-133 SDRAM||1,064 MB / s|
|DDR-200 or PC-1600||1,600 MB / s|
|DDR-266 or PC-2100||2,100 MB / s|
|DDR-333 or PC-2700||2,700 MB / s|
|DDR-400 or PC-3200||3,200 MB / s|
|Dual DDR-226||4,200 MB / s|
|Dual DDR-333||5,400 MB / s|
|Dual DDR-400||6,400 MB / s|
It is worth noting that these transfer values are theoretical, that is, they indicate the maximum reach.In practice, a number of factors can influence the transfer speed.But, even though it is theoretical, how is this calculation done?
It’s simple: in their operations, DDR memories can transfer up to 64 bits at a time, that is, 8 bytes.Then just multiply this value by the frequency of the memory plus the number of operations per cycle.Thus, the calculation of a DDR-400 module is as follows:
8 (64 bits) x 200 (frequency) x 2 (operations per cycle) = 3,200
The final result is given in megabytes per second.
Although very similar to SDR SDRAM memories, DDR memories have another considerable differential: they work with 2.5 V, against 3.3 V of the first.As such, they reduce energy consumption, an especially important aspect in portable devices, such as notebooks.
Physical aspects of DDR memories
Visually, it is easy to distinguish DDR memories from SDR SDRAM memories.The first have only one division in the module socket, between the contact terminals, while the second have two.In addition, DDR memories use 184 terminals, against 168 pins of the SDR SDRAM standard.
With regard to encapsulation (learn more about this in thearticle ROM and RAM memories), DDR chips generally use the TSOP (Thin Small Outline Package) standard, but it is also possible to find versions in CSP (Chip Scale Package), although more rare.
Dual-Channelcan be consideredas a solution that mitigates the fact that memories do not keep up with the speed of processors.For this, the scheme causes DDR memories to transfer twice as much data at a time.Thus, 3,200 MB per second can become 6,400 MB per second.
This is possible because in the motherboard chipset – or even inside processors, in the case of some more current models – there is a special circuit calleda memory controller, which is responsible for all aspects of access and use of this.In Dual-Channel, this controller allows DDR memories to transfer twice as much data at a time, that is, instead of 64 bits, they transfer 128 bits (16 bytes).With that, the calculation of the previous topic becomes:
16 (128 bytes) x frequency x 2 (operations per cycle)
To activate the Dual-Channel scheme on a computer, it is necessary to have a compatible chipset (or, if applicable, a processor).In addition, it is recommended to have one or two pairs of identical memory modules (or at least with the same specifications).Equality decreases the risk of problems.At this point, an interesting tip is to purchase a kit for Dual-Channel, which offers two DDR memory sticks to work in this mode.
Consult the motherboard manual to find out which slots the modules should be installed in to activate Dual-Channel mode, as well as whether it is necessary to change any parameters in the BIOS setup.
DDR memories were widely accepted in the market, however, as the evolution of technology does not stop, especially with regard to processors, new standards had to be launched to keep up with the speeds of the latest chips: these are DDR2 memories and DDR3.