DDR SDRAM ( Double Data Rate English Synchronous Dynamic Random Access Memory) is a type of random access memory (RAM ), that is a special semiconductor memory that are currently available in four variants. DDR SDRAM with 184 contacts, DDR2 SDRAM with 240 contacts, DDR3 - SDRAM also with 240 contacts and DDR4 SDRAM with 284 contacts. They are used mainly for memory modules in the DIMM or SO- DIMM standards as working memory in PCs and laptops.

  • 2.5.1 Example Calculations
  • 4.1 Deviations from the specification


As the supported Intel Direct Rambus DRAM technology ( RDRAM technology) in mid-1999 was due to an error in the i820 chipset problems and by the P3- FSB their performance despite high prices could not play, the computer industry sat back on the further development of SDRAM in the form of DDR memory technology.

First memory chips and motherboards with support for DDR - SDRAM came the end of 1999 to the market. It was not until early 2002, they were able to prevail on the European consumer market.



During "normal" SDRAM modules at a clock rate of 133 MHz data transfer rates of 1.06 GB / s, as modules use DDR SDRAM (133 MHz) with almost twice the data rate. Which is characterized by a relatively simple trick possible: Both in the on - and in the falling edge of the clock signal, a data bit is transmitted instead of the current only when ascending.

So that the double-data rate leads to an acceleration process, the number of contiguous requested data ( = " burst length" ) must always be equal to or greater than twice the bus width. As this is not always the case, DDR SDRAM is compared to a simple SDRAM is not exactly twice as fast at the same clock. Another reason is that the address and control signals are supplied in contrast to the data signals with a clock edge.

DDR SDRAM memory modules (DIMM ) have 184 contacts / Pins (DDR2 SDRAM DIMM DIMM/DDR3-SDRAM: 240 SDRAM DIMM: 168 contacts). The operating voltage is typically 2.5 volts, DDR -400 2.6V

DDR - 200 to DDR-400 as well as the constructed PC -1600 to PC -3200 memory modules are standardized by the JEDEC JESD79 as. All deviating modules which are being created by the names of her on the standards, but each manufacturer uses in electrical characteristics - the modules often offered as " overclocker memory " - its own specifications and often works with excessive surge.

A gain in safety bring the memory modules often used for servers with ECC (Error Checking and Correction ) or Registered modules with signal buffer. This is true but only if these memory modules are explicitly supported, often work - ECC modules not in normal desktop motherboards. These memory modules are available in all standard clock frequencies and recognizable by the additional designation R, ECC or ECC R, eg PC - 1600R, PC -2100 ECC or PC - 2700R ECC.


DDR2 SDRAM is a further development of the concept of DDR SDRAM, which is performed at rather than with a dual - prefetch with a quad - prefetch.

DDR2 SDRAM Memory Module ( DIMM) for desktop computers have 240 contacts / pins. (DDR3- SDRAM DIMM: also 240, DDR - SDRAM DIMM: 184 SDRAM DIMM: 168 contacts). DDR2 modules are mechanically and electrically compatible with DDR modules. Different contact strips ( significantly more and smaller contacts) will prevent any confusion.

The dimensions of the final packaged memory chips are smaller ( 126 mm instead of 261 mm ² ²). FBGA (Fine - pitch Ball Grid Array) instead of TSOP ( Thin Small Outline Package ): the by another packaging technology is achieved.

DDR2 SDRAM of the I / O buffer with twice the frequency of the memory chip is clocked. Is obtained, as in the older DDR standard, both at rising and on falling edge of the clock signal valid data. When DDR SDRAM with a read command (at least) read two consecutive addresses in DDR2 SDRAM four, due to the prefetch method of the respective standards. From a 128 -bit DDR module that is 256 bits per read access read, from a comparable DDR2 module 512, the absolute amount of data remains in the same I / O clock, for example, 200 MHz, but the same as the DDR2 module two bars rather than a need to transmit the data. DDR2 supports only two possible burst length ( number of data words which can be read or written in a single command ), namely four (due to four- Prefetch ), or eight, GDR does support two, four or eight.

In order to reduce the electrical power signal and supply voltage of DDR2 SDRAM was reduced to 1.8 volts (for DDR SDRAM is 2.5 V). In addition, the reduced voltage to a lower heat generation, which in turn can lead to higher clock rates can be achieved leads.

DDR2 SDRAM chips working with " On-Die Termination " (ODT ). Thus, the memory bus must be terminated no longer on the module board ( or board ). The scheduling function has been integrated directly into the chips, which saves space and costs. ODT works as follows: The memory controller sends a signal to the bus, which causes all inactive DDR2 SDRAM chips to switch to termination. Thus, only the active signal is on the data line interference are virtually eliminated.


DDR2 modules can in principle be used in any motherboard with DDR2 slots without regard to their respective speed indication. The memory controller ensures that modules that can operate at higher clock speed than the motherboard, only be operated with the clock rate, which is the motherboard maximum possible. Slower memory modules can be operated with a maximum clock rate, for which they are designed. So it is not technically required, in a motherboard that is designed for example for DDR2 -533 memory modules to use with just the speed rating 533.

Also DDR2 modules with different clock rates can be combined. However, it depends on the design of the main circuit board, whether the individual modules each operate at the highest possible rate, or whether the whole memory is operating with the lowest speed, which is the case in general.

However, since the JEDEC specifications are inaccurate, it may be compatibility problems between certain motherboards and certain memory modules. Often these compatibility problems can be solved by a BIOS update. Only in memory, which is on the so-called QVL ( Qualified Vendor List ) on the motherboard, it can be assumed that it works in this motherboard in any case.

DDR2 - 400 to DDR2 -1066 as well as the built PC2- 3200 to PC2 -8500 memory modules are standardized by JEDEC. All deviating modules which are being created by the names of her on the standards, but each manufacturer uses in electrical characteristics - the modules often offered as " overclocker memory " - its own specifications and often works with excessive surge. As DDR-SDRAM is also available DDR2 - SDRAM in addition to the unregistred modules ( often referred to as PC2 XY00U ) also registered modules ( PC2 XY00R ) and ECC modules ( PC2 XY00E ) and FBDIMM modules (PC2 - XY00F ).


DDR3 SDRAM is an evolution of the concept of DDR2 SDRAM, in which instead of a quad - prefetch (4-bit ) with an eight- prefetch (8 bit) is working.

The chips with a capacity of at least 512 Mebibit process data with 8500 megabytes per second and are significantly faster than DDR -400 or DDR2 -800 SDRAM. However, the CAS latency is higher. In addition, DDR3 SDRAM also requires only 1.5 volts instead of 1.8 volts, making it better -suited for mobile use, in which it comes to long battery life. Low-voltage versions ( DDR3L ) can be operated on suitable motherboards with 1.35 volts. Ultra -low-voltage versions ( DDR3U ) are intended for use with 1.25V.

DDR3 SDRAM Memory Module ( DIMM) have 240 Contacts / Pins ( DDR2 SDRAM DIMM: 240, DDR SDRAM DIMM: 184 SDRAM DIMM: 168 contacts); they are despite the same number of pins not compatible with DDR2 SDRAM and have different notches. SODIMM modules for notebooks have 204 contacts over 200 contacts as DDR2 variant and as DDR1 variant.

In the field of video RAM is used for quite some time GDDR3. But this is based on DDR2 memory chips only the voltage was initially lowered ( VDD; VDDQ = 1.5 V, 1.5 V instead of 2.5 V, 1.8 V). The term GDDR3 has no official specifications but was chosen for marketing reasons ( in order to distinguish themselves from the less successful GDDR2 ). GDDR4 and GDDR5 is based on DDR3 technology, but are in some cases substantially modified for use as video memory.

The specifications of DDR3 - 800 to DDR3 - 2133 as well as the built PC3 -6400 PC3- 17000 - to - memory modules are described by the JEDEC standards organization. All deviating modules which are being created by the names of her on the standards, but each manufacturer uses in electrical characteristics - the modules often offered as " overclocker memory " - its own specifications and often works with excessive surge. As with DDR1 SDRAM is also available in DDR3 - SDRAM ECC and Registered modules, however they are currently (May 2011) only standardized up to PC3- 12800. They are provided with the additional identifier R, ECC or ECC R Analogous to previous standards. An identifier PC3L - designated low-voltage memory modules PC3U ultra-low -voltage memory modules. Similarly, the names DDR3L or DDR3U for corresponding memory chips.


The DDR4 SDRAM has 284 contacts and the notebook counterpart SO -DIMM 256 contacts. As DDR3 SDRAM and the storage of 8 -bit prefetch operation. This means there is no doubling, as was the case with the previous DDR SDRAM generations. Instead, the modules can be operated at higher clock rates.

In May 2012, Micron delivered first test copies of the next DDR4 SDRAM of which is expected to be available from 2014 and was originally intended to reach at least 50 percent of the market as soon as 2015, today it is anticipated that this goal can be achieved in 2016 at the earliest. This would be replaced after about 9 years of DDR3 standard introduced in 2007. The new memory modules are to be manufactured in 30 - nanometer process.

Calculating memory transfer rate

To calculate the theoretical maximum memory throughput, the following formula is used:

  • ( Clock of the memory cells (in MHz) × bits per transmission × prefetch ) / 8 bit = memory transfer rate in bytes / s Then divide each by powers of 1000 for KB / s, MB / s, GB / s or by powers of 1024 for KiB / s, MiB / s, GiB / s

Example calculations

  • DDR-400 ( 200 MHz × 64 bits × 2) / 8 = 3200 MB / s ( 3,052 MiB / s) = 3.2 GB / s ( 2.980 GiB / s)
  • DDR2 -800 (200 MHz × 64 bits × 4) / 8 = 6400 MB / s ( 6,104 MiB / s) = 6.4 GB / s ( 5.960 GiB / s)
  • DDR3 -1600 ( 200 MHz × 64 bits × 8) / 8 = 12,800 MB / s ( 12.207 MiB / s) = 12.8 GB / s ( 11.921 GiB / s)

DDR SDRAM transfers in rising and falling clock edge at DDR2 and DDR3, the external clock is increased compared to the clock of the memory chips by a factor of two or four, as it is read from a plurality of memory chips one by one. When using a multi-channel storage subsystem with dual-channel technology, the transfer rate doubled accordingly.

Based on this calculation can be estimated as a good match RAM and the rest of the system. The memory must be fast enough to process the requests of all bus masters including the CPU, hard disk controllers, and graphics cards.

"Myth" of the slowness of DDR2 SDRAM compared to DDR1

In connection with DDR2 SDRAM has often spoken of a disadvantage in the area of ​​access time compared to DDR SDRAM modules. But this is only partially correct. In practice, the latency of the real clock and the access time of the RAM is dependent. Since a, respectively at 533 MHz. 667 MHz DDR2 module specified with a real clock speed of 133 MHz, respectively. 166 MHz works, it can not compete in any area with a DDR400 module. In addition, DDR2 memory is almost exclusively operated with Command Rate 2T, DDR -400 is often 1T possible. That is in addition a strong performance penalty. The theoretical maximum data rate of DDR2 module is indeed higher, but missing 66 MHz, respectively. 33 MHz clock, which adds latency when accessing the RAM, which provides the advantage of the higher data rate expires to a great extent. Since a DDR2 module four data packets sent per clock, but not necessarily all be used later ( it will be to the requested package is simply the next three to read ), the increased data rate is also fully usable. Upon publication of DDR2 memory also was added to the problem that the early modules generally very had great access times what the "myth" nor reinforced.

That's why it makes sense to switch from DDR400 to DDR2 -800 or higher. A DDR2 -800 DDR - SDRAM can catch up on the latency here and also still provide the theoretically double the data rate. However, this is only recommended in full, if the board really can handle a memory clock of 800 MHz; a dual -RAM board ( with both DDR, and DDR2 slots, one of which, however, is only one type at the same time usable) has a bottleneck if it can indeed be equipped with DDR 400, but only DDR2 -533; in such a case is DDR400 ( 200 MHz memory clock ) while the " wider ", but slower DDR2 (only 133 MHz memory clock ) is preferable.

It is particularly noteworthy that the real clock speeds of SDRAM memory for a long time remained constant. In DDR4, however, an increase was recorded again.

Latency compared

The performance of memory modules is measured mainly in the " absolute latency ". The absolute latency results from the factors (effective ) clock and timing.

Since the memory in the three following cases work with each exactly 200 MHz, but the timings are relative to the effective stroke (400 MHz, 800 MHz and 1600 MHz), the latency remain the same (in the range of some nanoseconds = seconds), although the timings differ. It doubles each the theoretical data rate due to the fact that the I / O bus operates in each case with 200, 400 or 800 MHz:

The latency can be - as can be seen from the examples just mentioned - calculated as follows:

The total access time is at least tRCD tCL divided by the clock. The effective frequency is twice as high as the actual clock frequency, since two is read once per clock (hence the name DDR = Double Data Rate).

Deviations from the specification

Some storage vendors do not keep the official specifications of JEDEC and offer modules with much higher clock rates or better timings. While DDR3 -1600 CL9 -9-9 an official specification is, it is at DDR2 -1066 CL4 - 4.4.12 not standards-compliant memory modules. This fast memory are often referred to as memory modules for overclockers. Even with DDR3 is expected to significantly faster memory modules are offered as a result of the continuous improvement of production processes in the near future. These are initially but working outside the official specification. The JEDEC could take these memory modules in the official specification, but this happens often appear years after the first availability. Such modules can only work satisfactorily if their parameters are stored correctly and can be adjusted accordingly by the system. If this is not so, they are operated with standard-compliant specifications.