DDR SDRAM (double data rate synchronous dynamic random access memory) is a class of memory integrated circuit used in computers. It achieves nearly twice the bandwidth of the preceding [single data rate] SDRAM by double pumping (transferring data on the rising and falling edges of the clock signal) without increasing the clock frequency.
With data being transferred 64 bits at a time, DDR SDRAM gives a transfer rate of (memory bus clock rate) × 2 (for dual rate) × 64 (number of bits transferred) / 8 (number of bits/byte). Thus with a bus frequency of 100 MHz, DDR SDRAM gives a maximum transfer rate of 1600 MB/s.
JEDEC has set standards for speeds of DDR SDRAM, divided into two parts: The first specification is for memory chips and the second is for memory modules. As DDR-SDRAM is superseded by the newer DDR2 SDRAM, the older version is sometimes referred to as DDR1-SDRAM.
There is no architectural difference between DDR SDRAM designed for different clock frequencies, e.g. PC-1600 (designed to run at 100 MHz) and PC-2100 (designed to run at 133 MHz). The number simply designates the speed that the chip is guaranteed to run at, hence DDR SDRAM can be run at either lower or higher clock speeds than those for which it was made. These practices are known as underclocking and overclocking respectively.
DDR SDRAM for desktop computers DIMMs have 184 pins (as opposed to 168 pins on SDRAM, or 240 pins on DDR2 SDRAM), and can be differentiated from SDRAM DIMMs by the number of notches (DDR SDRAM has one, SDRAM has two). DDR for notebook computers SO-DIMMs have 200 pins which is the same number of pins as DDR2 SO-DIMMs. These two specifications are notched very similarly and care must be taken during insertion when you are unsure of a correct match. DDR SDRAM operates at a voltage of 2.5 V, compared to 3.3 V for SDRAM. This can significantly reduce power usage. Chips and modules with DDR-400/PC-3200 standard have a nominal voltage of 2.6 Volt.
Many new chipsets use these memory types in dual-channel configurations, which doubles or quadruples the effective bandwidth.
PC3200 is DDR SDRAM specified to operate at 200 MHz using DDR-400 chips with a bandwidth of 3,200 MB/s. As DDR stands for Double Data Rate this means that the effective clock rate of PC3200 memory is 400 MHz.
1 GB PC3200 non-ECC sticks are usually made with 16 512 Mb chips, 8 down each side (512 Mb x 16) / (8 bits per Byte) = 1024 MB. The individual chips making up a 1 GB memory stick are usually organised with 64M bits and a data width of 8 bits, commonly expressed as 64M x 8. Memory manufactured in this way is LOW DENSITY RAM and will usually be OK with any motherboard specifying PC3200 DDR-400 memory.
In the context of the 1 GB non-ECC PC3200 SDRAM stick there is very little visually to differentiate Low Density from High Density RAM. High Density DDR RAM sticks will, like their Low Density counterparts, usually be double sided with eight 512 Mb chips per side. The difference is that each chip, instead of being organised in a 64M x 8 configuration is organised with 128M bits and a data width of 4 bits, or 128M x 4. To further confuse the issue, some RAM is labelled as 128M x 8, and is also called high density.
Most High Density PC3200 modules are assembled using Samsung chips. These chips come in both the familiar 22 x 10mm (approx) TSOP2 and smaller squarer 12 x 9mm (approx) FBGA package sizes. High density Samsung chips can be identified by the numbers on each chip. If the sixth and seventh characters are "04" (for example K4H510438D-UCCC) then the chips are x 4 and High Density. If the sixth and seventh characters are "08" then the chips are x 8 and Low Density.
High Density RAM devices were designed to be used in registered memory modules for servers. As a result, Performance or response times may suffer when used on a desktop or workstation. JEDEC standards do not apply to high-density DDR RAM in desktop implementations. JEDEC's technical documentation however supports 128Mb X 4Mb semiconductors as such that contradicts 128X4 being classified as high density. As such "High Density" is a relative term which can be used to describe memory which is not supported by a particular motherboard's memory controller
