DDR SDRAM (double data rate synchronous dynamic random access memory) is a class of memory integrated circuit used in computers. SDRAM refers to synchronous Dynamic random access memory, a term that is used to describe dynamic random access memory that has a synchronous interface SDRAM refers to synchronous Dynamic random access memory, a term that is used to describe dynamic random access memory that has a synchronous interface In Electronic engineering, DDR3 SDRAM or double-data-rate three Synchronous dynamic random access memory is a Random access memory Direct Rambus DRAM or DRDRAM (sometimes just called Rambus DRAM or RDRAM) is a type of synchronous dynamic RAM, designed by the Rambus XDR DRAM or extreme data rate Dynamic random access memory is a high-performance RAM interface and successor to the Rambus RDRAM it XDR2 DRAM is a type of Dynamic Random Access Memory that is offered by Rambus. V irtual C hannel R andom A ccess M emory ( VC-RAM GDDR3, Graphics Double Data Rate 3, is a graphics card-specific memory technology designed by ATI Technologies with the collaboration of JEDEC GDDR4 SDRAM (Graphics Double Data Rate version 4 is a type of Graphics card memory specified by the JEDEC Semiconductor Memory Standard GDDR5 (Graphics Double Data Rate version 5 is a type of Graphics card memory the standards of which were set out in the GDDR5 specification by JEDEC Microchipsjpg|right|thumb|200px|Microchips ( EPROM memory with a transparent window showing the integrated circuit inside A computer is a Machine that manipulates data according to a list of instructions. 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. In Computer networking and Computer science, digital bandwidth or just bandwidth is the capacity for a given system to transfer data over a connection SDRAM refers to synchronous Dynamic random access memory, a term that is used to describe dynamic random access memory that has a synchronous interface In Computing, a Computer bus operating with double data rate transfers data on both the rising and falling edges of the Clock signal. In Electronics and especially synchronous Digital circuits a clock signal is a signal used to coordinate the actions of two or more circuits
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). A bit is a binary digit, taking a value of either 0 or 1 Binary digits are a basic unit of Information storage and communication Thus with a bus frequency of 100 MHz, DDR SDRAM gives a maximum transfer rate of 1600 MB/s. A megabyte is a unit of Information or Computer storage equal to either 106 (1000000 Bytes or 220 (1048576 bytes depending on The second ( SI symbol s) sometimes abbreviated sec, is the name of a unit of Time, and is the International System of Units
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. JEDEC Solid State Technology Association, formerly known as J oint E lectron D evice E ngineering C ouncil ( JEDEC) or Joint As DDR-SDRAM is superseded by the newer DDR2 SDRAM, the older version is sometimes referred to as DDR1-SDRAM.
|Standard name||Memory clock||Cycle time||I/O Bus clock||Data transfers per second||Module name||Peak transfer rate|
|DDR-200||100 MHz||10 ns ||100 MHz||200 Million||PC-1600||1600 MB/s|
|DDR-266||133 MHz||7. A megabyte is a unit of Information or Computer storage equal to either 106 (1000000 Bytes or 220 (1048576 bytes depending on 5 ns||133 MHz||266 Million||PC-2100||2100 MB/s|
|DDR-300||150 MHz||6. A megabyte is a unit of Information or Computer storage equal to either 106 (1000000 Bytes or 220 (1048576 bytes depending on 67 ns||150 MHz||300 Million||PC-2400||2400 MB/s|
|DDR-333||166 MHz||6 ns||166 MHz||333 Million||PC-2700||2700 MB/s|
|DDR-400||200 MHz||5 ns||200 MHz||400 Million||PC-3200||3200 MB/s|
Note: All above listed (except DDR-300 ) are specified by JEDEC as JESD79. A megabyte is a unit of Information or Computer storage equal to either 106 (1000000 Bytes or 220 (1048576 bytes depending on A megabyte is a unit of Information or Computer storage equal to either 106 (1000000 Bytes or 220 (1048576 bytes depending on A megabyte is a unit of Information or Computer storage equal to either 106 (1000000 Bytes or 220 (1048576 bytes depending on JEDEC Solid State Technology Association, formerly known as J oint E lectron D evice E ngineering C ouncil ( JEDEC) or Joint All RAM speeds in-between or above these listed specifications are not standardized by JEDEC — most often they are simply manufacturer optimizations using higher-tolerance or overvolted chips.
The package sizes in which DDR SDRAM is manufactured are also standardized by JEDEC.
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. Underclocking also known as downclocking is the practice of modifying a synchronous circuit's speed settings to run at a lower Clock speed than the manufacturer's
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). A DIMM, or dual in-line memory module, comprises a series of Dynamic random access memory Integrated circuits These modules are mounted on a Printed DDR for notebook computers SO-DIMMs have 200 pins which is the same number of pins as DDR2 SO-DIMMs. A SO-DIMM, or small outline dual in-line memory module, is a type of Computer memory built using Integrated circuits. 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. The volt (symbol V) is the SI derived unit of electric Potential difference or Electromotive force. 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. A chipset is a group of Integrated circuits or chips that are designed to work together and are usually marketed as a single product Dual-channel architecture describes a technology that theoretically doubles data throughput from RAM to the memory controller.
Module and chip characteristics are inherently linked. A DIMM, or dual in-line memory module, comprises a series of Dynamic random access memory Integrated circuits These modules are mounted on a Printed A SO-DIMM, or small outline dual in-line memory module, is a type of Computer memory built using Integrated circuits.
Total module capacity is a product of one chip's capacity by the number of chips. ECC modules multiply it by 8/9 because they use one bit per byte for error correction. A module of any particular size can therefore be assembled either from 36 small chips, or 18 or 9 bigger ones.
DDR memory bus width per channel is 64 bits (72 for ECC memory). Total module bit width is a product of bits per chip by number of chips. It also equals number of ranks (rows) multiplied by DDR memory bus width. Consequently a module with greater amount of chips or using x8 chips instead of x4 will have more ranks.
|Module size||Number of chips||Chip size||Chip organization||Number of rows (ranks)|
|1 GB||36||256 Mbit||64M x 4||2|
|1 GB||18||512 Mbit||64M x 8||2|
|1 GB||18||512 Mbit||128M x 4||1|
This example compares different real-world server memory modules with a common size of 1 GB. A gigabyte (derived from the SI prefix Giga-) is a unit of Information or Computer One should definitely be careful buying 1 GB memory modules, because all these variations can be sold under one price position without stating whether they are x4 or x8, single or dual ranked.
There is a common belief that number of module rows or ranks equals number of sides. As above data shows, this is not true. One can find (2-side, 1-rank) or (2-side, 4-rank) modules. One can even think of 1-side, 2-rank memory module having 16(18) chips on single side x8 each, but it's unlikely such a module was ever produced.
In this context High Density memory means non-ECC 184 pin SDRAM memory.
How RAM is organised
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.
What is High Density RAM?
Again 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.
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. Also as a note, JDEC standards do not apply to high-density DDR ram.
A WORD OF CAUTION As the price of Low Density PC3200 DDR RAM has soared, however, an increasing number of people - usually sellers on auction sites - have been offering High Density PC3200 DDR-400 non-ECC RAM for sale at temptingly low prices. Be aware that some sellers may attempt to scam buyers with vague descriptions, conning some to buy high-density ram which will not work in the consumer's machine.
Some older PC motherboard chipsets may not support the High Density DDR modules. You will effectively cut in half your actual DDR memory capacity (512MB will only appear as 256MB) if your motherboard chipset does not support High Density DDR modules. High Density DDR typically sells at lower prices than Low Density DDR for this reason. Don't go for the 'bargain price' without checking first.
Also, mixing High Density DDR and Low Density DDR on the same motherboard may reduce actual memory capacity.
DDR (DDR1) has been superseded by DDR2 SDRAM, which has some modifications to allow higher clock frequency, but operates on the same principle as DDR. Competing with DDR2 are Rambus XDR DRAM. Rambus Incorporated ( founded in 1990, is a provider of high-speed interface technology most notably their Rambus Dynamic RAM memory technology, which XDR DRAM or extreme data rate Dynamic random access memory is a high-performance RAM interface and successor to the Rambus RDRAM it DDR2 has become the standard, as XDR is lacking support. DDR3 SDRAM is a new standard that offers even faster performance and new features. In Electronic engineering, DDR3 SDRAM or double-data-rate three Synchronous dynamic random access memory is a Random access memory
DDR's prefetch buffer depth is 2 bits; DDR2 uses 4 bits. Although the effective clock speeds of DDR2 are higher than for DDR, the overall performance was no greater in the early implementations, primarily due to the high latencies of the first DDR2 modules. DDR2 started to be effective by the end of 2004, as modules with lower latencies became available. 
Memory manufacturers have stated that it is impractical to mass-produce DDR1 memory with effective clock rates in excess of 400 MHz. DDR2 picks up where DDR1 leaves off, and is available at clock rates of 400 MHz and higher.
RDRAM is a particularly expensive alternative to DDR SDRAM, and most manufacturers have dropped its support from their chipsets. Direct Rambus DRAM or DRDRAM (sometimes just called Rambus DRAM or RDRAM) is a type of synchronous dynamic RAM, designed by the Rambus
MDDR is an acronym that some enterprises use for Mobile DDR SDRAM, a type of memory used in some portable electronic devices, like mobile phones, handhelds, and digital audio players. A mobile device (also known as cellphone device, handheld device, handheld computer, "Palmtop" or simply handheld) is a pocket-sized A digital audio player, more commonly referred to as an MP3 player, is a Consumer electronics device that stores organizes and plays audio files Some While standard DDR SDRAM operates at a voltage of 2. Electrical tension (or voltage after its SI unit, the Volt) is the difference of electrical potential between two points of an electrical 5 V, MDDR operates at voltage of 1. 8 V, which allows a reduced power consumption.