RAM

(From Wikipedia)










Types of RAM
Modern types of writable RAM generally store a bit of data in either the state of a flip-flop, as in SRAM (static RAM), or as a charge in a capacitor (or transistor gate), as in DRAM (dynamic RAM), EPROM, EEPROM and Flash. Some types have circuitry to detect and/or correct random faults called memory errors in the stored data, using parity bits or error correction codes. RAM of the read-only type, ROM, instead uses a metal mask to permanently enable/disable selected transistors, instead of storing a charge in them.
As both SRAM and DRAM are volatile, other forms of computer storage, such as disks and magnetic tapes, have been used as "permanent" storage in traditional computers. Newer products such as PDAs and small music players (up to 16 GB in Jan 2007) may not have hard disks however, but often rely on flash memory to maintain data between sessions of use; the same can be said about products such as mobile phones, advanced calculators, synthesizers etc; even certain categories of PC have begun replacing magnetic disk with so called flash drives. There are two basic types of flash memory: the NOR type, which is capable of true random access, and the NAND type, which is not; the former is therefore often used in place of ROM, while the latter is used in most memory cards and solid-state drives, due to a lower price.

DDR2 (from wikipedia )







A 512 MiB DDR2 533 module with BGA chips. DDR2 is a 240-pin module
Like all SDRAM implementations, DDR2 stores memory in memory cells that are activated with the use of a clock signal to synchronize their operation with an external data bus. Like DDR before it, DDR2 cells transfer data both on the rising and falling edge of the clock (a technique called "dual pumping"). The key difference between DDR and DDR2 is that in DDR2 the bus is clocked at twice the speed of the memory cells, so four words of data can be transferred per memory cell cycle. Thus, without speeding up the memory cells themselves, DDR2 can effectively operate at twice the bus speed of DDR.
DDR2's bus frequency is boosted by electrical interface improvements, on-die termination, prefetch buffers and off-chip drivers. However, latency is greatly increased as a trade-off. The DDR2 prefetch buffer is 4 bits deep, whereas it is 2 bits deep for DDR and 8 bits deep for DDR3. While DDR SDRAM has typical read latencies of between 2 and 3 bus cycles, DDR2 may have read latencies between 4 and 6 cycles. Thus, DDR2 memory must be operated at twice the bus speed to achieve the same latency.
Another cost of the increased speed is the requirement that the chips are packaged in a more expensive and more difficult to assemble BGA package as compared to the TSSOP package of the previous memory generations such as DDR and SDRAM. This packaging change was necessary to maintain signal integrity at higher speeds.
Power savings are achieved primarily due to an improved manufacturing process through die shrinkage, resulting in a drop in operating voltage (1.8 V compared to DDR's 2.5 V). The lower memory clock frequency may also enable power reductions in applications that do not require the highest available speed.

Relation to GDDR memory
The first commercial product to claim using the "DDR2" technology was the NVIDIA GeForce FX 5800 graphics card. However, it is important to note that this GDDR-2 memory used on graphics cards is not DDR2 per se, but rather an early midpoint between DDR and DDR2 technologies. Using "DDR2" to refer to GDDR-2 is a colloquial misnomer. In particular, the performance-enhancing doubling of the I/O clock rate is missing. It had severe overheating issues due to the nominal DDR voltages. ATI has since designed the GDDR technology further into GDDR3, which is more true to the DDR2 specifications, though with several additions suited for graphics cards.
GDDR3 is now commonly used in modern graphics cards and some tablet PCs. However, further confusion has been added to the mix with the appearance of budget and mid-range graphics cards which claim to use "DDR2". These cards actually use standard DDR2 chips designed for use as main system memory. These chips cannot achieve the clock speeds that GDDR3 can but are inexpensive enough to be used as memory on mid-range cards.

DDR3 ( from wikipedia)
DDR3 memory comes with a promise of a power consumption reduction of 30% compared to current commercial DDR2 modules due to DDR3's 1.5 V supply voltage, compared to DDR2's 1.8 V or DDR's 2.5 V. This supply voltage works well with the 90 nm fabrication technology used for most DDR3 chips. Some manufacturers further propose to use "dual-gate" transistors to reduce leakage of current.
The main benefit of DDR3 comes from the higher bandwidth made possible by DDR3's 8 bit deep prefetch buffer, whereas DDR2's is 4 bits, and DDR's is 2 bits deep.
Theoretically, these modules could transfer data at the effective clock rate of 800–1600 MHz (using both edges of a 400–800 MHz I/O clock), compared to DDR2's current range of effective 400–800 MHz (200–400 MHz clock) or DDR's range of 200–400 MHz (100–200 MHz). To date, such bandwidth requirements have been mainly found in the graphics market, where fast transfer of information between framebuffers is required.
Prototypes were announced in early 2005, and products are appearing on the market as of mid-[2007], in the form of motherboards based on Intel's P35 "Bearlake" chipset and memory DIMMs at speeds up to DDR3-1600. AMD's roadmap indicates their own adoption of DDR3 to come in 2008.
DDR3 DIMMs have 240 pins, the same number as DDR2, and are the same size, but are electrically incompatible and have a different key notch location.
GDDR3 memory, with a similar name but an entirely dissimilar technology, has been in use for several years in high-end graphic cards such as ones from NVIDIA or ATI Technologies, and as graphics system memory on the Sony Playstation 3. It has sometimes been incorrectly referred to as "DDR3".

Features
DDR3 SDRAM Components:
Introduction of asynchronous RESET pin
Support of system level flight time compensation
On-DIMM Mirror friendly DRAM pin out
Introduction of CWL (CAS Write Latency) per speed bin
On-die IO calibration engine
READ and WRITE calibration
DDR3 Modules:
Fly-by command/address/control bus with On-DIMM termination
High precision calibration resistors
Advantages compared to DDR2
Higher bandwidth performance increase (up to effective 1600 MHz)
Performance increase at low power (longer battery life in laptops)
Enhanced low power features
Improved thermal design (cooler)
Disadvantages compared to DDR2
Commonly higher CAS Latency
Generally costs much more than equivalent DDR2 memory.