<b>Types of RAM</b>
SDRAM - Stands for Synconous Dynamic Random Access Memory. With the introduction of DDR RAM SDRAM has become obsolete, but can still be found in some older systems.
DDR SDRAM - Stands for Double Data Rate Synconous Dynamic Random Access Memory. As the name suggests, DDR RAM provides twice the bandwidth of conventional SDRAM running at the same clock speed. This is because DDR RAM sends data twice every clock cycle instead of the once every clock cycle that SDRAM does.
DDR2 - This is a new type of RAM that is set to overtake DDR RAM, while technically very similar to DDR RAM is has improvements that allow it to reach higher clock speeds. IT IS NOT BACKWARDS COMPATIBLE WITH DDR RAM.
Currently it is only supported bby intels 915/925 chipsets. AMD has no plans to support it this year.
RDRAM - Stands for Rambus Dynamic Random Access Memory. Released to co-incide with the introduction of the pentium4. It offered extreamly high clock speeds but required two matched pairs of 16bit modules to work (32bit modules were released later but too late to save RDRAM). Although very fast it was very expensive and a pain to upgrade, it was soon overtaken by DDR RAM which was far cheaper.
<b>FAQ</b>
<b>How fast is my DDR RAM? </b>- When DDR RAM was first released it ran at 133Mhz but could transfer twice as much data as 133Mhz SDRAM. In order to show that DDR was twices as fast as SD marketing departments decided to call it DDR266. This caused confusion because people started to think it ran at 266Mhz when actually it DIDNT. So to aleviate this confusion they decided to name it after is peak theoretical bandwidth 2,100MegaBytes per second or PC2100 as it came to be known.
<b>What is dual Channel mode?</b>
Dual Channel mode for P4/Athlon64 - Certain chipsets for the pentium4 and Athlon64 (socket 939 only) allow the use of dual channel mode. DDR memory transfers 64bits every clock cycle, one day some clever people realised that if you combined 2 64bit channels you could make one big 128bit channel and send 128bits everyclock cycle. This essentially doubles the maximum memory bandwidth. It requires each memory channel to be the same size, in a motherboard with 4 RAM slots two RAM slots make up one channel and the other RAM slots make up the second channel. YOU HAVE TO HAVE AN EQUAL NUMBER OF RAM MODULES FOR DUAL CHANNEL MODE TO WORK i.e. either 2 or 4.
Dual Channel mode AthlonXP - Since Athlon XP's do not A) Have the FSB to cope with dual channel mode (the connection from northbridge to CPU is 3.2GB/s) or B) Benefit much from increased bandwidth, Nvidia decided not to combine RAM channels. Instead they opted to stagger latencies (the chip with the lowest latency signals first). This provides a bit of a performance increase but only in the region of 4-7%. Dual channel mode with AthlonXP's works requires at least 2 modules, but does not need equal sized module and does not require equall numbers of modules.
<b>Can I use RAM that is rated higher than my motherboard supports?</b> All DDR RAM modules are back wards compatible. That is to say that PC3200 will work in a board that only supports PC2700, however it will lower its clock speed to 333Mhz rather than 400Mhz. Often to modules of different speeds will be physically identical, but will be labled based on what clock speeds they can achieve.
The same applys to SDRAM and DDR2 as long as the pin count is the same (two versions of SDRAM were released one with 144 pins and one with 168pins.).
<b>What is ECC and do I need it?</b> ECC stands for Error Correction Checking/Code, ECC RAM contains an extra Chip which stores the parity bit for each byte. This allows the RAM to identify when an error has occured and sometimes remedy it. It is normally used in big servers where data loss cannot be tolerated. The only time you would need it would be if you use huge ammounts of system RAM (2GB+) or if you have a socket 940 AthlonFX (required ECC RAM).
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