A buffered(a.k.a PARITY) RAM module is very similar to a registered module. Buffers re-drive the signals through the memory chips and allows for the module to be built with more chips. Buffered and unbuffered memory chips cannot be mixed. The design of the computer memory controller dictates whether memory must be buffered or unbuffered

Registered RAM modules contain a register chip that delays all information transferred to the module by 1 clock cycle. This type of memory is primarily used in servers and was designed for modules with 32 or more chips on them to help ensure that data is properly handled.

ECC (error checking and correcting) memory performs double bit detection and single bit correction. This means that if you have a single bit memory error, the chipset and memory will find and repair the error on the fly without you knowing that it happened. If you have a double bit memory error, it will detect and report it. Non-ECC, non-parity memory, on the other hand, has no error detection and correction capabilities.

Using ECC decreases your PC's performance by about 2%. Current technology DRAM is very stable and memory errors are rare, so unless you have a need for ECC, you are better served with non-ECC, non-parity memory

FB-DIMM (Fully Buffered DIMM) is a memory module technology targeted to servers developed recently created in order to increase the memory speed and the maximum memory capacity of a server. In this tutorial we will explain what are the main features of FB-DIMM modules and the difference between DIMM and FB-DIMM modules.

The main difference between FB-DIMM modules and regular DIMM modules is that on FB-DIMM the communication between the memory controller (chipset) and the module is serial, in the same way that occurs with PCI Express, while on standard DIMM modules this communication is parallel.

Using serial communication the number of wires needed to connect the chipset to the memory module is lower and also allows the creating of more memory channels, what increases memory performance. With FB-DIMM technology it is possible to have up to eight modules per channel and up to six memory channels. So this technology increases both memory capacity and speed.

Each extra memory channel that is added to the system increases the memory subsystem transfer rate. For example, if you use a single DDR2-533 channel, the memory transfer rate will be of 4,264 MB/s. If you use two DDR2-533 channels, as it happens on dual channel configuration available today, the memory transfer rate will be of 8,528 MB/s. With four channels, the memory transfer rate jumps to 17,056 MB/s, and so on.

http://www.hardwaresecrets.com/article/266

CAN I MIX AND MATCH?

BUFFERED AND UNBUFFERED
The buffered and unbuffered modules are not interchangeable and even use slightly different printed circuit boards (PCB's). If you try to install the wrong type, the first notch on the bottom of the module will be offset. You can determine if the module is buffered by looking at the leads next to the first notch. If the leads are evenly spaced, the module is buffered. If the leads are not evenly spaced (a larger PCB area next to the lead) the module is unbuffered.

ECC AND NON-ECC
Parity modules have an extra chip that detects if data was correctly read or written by the memory module, depending on the type of error. However, a parity module will not correct the error. ECC modules can detect double bit errors and correct single bit errors. Most motherboards that do not have an ECC function within the BIOS are still able to use a module with ECC, but the module will run in non-ECC mode. Keep in mind, there are some cases where the motherboard will not accept an ECC module, depending on the BIOS programming. The only sure-fire way to test this is to place the module in the motherboard and see if the BIOS will recognize the memory addition.

Source: http://www.datamemorysystems.com/faq.asp

HOW CAN IT WORK?
Highly dependant on the chipset of your motherboard and BIOS! If either one of them don't detect that you can use ECC RAM then you cannot use them. Or can use them but it the ECC function would be useless. ECC is much slower than the non-ECC. They are not as fast as the non-ECC ones but I think technology will cut or closer that gap in the near future.

This post has been edited by viqq: May 12 2007, 04:22 PM

Why DDR2 price drop so significantly? hmm..let me have a guess

IMHO, first is that DDR2 are too many on the market because of the new manufacturing & packaging technology, they can make DDR2 super duper fast..I mean faster than DDR or it could be that they have stopped DDR manufacturing and full force DDR2 now before DDR3 comes out..hehe..