Re: The difference between RAMs ( RDRAM, SDRAM, SRAM, DRAM),which is fastest?

Hello, Jon.  First off, there are so many different types of memories because the market for memories is so huge, and because different types of memories can offer different 
advantages that are appropriate to different applications, corresponding to different market "niches" for memories.  If a type of memory came out that was the fastest AND 
cheapest, then it might dominate.  There are some new types of memories that might do this in the future - time will tell.

In terms of RAM's (Random Access Memories), there are two basic types, SRAM's and DRAM's.  RDRAM's and SDRAM's are types of DRAM's.

SRAM's store bits (1's or 0's) in memory cells that are basically flip flops.  In a logic diagram, an SRAM memory cell may look like two cross-connected NAND or NOR gates 
- that is, the output of each NAND or NOR gate is connected to one input of the other NAND or NOR gate.This is a "flip flop", which can store a 1 or a zero for as long as 
you apply power - but not very much power.  Very little power consumption is experienced except during transitions – writing into the memory cell. Static power consumption 
(without transitions) is very low because the flip flop is based on CMOS NAND or NOR gates where either a P-channel or an N-channel transistor conducts, but not both – 
and the P and N channel transistors are connected in such a way that there is no conduction path from the power supply to ground so power is not consumed in the static state 
(except through leakage). The advantages of SRAM's are their very high speed and very low power consumption - but SRAM memory cells generally require 6 transistors 
(there are efforts towards memory cells based on fewer transistors, such as 4 transistor cells). So, the simple answer to your question is that SRAM’s, Static Random Access 
Memories, are the fastest.

DRAM’s store bits in memory cells that are basically capacitors under transistors. The transistor is used as a relay to the capacitor, and the capacitor stores a bit (a "1" or a "0") 
as either storing charge on the capacitor or not storing charge. However, the charge on a capacitor gradually leaks out over time – so, periodically (generally several thousand 
times per second), the contents of a DRAM need to read out and then rewritten back in – this is called a  "refresh cycle", or a "refresh".  As you might expect, the refresh 
consumes a lot of power.  DRAM’s consume more power, and are slower than SRAM’s – but because a DRAM cell uses just one transistor (with a capacitor) compared to 6 
transistors for an SRAM cell, a DRAM memory is much cheaper than an SRAM memory.

RDRAM’s are special types of DRAM’s developed by Rambus. 

Here is a webpage that explains more about their technology. 
RDRAM’s are faster than other types of DRAM’s, as discussed at the Rambus website.  Recent court cases involving Rambus’ intellectual 
property have cast a shadow over RDRAM’s, as discussed in  this article 

SDRAM’s are synchronous DRAM’s, as discussed in this Webopedia page.