System memory technology has taken quite a few jumps over the last 10 years, we have seen the demise of SIMM memory modules, the rise of DIMM technology, the attempts to push new technologies that did not really make any sense and now evolution has pushed us even further than we could imagine.
Back when the 386 was king of desktop computers, the memory modules of choice (well in fact they were the only choice) were SIMM or Single In-line Memory Modules. SIMMs worked similar to how dual-channel memory works, only on a different process. Because a SIMM is a single module, in order to get a system to work you need two identical modules. SIMMs are 32-bit modules so two of these give you a 64-bit memory bus.
Socket 7 brought a new memory technology to the table, DIMM or Dual In-line Memory Modules. DIMMs marked the first breakthrough in memory technology where you did not need two modules to run the system, only one, this was accomplished by putting the full amount of SIMM modules onto a single DIMM module. Latencies were reduced and speeds were increased to 100MHz and eventually 133MHz thanks to VIA's big push on the SDRAM market.
When Pentium 3 Coppermine first saw the light of day, Intel tried to move the market to a new memory technology, called RIMMs or Rambus In-line Memory modules. RIMMs were a step back in terms of their use as they had to be installed in pairs like SIMMs. Also if you had more than one Rambus channel you needed to have terminators in the second channel, making it just that much harder for users to work the technology out. Intel only kept the push on Rambus for its Pentium 3 and early Pentium 4 processors because of its licence agreement with Rambus Corporation which meant that using competing technologies could have resulted in the termination of the contract and Intel being sued for quite a lot of money
At the same time Rambus was coming out for the Pentium 4, JEDEC was ratifying a new technology based on the SDRAM architecture. This was the first time DDR came into the picture for the desktop market. DDR memory took existing SDRAM architecture and doubled the bandwidth by allowing information to be sent on the rising and falling edge of the clock cycle.
Following from the DDR architecture, JEDEC took this design and ratified dual-channel DDR memory. Dual-channel increased the bandwidth by increasing the bus width from 64-bit to 128-bit but to do this you needed two identical DDR memory modules.
DDR-2 evolved from the DDR-1 standard by reducing the transistor size, voltage requirements and speeds along with quite a few extras to the DDR architecture.
DDR-3 is now coming to the desktop thanks again to Intel. DDR-3 in its early form has been shown to not only outperform DDR-2 at clock for clock speeds (something DDR-2 could not do over DDR-1), but DDR-3 is able to clock higher than DDR-2 could ever dream of, in fact we managed 1500MHz DDR-3 speeds in our recent tests.
Today we are stepping back from DDR-3 and looking at some DDR-2 modules from Crucial Technology. While DDR-3 is the future for Intel platforms, DDR-2 is still the mainstream product for Intel as well as the only memory technology currently supported by AMD and its Athlon 64 processors which have memory controllers on the CPU.