The Evolving Memory Landscape
PC makers are starting to push DDR2 as a replacement for today's DDR memory. Meanwhile, Rambus dreams of an XDR world, while quad data rate seems to have vanished. We try to make sense of the changes in PC memory.Robert Heinlein is famous for writing some of science fictions classic novels. We remember him today, however, for his famous acronym: TANSTAAFL ("There Aint No Such Thing As A Free Lunch.") Heinlein was by no means the first writer to express that sentiment, but he used the acronym often enough that it stuck. TANSTAAFL is really a restatement of the second law of thermodynamics: systems tend towards greater disorder unless you feed the system energy from an outside source. In the case of PC performance, we can see that increasing amounts of energy need to be fed into the system to increase performance. Processors are getting faster -- and hotter. We now have graphics hardware that can eat up to 75W or more of power, and generate substantial heat in their own right. Memory is no exception. People are increasingly loading up their PCs with more and faster memory. The net result is added power draw and heat consumption.
Most new PC systems sold in the past year used DDR (double data rate) memory. DDR memory pumps out two data samples per clock cycle. Youll typically see DDR memory rated at the effective clock rate of the memory, as if it were sending one data sample per clock cycle. For example, DDR400 memory really runs at 200MHz. Youll also see this listed as "400MHz effective."
An alternative naming scheme uses the effective data rate of the memory. So DDR400, which is capable of shipping out 3.2 gigabytes per second of data, is also called PC3200 (for 3200MB per second). That seems pretty fast, but by todays processor standards, its not. A 3.4GHz Pentium 4 or a 2.4GHz Athlon 64 FX-53 still spends a lot of time waiting for memory. Thats one reason cache sizes inside the CPU have been increasing to reduce the time spent waiting for data to be retrieved from or written to memory.
DDR has other issues that affect its stability and performance. Memory termination, for example, isnt built into DDR memory, so its actually a set of resistor packages built onto the motherboard itself. Termination is needed to minimize signal reflections which degrade stability. This adds some cost, and also increases risk of instability as clock rates go higher, since the termination resistors are far away (relatively speaking) from the DRAM chips. Another escalating problem is heat and power draw as memory clock rates go up. So a new type of DDR memory has arrived on the scene.