Back in the ‘olden’ days it was simple, you had one type of memory to choose from – Fast Page (FP). Then along came Extended Data Output (EDO) followed by SDRAM (Synchronous DRAM), and that’s when things started to get confusing. At first, SDRAM was rated for 66MHz, then came PC100 for 100MHz and finally PC133 for 133MHz. Next came RDRAM (Direct Rambus – which has all kinds of issues I wont even get into here) followed by DDR SDRAM (Double Data Rate). Each time the peak bandwidth increased, something that was needed by ever faster CPU’s. Let’s take a look at a simple table showing bandwidth of the different memory technology:
Clock Rate (MHz)
Bus Width (bits)
PC1600 DDR SDRAM
PC2100 DDR SDRAM
Notice that the bandwidth has increased by a factor of 10+! Also memory clock speeds are over 5 times faster, and we have technologies that now allows for more than one operation per cycle (DDR & RDRAM). With each new memory technology, we did not see an immediate significant gain in performance, but can you imagine trying to run a 1.4GHz Athlon CPU using a 266MHz FSB (133MHz HostClk x2 using DDR) and yesterday’s FP memory? What we have seen is technology that has kept up with ever faster CPU speeds (and the rest of the system), and with the introduction of each new memory we have seen the memory speed that is needed for the next gain in CPU speed and technology. Try and keep that in mind: While PC133 SDRAM may be just as fast as DDR SDRAM with current CPU’s and technology, it may not keep up with what is just ahead. When SDRAM first came out it was not faster than the current EDO memory, but as CPU speeds (along with higher FSB speeds) increased, PC100 and then PC133 memory was needed to keep up. The same will apply with DDR SDRAM (and maybe even RDRAM).
One thing I haven’t covered is VCM – Virtual Channel Memory – SDRAM. It is not very common (I don’t even know of a supplier that carries it). The bandwidth of VCM is the same as SDRAM at the same speed, but its the way it access memory that is different. VCM should make the biggest difference in applications that use a lot of memory access and multi-tasking. If you’d like to read up on VCM take a look here (it’s in German so you’ll need to translate).
Want to know more about memory? Check some of the links here at Real World Technologies (Dean Kent has written a number of different articles) or take a look at Crucial or Kingston – they both have pretty good primers.
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