DDR1 DDR2 RAM

We have seen computer technologies improve greatly over the past few years. The memory of computers have also evlolved a lot from RAM, DRAM, SDRAM. Then came DDR-SDRAM and now DDR2-SDRAM. We should not concern ourselves with the very old models of RAM since those are no longer in use in most parts of the world.

SDRAM (Sychronous Dynamic Random Access Memory) is a type of memory that requires power to hold its data. This is the prevalent memory used in computers right before the intoduction of DDR-SDRAM (Double Data Rate SDRAM). DDR improves upon the architecture of SDRAM by a process known as “double pumping”. Instead of transferring data once every clock cycle, DDR changes its data state twice every cycle. Once on the rising edge, then another on the falling edge. This is true for both ddr and ddr2. So why is ddr2 better than ddr?

Actually when ddr2 came out, it was actually worse than ddr. The original ddr memory has its memory clock synchronized with the bus clock, allowing 2 bits to be transferred every clock cycle. Ddr2 changes this by doubling the bus speed while still holding the memory clock at the same frequency. So that at every memory cycle, 4 bits of data is transferred. The trade-off to this is the higher latency of ddr2 memory compared to ddr memory when operating at the same bus frequency.



In order to obtain the same performance as a ddr running at 100Mhz bus speed, ddr2 memory must be run at 200Mhz. But if we look at the clock speed, the ddr is also working at 200Mhz while the ddr2 is only at 100Mhz. If we have a ddr2 chip that operates at the same clock speed as ddr1 then we can see that it has double the throughput.

Memory clock speed is very important because producing chips with higher clock speeds is very expensive since in every batch of chips produced, only a small amount of it are capable of higher clock speeds. So if we compare two chips that are similarly clocked, the ddr2 would be cheaper. And if we compare two chips that are similarly priced, the ddr2 would be faster. The technology of ddr also ends where ddr2 begins, meaning that ddr chips can no longer be improved further due to restrictions in cost while ddr2 goes far beyond the capabilities of ddr. The capabilities of ddr2 also has its limits, mainly as the clock speed gets higher, thats where ddr3 comes in. But that’s another story altogether.