Introduction to DDR SDRAM


(or Double Data Rate SDRAM)


We have no intention of getting into the disputes over which technology is better, Rambus or DDR SDRAM. Both have their place in the user environment, and they both have their positive, as well as negative, aspects. If you are building your own system, or having us do it for you, you will be well served to decide first what it is that you want to do with your computer and then select the components based upon that use. The number of people that are unhappy with their computers is staggering, and it is for no other reason then they purchased their computer before deciding what it is they want to do with it.

It seems as though every time we visit memory related issues, we find something new being developed, or existing technology is being redeveloped. In any event, DDR SDRAM (Double Data Rate SDRAM, “DDR” for short, is the new kid on the block. Released earlier this year, DDR appears to be fulfilling most of its developers promises as far as the speed to cost ratio. Although new technologies take time to fully develop, we expect to find that DDR will be appearing not only in mid-range work stations, but also in servers and high-end work stations.

DDR derives its name from its functional characteristic of using the rising and falling edge of the memory bus clock for timing. Whereas traditional memory modules use only the rising edge of this clock for timing, DDR can take a 100 MHz or 133 MHz clock and effectively run at 200 MHz or 266 MHz. This greatly increases the effectiveness of the memory bus for data transfer. At this speed, one industry leader, Hitachi, says that the 133 MHz DDR module will provide a peak data bandwidth of 2.1 GB/s (GigaBytes per second). In comparison, a 133 MHz bus with SDRAM provides a peak bandwidth of 1.066 GB/s and a RAMBUS 800 MHz module provides 1.6 GB/s peak bandwidth. You can read more about Memory Speeds here.

Peak bandwidth should not be confused with overall performance. The average bandwidth must be considered also. The average bandwidth is the amount of data actually traveling on the memory bus over a prescribed period of time. DDR, like SDRAM, has inherent Latencies (or wait cycles) when the processor is not reading or writing data to the memory bus. CAS (Column Address Strobe) Latencies occur after a read or write command is issued from the processor to the memory. The Latency, specified in clock cycles, is the number of cycles the memory requires after a command, but before it is ready to perform that command. Typically SDRAM has a Latency of 2 or 3 cycles. These Latencies effect the average bandwidth of the memory module. DDR running at 266 MHz, should have an average bandwidth of about 1.2 GBs with an efficiency of 60%. In comparison, RAMBUS running at 800 MHz has an average bandwidth of 1.5 GBs with an efficiency of 95%.

Unfortunately, you will not be able to transfer your existing SDRAM memory modules to a computer that utilizes DDR or RAMBUS memory. Will you be able to use DDR in your present computer? Unfortunately no you cannot, as DDR is not backwards compatible with current SDRAM modules, and neither is RAMBUS. Both DDR and RAMBUS use different voltages and a different number of pins on their edge connectors than SDRAM modules.

DDR is an extremely fast type of memory. It tops RAMBUS in peak bandwidth performance. However, the average bandwidth is slower. Both RAMBUS and DDR are currently in production, and RAMBUS is presently more costly than SDRAM and DDR. What you will need to determine is whether your needs are for consistent average high capacity bandwidth, or fast short strobe peak bandwidth.

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