DDR Has an Edge over SDRAM
Much has been written about the benefits of higher memory bandwidth, starting with DRDRAM and now DDR. Most have been looking for the reason that one or the other of these memory technologies will become the ‘next standard’ in PCs. To this point, the primary focus has been on performance, with many showing that the extra bandwidth provides no real performance difference with either memory type. With the introduction of the KT133A chipset and its ability to run the memory at 133MHz while the memory to CPU interface runs at 266MHz, it has become even more difficult to make any compelling argument for either of these new memory types.
Power dissipation has been discussed mostly as an aside, yet it may be the main reason it becomes widely accepted in the industry. DRDRAM devices can dissipate a significant amount of power when active (see Direct Rambus DRAM, Part 2 – Operation and Performance by Paul DeMone), which limits the number of pages that can be left open and active. This, in turn, results in greater latency so that performance improvements are somewhat limited. For this reason, as well as higher cost, DRDRAM implementations have been limited to workstations and high-end desktops.
A recent test using Micron DDR SDRAM modules shows that the power dissipation for PC1600 modules is about half that of PC133 SDRAM, using the same motherboard and other components (note that the SDRAM was plugged into a riser card so it would fit into the 184 pin DIMM slot). This is quite significant, and is particularly important for the mobile market and server markets. During the past several months, there has been much discussion about the power requirements of modern processors, and what that will mean for both servers and mobile systems in the next few years. This new information seems to be the most compelling reason why DDR is the smart choice for both server and mobile systems, if not desktops.
Figure 1 Power Consumption for DDR and SDRAM – Courtesy Micron Technology, Inc.
While performance makes for good marketing material, in the end what wins in the marketplace is the cost of implementation. DDR SDRAM incurs a very small die size penalty vs. SDRAM in the same process, and can be manufactured on the same production line as SDRAM, so it is almost at price parity with SDRAM. By reducing the requirements for expensive thermal management in servers, and additional battery life in notebooks, it actually provides a lower cost solution than DRDRAM with similar performance in all but the most memory intensive applications. Though the memory battle continues to be waged for the desktop, DDR currently appears to have the upper hand as the memory of choice for mobile and server systems.
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