SDRAM, DDR and Rambus Memory

SDRAM, DDR SDRAM and Rambus Memory
What’s all the fuss about?


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As we move through yet another memory development period, more and more customers ask SDRAM and whether or not it has become obsolete with the release of DDR SDRAM and RDRAM, better known as Rambus. A few have even asked whether or not they can upgrade to either DDR SDRAM or Rambus. The following should answer these questions for you, plus provide you with a few of the basics.

Indeed, DDR SDRAM and Rambus DRAM are the new kids on the block when it comes to memory development. During development, both types promised to make computers run faster, and after substantial testing, both actually delivered on their promises. Neither one, however, will make SDRAM obsolete anytime in the near future.

Over the last few years SDRAM (synchronous DRAM) has become the standard type of memory installed in all personal computers. This is mainly due to the fact that SDRAM is tied to the front-side bus clock of the PC. SDRAM and the system bus execute instructions at the same time rather than one of them having to wait for the other to perform their function. As bus speeds have increased to 133MHz (and beyond), this tie to the system bus has improved system performance substantially compared to previous memory types. Need more information on memory speeds?

DDR SDRAM
The next generation of SDRAM is DDR, or Double Data Rate. Like SDRAM, DDR is synchronous with the system clock. Although similar, in that both are synchronous, the big difference between DDR and SDRAM is that DDR reads data on both the rising and falling edges of the clock signal, while SDRAM only carries information on the rising edge of a signal. This improvement allows the DDR module to transfer data twice as fast as SDRAM. As an example, instead of a data rate of 133MHz, DDR memory transfers data at 266MHz.

DDR modules, like their SDRAM predecessors, arrive in their DIMM form factor. Although motherboards designed to implement DDR are similar to those that use SDRAM, they are not backward compatible with motherboards that support SDRAM. You cannot use DDR in earlier SDRAM based motherboards, nor can you use SDRAM on motherboards that are designed for DDR. DDR memory is available in either ECC (Error Correction Code) or non-ECC.

Rambus DRAM
Rambus memory (RDRAM®) is a revolutionary break from the engineering designs implemented in SDRAM. Rambus is an entirely new memory design with changes to the bus structure and how signals are carried. Although Rambus memory sends less information on the data bus, which is 16 bits wide as opposed to the standard 32 or 64 bits, however it sends data more frequently. Rambus, like DDR, also reads data on both the rising and falling edges of the clock signal. As a result, Rambus memory is able to achieve effective data transfer speeds of 800MHz and higher.

Unlike SDRAM, motherboards that support Rambus memory require that all memory slots in the motherboard must be populated. Not with memory modules necessarily, but with a combination of memory modules and Continuity Modules, more commonly referred to as CRIMMs. As an example, on SDRAM based motherboards, you could have 4 memory slots and fill only one with a memory module, and the system would operate. On Rambus based motherboards, all blank or unfilled memory sockets must be populated with either a memory module (in this case a RIMM™ Rambus Inline Memory Modules) or a Continuity Module, (CRIMM) to complete the memory path to the bus. Rambus memory is available in either ECC or non-ECC format.

Production Challenges
One of the most daunting challenges development of Rambus memory faced was the high cost of manufacturing as compared to SDRAM and later, DDR. Rambus memory is a proprietary technology of Rambus Inc., therefore manufacturers that want to produce it are required to pay a royalty to Rambus Inc. DDR designs, on the other hand, are open architecture. Since Rambus is an entirely new design, there are other cost factors to be added in, such as an entirely new module manufacturing and testing process and larger die size. Rambus chips are considerably larger than SDRAM or DDR. This larger die size means that fewer parts can be produced on a wafer. If you would like to review additional details about the memory manufacturing process, visit: Micron Manufacturers Memory: Here’s How. As Rambus technology moved forward and more manufacturers began using it, pricing dropped dramatically. This is the case with most new technologies though isn’t it?

Many major computer manufacturers, including us, announced the production of Rambus based systems during 1999. Initially these systems used motherboards built around the Intel 820 (Camino) chipset, which was a complete debacle. Intel quickly resolved the problems with the release of the 840 and 850 chipsets, and now the 860 chipset. Although you will read arguments from proponents of DDR that it out performs Rambus, you will also find articles from Rambus proponents arguing that Rambus is the leader. We’ve run the tests, and frankly it’s our opinion that both memory types have their place. You can read more about this testing here: DDR vs. Dual Channel RDRAM.

Performance
This should be reasonably easy to determine, data flow at 800MHz should be faster than data flow at 266MHz, right? Well, it isn’t all that simple. Because EDO DRAM, SDRAM, and RDRAM are based on the same core-memory technology, their internal device timings are nearly identical. Thus, the differences among memory subsystems that affect the latency include the rate at which the system can move the address and control information to the DRAM and the rate at which the DRAM can move data from the DRAM to the memory controller. We know what you’re thinking .. huh? The simplest and easiest explanation is that the speed is entirely dependent upon how the systems, consisting of the motherboard components such as the memory controller etcetera, are designed and constructed.

In a DDR or SDRAM system, each DIMM is connected, individually and in parallel, to the data bus. So whether you have a single DIMM or multiple DIMMs, the amount of time it takes to initiate a data transfer is effectively unchanged.

In a Rambus system, RIMM modules are connected to the bus in a series. The first data item transferred must pass through each RIMM module (and/or CRIMM module) before it reaches the bus. This makes for a much longer distance for the signal to travel, but remember it is also traveling 3+ times faster. Again, which memory you choose is entirely dependent upon how you intend to use the system. All too often people purchaser computers without first determining whether their choice will meet their needs. You can read more about Rambus here: High-Speed DRAMs keep pace with high-speed systems and here: Rambus Dram Performance. And you’ll find more about DDR SDRAM here: The Technical Aspects of DDR and RDRAM. Each of these links will open in a new window.

What Does The Future Hold?
So, we arrive at the big question, which technology will become the memory of choice for the computer industry in the very near future? We took a look at Memory Trends in the Year 2001 in February of 2001, and nothing has really changed since then. When Rambus was emerging in 1999, it was touted as the biggest breakthrough yet in memory technology. Shortly thereafter, DDR SDRAM emerged, and its proponents said that DDR was the true king of the hill. We stand by our opinions: Both are extremely fast; there is no clear winner, as we believe that adequate benchmarks for this technology do not yet exist; and finally we believe that both technologies have their application environments. Remember that most platforms, whether it be Windows, Linux or what have you, they have been designed around existing technologies, including PC-100 and PC-133. In order to adequately evaluate either Rambus or DDR, you must have a hardware platform that takes full advantage of this memory technology, and then design the software platform around the hardware platform. Once you have achieved this much, you can then design the benchmark software around both!

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