What is the difference between
SDRAM and RDRAM?

What is the difference between SDRAM and RDRAM?

Rambus® and DDR SDRAM

Question: I just bought a computer that has SDRAM and now I'm hearing that DDR and Rambus memory are better. What are these new types of memory and how do they work? Did I buy the wrong thing? Help!

Answer: You heard right, DDR and Rambus DRAM are two new types of memory promising to make computers run faster. But before we discuss them, relax! They will NOT make SDRAM obsolete in the near future.

First, let's start with a quick review. In the last few years, SDRAM
(synchronous DRAM) has become the standard type of memory for PCs. The main reason for this is that SDRAM is tied to the front-side bus clock in your system. SDRAM and the bus execute instructions at the same time rather than one of them having to wait for the other. As bus speeds have increased beyond 100MHz, this has improved system performance.

DDR SDRAM
DDR (double data rate) memory is the next generation SDRAM. Like
SDRAM, DDR is synchronous with the system clock. The big difference between DDR and SDRAM memory is that DDR reads data on both the rising and falling edges of the clock signal. SDRAM only carries information on the rising edge of a signal. Basically this allows the DDR module to transfer data twice as fast as SDRAM. For example, instead of a data rate of 133MHz, DDR memory transfers data at 266MHz.

DDR modules, like their SDRAM predecessors, are called DIMMs. They use motherboard system designs similar to those used by SDRAM; however, DDR is not backward compatible with SDRAM-designed motherboards. DDR memory supports both ECC (error correction code, typically used in servers) and non-parity (used on desktops/laptops.)

If your system or motherboard requires DDR, you can purchase the upgrades you need through Crucial's Memory Selector™.

Rambus DRAM
Rambus memory (RDRAM®) is a revolutionary step from SDRAM. It's a new memory design with changes to the bus structure and how signals are carried. Rambus memory sends less information on the data bus (which is 18 bits wide as opposed to the standard 32 or 64 bits) but it sends data more frequently. It also reads data on both the rising and falling edges of the clock signal, as DDR does. As a result, Rambus memory is able to achieve effective data transfer speeds of 800MHz and higher.

Another difference with Rambus memory is that all memory slots in the motherboard must be populated. Even if all the memory is contained in a single module, the "unused" sockets must be populated with a PCB, known as a continuity module, to complete the circuit.

Rambus DRAM modules are known as RIMM™ modules (Rambus inline memory modules). Rambus memory supports both ECC and non-ECC applications.

Production Challenges
One of the challenges Rambus memory currently faces is that it is expensive to produce compared to SDRAM and DDR. Rambus memory is proprietary technology of Rambus Inc. Manufacturers that want to produce it are required to pay a royalty to Rambus Inc., whereas DDR designs are open architecture. Other cost factors for Rambus memory include additional module manufacturing and testing processes and a larger die size. Rambus die (chips) are much larger than SDRAM or DDR die. That means fewer parts can be produced on a wafer.

Performance
Now for the million-dollar question: How do DDR and Rambus memory compare performance wise? Sorry, I know you don't want to hear this — that depends. Both technologies have their own ardent supporters and we have seen several different benchmarks to date that provide conflicting results.

On the surface, it seems simple: Data flow at 800MHz is faster than data flow at 266MHz, right? Unfortunately, it isn't that simple. While Rambus modules may have the ability to transfer data faster, it appears to have higher latency (the amount of time you have to wait until data flows) than that of a DDR system. In other words, the first data item transferred in a Rambus transaction takes longer to initiate than the first data item moved in a DDR system. This is due in part to how the systems are 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 before it reaches the bus. This makes for a much longer distance for the signal to travel. The result is higher latency. That's not necessarily a problem in an environment where data transactions involve lengthy streams of data, such as gaming. But it can become an issue in environments where many small transactions are initiated regularly, such as a server.

To further explain, here's an example that we can all relate to — driving your car to the store. You can take the roundabout freeway and drive 20 miles at 70 MPH. Or, you can take a more direct route and drive just 5 miles at 50 MPH. You might go faster on the freeway but you'll get to the store (Memory Controller) faster on the straight-line route.

Looking to the Future
So which technology will become the memory of choice for the computer industry? That probably won't be clear until sometime in the near future. However, it really doesn't matter to a certain extent.

Generally speaking, motherboards are built to support one type of memory. You cannot mix and match more than one type of SDRAM, DDR, or Rambus memory on the same motherboard in any system. They will not function and will not even fit in the same sockets. The right type of memory to use is the one that your motherboard takes! And no matter what type of memory you use, more is typically better. A memory upgrade is still one of the most cost-effective ways to improve system performance.

At this point in time, the market for DDR and Rambus memory is relatively small. However, it is growing. Crucial currently offers DDR and will offer other new technologies as the market dictates.

BACK TO INDEX

Memory Modules 101 - glossary of terms

Do I really need ECC SDRAM for my server?

Do I need ECC or non-ECC Memory?

Which is better, DDR or RDRAM?

What does PC100 stand for?

Installing RAM