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,
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.
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.
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
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.