It’s been quiet a while since Rambus released RDRAM. However, adoption of this new technology has been somewhat slow, despite support from Intel in its 820 chipset-based motherboards. One of the reasons for this has been the much higher cost of RDRAM compared to SDRAM.

The major difference between RDRAM and other kinds of memory is the clock speed. Whereas the memory used in most computers today runs at 100 MHz or 133 MHz, RDRAM has the capability to scale up to 800 MHz. It’s available in three different speeds—600 MHz, 700 MHz, and the fastest and costliest version at 800 MHz. It may seem like 800 MHz RDRAM would be six times faster than 133 MHz SDRAM, but this is not true, since the true measure of the capabilities of RAM is not just its frequency, but also factors like bandwidth, latency, etc. These are some vital areas where RDRAM doesn’t outperform SDRAM as much as one would expect.

Memory bandwidth

To understand how data is transferred from the memory using a bus, imagine a highway with multiple lanes. Each lane has the capability to accommodate one car. Depending on the number of lanes, a highway will have the capacity to handle more and more traffic. This is exactly how a bus works as well. In our case, these "cars" are nothing but single bits coming from memory. The frequency of the memory decides how quickly traffic exits the highway. The number of "lanes" differs between different memories. SDRAM has a 64-bit bus width. This means that at 133 MHz, its bandwidth stands at 1.064 GB/sec (64x133/8). In contrast, RDRAM has a 16-bit bus width but runs at 800 MHz giving it a theoretical bandwidth of 1.6 GB/sec (16x800/8). Thus, these figures show that RDRAM will not work all that fast compared to SDRAM.

To say that RDRAM has 800 MHz clock is not entirely accurate as well. Actually, it’s fed with a 400 MHz clock, but transfers data on the rising as well as falling edge of the clock pulse, thus effectively making it work at 800 MHz. Though this works out well in the case of PC800 RAM, it creates problems in slower versions. For example, in the case of PC600 RDRAM, the external clock speed is actually a measly 266 MHz, with effective clock speed at 532 MHz. A little mathematics, and the bandwidth for PC600 RDRAM comes out to be 1.064 GB/sec, which happens to be exactly the same as PC133 SDRAM.

Memory organization

SDRAM is usually mounted on a small PCB (printed circuit board) that’s called a DIMM (Dual Inline Memory Module). RDRAM, on the other hand, is mounted on a different slot called RIMM (Rambus Inline Memory Module). The number of SDRAM or RDRAM chips in a module differ, depending on the bus width supported by individual RAM chips. For example, one SDRAM module —which has a 64-bit data bus—could have sixteen 4-bit wide or eight 8-bit wide SDRAM chips.

SDRAM is parallel in nature. This means that all memory chips in a module of SDRAM are connected in parallel to the data bus that’s used to read and write data. On the other hand, RDRAM has a serial nature in that individual chips are connected in serial to the data bus. So, it’s arranged logically as a single strip of RDRAMs through which the common 16-bit bus runs. As a result of this, no RIMM slot on a motherboard can be left empty and must be filled up by a special kind of PCB, called a continuity module.

Latency

All memories take some time to process a request for data and transfer the same. This is called latency. Due to the serial nature of RDRAM, the chips closest to the memory controller take much less time to respond to the controller, compared to those that are located further away. This difference in time can be quite a lot, since the farthest RDRAM chip can be about a foot away from the memory controller. Hence, the controller must find a way to manage all these different latencies. To do this, the controller finds out the highest latency value in all the RDRAM chips during the boot phase, and then programs the rest to work at that latency. Thus, even though the actual latency for RDRAM may be very low, more often that not, the RIMM ends up working at a much higher latency value.

Both the latest SDRAM as well as RDRAM have 20 nanoseconds latency. But because of the reasons given above, RDRAM always has latency greater than published figures.

To the benchmarks

We decided to see for ourselves how the two technologies performed in comparison to each other. Our test setup included 128 MB of PC800 RDRAM on an Intel VC820 motherboard and 128 MB of PC100 SDRAM on an Asus CUBX motherboard. All testing was done using a GeForce display adapter, PIII/700 MHz CPU and 5,400 rpm Seagate hard drives.

First up, Business Winstone 99. This benchmark is a popular suite of tests used for evaluating performance in business applications like MS Office, Lotus SmartSuite, etc. This is an area where the primary advantage of RDRAM—its bandwidth—is not stressed very much. The results favored SDRAM slightly, as it managed to score 29.6 in comparison to RDRAM’s 28.8.

Next came Quake III Arena. Games are known to be very heavy applications and quickly saturate whatever bandwidth and processing power you throw at them. However, with the continued development of more and more advanced AGP cards, the focus has been shifting away from the RAM and processor to the display card. Our results showed as much, as both our test systems consistently gave us the same frame rates. So, we thought we’d use CAD applications for testing, thinking that we might be able to find some difference. For this, we used the Indy3D benchmark to stress the PC. The benchmark has highly customizable tests to stress your PC to the limit. No luck here either. Everywhere we would get the same scores, no matter what we’d try.

The verdict

RDRAM has certainly been a step in the right direction. All indicators in the current-day scenario point towards a future where more and more bandwidth will be required. SDRAM simply cannot satisfy this insatiable appetite. However, one has to think whether RDRAM is the right solution. The most prohibitive factor in the whole RDRAM affair is its exorbitant cost. One simply cannot be expected to pay the same amount for RAM, as for a whole new computer. Then there are performance issues as well. A new memory called DDR SDRAM is just around the corner. Expected to cost a little more than the current SDRAM, DDR (Double Data Rate) memory will have even higher bandwidth than the best RDRAM available today. Hence, for now, RDRAM seems to be a waste of money.