While the CPUs has become around 200 times faster in
ten years, the RAM performance has only gone up 20 times. So we need new
RAM types. But which?
Many vendors decided to go for DDR RAM as described in . Where DDR RAM is a development of the existing SDRAM technology, Intel choose RDRAM, which represents a much more revolutionary change in RAM design.
Intel and RDRAM without succesIntel is comitted to the Rambus RAM, which also is called RDRAM (Rambus Direct RAM), nDRAM, or RIMM (Rambus Inline Memory Modules).
RDRAM is an advanced technology patented by a company, who sells the technology to other chip manufactures for a 2% in license. In 1997 Intel signed a contract that apparently commits them to support RDRAM only in all new chipset up to 2002.
Originally AMD also expected to support the Rambus RAM for its Athlon processor. But having seen all Intel's problems with the technology, AMD is not so keen on the Rambus anymore. However, RDRAM is already used in Sony PlayStation 2 and in Nintendo64 machines. In the Sony PlayStation 2 you find 32 MB of RDRAM delivering a bandwidth of 3.2 GB/sec.
During 1999 and 2000, Rambus was not very successful. In fact, Intel has suffered a serious set-back due to their commitment to the Rambus design. The chip set i820 "Camino" became a little disaster.
Intel failed to produce a reliable way to interface SDRAM to the 820 chipset. The MTH (Memory Translator Hub - which translated RDRAM bus to SDRAM modules) had some timing or noise issues that caused unreliable operation. Intel replaced CC820 boards with VC820 boards (with 128MB RDRAM included) as the CC820 use the MTH and SDRAM while the VC820 used RDRAM.
But, on the paper, Rambus sounds great:
Intelligent Rambus designRDRAM is developed from the traditional DRAM, but the architecture is completely new. It has been streamed and optimized to yield new performance.
The RAMBUS-design gives a more intelligent access to the RAM, meaning that units can "prefetch" data and this way free the CPU some work. The idea is that data is read in small packets at a very high clock speed.
The RIMM modules are only 16 bit wide compared to the traditional 64 bit SDRAM DIMMs, but they work at a much higher clock frequency:
The Rambus modules work on 2.5 volts, which internally is reduced down to 0.5 volt when possible. This helps to reduce heat and radio signals.
The RIMMs hold controlling chips that turns off the power to sections not in use. They can also reduce the memory speed if thermal sensors report of overheating.
CRIMMsAll RAM slots have to be full; this is new, with RAMBUS we have to fill in blank modules in slots which are not in use. The blank modules are called CRIMMs (with a 'C' for continuity).
The RIMM modules hold 184 pins.
The RDRAM chips have to be placed very close to the CPU to reduce radio noise. This indicates, that RIMM technology is rather sensitive; Intel seems to have made that discovery as well.
High clock rates
As mentioned, the modules are only 16 bit wide, but they work at 600, 700 and 800 MHz. Actually a PC800 RIMM runs on a 400 MHz clock using both rising and falling edges, being clockdoubled just as DDR RAM.
More confusing the PC600 RIMM actually runs on a 266/532 MHz clock, and the PC700 works at 366/712 MHz.
This gives the bandwidth of up to 1.6 GB per second - compared to the 500-800 MB/sec of PC100 SDRAM - of a single Rambus channel. You may find a chart comparing the bandwiths of different RAM types in the next page.
Multichannel memory designYou may bundle four channels to a 64 bit wide RAM bus giving 6.4 GB/sec:
This is not possible using the existing RAMBUS-based chip sets like i820. They only operate with one RAMBUS channel onboard. The high-end chip set i840 operates with dual RDRAM channels, as the up-coming i850 will.
RIMM in the future, says Intel
GigaHertz versions of Rambus RAM will probably follow, so the technology has potential for much higher bandwidths.
In 1999 it seemed that Intel was having big problems with the Rambus technology in the ill-faited i820 chip set (the so-called "Caminogate" tragedy). Hence they were forced to support PC133 RAM as seen in the i815 chipset.
Poor performance so farUnfortunately it was soon obvious that the i815 chip set with its PC133 RAM was performing slightly better than the i820 chip set with its still very expensive RDRAM. You have to use dual Rambus channels (as in upcomming Intel chip set i850 "Tehema") to benefit from a higher bandwidth. But this doubling is also possible from using DDR RAM.
A test between a i840-based dual Rambus PC and a Micron DDR-based PC gave the same result; all benchmarks were better on the DDR system.
So far Rambus RAM is of no big interest. It is too expensive, and there is nothing to gain from it. However the Rambus technology stil is quite promising, but the prices has to come down, and it better be soon. DDR RAM is closing in.
Intel claims that DDR is to slow for the new Pentium 4 processor. It would require dual channel DDR RAM to get the required bandwidth. And dual channel DDR RAM meens a 128 bit wide bus, which is no good solution. The north bridge and the motherboard would be loaded with hundreds of signal lines.
In 2001 RDRAM is being used with great success on the GB850 Pentium 4 board and RDRAM prices are tumbling steadily.
RDRAM 2.0Rambus plans to speed up the bandwith a factor two using a Quad Rambus Signaling Level. This should happen without any increase in clock frequency.
Read module 5a about expansion cards, where we evaluate the I/O buses from the port side.