Memory

News SDRAM DDRRAM Rambus Etc

Much of what you hear regarding DDRAM technology is marketing propaganda. For example, SDRAM manufacturers are naming their memory after the peak bandwidth, not bus frequency. PC1600 DDR SDRAM is really PC100 DDR SDRAM, because the 100MHz bus transfers two bits of data per wire per clock (double data rate), which yields 1600MB (1.6GB) per second over a 64-bit bus (eight bits per byte). PC2100 is really PC133 DDR SDRAM because 133MHz multiplied by two bits per clock multiplied by eight bytes equals 2100MB (2.1GB) per second. PC1600 and PC2100 coincidently sound more inspiring than PC100 or PC133.

If your last name happens to be Rockefeller or Forbes, you wont be disappointed. For students and the rest of the penny-pinching public, the cost associated with a full upgrade to an AMD 761-based motherboards are probably still excessive :-)

Theoretically Double Data Rate SDRAM should be able to provide twice the bandwidth of Single Data Rate by transferring data both on the rising and the falling edge of the clock signal. But in reality the most of benchmarks is so close that you almost cannot tell the difference. The performance advantage of DDR memory over SDR is usually between 5% or even less... 

RAM from Crucial (a division of Micron) is heralded as some of the best RAM in the industry. They have great prices on PC133, CAS2 (excellent for overclockers) RAM. They also have free 2-day shipping and 10% off through the links below. No rebates involved!

Nikolai Bezroukov


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Slashdot PC1066 RDRAM vs. DDR SDRAM

Intel's i850 does not support PC1066 officially, and parts of that speed have only been validated since the release of i850E. Officially, the chipset simply supports a FSB that would complement that speed, if the two busses ran synchronously. Seen here:
http://www.theinquirer.net/24050203.htm [theinquirer.net]

That said, PC1066 has been tested before (can't find the article at Ace's Hardware), and the bandwidth of DRDRAM appears to compensate quite nicely for the P4's generally lousy architecture, as does its increased cache size (now 512k L2).

Why didn't they show us any Quake III comparison benches? We all know that at lower resolutions the processor drives Quake III and that its extremely sensitive to memory bandwith capabilities. Anyway it appears that RDRAM 1066 is a definite improvement over RDRAM 800. Its good to see that Intel is still continually raising the bar.

Also I believe there were some initial benches (better ones) on http://www.tomshardware.com

 

www.micron.com Micron to Demonstrate DDR333 SDRAM Platforms At VIA Tech Forum

Micron will demonstrate a Micron chipset and system, which utilizes DDR333 SDRAM memory modules and features a PCI-X bus, as well as a 64-bit PCI 2.2 bus. Micron designed this as a development chipset to enable system developers and designed the chipset, motherboard, and DDR SDRAM components and modules.

With DRAM technology becoming increasingly complex, Micron is focusing on ways to help its customers implement new high-performance DRAM products. ``We developed our DDR SDRAM platforms as reference designs that our customers can use in designing DDR SDRAM systems,'' said Mike Seibert, Micron's Strategic Marketing Manager. ``Micron is the only memory company conducting system technology development to help enable our customers. We are providing customers with the various tools and resources they need to develop system-level products using cutting-edge memory technology. We are also working with customers on design guidelines for DDR333 technology.''

Micron was the first company to demonstrate a DDR266 system and continues to collaborate closely with key suppliers to drive leading-edge system technology. The company worked with multiple motherboard suppliers to develop DDR333 systems. Micron recently demonstrated its DDR333 platform at the Intel Developers Forum. ALI is also demonstrating platforms using Micron DDR333 SDRAMs.

New Pentium 4-Rambus chipset on way - Tech News - CNET.com

September is key

September in many ways will likely be a crucial month for Rambus. PC executives and analysts have predicted that Pentium 4-SDRAM computers could end up selling for $50 to $150 less than their Pentium 4-RDRAM counterparts, depending on the configuration, because of cheaper memory and motherboards.

Some analysts were surprised at the prospect of another Rambus chipset. "It (Rambus) is a sunset industry," said Ashok Kumar, an analyst at US Bancorp Piper Jaffray. "I don't know what the market opportunity is."

Corporate customers are also expected to snap up the Pentium 4-SDRAM systems.

"SDRAM is what they are desiring," said Jeff Austin, a marketing manager for Intel. Corporate buyers have been reluctant to embrace RDAM because, as with all new technologies, "they want to see it adopted somewhere else first," he said.

By contrast, Tom Quinn, vice president of marketing at Samsung, said that the price difference between PCs with RDRAM and those with SDRAM will be relatively small, especially when compared with overall PC prices. Although SDRAM has been dropping in price, so has the Rambus memory. Pentium 4-RDRAM computers will also provide more performance.

"How is the market going to react to (Pentium 4/845) systems?" Quinn said. "The difference in cost will be less than the sales tax."

Both Intel and Rambus declined to comment on any upcoming third-party chipsets. An Intel representative said that three companies currently have licenses to manufacture Pentium 4 chipsets: Acer Labs, SiS and ATI Technologies. These companies are all expected to come out with Pentium 4-SDRAM chipsets.

A Rambus representative said that Acer also has a Rambus license.

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Rambus

New Pentium 4-Rambus chipset on way - Tech News - CNET.com

September is key

September in many ways will likely be a crucial month for Rambus. PC executives and analysts have predicted that Pentium 4-SDRAM computers could end up selling for $50 to $150 less than their Pentium 4-RDRAM counterparts, depending on the configuration, because of cheaper memory and motherboards.

Some analysts were surprised at the prospect of another Rambus chipset. "It (Rambus) is a sunset industry," said Ashok Kumar, an analyst at US Bancorp Piper Jaffray. "I don't know what the market opportunity is."

Corporate customers are also expected to snap up the Pentium 4-SDRAM systems.

"SDRAM is what they are desiring," said Jeff Austin, a marketing manager for Intel. Corporate buyers have been reluctant to embrace RDAM because, as with all new technologies, "they want to see it adopted somewhere else first," he said.

By contrast, Tom Quinn, vice president of marketing at Samsung, said that the price difference between PCs with RDRAM and those with SDRAM will be relatively small, especially when compared with overall PC prices. Although SDRAM has been dropping in price, so has the Rambus memory. Pentium 4-RDRAM computers will also provide more performance.

"How is the market going to react to (Pentium 4/845) systems?" Quinn said. "The difference in cost will be less than the sales tax."

Both Intel and Rambus declined to comment on any upcoming third-party chipsets. An Intel representative said that three companies currently have licenses to manufacture Pentium 4 chipsets: Acer Labs, SiS and ATI Technologies. These companies are all expected to come out with Pentium 4-SDRAM chipsets.

A Rambus representative said that Acer also has a Rambus license.

 

Tom's Hardware Guide Mainboard Guide - Dissecting Rambus

The second important thing to understand about RDRAM is that clock-speed cannot always be trusted. Yes, the fastest grade of PC800 does run at 800 MHz, but the bus width (which determines the amount of data that can be transferred at any instant in time) is only 16 bits (8 bits = 1 byte so 16 bits = 2 bytes) wide. This means that PC800 RDRAM is capable of transferring 2 bytes x 800 MHz = 1.6 GB/sec. The number 1.6 GB/sec is called the "bandwidth" for PC800 and represents the maximum possible rate that data can be transferred using this technology.

By the way RDRAM's bus is double-pumped meaning that it transfers data on the rising and falling edges of clock pulses. Accordingly PC800 is fed by a 400 MHz clock. Less intuitively, as stated in our recent article, PC700 actually runs at 712 MHz double-pumped from a 356 MHz clock. To insure that there is no logic to RDRAM speed ratings, PC600 real speed is 532 MHz triggered by 266 MHz clock.

SDRAM has a much larger 64-bit (8 bytes) bus width. Currently the fastest DIMMs (sticks of SDRAM) run at 133 MHz. This gives PC133 (SDRAM running at 133 MHz) a bandwidth of 8 bytes x 133 MHz = 1.064 GB/sec. What should be apparent is that due to their differing bus widths, RDRAM must run four times faster than SDRAM to provide the same bandwidth. So when comparing these memory technologies, be sure to take this important fact into consideration.

Positioned by Micron and many other manufacturers to replace SDRAM, DDR SDRAM also has a 64-bit bus, but like RDRAM, DDR SDRAM transfers data at the rising and falling edges of clock signals, effectively doubling its bandwidth. Therefore DDR SDRAM feed by a 133 MHz clock has a bandwidth of 2 x PC133 bandwidth = 2 x 1.064 GB/sec = 2.128 GB/sec. According to Micron, DDR SDRAM should be priced similarly to current SDRAM making it greatly cheaper than RDRAM. Although DDR SDRAM is currently in certain Nvidia graphics cards, it has not been released yet for use as main memory. DDR SDRAM should make its widespread debut around July and should first appear in systems built around AMD's Athlon.

.............

RDRAM is a protocol based memory system. This means that it works similarly to a tiny network passing 16-bit wide data packets from one RDRAM device to another until these packets are assembled into 64-bit wide chunks that the processor can digest. In this process, data may actually have to worm its way through all of the RIMMs in a system. This is radically different from the grid access used in SDRAM. This also adds greatly to RDRAM's complexity causing several problems including increased latencies.

Unlike SDRAM, RDRAM's latency varies considerably as more memory is added. Remember that data has to pass serially through each of the RIMMs before it can exit to the memory controller. Without going into great detail (Paul DeMone has written an excellent article for those interested in more information - thanks go to JC's for providing the link) imagine first a system with only one RIMM containing only one RDRAM chip (this is only for illustration - a RIMM contains many chips, by the way). Whenever a read (request for data) is made, data will take a certain amount of time to travel from this one RIMM to the RDRAM controller.

When another identical RIMM is added to the system, it brings with it parasitic input capacitances and impedance mismatches that slow down signal propagation. Due to this and the fact that the second RIMM is simply further away from the memory controller, data from the second RIMM necessarily takes longer to reach the RDRAM controller. In other words, this second RIMM will have a greater latency than the first RIMM.

Continuously managing multiple latencies would be a nightmare for the memory controller. In order to work around this, when a system is booted the RDRAM subsystem performs an involved initialization process to determine what the greatest latency is for the entire RDRAM system and adjusts all RDRAM devices to have the same latency as the slowest RDRAM device on the system. And remember that in a real world system each RIMM will have many RDRAM devices so this latency balancing is quite complex.

An RDRAM chip typically has a normal 20 ns page read access latency. To balance latencies, these chips have a TPARM control register that can be programmed with a 2.5, 5.0, 7.5 or 10.0 ns of artificial compensating latency. This means that the normal chip latency can be as much as 50% higher than the minimal 20 ns often quoted as RDRAM's page read latency. Compare this with the fastest PC100 SDRAM with a latency of only 20 ns, but again remember that RDRAM has even other issues that bring its total latency much higher still.

... ... ...

A quick check of www.pricewatch.com shows that the cheapest 128 MB RIMM is a little over $600 while a 128 MB PC133 DIMM is right at $80. This is a whopping 750% price differential and this doesn't include the price premium for motherboards utilizing RDRAM.

All together the total of these delays amounts to as much as a 100% average page read latency increase over the latency of the fastest PC100 SDRAM.

Although not completely analogous, both SDRAM and RDRAM can be envisioned as having tiny caches of the most recently accessed data in the form of active pages. If this data is not what was requested (a page miss), then this data must be activated through a bank read causing an additional latency hit. Although it is possible to have more active pages in RDRAM, the latency penalty for RDRAM during a bank read can be many, many times higher than SDRAM. This can make an additional massive contribution to the latency of an RDRAM system.

 

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