RDRAM

Direct Rambus DRAM or DRDRAM (sometimes just called Rambus DRAM or RDRAM) is a type of synchronous dynamic RAM. RDRAM was developed by Rambus Inc., in the mid-1990s as a replacement for then-prevalent SDRAM memory architecture.

RDRAM was initially expected to become the standard in PC memory, especially after Intel agreed to license the Rambus technology for use with its future chipsets. Further, RDRAM was expected to become a standard for VRAM. However, RDRAM got embroiled in a standards war with an alternative technology—DDR SDRAM— and quickly lost out on grounds of price, and later, performance. By around 2003, RDRAM was no longer supported by any personal computer.

PC main memory

The first PC motherboards with support for RDRAM debuted in late 1999, after two major delays. RDRAM was controversial during its widespread use by Intel for having high licensing fees, high cost, being a proprietary standard, and low performance advantages for the increased cost. RDRAM and DDR SDRAM were involved in a standards war. PC-800 RDRAM, which operated at 400 MHz and delivered 1600 MB/s of bandwidth over a 16-bit bus. It was packaged as a 184-pin RIMM (Rambus in-line memory module) form factor, similar to a DIMM (dual in-line memory module). Data is transferred on both the rising and falling edges of the clock signal, a technique known as DDR. To emphasize the advantages of the DDR technique, this type of RAM was marketed at speeds twice the actual clock rate, i.e. the 400 MHz Rambus standard was named PC-800. This was significantly faster than the previous standard, PC-133 SDRAM, which operated at 133 MHz and delivered 1066 MB/s of bandwidth over a 64-bit bus using a 168-pin DIMM form factor.

RDRAM memory with integrated heat spreader

A Samsung RDRAM 6400 128 MB

A Samsung RDRAM Installed with Pentium 4 1.5 GHz

Moreover, if a mainboard has a dual- or quad-channel memory subsystem, all of the memory channels must be upgraded simultaneously. 16-bit modules provide one channel of memory, while 32-bit modules provide two channels. Therefore, a dual channel mainboard accepting 16-bit modules must have RIMMs added or removed in pairs. A dual channel mainboard accepting 32-bit modules can have single RIMMs added or removed as well. Note that some of the later 32 bit modules had 232 pins as compared to the older 184 pin 16 bit modules.

Module specifications

Designation	Bus width (bits)	Channels	Clock rate (MHz)	Bandwidth (MB/s)
PC600	16	Single	266	1066
PC700	16	Single	355	1420
PC800	16	Single	400	1600
PC1066 (RIMM 2100)	16	Single	533	2133
PC1200 (RIMM 2400)	16	Single	600	2400
RIMM 3200	32	Dual	400	3200
RIMM 4200	32	Dual	533	4200
RIMM 4800	32	Dual	600	4800
RIMM 6400	32	Dual	800	6400

Continuity modules

File:RAMBUS-Terminator.jpg

A Rambus continuity RIMM (CRIMM), also known as terminator or dummy

The design of many common Rambus memory controllers dictated that memory modules be installed in sets of two. Any remaining open memory slots must be filled with continuity RIMMs (CRIMMs.) These sticks provide no extra memory, and only served to propagate the signal to termination resistors on the motherboard instead of providing a dead end where signals would reflect. CRIMMs appear physically similar to regular RIMMs, except they lack integrated circuits (and their heat-spreaders).

Performance

Compared to other contemporary standards, Rambus showed a significant increase in latency, heat output, manufacturing complexity, and cost. Because of the way Rambus designed RDRAM, RDRAM's die size is inherently larger than similar SDRAM chips. RDRAM's die size is larger in order to house the added interface and results in a 10-20 percent price premium at 16-megabit densities (adding about a 5 percent penalty at 64M).[1]

PC-800 RDRAM operated with a latency of 45 ns, which was more latency than other comparable DRAM technologies of the time. RDRAM memory chips also put out significantly more heat than SDRAM chips, necessitating heatspreaders on all RIMM devices. RDRAM includes a memory controller on each memory chip, significantly increasing manufacturing complexity compared to SDRAM, which used a single memory controller located on the northbridge chipset. RDRAM was also two to three times the price of PC-133 SDRAM due to a combination of high manufacturing costs and high license fees. PC-2100 DDR SDRAM, introduced in 2000, operated with a clock rate of 133 MHz and delivered 2100 MB/s over a 64-bit bus using a 184-pin DIMM form factor.

When more than one RIMM is installed on a memory channel, the performance impact is greater than for SDRAM because the data in the memory module furthest from the memory controller has to travel across all other memory chips, instead of just one or two chips in production SDRAM motherboards.

With the introduction of the Intel 840 (Pentium III), Intel 850 (Pentium 4), Intel 860 (Pentium 4 Xeon) chipsets, Intel added support for dual-channel PC-800 RDRAM, doubling bandwidth to 3200 MB/s by increasing the bus width to 32-bit. This was followed in 2002 by the Intel 850E chipset, which introduced PC-1066 RDRAM, increasing total dual-channel bandwidth to 4200 MB/s. In 2002, Intel released the E7205 Granite Bay chipset, which introduced dual-channel DDR support - for a total bandwidth of 4200 MB/s - at a slightly lower latency than competing RDRAM. Granite Bay matched i850E + PC-1066 RDRAM bandwidth with considerably lower latency.

To achieve RDRAM's 800 MHz clock rate, the memory module runs on a 16-bit bus instead of a 64-bit bus in contemporary SDRAM DIMM. At the time of the Intel 820 launch some RDRAM modules operated at rates less than 800 MHz.

Benchmarks

Benchmark tests conducted in 1998 and 1999 showed most everyday applications to run minimally slower with RDRAM. In 1999, benchmarks comparing the Intel 840 and Intel 820 RDRAM chipsets with the Intel 440BX SDRAM chipset led to the conclusion that the performance gain of RDRAM did not justify its cost over SDRAM except for use in workstations. In 2001, benchmarks pointed out that single-channel DDR266 SDRAM modules could closely match dual-channel 800 MHz RDRAM in everyday applications.

Marketing history

In November 1996, Rambus entered into a development and license contract with Intel. Intel announced to the Wintel development community that it would only support the Rambus memory interface for its microprocessors, Intel was granted rights to purchase 1M shares of Rambus' stock at $10 per share.

As a transition strategy, Intel planned to support PC-100 SDRAM DIMM on future Intel 82x chipsets using Memory Translation Hub (MTH). In 2000, Intel recalled the Intel 820 motherboard with memory translation hub (MTH) because the MTH can, while doing simultaneous switching, produce noise that may cause the computer to hang mysteriously or to spontaneously reboot. Since then, no production Intel 820 motherboards contain MTH.

In 2000, Intel subsidized RDRAM by bundling retail boxes of Pentium 4 CPUs with 2 RIMMs. Intel began to phase out Rambus subsidies in 2001.

In 2003, Intel introduced the 865 and 875 chipsets with dual-channel DDR RAM support, which were marketed as high end replacements of the 850 chipset. Furthermore, the future memory roadmap did not include Rambus memory.

Other uses

Video game consoles

RDRAM18-NUS on Nintendo 64

Rambus's RDRAM saw use in three video game consoles, beginning in 1996 with the Nintendo 64. The Nintendo console utilized 4 MB RDRAM running with a 500 MHz clock on a 9-bit bus, providing 500 MB/s bandwidth. RDRAM allowed N64 to be equipped with a large amount of memory bandwidth while maintaining a lower cost due to design simplicity. RDRAM's narrow bus allowed circuit board designers to use simpler design techniques to minimize cost. The memory, however, was disliked for its high random access latencies. In the N64, the RDRAM modules are cooled by a passive heatspreader assembly.[2]

The Sony PlayStation 2 was equipped with 32 MB of RDRAM, and implemented a dual-channel configuration resulting in 3200 MB/s available bandwidth. The PlayStation 3 utilizes 256 MB of Rambus's XDR DRAM, which could be considered a successor to RDRAM, on a 64-bit bus at 400 MHz with an octal data rate (cf. double data rate) providing a clock rate of 3.2 GHz, allowing a large 204.8 Gbit/s (25.6 GB/s) bandwidth.

Texas Instruments DLP

RDRAM was used in Texas Instruments DLP systems

Video cards

Cirrus Logic implemented RDRAM support in their Laguna graphics chip, with two members of the family; the 2D-only 5462 and the 5464, a 2D chip with 3D acceleration. RDRAM offered a potentially faster user experience than competing DRAM technologies with its high bandwidth. The chips were used on the Creative Graphics Blaster MA3xx series, among others.

See also

• List of device bandwidths
• SLDRAM, an alternative open standard

External links

• RDRAM on the Rambus website
• Reference where information about dual-channel RDRAM in the PS2 can be found
• How to install RAMBUS memory
• Illustrated Guide for Installing RDRAM