Rambus - Technology

Technology

The first PC motherboards with support for RDRAM were released in 1999. They supported PC800 RDRAM, which operated at 400 MHz but presented data on both the rising and falling edge of the clock cycle resulting in effectively 800 MHz, and delivered 1600 MB/s of bandwidth over a 16-bit bus using a 184-pin RIMM form factor. This was significantly faster than the previous standard, PC133 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.

Some disadvantages of RDRAM technology, however, included significantly increased latency, power dissipation as heat, manufacturing complexity, and cost. PC800 RDRAM operated with a minimum latency of 45 ns, compared to 15 ns for PC133 SDRAM. RDRAMs can also be told to increase their latencies in order to prevent the possibility of two or more chips transmitting at the same time and causing a collision. However, SDRAM latency depends on the current state of memory so its latency can vary widely depending on what happened earlier and the strategy used by the SDRAM controller, while RDRAM latency is constant once it has been established by the memory controller. RDRAM memory chips also put out significantly more heat than SDRAM chips, necessitating heatsinks on all RIMM devices. RDRAM also 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 PC133 SDRAM due to manufacturing costs, license fees and other market factors.

With the introduction of the Intel 840 chipset, dual-channel support was added for PC800 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 PC1066 RDRAM, increasing total dual-channel bandwidth to 4200 MB/s. Also in 2002, Intel released the E7205 chipset, which introduced dual-channel DDR support for a total bandwidth of 4200 MB/s, but at a much lower latency than competing RDRAM. In 2003, Intel released the 875P chipset, and along with it dual-channel DDR400. With a total bandwidth of 6400 MB/s, it marked the end of RDRAM as a technology with competitive performance.

Rambus also developed and licensed its XDR RAM technology, notably used in the PlayStation 3, and more recently XDR2 DRAM.