RAMBUS Dynamic RAM

Currently all but extinct in the standard desktop PC market, RDRAM is a proprietary memory standard, developed by the RAMBUS company. RDRAM originally made a big splash in 1998 with its adoption by Intel to provide memory support for their high-end Pentium III boards and the early Pentium 4 models.

Sadly for the company, this was closely followed by a protracted series of court battles with major memory manufacturing companies such as Infineon and Hyundai over alleged patent violations among other things. The comparatively high price of RDRAM, its early stranglehold on the Pentium 4 processor market, and the perception that the RAMBUS company's series of lawsuits might well drive up conventional SDRAM and DDR-SDRAM prices if they succeeded combined to make RDRAM rather unpopular with the home user and PC enthusiast markets.

RDRAM failed to decrease appreciably in price, so Pentium 4 chipsets supporting SDRAM were introduced by Intel in 2001 to attract the lower-end market. SDRAM did not deliver the necessary performance for the Pentium 4, so Intel introduced Pentium 4 motherboard chipsets supporting DDR-SDRAM in 2002, all but eliminating RAMBUS memory from the home and small business PC market. There have been some signs of a resurgence in RAMBUS fortunes recently, most notably the January overturning of a court ruling against them in favour of Infineon, so who knows what the future holds.

In design and operation, RDRAM differs considerably from SDRAM and DDR-SDRAM memory in several ways. First of all, RDRAM transfers only 16 bits, or 2 bytes of data at a time, as compared to SDRAM/DDR-SDRAM's 64-bit data channel, but it transmits those 16 bits at a considerably higher frequency, 400Mhz for basic PC800 RDRAM. Also, RDRAM transmits data twice per clock cycle just like DDR-SDRAM, so the effective data transfer rate starts at 800Mhz. Using the formula

To determine maximum memory bandwidth gives RDRAM a theoretical maximum bandwidth of 1.6 GB per second. This gave it a considerable on-paper advantage to SDRAM at the time, and was one of the main reasons why Intel decided to use RDRAM to launch its Pentium 4 processor line.

In reality, while RDRAM's high (at that point in time) bandwidth gave it an advantage over SDRAM in Pentium 4 chipsets, the longer time required for RDRAM to initially locate memory cells to be written or read from, as compared to SDRAM or DDR-SDRAM resulted in it actually performing worse than SDRAM when used with the Pentium 3.

RDRAM's advantage is the speed of the burst or sequential memory transfers after the initial delay due to the higher frequency of the memory. Newer RDRAM modules can run at 533/1066Mhz and 600/1200Mhz with a corresponding increase in bandwidth. RDRAM numbers still trail those of the fastest DDR-SDRAM however.

Just like DDR-SDRAM, some RDRAM chipsets may be dual-channel, requiring two identical chips to function. RDRAM memory chips can generate considerable heat, and require a metal heat spreader to help dissipate the excess. RDRAM is available in PC800, PC1066 and PC1200 types (where 'PC800' indicates the speed in Mhz after taking into account the doubled data transfer rate), at sizes from 64MB to 512MB. RDRAM modules are 184-pin packages called RIMMs (RAMBUS inline memory modules).