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Page 4.5.10-7 defines 2.5V (left), 1.8V (centre), TBD (right), while page 4.20.5–40 nominates 3.3V for the right notch position. The orientation of the module for determining the key notch position is with 52 contact positions to the left and 40 contact positions to the right.
DDR SDRAM operating with a 100 MHz clock is called DDR-200 (after its 200 MT/s data transfer rate), and a 64-bit (8-byte) wide DIMM operated at that data rate is called PC-1600, after its 1600 MB/s peak (theoretical) bandwidth. Likewise, 12.8 GB/s transfer rate DDR3-1600 is called PC3-12800.
The key difference between DDR2 and DDR SDRAM is the increase in prefetch length. In DDR SDRAM, the prefetch length was two bits for every bit in a word; whereas it is four bits in DDR2 SDRAM. During an access, four bits were read or written to or from a four-bit-deep prefetch queue.
A third bank group bit (BG2) was added, allowing up to eight bank groups (2 → 3). The maximum number of banks per bank group remains at four (2 → 2), The number of row address bits remains at 17, for a maximum of 128K rows (17 → 17). One more column address bit (C10) is added, allowing up to 8192 columns (1 KB pages) in ×4 chips (11 → 12).
The number of physical DRAMs depends on their individual widths. For example, a rank of ×8 (8-bit wide) DRAMs would consist of eight physical chips (nine if ECC is supported), but a rank of ×4 (4-bit wide) DRAMs would consist of 16 physical chips (18, if ECC is supported). Multiple ranks can coexist on a single DIMM.
GDDR SDRAM is distinct from the more widely known types of DDR SDRAM, such as DDR4 and DDR5, although they share some of the same features—including double data rate (DDR) data transfers. As of 2025 [update] , GDDR SDRAM has been succeeded by GDDR2 , GDDR3 , GDDR4 , GDDR5 , GDDR5X , GDDR6 , GDDR6X , GDDR6W and GDDR7 .
To alleviate this issue, the next standards of DDR DIMMs were created with a "low profile" (LP) height of around 1.2 inches (30 mm). These fit into vertical DIMM sockets for a 1U platform. With the advent of blade servers , angled slots have once again become common in order to accommodate LP form factor DIMMs in these space-constrained boxes.
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 ...