Rambus DRAM

At the time that SDRAM was popular, there was a high-speed flavor of DRAM on the market called Rambus DRAM (RDRAM), which runs at speeds around 800 MHz! The RDRAM chips have a 16-bit internal bus width and are packaged together in a 184-pin, gold-plated memory module called a Rambus Inline Memory Module (RIMM). In order to take advantage of this type of memory, you need a motherboard and chipset that support RDRAM.

DDR

Double Data Rate (DDR) memory gets its name from the fact that it can transfer data twice during each clock cycle, whereas SDRAM can transfer data only once per clock cycle. DDR memory ships in 184-pin DIMM modules (see the section "DIMMs," later in this tutorial) for desktop computers and 200-pin SO-DIMMs for laptop systems.

The speed of DDR memory is measured in MHz, like SDRAM is, and is labeled to indicate the speed. The labeling of DDR memory may look obscure at first because it also indicates the bandwidth by taking the speed and multiplying it by 8 bytes of data (64 bits). So if DDR memory is labeled PC1600, that label breaks down like this: If you divide the 1600 by 8 bytes, you get the speed of the memory; in this case, you're looking at 200 MHz memory. PC2700 runs at 333 MHz, while PC3200 runs at 400MHz. When you upgrade memory on systems that require DDR memory, you need to know the speed of the DDR memory.

DDR2

Improvements to DDR memory have already started with DDR2 memory. DDR2 memory runs at speeds 400 MHz and higher, which is where DDR memory left off. DDR2 memory uses 240-pin memory modules and runs at 1.8 volts (as opposed to 2.5 volts for DDR memory). This results in less power consumption for more memory - which is great for laptop users. Popular modules of DDR2 memory at the time of this writing are PC3200 (400 MHz), PC4200 (533 MHz), PC5300 (666 MHz), and PC6400 (800 MHz).