Navigating the Memory Upgrade Jungle

Thomas Soderstrom
March 24, 2006 10:30
Memory Speeds Explained

RAM is normally rated at its highest tested stable speed, first by the chip manufacturer in cycle time (measured in nanoseconds, abbreviated ns), then by the module producer in frequency (megahertz, or MHz). Because of the inverse relationship between cycle time and frequency, knowing the rated frequency for a single chip is as easy as inverting cycle time and moving the decimal place. For example, 200 MHz SDRAM would be equivalent to 5 ns, because 5 ns equals 0.005 microseconds, and 1 divided by 0.005 equals 200.



By design, DRAM maintains its data only as long as a charge is applied to the cells, so there is no maximum cycle time. For example, 133 MHz SDRAM could operate at 133 MHz, 100 MHz, 66 MHz, or even at speeds of less than 1 MHz, depending on how quickly the system accesses it. This allows a wide range of compatibility for higher speed modules in older systems; it is common practice for manufacturers to re-label faster RAM at slower speeds whenever the slower RAM ceases production. (This explains the popularity of "PC100" modules that use 7 ns chips.)

Double Data Rate (DDR) technology allows data to be transmitted twice per clock cycle, so DDR SDRAM with a 200 MHz clock rate actually has a 400 MHz data rate, and is referred to as DDR400. The naming convention for finished modules has become its bandwidth, with each module providing 64 data connections for 8 bytes of data per transfer. A 400 MHz data rate multiplied by 8 bytes per transfer provides 3,200 MB/s bandwidth, hence the name PC3200.