RAM Types

Although today’s systems use memory modules built from a combination of chips, rather than individual chips plugged into the motherboard as with early PC systems, it’s still necessary to understand the different types of memory chips that have been and are used to build memory modules.

DRAM

Virtually all memory modules use some type of dynamic RAM, or DRAM chips. DRAM requires frequent recharges of memory to retain its contents. Early types of DRAM, including variations such as fast-page mode (FPM) and extended data-out (EDO), were speed rated by access time, measured in nanoseconds (ns; smaller is faster). Typical speeds for regular DRAM chips were 100ns or slower; FPM memory, used primarily in 30-pin and 72-pin SIMM modules, ran at speeds of 70ns, 80ns, and 100ns. EDO DRAM, which was used primarily in 72-pin SIMM modules and a few 168-pin DIMM modules, typically ran at 60ns.