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PC100 is a standard for internal removable computer random access memory, defined by the JEDEC. PC100 refers to Synchronous DRAM operating at a clock frequency of 100 MHz, on a 64-bit-wide bus, at a voltage of 3.3 V. PC100 is available in 168-pin DIMM and 144-pin SO-DIMM form factors. PC100 is backward compatible with PC66 and was superseded by the PC133 standard.
A module built out of 100 MHz SDRAM chips is not necessarily capable of operating at 100 MHz. The PC100 standard specifies the capabilities of the memory module as a whole. PC100 is used in many older computers; PCs around the late 1990s were the most common computers with PC100 memory.