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Asynchronous and Synchronous DRAM
Conventional DRAM, of the type that has been used in PCs since the original IBM PC days, is said to be asynchronous. This refers to the fact that the memory is not synchronized to the system clock. A memory access is begun, and a certain period of time later the memory value appears on the bus. The signals are not coordinated with the system clock at all, as described in the section discussing memory access. Asynchronous memory works fine in lower-speed memory bus systems but is not nearly as suitable for use in high-speed (>66 MHz) memory systems.
A newer type of DRAM, called "synchronous DRAM" or "SDRAM", is synchronized to the system clock; all signals are tied to the clock so timing is much tighter and better controlled. This type of memory is much faster than asynchronous DRAM and can be used to improve the performance of the system. It is more suitable to the higher-speed memory systems of the newest PCs.
Note that there are several different flavors of both asynchronous DRAM and synchronous DRAM; they are discussed on the page covering DRAM technologies.
Next: The Memory Bus