Dynamic RAM (DRAM)
The most common type of system memory used in modern PC's is Dynamic RAM, or DRAM. Unlike SRAM that uses transistors to store its 1's and 0's, DRAM uses capacitors. If you recall the lesson from Chapter 0001, capacitors are electronic components that store an electrical charge. If the capacitor is holding an electrical charge, it is a 1. If it holds no charge, it is a 0. Because the capacitors are so small, they lose their charge very fast. To keep from losing data, the memory controller is constantly refreshing the cells. This contributes to the fact that DRAM is slower that SRAM and consumes more power.
Standard DRAM operates in what is known as Asynchronous mode. This means that the read / write cycles of the memory is not synchronized with the system clock. When the CPU sends a signal requesting data from the system memory, the data appears on the data bus (described later) a certain period of time later. The two signals, the request signal and the data return signal, are not coordinated with the system clock at all. This type of transfer was the standard in lower speed systems (less than 66 MHz), but as computers became faster, asynchronous data transfers were significantly impacting the overall performance of the system.
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