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Unformatted text preview: red in 1995 in high-end systems, and by 2001 most PC’s were built with SDRAMs. End Aside. Nonvolatile Memory
DRAMs and SRAMs are volatile in the sense that they lose their information if the supply voltage is turned off. Nonvolatile memories, on the other hand, retain their information even when they are powered off. There are a variety of nonvolatile memories. For historical reasons, they are referred to collectively as read-only memories (ROMs), even though some types of ROMs can be written to as well as read. ROMs are distinguished by the number of times they can be reprogrammed (written to) and by the mechanism for reprogramming them. A programmable ROM (PROM) can be programmed exactly once. PROMs include a sort of fuse with each memory cell that can be blown once by zapping it with a high current. An erasable programmable ROM (EPROM) has a small transparent window on the outside of the chip that exposes the memory cells to outside light. The EPROM is reprogrammed by placing it in a special device that shines ultraviolet light onto the storage cel...
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