masked ROMs The RAM family includes two important memory devices: static (SRAM) and dynamic (DRAM). primary difference between them is the lifetime of data they store. SRAM retains its contents as long electrical power applied to chip. If turned off or lost temporarily will be forever. DRAM on other hand has an extremely short lifetime-typically about four milliseconds. This true even when constantly. In short all properties you think hear word RAM. Compared that DRAM seems kind useless. By itself it is. However a simple piece hardware called controller can used make behave more like SRAM. job periodically refresh stored in DRAM. refreshing before expires kept alive for are needed. So useful after all. When deciding which type use system designer must consider access time cost. devices offer fast times (approximately faster than DRAM) but much expensive produce. Generally only where speed important. A lower cost-per-byte makes attractive whenever large amounts required. Many embedded systems include both types: small block (a few kilobytes) along critical path larger (perhaps Megabytes) everything else. Types ROM Memories distinguished by methods write new (usually programming) number rewritten. classification reflects evolution from hardwired programmable erasable-and-programmable. common feature these their ability retain programs forever during failure. very first ROMs were that contained preprogrammed set instructions. had specified chip production so actual could arrange transistors inside Hardwired memories still used though now distinguish types ROM. advantage masked low production Unfortunately cost quantities same One step up PROM (programmable ROM) purchased unprogrammed state. look at PROM would see made entirely 1's. process writing your involves special equipment device programmer. programmer writes one applying charge input pins Once been programmed this way never changed. code changed current discarded. As result PROMs also known one-time (OTP) devices. An EPROM (erasable-and-programmable ROM) exactly manner PROM. EPROMs erased reprogrammed repeatedly. To erase EPROM simply expose strong source ultraviolet light. (A window top allows light reach silicon.) doing this essentially reset entire initial--unprogrammed--state. Though PROMs essential part software development testing process. Hybrids technology matured recent years line blurred. Now several combine features both. These do not belong either group collectively referred hybrid Hybrid read written desired RAM maintain without power just Two devices EEPROM flash descendants typically store code. third hybrid NVRAM modified version NVRAM usually holds persistent data. EEPROMs electrically-erasable-and-programmable. Internally similar EPROMs operation accomplished electrically rather exposure Any byte within may written remain forever--or least until electrically erased. tradeoff improved functionality higher cost cycles significantly longer wouldn't want use main memory.