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Memory is characterized by whether values can be extracted, updated, or both.The primary form of memory used in a conventional computer system permits an arbitraryvalue in memory to be accessed (read) or updated (written) at any time. However,other forms of memory provide more permanence. For example, some memory ischaracterized as Read Only Memory (ROM) because the memory contains data valuesthat can be accessed, but cannot be changed.A form of ROM, Programmable Read Only Memory (PROM), allows data valuesto be entered once, and then accessed many times. Typically, values are initially placedin PROM by using high voltage to alter the physical circuits on the chip (e.g., to destroythe electrical path that corresponds to a zero bit). Informally, we say that valuesare burned into the memory.Intermediate forms of permanence also exist. For example, an Electrically ErasableProgrammable Read Only Memory (EEPROM) is a form of nonvolatile memorythat permits values to change. However, storing a value in EEPROM memory requiresactivation of special circuits, and takes much longer than reading a value. Thus,EEPROMs are used in situations where nonvolatility is desired, but values change infrequently.A popular variant of EEPROM technology known as Flash memory or Flash ROMis commonly used in digital cameras — although it takes longer to store an image inFlash, the delay is not critical because it happens in less time than is required for a humanto aim and focus the camera.
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