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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 arbitrary
value in memory to be accessed (read) or updated (written) at any time. However,
other forms of memory provide more permanence. For example, some memory is
characterized as Read Only Memory (ROM) because the memory contains data values
that can be accessed, but cannot be changed.
A form of ROM, Programmable Read Only Memory (PROM), allows data values
to be entered once, and then accessed many times. Typically, values are initially placed
in PROM by using high voltage to alter the physical circuits on the chip (e.g., to destroy
the electrical path that corresponds to a zero bit). Informally, we say that values
are burned into the memory.
Intermediate forms of permanence also exist. For example, an Electrically Erasable
Programmable Read Only Memory (EEPROM) is a form of nonvolatile memory
that permits values to change. However, storing a value in EEPROM memory requires
activation 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 ROM
is commonly used in digital cameras — although it takes longer to store an image in
Flash, the delay is not critical because it happens in less time than is required for a human
to aim and focus the camera.
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