1.4 Memory Types
The physical memory locations can be represented by various types of memory devices.
Two different types of memory made from semiconductor materials will be described:
volatile and nonvolatile. Volati le memory is a memory that retains the data only when
the power is applied. If the power is removed, the data stored in the memory locations
will be lost. As the name implies, nonvolatile memory is not volatile. A nonvolatile
memory retains the data with or without power being applied to the device.
RAM
RAM stands for Random Access Memory; however, it is better described as read/write
memory. Most memory on modern computers is random access, which means that it
can be randomly addressed (the data is accessed in any order the processor chooses).
Sequentially accessed memory is used only for special processing and timing applica-
tions and will not be described in this text.
RAM is volatile, retaining data only when the power is applied. If the power going to
RAM is turned off, all data contained in it is lost. When power is reapplied, the content
of RAM is undefined, which means the data can be any value. RAM is used for
temporary storage of programs while they are being executed, as well as for temporary
storage of data while it is being processed. RAM is relatively fast memory when
compared to the whole family of various memory devices. RAM is also relatively less
expensive, uses less power and is smaller than most memory devices.
ROM
ROM stands for Read Only Memory. ROM is used for permanent storage of programs
and data. It is a nonvolatile memory; therefore, it always retains its data, whether power
is applied or not. Standard ROM must be loaded with the permanent programs and
data during the manufacturing process. Other types of ROM have been developed that
allow programs and data to be written to them after the manufacturing process.
Programmable Read Only Memory or PROM is designed to be programmed in the
field, after the manufacturing process. This provides the user the ability to purchase
one chip and use it for various purposes dependent upon the field application. PROM
can be programmed one time. If a program or data contained in a PROM needs to be
modified, the PROM must be discarded and a new one must be programmed.
Erasable Programmable Read Only Memory, EPROM, is a type of PROM that can be
erased by applying an intense ultraviolet light to the memory circuitry. Once it has been
erased, it can be reprogrammed using a special programming fixture. Chips that
contain EPROM have a small window on the chip package that allows the light to reach
the memory circuitry. Once an EPROM is programmed, this window is covered to stop
light from entering the device and inadvertently erasing the memory locations.
Electrically Erasable Programmable Read Only Memory, EEPROM, can be erased with
electrical signals and then reprogrammed. EEPROM is used in various devices that
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Introduction to Computer Hardware