Editing Semiconductor memory (section)

===Non-volatile memory===
[[Non-volatile memory]] (NVM) preserves the data stored in it during periods when the power to the chip is turned off.  Therefore, it is used for the memory in portable devices, which don't have disks, and for removable [[memory cards]] among other uses. Major types are:<ref name="Godse" /><ref name="Arora" /> 

* '''ROM''' (''[[Read-only memory]]'') {{ndash}} This is designed to hold permanent data, and in normal operation is only read from, not written to.   Although many types can be written to, the writing process is slow and usually all the data in the chip must be rewritten at once.  It is usually used to store [[system software]] which must be immediately accessible to the computer, such as the [[BIOS]] program which starts the computer, and the software ([[microcode]]) for portable devices and embedded computers such as [[microcontroller]]s.
** '''MROM''' ([[Mask ROM|''Mask programmed ROM'' or ''Mask ROM'']]) {{ndash}} In this type the data is programmed into the chip when the chip is manufactured, so it is only used for large production runs.  It cannot be rewritten with new data.
** '''PROM''' (''[[Programmable read-only memory]]'') {{ndash}} In this type the data is written into an existing PROM chip before it is installed in the circuit, but it can only be written once.  The data is written by plugging the chip into a device called a PROM programmer.
** '''EPROM''' (''[[Erasable programmable read-only memory]]'' or UVEPROM) {{ndash}} In this type the data in it can be rewritten by removing the chip from the circuit board, exposing it to an [[ultraviolet light]] to erase the existing data, and plugging it into a PROM programmer. The IC package has a small transparent "window" in the top to admit the UV light. It is often used for prototypes and small production run devices, where the program in it may have to be changed at the factory. [[Image:4Mbit EPROM Toshiba TC574200D (2).jpg|thumb|4M EPROM, showing transparent window used to erase the chip]]
** '''EEPROM''' (''[[Electrically erasable programmable read-only memory]]'') {{ndash}} In this type the data can be rewritten electrically, while the chip is on the circuit board, but the writing process is slow.  This type is used to hold [[firmware]], the low level microcode which runs hardware devices, such as the [[BIOS]] program in most computers, so that it can be updated.
* '''NVRAM''' (''[[Non-volatile random-access memory]]'')
** '''FRAM''' (''[[Ferroelectric RAM]]'') {{ndash}} One type of nonvolatile RAM.
* '''[[Flash memory]]''' {{ndash}} In this type the writing process is intermediate in speed between EEPROMS and RAM memory; it can be written to, but not fast enough to serve as main memory.  It is often used as a semiconductor version of a [[hard disk]], to store files.   It is used in portable devices such as PDAs, [[USB flash drives]], and removable [[memory card]]s used in [[digital camera]]s and [[cellphone]]s.