Editing Semiconductor memory (section)

==Types==
===Volatile memory===
[[Image:RAM n.png|thumb|[[Random-access memory|RAM]] chips for computers usually come on removable [[memory module]]s like these.  Additional memory can be added to the computer by plugging in additional modules. ]]

[[Volatile memory]] loses its stored data when the power to the memory chip is turned off. However it can be faster and less expensive than non-volatile memory.  This type is used for the main memory in most computers, since data is stored on the [[hard disk]] while the computer is off.  Major types are:<ref name="Godse">{{cite book
 |last        = Godse
 |first       = A.P.
 |author2     = D.A.Godse
 |title       = Fundamentals of Computing and Programing
 |publisher   = Technical Publications
 |date        = 2008
 |location    = India
 |pages       = 1.35
 |url         = https://books.google.com/books?id=3XEIY1bjLAUC&q=%22semiconductor+memory%22+RAM+%22random+access%22+ROM+PROM&pg=SA1-PA35
 |isbn        = 978-8184315097
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}}</ref><ref name="Arora">{{cite book
 |last        = Arora
 |first       = Ashok
 |title       = Foundations of Computer Science
 |publisher   = Laxmi Publications
 |date        = 2006
 |pages       = 39–41
 |url         = https://books.google.com/books?id=CrcoszZBMowC&q=%22semiconductor+memory%22+RAM+%22random+access%22+ROM+PROM&pg=PA39
 |isbn        = 8170089719
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}}</ref>

'''RAM''' (''[[Random-access memory]]'') {{ndash}} This has become a generic term for any semiconductor memory that can be written to, as well as read from, in contrast to ROM ''(below)'', which can only be read.  All semiconductor memory, not just RAM, has the property of [[random access]].
* '''DRAM''' (''[[Dynamic random-access memory]]'') {{ndash}} This uses [[memory cell (computing)|memory cells]] consisting of one [[MOSFET]] (MOS field-effect transistor) and one [[MOS capacitor]] to store each bit. This type of RAM is the cheapest and highest in density, so it is used for the main memory in computers.  However, the [[electric charge]] that stores the data in the memory cells slowly leaks out, so the memory cells must be periodically [[memory refresh|refreshed]] (rewritten) which requires additional circuitry.  The refresh process is handled internally by the computer and is transparent to its user.
** '''FPM DRAM''' (''[[FPM DRAM|Fast page mode DRAM]]'') {{ndash}} An older type of asynchronous DRAM  that improved on previous types by allowing repeated accesses to a single "page" of memory to occur at a faster rate.  Used in the mid-1990s.
** '''EDO DRAM''' (''[[Extended data out DRAM]]'') {{ndash}} An older type of asynchronous DRAM which had faster access time than earlier types by being able to initiate a new memory access while data from the previous access was still being transferred.  Used in the later part of the 1990s.
** '''VRAM'''  (''[[Video RAM|Video random access memory]]'') {{ndash}} An older type of [[dual-ported RAM|dual-ported]] memory once used for the [[frame buffer]]s of [[video adapters]] (video cards).
** '''SDRAM''' (''[[Synchronous dynamic random-access memory]]'') {{ndash}} This added circuitry to the DRAM chip which synchronizes all operations with a clock signal added to the computer's [[memory bus]].  This allowed the chip to process multiple memory requests simultaneously using ''[[Pipeline (computing)|pipelining]]'', to increase the speed. The data on the chip is also divided into ''banks'' which can each work on a memory operation simultaneously.  This became the dominant type of computer memory by about the year 2000.
*** '''[[DDR SDRAM]]''' (''Double data rate SDRAM'') {{ndash}} This could transfer twice the data (two consecutive words) on each clock cycle by [[double pumping]] (transferring data on both the rising and falling edges of the clock pulse).  Extensions of this idea are the current (2012) technique being used to increase  memory access rate and throughput.  Since it is proving difficult to further increase the internal clock speed of memory chips, these chips increase the transfer rate by transferring more data words on each clock cycle
**** '''[[DDR2 SDRAM]]''' {{ndash}} Transfers 4 consecutive words per internal clock cycle
**** '''[[DDR3 SDRAM]]''' {{ndash}} Transfers 8 consecutive words per internal clock cycle.
**** '''[[DDR4 SDRAM]]''' {{ndash}} Transfers 16 consecutive words per internal clock cycle.
*** '''RDRAM''' (''[[Rambus DRAM]]'') {{ndash}} An alternate double data rate memory standard that was used on some Intel systems but ultimately lost out to DDR SDRAM.
**** '''[[XDR DRAM]]''' (''Extreme data rate DRAM'')
*** '''SGRAM''' (''[[Synchronous graphics RAM]]'') {{ndash}} A specialized type of SDRAM made for [[graphics adaptor]]s (video cards).  It can perform graphics-related operations such as [[bit masking]] and block write, and can open two pages of memory at once.
**** '''[[GDDR SDRAM]]''' (''Graphics DDR SDRAM'')
***** '''[[GDDR2]]'''
***** '''[[GDDR3 SDRAM]]'''
***** '''[[GDDR4 SDRAM]]'''
***** '''[[GDDR5 SDRAM]]'''
***** '''[[GDDR6 SDRAM]]'''
*** '''HBM''' (''[[High Bandwidth Memory]]'') {{ndash}} A development of SDRAM used in graphics cards that can transfer data at a faster rate.  It consists of multiple memory chips stacked on top of one another, with a wider data bus. 
** '''PSRAM''' (''[[Pseudostatic RAM]]'') {{ndash}} This is DRAM which has circuitry to perform [[memory refresh]] on the chip, so that it acts like SRAM, allowing the external memory controller to be shut down to save energy.  It is used in a few [[game console]]s such as the [[Wii]].
* '''SRAM''' (''[[Static random-access memory]]'') {{ndash}} This stores each [[bit]] of data in a circuit called a [[flip-flop (electronics)|flip-flop]], made of 4 to 6 transistors.  SRAM is less dense and more expensive per bit than DRAM, but faster and does not require [[memory refresh]].  It is used for smaller [[cache memory|cache memories]] in computers. 
* '''CAM''' (''[[Content-addressable memory]]'') {{ndash}} This is a specialized type in which, instead of accessing data using an address, a data word is applied and the memory returns the location if the word is stored in the memory.  It is mostly incorporated in other chips such as  [[microprocessor]]s where it is used for [[cache memory]].

===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.