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{{More citations needed|date=December 2019}}
[[Image:Pair32mbEDO-DRAMdimms.jpg|thumb|A pair of 32&nbsp;[[Megabyte|MB]] RAM modules from 1995]]'''Computer memory''' is a [[temporary]] [[storage]] [[area]]. It holds the [[Information|data]] and [[Instruction (computer science)|instructions]] that the Central Processing Unit ([[CPU]]) needs. Before a [[computer program|program]] can run, the program is [[load]]ed from [[Computer data storage|storage]] into the memory. This allows the CPU direct access to the computer program. Memory is needed in all [[computer|computers]].

A computer is usually a [[binary]] [[digital electronics]] device. Binary means it has only two states. On or Off. Zero or One. In a binary digital computer [[transistors]] are used to switch the electricity on and off. The computer's memory is made from lots of transistors. 

Each on/off setting in the computer's memory is called a binary [[digit]] or [[bit]]. A group of eight bits is called a [[byte]]. A byte is made from two [[nibble]]s of four bits each. Computer scientists made up the words ''bit'' and ''byte''. The word ''bit'' is short for ''binary digit''. It takes ''bi'' from binary and adds the ''t'' from digit. A collection of bits was called a bite. The computer scientists changed the spelling to ''byte'' to avoid confusion. When the computer scientists needed a word for half a byte, they thought ''nibble'', as in ''half a bite'', would be a fun word to choose.<ref>{{cite web |url=https://whatis.techtarget.com/definition/nibble |title=Definition of nibble |author=<!--Not stated--> |date= |website=techtarget.com |publisher=TechTarget |access-date=5 December 2019 |quote=nibble carries on the "edible data" metaphor established with bit and byte}}</ref>
== Characters in memory ==
A byte of memory is used to store a [[code]] to represent a character such as a [[number]], a [[text|letter]] or a [[symbol]]. Eight bits can store 256 different codes. This was thought enough and a byte became fixed at eight bits. 
This allows the ten [[decimal]] digits, 26 [[lower-case]] letters, 26 [[Capitalization|upper-case]] letters and many symbols. 
Early computers used six bits to a byte. This gave them 64 different codes. These computers did not have lower-case letters.<ref>{{cite web |url=http://www.ourcomputerheritage.org/ccs-t3x2.pdf |title=The ICT/ICL 1900 Range |publisher=ourcomputerheritage.org |format=PDF |date=16 December 2003 |access-date=5 December 2019 |archive-date=4 April 2020 |archive-url=https://web.archive.org/web/20200404210745/http://www.ourcomputerheritage.org/ccs-t3x2.pdf |url-status=dead }}</ref> 

Computer scientists had to agree on which code would represent each character. Most modern computers use [[ASCII]], the ''American Standard Code for Information Interchange''. In ASCII each code is eight bits – any combination of 0s and 1s – and form one character. The letter '''A''' is denoted by the code 01000001.

To allow for all the different characters in all the worlds languages, modern computers need more than 256 different characters. Another code system called [[Unicode]] allows for 1,112,064 different characters by using from one to four bytes for each character.
== Memory address ==
The computer's CPU can access each individual byte. It uses an [[computer address|address]] for each byte. Computer memory addresses start at zero and go up to the biggest number the computer can use. Older computers were limited in how much memory they could address. 32-bit computers can address up to 4[[GB]] of memory. Modern computers use 64 bits and could address up to 18,446,744,073,709,551,616 bytes = 16 [[exabytes]] of memory.

The numbers that computers use can get very large. To make it easier, the [[unit]] K (for [[kilobyte]]) or Ki (for [[kibibyte]]) can be used. In computer memory numbers are powers of two. One Kibibyte is two to the power of 10, that is 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 and written as 2<sup>10</sup> = 1024 bytes. For example, 64 Kibibytes, written as 64KiB or 64KB, of memory is the same as 65,536 bytes (1,024 × 64 = 65,536). For larger memory capacities, the units ''megabyte'' (MB) or ''mebibyte'' (MiB) and ''gigabyte'' (GB) or ''gibibyte'' (GiB) are used. One [[megabyte]] of computer memory means 2<sup>20</sup> bytes or 1024KB, which is 1,048,576 bytes. One [[gigabyte|gibibyte]] means 2<sup>30</sup> bytes or 1024MB. 

The numbers are multiples of two. This is why a kilobyte of memory is 1024 bytes and not 1000 as would be the case for [[kilogram]]. To try to avoid this confusion the [[International Electrotechnical Commission]] (IEC) use the names kibibyte, mebibyte, and gibibyte for binary powers. They use kilobyte, megabyte, and gigabyte to mean powers of 10. The [[Joint Electron Device Engineering Council]] (JEDEC) have kept the older names. To make it worse the sizes of [[Computer data storage|computer storage]], such as hard disk drives ([[HDD]]), are measured in powers of ten. So a 500GB disk drive is 500 x 1000 x 1000 x 1000 bytes. This is a lot less that 500GB of memory which is 500 x 1024 x 1024 x1024. Most computer scientists still use the old names and have to remember that the units are different when talking about memory and storage devices.
== Read only memory ==
There are some programs and instructions which the computer will always need. [[Read only memory]] (ROM) is the [[permanent|permanent]] memory which is used to store these important control programs and systems software to perform functions such as [[booting]] up or starting up programs. ROM is [[Non-volatile memory|non-volatile]]. That means the contents are not lost when the power is switched off. Its contents are written when the computer is built, but in modern computers, the user can change the contents using special software.
== Random access memory ==
[[Random access memory]] (RAM) is used as the working memory of a computer system.  It stores input data, intermediate results, programs, and other information temporarily.  It can be read and/or written.  It is usually volatile, which means that all data will be lost when the power is turned off. In most cases it is loaded again from the [[hard disk]] which is used as [[data storage]].
== Non-volatile memory ==
[[File:Intel X25-M Solid-State Drive.jpg|thumb|[[Solid-state drive]]s are one example of non-volatile storage.]]
{{Main|Non-volatile memory}}

Non-volatile memory is computer memory that keeps the stored information when not powered.<br/>
Examples of non-volatile memory include:
* [[read-only memory]] 
* [[flash memory]]

It can sometimes refer to [[Computer data storage|computer storage]]. These are always non-volatile.<br/> 
Examples inlcude:
* Solid state devices that use flash memory, like Solid State Drives ([[SSD]]) and [[USB flash drive]]s.
* Magnetic computer storage devices like [[hard disk drive]]s (HDD), [[floppy disk]]s and [[magnetic tape]]
* [[optical disc]]s such as [[CD-ROM]], [[DVD-ROM]] and [[Blu-ray]]
* paper storage such as [[punched tape|paper tape]] and [[punched card]]s

== Computer Memory Media ==
<gallery widths='160px' heights='100%' mode='traditional' caption=''>
File:RAM Module (SDRAM-DDR4).jpg|[[DDR4 SDRAM]] module. {{As of|2021}}, over 90 percent of computer memory used in PCs and servers was of this type.

File:Historical cost of computer memory and storage.svg|Historical lowest retail price of computer memory and [[computer storage|storage]]

File:Memoria elettromeccanica per il calcolatore IBM 602A - Museo scienza tecnologia Milano D1191.jpg|Electromechanical memory used in the [[IBM 602]], an early punch multiplying calculator

File:ENIAC Penn2.jpg|Detail of the back of a section of [[ENIAC]], showing [[vacuum tubes]]

File:James Pomerene IAS machine.jpg|[[Williams tube]] used as memory in the [[IAS computer]] {{circa|1951}}

File:8 bytes vs. 8Gbytes.jpg|8{{nbsp}}[[Gibibyte|GB]] [[microSDHC]] card on top of 8{{nbsp}}[[bytes]] of [[magnetic-core memory]] (1{{nbsp}}core is 1{{nbsp}}[[bit]].)

File:Kinds-of-RAM.JPG|Various memory modules containing different types of DRAM (from top to bottom): DDR SDRAM, SDRAM, EDO DRAM, and FPM DRAM
</gallery>
== References ==
{{reflist|30em}}

[[Category:Computer memory| ]]