Computer data storage: Difference between revisions

Revision as of 13:11, 15 July 2025

Latest revision as of 17:25, 11 August 2025

(4 intermediate revisions by 4 users not shown)

Line 40:

=== Primary storage ===

<!-- Additional DIRECT links point to this section by its name. -->

{{Main|Computer memory}}

''Primary storage'' (also known as ''main memory'', ''internal memory'', or ''prime memory''), often referred to simply as ''memory'', is the only one directly accessible to the CPU. The CPU continuously reads instructions stored there and executes them as required. Any data actively operated on is also stored there in a uniform manner.

Historically, [[History of computing hardware|early computers]] used [[delay-line memory|delay lines]], [[Williams tubestube]]s, or rotating [[drum memory|magnetic drums]] as primary storage. By 1954, those unreliable methods were mostly replaced by [[magnetic-core memory]]. Core memory remained dominant until the 1970s, when advances in [[integrated circuit]] technology allowed [[semiconductor memory]] to become economically competitive.<!-- Please do not expand above text! Change but no expand any further. Anyone desiring to know more types, let it be [[twistor memory]] or [[bubble memory]] etc, will click the History link. Do not confuse other readers. -->

This led to modern [[random-access memory]] (RAM). It is small-sized, light, but quite expensive at the same time. The particular types of RAM used for primary storage are [[volatile memory|volatile]], meaning that they lose the information when not powered. Besides storing opened programs, it serves as [[Page cache|disk cache]] and [[write buffer]] to improve both reading and writing performance. Operating systems borrow RAM capacity for caching so long as it's not needed by running software.<ref>{{cite web| url = https://www.kernel.org/doc/html/latest/admin-guide/sysctl/vm.html| title = Documentation for /proc/sys/vm/ — The Linux Kernel documentation}}</ref> Spare memory can be utilized as [[RAM drive]] for temporary high-speed data storage.

As shown in the diagram, traditionally there are two more sub-layers of the primary storage, besides main large-capacity RAM:

* [[Processor registersregister]]s are located inside the processor. Each register typically holds a [[Word (computer architecture)|word]] of data (often 32 or 64&nbsp;bits). CPU instructions instruct the [[arithmetic logic unit]] to perform various calculations or other operations on this data (or with the help of it). Registers are the fastest of all forms of computer data storage.

* [[Processor cache]] is an intermediate stage between ultra-fast registers and much slower main memory. It was introduced solely to improve the performance of computers. Most actively used information in the main memory is just duplicated in the cache memory, which is faster, but of much lesser capacity. On the other hand, main memory is much slower, but has a much greater storage capacity than processor registers. Multi-level [[Memory hierarchy|hierarchical cache]] setup is also commonly used—''primary cache'' being smallest, fastest and located inside the processor; ''secondary cache'' being somewhat larger and slower. <!-- Please DO NOT EXPAND above text, especially with L1/L2/etc variants - reader can always click the link. -->

Main memory is directly or indirectly connected to the central processing unit via a ''memory bus''. It is actually two buses (not on the diagram): an [[address bus]] and a [[data bus]]. The CPU firstly sends a number through an address bus, a number called [[memory address]], that indicates the desired ___location of data. Then it reads or writes the data in the [[Memory cell (computing)|memory cells]] using the data bus. Additionally, a [[memory management unit]] (MMU) is a small device between CPU and RAM recalculating the actual memory address, for example to provide an abstraction of [[virtual memory]] or other tasks.

As the RAM types used for primary storage are volatile (uninitialized at start up), a computer containing only such storage would not have a source to read instructions from, in order to start the computer. Hence, [[Non-volatile memory|non-volatile primary storage]] containing a small startup program ([[BIOS]]) is used to [[Bootstrapping#Computing|bootstrap]] the computer, that is, to read a larger program from non-volatile ''secondary'' storage to RAM and start to execute it. A non-volatile technology used for this purpose is called ROM, for [[read-only memory]] (the terminology may be somewhat confusing as most ROM types are also capable of ''[[random access]]'').

Many types of "ROM" are not literally ''read only'', as updates to them are possible; however it is slow and memory must be erased in large portions before it can be re-written. Some [[embedded systemssystem]]s run programs directly from ROM (or similar), because such programs are rarely changed. Standard computers do not store non-rudimentary programs in ROM, and rather, use large capacities of secondary storage, which is non-volatile as well, and not as costly.

Recently, ''primary storage'' and ''secondary storage'' in some uses refer to what was historically called, respectively, ''secondary storage'' and ''tertiary storage''.<ref>{{cite web|url=http://searchstorage.techtarget.com/topics/0,295493,sid5_tax298620,00.html|title=Primary storage or storage hardware (shows usage of term "primary storage" meaning "hard disk storage")|url-status=dead|archive-url=https://web.archive.org/web/20080910151628/http://searchstorage.techtarget.com/topics/0,295493,sid5_tax298620,00.html|archive-date=10 September 2008|website=searchstorage.techtarget.com|access-date=18 June 2011}}</ref>

The primary storage, including [[Read-only memory|ROM]], [[EEPROM]], [[NOR flash]], and [[Random access memory|RAM]],<ref>{{cite book | url=https://books.google.com/books?id=QGPHAl9GE-IC&dq=size+of+a+memory+address&pg=PA321 | isbn=978-0-7637-3769-6 | title=The Essentials of Computer Organization and Architecture | date=2006 | publisher=Jones & Bartlett Learning }}</ref> are usually [[byte-addressable]].

=== Secondary storage ===

Line 229 ⟶ 232:

=== Energy use ===

* Storage devices that reduce fan usage automatically shut-down during inactivity, and low power hard drives can reduce energy consumption by 90 percent.<ref>{{cite web|url=http://www.springlightcfl.com/consumer/energy_savings_calculator.aspx|title=Energy savings calculator|url-status=dead|archive-url=https://web.archive.org/web/20081221131054/http://springlightcfl.com/consumer/energy_savings_calculator.aspx|archive-date=21 December 2008}}</ref><ref>{{Cite web|url=http://www.simpletech.com/content/eco-friendly-redrive|url-status=dead|archive-url=https://web.archive.org/web/20080805092907/http://www.simpletech.com/content/eco-friendly-redrive|archive-date=5 August 2008|title=How much of the [re]drive is actually eco-friendly?|website=Simple tech}}</ref>

* 2.5-inch hard disk drives often consume less power than larger ones.<ref>{{cite web|title=IS the Silent PC Future 2.5-inches wide?|url=http://www.silentpcreview.com/article145-page1.html|access-date=2 August 2008|author=Mike Chin|date=8 March 2004|url-status=live|archive-url=https://web.archive.org/web/20080720000101/http://www.silentpcreview.com/article145-page1.html|archive-date=20 July 2008}}</ref><ref>{{cite web|url=http://www.silentpcreview.com/article29-page2.html|title=Recommended hard drives|access-date=2 August 2008|author=Mike Chin|date=18 September 2002|url-status=live|archive-url=https://web.archive.org/web/20080905085853/http://www.silentpcreview.com/article29-page2.html|archive-date=5 September 2008}}</ref> Low capacity [[solid-state drive]]s have no moving parts and consume less power than hard disks.<ref>{{Cite web|url=http://techreport.com/articles.x/10334/13|title=Super Talent's 2.5" IDE flash hard drive|website=The tech report|date=12 July 2006|page=13|archive-url=https://web.archive.org/web/20120126045422/http://techreport.com/articles.x/10334/13|archive-date=26 January 2012|access-date=18 June 2011}}</ref><ref>{{Cite web|url=httphttps://www.tomshardware.com/reviews/conventional-hard-drive-obsoletism,1324-5.html|title=Power consumption – Tom's hardware : Conventional hard drive obsoletism? Samsung's 32 GB flash drive previewed|website=tomshardware.com|date=20 September 2006|access-date=18 June 2011}}</ref><ref name=xbitSSDvsHD/> Also, memory may use more power than hard disks.<ref name=xbitSSDvsHD>{{cite web|title=SSD, i-RAM and traditional hard disk drives|date=23 April 2008|url=http://www.xbitlabs.com/articles/storage/display/ssd-iram.html|author=Aleksey Meyev|publisher=X-bit labs|url-status=dead|archive-url=https://web.archive.org/web/20081218134355/http://www.xbitlabs.com/articles/storage/display/ssd-iram.html|archive-date=18 December 2008}}</ref> Large caches, which are used to avoid hitting the [[memory wall]], may also consume a large amount of power.

=== Security ===

Line 402 ⟶ 405:

== Further reading ==

* {{cite journal|title=The history of storage systems|journal=[[Proceedings of the IEEE]]|author1=Goda, K. |author2=Kitsuregawa, M. |year=2012|pages=1433–1440|volume=100|doi=10.1109/JPROC.2012.2189787|doi-access=free}}

* [httphttps://www.computerhistory.org/revolution/memory-storage/8 Memory & storage], Computer history museum

{{Basic computer components}}