Alt code: Difference between revisions

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{{Short description|MethodInput for entering characters into a computermethod}}

{{More citations needed|date=March 2022}}

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==MS-DOS==

On [[IBM PC compatible]] [[personal computer]]s from the 1980s, the [[BIOS]] allowed the user to hold down the {{keypress|Alt}} key and type a decimal number on the keypad. It would place the corresponding code into the keyboard buffer so that it would look (almost) as if the code had been entered by a single keystroke. Applications reading keystrokes from the BIOS would behave according to what action they associate with that code. Some would interpret the code as a command, but often it would be interpreted as an 8-bit character from the current [[code page]] that was inserted into the text the user was typing.<ref name="AltModulo">If the entered value exceeds 255, only the remainder after dividing by 256 (i.e., the value [[modulo]] 256) was used by the BIOS. For example, 520 is interpreted as 520 mod 256 = 8, so it produces the same character as for value 8.</ref> On the original [[IBM PC]] the code page was [[CP437]].

Some Eastern European, Arabic and Asian computers used other hardware [[code page]]s, and MS-DOS was able to switch between them at runtime with commands like <code>KEYB</code>, <code>[[List of DOS commands#CHCP|CHCP]]</code> or <code>[[List of DOS commands#MODE|MODE]]</code>. This causes the Alt combinations to produce different characters (as well as changing the display of any previously-entered text in the same manner). A common choice in locales using variants of the Latin alphabet was [[CP850]], which provided more Latin character variants. (There were, however, many more code pages{{Crossreference|text=; for a more complete list, see ''[[code page]]''}}).

PC keyboards designed for non-English use included other methods of inserting these characters, such as national [[keyboard layout]]s, the [[AltGr key]] or [[dead key]]s, but the Alt key was the only method of inserting some characters, and the only method that was the same on all machines, so it remained very popular.{{where|date=October 2020}}{{clarify|reason = Alt codes worked on on computers set to US layout, and also many characters were still not accessible using AltGr|date=October 2020}} This input method is emulated by many pieces of software (such as later versions of MS-DOS and Windows) that do not use the BIOS keyboard decoding.

In the ASCII standard, the numbers 0-31 and 127 are assigned to [[Control character#In Unicode|control characters]], for instance, [[code point]] 7 is typed by {{keypress|Ctrl|G}}. While some (most?) applications would insert a [[Bullet (typography)|bullet]] character {{char|•}} (code point 7 on [[code page 437]]), some would treat this identical to {{keypress|Ctrl|G}} which often was a command for the program.{{citation needed|date=October 2020}}

Typing a number greater than 255 causes an [[integer overflow]], producing a character whose code is the input number [[modulo operator|modulo]] 256.<ref name="WannaBuildASnowman" />

==Windows==

The Alt codes had become so well known and memorized by users that Microsoft decided to preserve them in [[Microsoft Windows]], even though the OS features a newer and different set of code pages, e.g.,such as [[CP1252]]. As such, Windows supportsincludes the following processing algorithm for Alt code, which supports both algorithmsmethods:

* The familiar {{key press|Alt}}+{{key press|#}}{{key press|## combination (where}}{{key press|###}} is from 0 to 255)combination retains the old [[MS-DOS]] behavior, i.e., generates characters from the legacy code pages now called "[[Windows code page#OEM code page|OEM code pages]]".<ref name="WindowsAltAlgo" /> For instance, the combination {{key press|Alt}}+{{key press|1}}{{key press|6}}{{key press|3}} would result inproduces {{charunichar|ú00fa}}. (Latincode letterpoint u163 within [[acuteCP437]] accentor [[CP850]]). whichThe is"OEM atcode 163page" selection served little or no other purpose other than to control what Alt codes produced, if other code pages were selected then these numbers could produce different characters, but it did not change the display. It was impossible to type characters that were not in the OEMWindows code page, ofsuch CP437as orbox-drawing CP850symbols.<ref name="WindowsAltAlgo" />

* The new Alt+0### combination (which prefixes a zero to each Alt code), produces characters from the newer "[[Windows code page#ANSI code page|Windows code pages]]."{{efn|Microsoft initially referred to them as "ANSI" code pages, but later acknowledged that this as a misnomer.}} For example, {{key press|Alt}}+{{key press|0}}{{key press|1}}{{key press|6}}{{key press|3}} yields the character {{char|£}} (symbol for the [[pound sterling]]) which is at 163 in CP1252.<ref name="WindowsAltAlgo">{{Cite web |date=2016-07-22 |title=To input characters that are not on your keyboard |url=http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/lang_char_code_input.mspx |url-status=dead |archive-url=https://web.archive.org/web/20160722031546/http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/lang_char_code_input.mspx?mfr=true |archive-date=2016-07-22 |access-date=2022-12-30 |website=Microsoft }}</ref>

* The built-in Windows [[RichEdit]] control (and by extension, all apps that use it, e.g., [[WordPad]]) override the above behavior and instead accept Alt+#### where #### is a decimal [[Unicode]] code point between 0 to 65,535.<ref name="RichEdit">{{Cite web |last=Walker |first=Jim |display-authors=etal |title=About Rich Edit Controls |work=Windows App Development |publisher=Microsoft |via=[[Microsoft Learn]] |url=https://learn.microsoft.com/windows/win32/controls/about-rich-edit-controls |date=27 April 2022}}</ref> For instance, {{key press|Alt}}+{{key press|9}}{{key press|7}}{{key press|3}}{{key press|1}} in WordPad produces the {{unichar|2603}}.

* The new {{key press|Alt}}+{{key press|0}}{{key press|#}}{{key press|#}}{{key press|#}} combination (which prefixes a zero to each Alt code), produces characters from the newer "[[Windows code page#ANSI code page|WindowsANSI code pages]]"."{{efn|Microsoft initially referredacknowledged to them asthat "ANSI" code pages," but later acknowledged that this asis a misnomer.}} Forterm example,and {{keybetter press|Alt}}+{{key press|0}}{{key press|1}}{{key press|6}}{{key press|3}} yields the character {{char|£}} (symbol for the [[pound sterling]]) whichname is at"Windows 163code in CP1252page".}}<ref name="WindowsAltAlgo">{{Cite web |date=2016-07-22 |title=To input characters that are not on your keyboard |url=http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/lang_char_code_input.mspx |url-status=dead |archive-url=https://web.archive.org/web/20160722031546/http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/lang_char_code_input.mspx?mfr=true |archive-date=2016-07-22 |access-date=2022-12-30 |website=Microsoft }}</ref> For instance {{key press|Alt}}+{{key press|0}}{{key press|1}}{{key press|6}}{{key press|3}} results in {{unichar|00a3}} (when [[CP1252]] is active, because it occupies position 163 in that code page). The active Windows code page could be different, which could change the effect of these Alt codes.

===Overflow behavior===

==Unicode==

As stated above, the valid value for an Alt code (except the RichEdit cases) is between 0 to 255. However, the user may break this convention and type a larger number. In this case, Windows interprets all numbers greater than 255 [[Modulo operator|modulo]] 256.<ref name="WannaBuildASnowman">{{Cite web |first=Raymond |last=Chen |title=The history of Alt+number sequences, and why Alt+9731 sometimes gives you a heart and sometimes a snowman |work=The Old New Thing |publisher=Microsoft |date=2 July 2024 |url=https://devblogs.microsoft.com/oldnewthing/20240702-00/?p=109951}}</ref> In computer science, this behavior is called [[integer overflow]].

Later versions of Windows and applications such as Microsoft Word supported Unicode. As Unicode included all the characters in all the MSDOS code pages, this had the immediate benefit that all the old MSDOS Alt combinations worked, not just the ones that existed in the Windows Code Page. And far more software stopped changing how documents displayed or printed when the selected code page was changed.

In the IBM PC Bios typing an Alt code greater than 255 produced the same as that number [[Modulo operator|modulo]] 256.<ref name="AltModulo"/><ref name="WannaBuildASnowman">{{Cite web |first=Raymond |last=Chen |title=The history of Alt+number sequences, and why Alt+9731 sometimes gives you a heart and sometimes a snowman |work=The Old New Thing |publisher=Microsoft |date=2 July 2024 |url=https://devblogs.microsoft.com/oldnewthing/20240702-00/?p=109951}}</ref> Some applications retained this behavior, while others (in particular applications using the Windows [[RichEdit]] control, such as [[WordPad]] and [[PSPad]]) made numbers from 256 to 65,535 produce the corresponding Unicode character.<ref name="RichEdit">{{Cite web |last=Walker |first=Jim |display-authors=etal |title=About Rich Edit Controls |work=Windows App Development |publisher=Microsoft |via=[[Microsoft Learn]] |url=https://learn.microsoft.com/windows/win32/controls/about-rich-edit-controls |date=27 April 2022}}</ref> For instance, {{key press|Alt}}+{{key press|9}}{{key press|7}}{{key press|3}}{{key press|1}} in WordPad produces the {{unichar|2603}}. If the Windows Code Page was set to CP1252 then all Unicode BMP characters except [[C0 and C1 control codes|control characters]] could be typed this way.

Since the Windows RichEdit control implements a different Alt code algorithm that supports [[Unicode]] characters,<ref name="RichEdit" /> integer overflow occurs after 65,535, not 255, i.e., RichEdit interprets them modulo 65,536.<ref name="WannaBuildASnowman" />

===HexadecimalHex Alt codesCodes===

Because most Unicode documentation and the [[Character Map (Windows)|Character Map]]character utilitytables show the code points in [[base 16|hex]], not decimal, a variation of Alt codes was developed to allow the typing of numbers in hex (using the main keyboard for {{keypress|A}}–{{keypress|F}}). To enable it, a user must set or create a string type ({{code|REG_SZ}}) value called <code>EnableHexNumpad</code> in the [[Windows Registry|registry]] key <code>HKEY_CURRENT_USER\Control&nbsp;Panel\Input&nbsp;Method</code>, assign the value data <code>1</code> to it, and then reboot or log out/in. A leading {{keypress|+}} then indicates hex input, for example {{key press|Alt|+}}{{key press|1|1|B|chain=}} will produce {{char|ě}}&nbsp;(e with [[caron]]).

There are many [[Unicode input|other ways]] of typing arbitrary Unicode characters, such as the [[Character Map (Windows)|Character Map]] utility.

== Other operating systems ==

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==Limitations==

If {{key press|[[Num lock]]}} is disabled, attempting an Alt code may cause unexpected results in some applications, due to the controls used on the same key. For example, {{keypress|Alt|4}} can be taken as {{keypress|Alt|←}}, causing a web browser to go back one page.

==Alternatives==

{{Main|Input method|Unicode input}}

[[Windows NT 3.1]] introduced the [[Character Map]] utility that became a mainstay of the OS. The utility enables users to visually browse a typeface and copy its characters.

[[Windows 8]] introduced the touch keyboard, an on-screen keyboard for touchscreen devices without physical keyboards. The touch keyboard can input more characters than most physical keyboards. [[Windows 10]] greatly enhanced the number of characters it can type. Also introduced in Windows 10 is the emoji panel, invoked with the {{keypress|Win|.}} combination. It allows users to find and type emoji and Unicode symbols.

[[Microsoft Word]] comes with an Input Symbol command (similar to Character Map), a Unicode input method, and an auto-correct feature for converting rudimentary notations to their official Unicode equivalents.

== List of codes ==

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* [[List of Unicode characters]]

* [[Numeric character reference]]

* [[Unicode input]]

==Notes==