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On IBM compatible personal computers, many characters not directly associated with a key can be entered using the Alt Numpad input method or Alt code: pressing and holding the Alt key while typing the a number identifying the character with the keyboard's numeric keypad.[nb 1] Similar or extended forms of this feature are also available in many operating systems, including DOS and Microsoft Windows.
Often this is the best or only method many computer users know for entering non-ASCII characters. Many Wikipedia articles on various characters will include how to type that character using Alt codes for codepage 437.
Originally on IBM PCs, the user could hold down the Alt key and type a decimal number on the keypad. The system BIOS would place the corresponding code into the keyboard buffer so that, for software using the BIOS for character input, 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 a code to be placed on the screen at the location of the cursor, thus displaying the corresponding 8-bit character from the current codepage. Without other software loaded, this is the so-called hardware codepage, which is codepage 437 for original IBM PCs and most other computer systems using English. Some Eastern European, Arabic and Asian computers use other hardware codepages, however.
When an operating system like DOS was used, it became possible to override the hardware codepage with a variety of other codepages and even switch between them at runtime with commands like
MODE. While most English systems still used codepage 437 under DOS, another very common choice in locales using variants of the Latin alphabet was codepage 850, which provided more Latin character variants than codepage 437. There were, however, many more codepages; for a more complete list, see codepage.
These numbers became so well known and memorized by computer users that Microsoft was forced to preserve them even though it used a new and different set of codepages for Windows, such as Codepage 1252. These new codepages were called ANSI codepages by Microsoft, while the old ones were called OEM codepages. Holding Alt and typing three digits (first one non-zero) would attempt to translate the code from the 8-bit OEM codepage (for example, codepage 850) to a matching glyph in the ANSI codepage. A leading zero (0) and then a number would produce the character directly from the ANSI codepage.
For instance, the combination⎇ Alt+161 for codepoint 161 would result in "í" (Latin letter i with acute accent) with an OEM codepage setting of e.g. 437 or 850. On a Windows system using codepage 1252 as ANSI codepage, ⎇ Alt+0161 yields the character "¡" (inverted exclamation) which is at 161 in ANSI codepage 1252.
When Windows later transitioned to Unicode, the 0-leading codes had become so well known as well that a third method needed to be invented to produce Unicode codepoints. Although codepage 1252 is quite close to the start of Unicode and it would seem logical to just allow larger numbers to be typed, other international sets did not match, and some users were accustomed to the composition exiting after the third digit without releasing Alt, or numbers larger than 255 being translated modulus 256. To enable the third method, a user must set or create the registry key
HKCU\Control Panel\Input Method\EnableHexNumpad with type REG_SZ to value 1 and reboot (logging out and logging back in is sufficient). Once the registry key is set, the following method can be used to enter Unicode codepoints:
For example, ⎇ Alt++11b will produce "ě" (e with caron).
The transition to Unicode actually made the older legacy Alt codes (the ones with no leading zero) more reliable, as all the glyphs in the legacy codepages have matching glyphs in Unicode, so they all work.
If Num Lock is disabled, attempting an Alt code may cause unexpected results in some applications. For example, ⎇ Alt+4 can be taken as ⎇ Alt+←, causing a web browser to go back one page.
Many laptops do not have a separate numeric keypad; however, the Fn key can be used to turn certain keys into a numeric keypad. There usually is a key called Num ⇩ to "shift" certain keys to act as if they were the numeric keypad keys. For example, on keyboards where there is no dedicated num-pad (mostly found on small-medium laptops and netbooks), the following scheme is very common:
The Alt key method does not work on Linux systems. There are two alternative methods for direct input of Unicode characters in Linux:
GTK-based applications sport a similar input method:
In the Opera browser, Unicode characters can be entered by typing the four hex-digits and then press ^ Ctrl+⇧ Shift+X.
The alternative DOS keyboard and console drivers K3PLUS and FreeKEYB offered a number of extensions to the Alt Numpad input method compared to the default behaviour of the standard keyboard driver KEYB. For example, codepoints could be entered in various numeral systems:
As with a standard keyboard driver, in decimal input mode the first digit had to be entered through the Numpad in order to avoid conflicts with standard key definitions for the ⎇ Alt layer. However, once in Alt input mode, these keyboard drivers accepted key presses for digits 0-9 also from the numeric row, and the ⇧ Shift key could be released again. Leading zeros were accepted as well. Pressing Space instead of releasing the ⎇ Alt key would output the character without leaving the Alt input method, so that characters could be repeated. s was used to emulate keyboard scancodes rather than only compose characters. In addition to this, integer calculations and combinational logic evaluation could be carried out in a small resident RPN-like calculator embedded into the Alt Numpad input method. This included a base converter into other numeral systems by pressing D, H, O, or B before releasing the ⎇ Alt key. Pressing # would output the current value as text. Numbers could be stored in a set of internal memories for later use. If invoked, another feature of these keyboard drivers, a freely moveable second cursor called CopyCursor, could be used to take the character under the CopyCursor as input into the Alt Numpad input method, so that characters on the screen could be converted back into an expression of its codepoint, stuffed into the keyboard buffer and read by applications like editors. Since these functions were an integral part of the keyboard driver, they were fully transparent to running software and therefore worked with virtually any DOS programs when using these drivers. Examples:
⎇ Alt+42 or ⎇ Alt+⇧ Shift+$+2a would both produce the character "*", ⎇ Alt+42h would produce the string "2Ah", whereas ⎇ Alt+⇧ Shift+$+2a⇧ Shift+D would result in the string "42d" to be spooled into the keyboard buffer instead.
K3PLUS.DOCis part of the