On personal computers with numeric keypads that use Microsoft , such as Windows, many characters that do not have a dedicated key combination on the keyboard may nevertheless be entered using the Alt code (the Alt numpad input method). This is done by pressing and holding the key, then typing a number on the keyboard's numeric keypad that identifies the character and then releasing .
MS-DOS
On IBM PC compatible personal computers from the 1980s, the
BIOS allowed the user to hold down the 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.
[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.] On the original
IBM PC the code page was CP437.
Some Eastern European, Arabic and Asian computers used other hardware , and MS-DOS was able to switch between them at runtime with commands like KEYB, CHCP or MODE. 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).
PC keyboards designed for non-English use included other methods of inserting these characters, such as national , the AltGr key or , 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. 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 characters, for instance, code point 7 is typed by . While some (most?) applications would insert a bullet character (code point 7 on code page 437), some would treat this identical to which often was a command for the program.
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 such as CP1252. Windows includes the following processing algorithm for Alt code, which supports both methods:
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The familiar + combination retains the old MS-DOS behavior, i.e., generates characters from the legacy code pages now called "OEM code pages".
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The new + combination (which prefixes a zero to each Alt code), produces characters from the newer "ANSI code pages".
Unicode
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 256.
Hex Alt Codes
Because most Unicode documentation and character tables show the code points in hex, not decimal, a variation of Alt codes was developed to allow the typing of numbers in hex (using the main keyboard for –). To enable it, a user must set or create a string type () value called EnableHexNumpad in the
Windows Registry key HKEY_CURRENT_USER\Control Panel\Input Method, assign the value data 1 to it, and then reboot or log out/in. A leading then indicates hex input, for example will produce (e with
caron).
There are many Unicode input of typing arbitrary Unicode characters, such as the Character Map utility.
Other operating systems
The Alt key method does not work on
ChromeOS,
macOS,
Linux or other
and there is no evidence of interest in replicating it. However, numeric entry of Unicode characters is possible in most
Unix or
Unix-like OSs by pressing and releasing , and typing the hex number followed by the space bar or enter key. For example,
-
For the registered trademark symbol , type , , .
-
For the no entry sign , type , , .
Limitations
If is disabled, attempting an Alt code may cause unexpected results in some applications, due to the controls used on the same key. For example, can be taken as , causing a web browser to go back one page.
List of codes
See also
Notes
