Alt code

On personal computers with numeric keypads that use Microsoft operating systems, 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 Alt key, then typing a number on the keyboard's numeric keypad that identifies the character and then releasing Alt.

MS-DOS

On IBM PC compatible personal computers from the 1980s, the BIOS allowed the user to hold down the 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. On the original IBM PC the code page was CP437.

Some Eastern European, Arabic and Asian computers used other hardware code pages, 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 keyboard layouts, the AltGr key or dead keys, 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?][clarification needed] 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 Ctrl+G. While some (most?) applications would insert a bullet character • (code point 7 on code page 437), some would treat this identical to Ctrl+G which often was a command for the program.[citation needed]

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:

• The familiar Alt+### combination retains the old MS-DOS behavior, i.e., generates characters from the legacy code pages now called "OEM code pages". For instance Alt+163 produces ⟨ú⟩ from code point 163 in CP437 or CP850. The "OEM code page" 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 Windows code page, such as box-drawing symbols.
• The new Alt+0### combination (which prefixes a zero to each Alt code) produces characters using the Windows code page. For instance Alt+0163 results in ⟨£⟩ (when CP1252 is active) because it occupies position 163 in CP1252. The active Windows code page depends on the current input locale (keyboard layout) and may vary, which can change the effect of these Alt codes.

Unicode

Later versions of Windows and applications such as Microsoft Word added support for Unicode. In some applications this had the benefit that all the "old" Alt combinations, for instance for line-drawing characters, worked again.

Setting the Windows Code Page to CP1252 made the zero-prefixed numbers 32-126 and 160-255 produce the matching code point from Unicode.

In the IBM PC BIOS, typing an Alt code greater than 255 produced the same as that number modulo 256. 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. For instance, Alt+9731 in WordPad produces the U+2603☃ SNOWMAN.

Hex Alt Codes

Because most Unicode documentation and character tables show code points in hexadecimal rather than decimal, Windows added a mechanism to input Alt codes in hexadecimal form. To enable it, a user must set or create a string (REG_SZ) value called EnableHexNumpad in the registry key HKEY_CURRENT_USER\Control Panel\Input Method, assign the value data 1, and then reboot or log out/in.

When enabled, holding Alt and pressing + on the numeric keypad enters hexadecimal Unicode input mode. The digits are entered using the numeric keypad, while the letters A–F are typed on the main keyboard. For example, Alt++11B produces U+011Bě LATIN SMALL LETTER E WITH CARON. The input value is accumulated into a 16-bit value, so only the lower 16 bits are used; effectively, this limits input to Unicode BMP characters (up to four hexadecimal digits), with excess digits being truncated. This did not work in all applications or user interface elements.

Windows also supports an alternative hexadecimal input mode (less commonly documented) where Alt+. on the numeric keypad enters a mode that interprets the value using the Windows code page.

There are many other ways 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 operating systems 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 Ctrl+⇧ Shift+U, and typing the hex number followed by the space bar or enter key. For example,

• For the registered trademark symbol ®, type Ctrl+⇧ Shift+U, AE, ↵ Enter.
• For the no entry sign ⛔, type Ctrl+⇧ Shift+U, 26D4, ↵ Enter.

Limitations

If 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, Alt+4 can be taken as Alt+←, causing a web browser to go back one page.

List of codes

See also

• Combining character
• Compose key for other operating systems
• Keyboard layout
• List of Unicode characters
• Numeric character reference
• Unicode input

Notes