On personal computers with numeric keypads that use Microsoftoperating 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.
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On IBM PC compatiblepersonal 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 a code to be placed on the screen at the location of the cursor, thus displaying the corresponding 8-bit character from the current code page. On the original IBM PC this was CP437.[lower-alpha 1]. In most cases typing a number greater than 255 produced the character associated with the remainder after the number is divided by 256.
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; 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 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, but MS DOS did not interpret the numbers this way. For instance, code point 7 is assigned to BEL. However with some applications, Alt+7 may yield a bullet character • (code point 7 on code page 437), but in others would treat this input as identical to Ctrl+G (which on a terminal produces a control character with a value of 7).[citation needed]
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