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The history of Alt+number sequences, and why Alt+9731 sometimes gives
you a heart and sometimes a snowman

[png]

Raymond Chen

July 2nd, 20242 0

Once upon a time, the IBM PC was released.

In the IBM PC BIOS, you could enter characters that weren't present
on the keyboard by holding the Alt key and typing the decimal value
on the numeric keypad. For example, you could enter n by holding Alt
and typing Numpad1 Numpad6 Numpad4, then releasing the Alt key.

For expository simplicity, I will henceforth use the notation Alt+164
to indicate that you press the Alt key, then type the specified
digits in sequence on the numeric keypad, then release the Alt key.

Okay, so in the IBM PC BIOS, when you typed Alt+..., the code numbers
were treated as decimal byte values, and the result on the screen
came from your video card's character generator. In the United
States, the character generator's ROM showed what we today call Code
Page 437.

When it was introduced, Windows in the United States used Code Page
1252 as its 8-bit character set, which it called the "ANSI character
set"; the old BIOS character set was retroactively named the OEM
character set. To preserve compatibility with MS-DOS, if you used the
Alt key in conjunction with the numeric keypad, the number you typed
was still looked up in OEM character set, so that your muscle-memory
code numbers still worked. You could still type Alt+164 to get your
n, even though the code number for n in Code Page 1252 is 241, not
164.

If you wanted to type a character that had no OEM equivalent, you
could prefix a numeric keypad 0 to indicate that you wanted the value
looked up in the ANSI code page. Therefore, you could type Alt+0169
to get a (c), which did not exist in the OEM code page. You could also
type Alt+0241 to get your precious n, using the ANSI code point
number rather than the OEM code point number.

If you entered a number larger than 255, both Windows and the IBM PC
BIOS took your value mod 256, so typing Alt+259 was the same as
typing Alt+3. Both gave you OEM code point 3, which for Code Page 437
is a heart .

If you ask the Internet how to type some of these non-ASCII
characters on Windows, you may see people (and large language models)
that tell you to type, say, Alt+9731 to get a Unicode snowman .
Unfortunately, from what we've learned above, this doesn't work. You
instead get the OEM character whose value is 9731 mod 256 = 3, or the
aforementioned heart .

A customer reported that a recent Windows update broke their ability
to type a snowman by using Alt+9731. We explained that the update was
not at fault; rather, Alt+9731 was never supposed to produce a
snowman at all! But the customer insisted that it used to work.

A closer investigation of the issue revealed the reason.

You see, while it's true that the Alt+... decimal value is taken mod
256, that is just the default behavior of the Windows input system.
But some controls (most notably the RichEdit control) override the
default handling of the Alt+... sequence and parse out the decimal
value mod 65536 rather than mod 256.

This means that whether the Alt+... value is taken mod 256 depends on
what kind of control you are typing into.

By default, the value is taken mod 256, and Alt+9731 gives you a
heart.

But if you happen to be using a RichEdit control, then the Alt+...
value is taken mod 65536, and Alt+9731 gives you a snowman.

(I don't know of anybody who takes the value mod 2097151, to support
direct entry of code points outside the Basic Multilingual Plane.)

[png]

Raymond Chen

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2 comments

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  * [png]
    Mason Boswell July 2, 2024 11:22 am  0
    collapse this comment copy link to this comment
   
    I was recently helping my daughter with her Spanish homework in
    Word. She asked me how to type an accented "e" and I did what
    always used to work with Alt-num sequences only to find out it no
    longer works. Not only do a lot of modern laptops not provide
    NumLock anymore, but Word interprets Alt+0 and other numbers as
    its own shortcuts. I guess the modern world has no use for
    quicker ways of doing things with the keyboard. I can barely get
    my kids to learn to type:).

    Log in to Vote or Reply
      + [png]
        Aged .Net Guy July 2, 2024 2:07 pm  0
        collapse this comment copy link to this comment
       
        Actually, modern tools (be they laptops or keyboards or
        Windows) de-emphasized the utterly non-mnemonic, primitive,
        and clumsy alt sequences for a darn good reason. And replaced
        them with something far easier and better. Because the need
        for everyone everywhere to enter international characters is
        only growing as the world gets smaller and more multilingual.

        If you are a) an American, and b) need to type characters not
        commonly found on a US hardware keyboard, the correct answer
        is to enable the English United States International
        keyboard. It's in Settings under Keyboards. Once enabled, you
        can toggle back and forth between the domestic and
        international versions of the keyboard with a single chord:
        [Windows]+[Spacebar].

        With the keyboard in international mode, typing n is the far
        _more_ mnemonic ~n. The various other diacritics are equally
        simple. Type a : for umlaut, a `, ^ or ' for the three kinds
        of accent, then the letter to be modified and suddenly
        Roberto es tu tio. I have no doubt your daughter will find ~n
        far more memorable and easy than she will find alt+194.

        Log in to Vote or Reply

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