Understanding ASCII and ANSI Characters

Virtually everyone knows that a computer doesn't understand characters, it understands numbers. Thus, each character you see on the screen in a program such as Word is maintained internally as a number. The "mapping" of characters to numbers is known as a character set.

For the most part, Word relies on the character set used by whatever version of Windows you are using. Don't confuse the character set used by Windows with the character set used by the computer itself, as they are not the same. For instance, when you first boot your computer, you may see some start-up information on the screen. This information uses a character set maintained internally by the computer on ROM. Since Windows is not running at the time this information is displayed, the character set used by Windows cannot be in use. Once Windows is up and running, then the character set used by the computer itself is no longer used and the one maintained by Windows is relied upon.

This may sound confusing, but it is not meant to be. In the relatively short history of computers, there have been several different character sets used. The first character set used in small computers was ASCII, which is an acronym for American Standard Code for Information Interchange. It started as a code of 128 characters, using seven bits to represent all the characters. (A bit is a binary digit; it can have either two values: on or off. Thus, seven bits can have 2^7 or 128 possible unique values.)

ASCII was first developed for machines that used only seven bits of each byte (such as teletypes). Early personal computers, however, used eight bits, and thus could utilize 2^8 or 256 possible values for a character code. This led to what was known as extended ASCII, where the first 128 characters matched those in ASCII, but the second 128 were left up to the computer manufacturer. In early IBM PC models, the extended ASCII character set included some foreign-language symbols and many line-drawing characters, used for rudimentary graphics.

It is interesting to note that ASCII has not given way to extended ASCII in all computer systems. Indeed, many mainframe and mini computer systems still support strictly seven-bit ASCII. Thus, I have received complaints from some WordTips readers if an extended ASCII character has been included in the weekly newsletter. These characters are not displayed the same on all systems, and some programs used on those systems strip out any non-ASCII values automatically.

Microsoft calls the character set utilized by your computer (as pointed out earlier in this tip) the OEM character set. (OEM means "original equipment manufacturer.") Windows versions through Windows 95 utilize what is called an ANSI character set. This is a single-byte character set that can represent up to 256 characters. The original ASCII character set occupies the first 128 characters of the ANSI set used in Windows. All later versions of Windows, on the other hand, utilize the Unicode character set, which is described in other issues of WordTips.

Remember that this discussion of what the various versions of Windows use refers to what they use internally. Externally, for a typical Word user, there isn't much effect.