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Description.
Many variations of ASCII have been developed by corporations and standards bodies. The goal is to facilitate the expression of non-English languages still using Roman-based alphabets. Some of these variations are considered ASCII extensions. However, the term is sometimes misapplied to cover all variants. This includes those not preserving ASCII's character map, in the 7-bit range.
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ISO 646 (1972) was the first attempt to remedy the English bias. It created compatibility problems, since it was still a seven-bit character set. No additional codes were available, so some were re-assigned, in language-specific variants. This made it impossible to know what character was represented, by a code, without knowing what variant was in use. Text processing systems were generally able to cope with only one variant, anyway.
Eventually, improved technology brought, out-of-band, means. They represent the information, formerly encoded in the eighth bit of each byte. It freed this bit, to add another 128 additional character codes, for new assignments.
For example, IBM developed eight-bit code pages, such as code page 437. This replaced the control characters, with graphic symbols such as smiley faces. It mapped additional graphic characters, to the upper 128 bytes. These code pages were supported, in hardware, by IBM PC manufacturers. This included operating systems, such as DOS. excel formulas
8-bit standards.
Eight-bit standards, such as ISO/IEC 8859, were true extensions of ASCII. They left the original character mapping intact, and added additional values, above the 7-bit range. This enabled a broader range of languages, to be represented. However, these standards were still plagued with incompatibilities and limitations. Still, ISO/IEC 8859-1, and original 7-bit ASCII, are the most common character encodings in use today.
Unicode and ISO.
Unicode and ISO/IEC 10646: the Universal Character Set, have a much wider array of characters, and their various encoding forms are rapidly supplanting ISO/IEC 8859 and ASCII in many environments. While ASCII is defined in terms of 7-bit codes, Unicode and the UCS are defined in terms of relatively abstract "code points": non-negative integer numbers that can be mapped, using different encoding forms and schemes, to sequences of 1 or more 8-bit bytes. excel formulas
To permit backward compatibility, Unicode and the UCS assign the first 128 code points to the same characters as ASCII. ASCII can therefore be thought of as being a 7-bit encoding scheme for a very small subset of Unicode and the UCS. The popular UTF-8 encoding form prescribes the use of one to four 8-bit code values for each code point character, and is identical to ASCII for the code values below 128.
More variations.
Other encoding forms such as UTF-16 resemble ASCII in how they represent the first 128 characters of Unicode, but tend to use 16 or 32 bits per character, so they are not entirely compatible without conversions.
The portmanteau word ASCIIbetical has evolved to describe the collation of data in ASCII code order rather than "standard" alphabetical order (which requires some tricky computation, and varies with language). excel formulas
ASCII contains many characters which were not commonly used, or at least spoken of, outside of the computing context; the ";popularization" of these characters required that names be agreed upon for them. See the pronunciation guide in the external links, below.
ASCIIZ or ASCIZ is an abbreviation used to refer to a null-terminated ASCII string.
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