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Arrangement of the Encoding Space

Unicode's designers tried to assign the characters to numeric values in an orderly manner that would make it easy to tell something about a character just from its code point value. As the encoding space has filled up, this has become more difficult to do, but the logic still comes through reasonably well.

Unicode was originally designed for a 16-bit encoding space, consisting of 256 rows of 256 characters each. ISO 10646 was designed for a 32-bit encoding space, consisting of 128 groups of 256 planes containing 256 rows of 256 characters. Thus the original Unicode encoding space had room for 65,536 characters, and ISO 10646 had room for an unbelievable 2,147,483,648 characters. The ISO encoding space is clearly overkill (experts estimate that perhaps 1 million or so characters are eligible for encoding), but it was clear by the time Unicode 2.0 came out that the 16-bit Unicode encoding space was too small.

The solution was the surrogate mechanism, a scheme whereby special escape sequences known as surrogate pairs could be used to represent characters outside the original encoding space. It extended the number of characters that could be encoded to 1,114,112, leaving ample space for the foreseeable future (only 95,156 characters are actually encoded in Unicode 3.2, and Unicode has been in development for 12 years). The surrogate mechanism was introduced in Unicode 2.0 and has since become known as UTF-16. It effectively encodes the first 17 planes of the ISO 10646 encoding space. The Unicode Consortium and WG2 have agreed never to populate the planes above plane 16, so for all intents and purposes, Unicode and ISO 10646 now share a 21-bit encoding space consisting of 17 planes of 256 rows of 256 characters. Valid Unicode code point values run from U+0000 to U+10FFFF.

Organization of the Planes

Figure 3.1 shows the Unicode encoding space.

Figure 3.1Figure 3.1 The Unicode Encoding Space

Plane 0 is the Basic Multilingual Plane (BMP). It contains the majority of the encoded characters, including all of the most common ones. In fact, prior to Unicode 3.1, no characters were encoded in any of the other planes. The characters in the BMP can be represented in UTF-16 with a single 16-bit code unit.

Plane 1 is the Supplementary Multilingual Plane (SMP). It is intended to contain characters from archaic or obsolete writing systems. Why encode them at all? They are here mostly for the use of the scholarly community in papers where they write about these characters. Various specialized collections of symbols will also go into this plane.

Plane 2 is the Supplementary Ideographic Plane (SIP). This extension of the CJK Ideographs Area from the BMP contains rare and unusual Chinese characters.

Plane 14 (E) is the Supplementary Special-Purpose Plane (SSP). It's reserved for special-purpose characters—generally code points that don't encode characters as such but are instead used by higher-level protocols or as signals to processes operating on Unicode text.

Planes 15 and 16 (F and 10) are the Private Use Planes, an extension of the Private Use Area in the BMP. The other planes are currently unassigned, and will probably remain that way until Planes 1, 2, and 14 start to fill up.

The Basic Multilingual Plane

The heart and soul of Unicode is plane 0, the BMP. It contains the vast majority of characters in common use today, and those that aren't yet encoded will go here as well. Figure 3.2 shows the allocation of space in the BMP.

Figure 3.2Figure 3.2 The Basic Multilingual Plane

The characters whose code point values begin with 0 and 1 form the General Scripts Area. This area contains the characters from all of the alphabetic writing systems, including the Latin, Greek, Cyrillic, Hebrew, Arabic, Devanagari (Hindi), and Thai alphabets, among many others. It also contains a collection of combining marks that are often used in conjunction with the letters in this area. Figure 3.3 shows how the General Scripts Area is allocated.

Figure 3.3Figure 3.3 The General Scripts Area

There are a few important things to note about the General Scripts Area. First, the first 128 characters (those from U+0000 to U+007F) are exactly the same as the ASCII characters with the same code point values. Thus you can convert from ASCII to Unicode simply by zero-padding the characters out to 16 bits (in fact, in UTF-8, the 8-bit version of Unicode, the ASCII characters have exactly the same representation as they do in ASCII).

Second, the first 256 characters (those from U+0000 to U+00FF) are exactly the same as the characters with the same code point values from the ISO 8859-1 (ISO Latin-1) standard. (Latin-1 is a superset of ASCII; its lower 128 characters are identical to ASCII.) You can convert Latin-1 to Unicode by zero-padding out to 16 bits. (Note, however, that the non-ASCII Latin-1 characters have two-byte representations in UTF-8.)

For those writing systems that have only one dominant existing encoding, such as most of the Indian and Southeast Asian ones, Unicode keeps the same relative arrangement of the characters as their original encoding had. Conversion back and forth can be accomplished by adding or subtracting a constant.

We'll be taking an in-depth look at all of these scripts in Part II. The Latin, Greek, Cyrillic, Armenian, and Georgian blocks, as well as the Combining Diacritical Marks, IPA Extensions, and Spacing Modifier Letters blocks, are covered in Chapter 7. The Hebrew, Arabic, Syriac, and Thaana blocks are covered in Chapter 8. The Devanagari, Bengali, Gurmukhi, Gujarati, Oriya, Tamil, Telugu, Kannada, Malayalam, Sinhala, Thai, Lao, Tibetan, Myanmar, Khmer, and Philippine blocks are covered in Chapter 9. The Hangul Jamo block is covered in Chapter 10. The Ethiopic, Cherokee, Canadian Aboriginal Syllables, Ogham, Runic, and Mongolian blocks are covered in Chapter 11.

The characters whose code point values begin with 2 (with a few recent exceptions) form the Symbols Area. This area includes all kinds of stuff, such as a collection of punctuation that can be used with many different languages (this block actually supplements the punctuation marks in the ASCII and Latin-1 blocks), collections of math, currency, technical, and miscellaneous symbols, arrows, box-drawing characters, and so forth. Figure 3.4 shows the Symbols area, and Chapter 12 covers the various blocks.

The characters whose code point values begin with 3 (with Unicode 3.0, this group has now slopped over to include some code point values beginning with 2) form the CJK Miscellaneous Area. It includes all of the characters used in the East Asian writing systems, except for the three very large areas immediately following. For example, it includes punctuation used in East Asian writing, the phonetic systems used for Japanese and Chinese, various symbols and abbreviations used in Japanese technical material, and a collection of "radicals," component parts of Han ideographic characters. These blocks are covered in Chapter 10 and shown in Figure 3.4.

Figure 3.4Figure 3.4 The Symbols and CJK Miscellaneous Areas

The characters whose code point values begin with 4, 5, 6, 7, 8, and 9 (in Unicode 3.0, this area has slopped over to include most of the characters whose code point values begin with 3) constitute the CJKV Unified Ideographs Area. This is where the Han ideographs used in Chinese, Japanese, Korean, and (much less frequently) Vietnamese are located.

The characters whose code point values range from U+A000 to U+A4CF form the Yi Area. It contains the characters used for writing Yi, a minority Chinese language.

The characters whose code point values range from U+AC00 to U+D7FF form the Hangul Syllables Area. Hangul is the alphabetic writing system used (sometimes in conjunction with Han ideographs) to write Korean. Hangul can be represented using the individual letters, or jamo, which are encoded in the General Scripts Area. The jamo are usually arranged into ideograph-like blocks representing whole syllables, and most Koreans look at whole syllables as single characters. This area encodes all possible modern Hangul syllables using a single code point for each syllable.

We look at the CJKV Unified Ideographs, Yi, and Hangul Syllables Areas in Chapter 10.

The code point values from U+D800 to U+DFFF constitute the Surrogates Area. This range of code point values is reserved and will never be used to encode characters. Instead, values from this range are used in pairs as code-unit values by the UTF-16 encoding to represent characters from planes 1 through 16.

The code point values from U+E000 to U+F8FF form the Private Use Area (PUA). This area is reserved for the private use of applications and systems that use Unicode, which may assign any meaning they wish to the code point values in this range. Private-use characters should be used only within closed systems that can apply a consistent meaning to these code points; text that is supposed to be exchanged between systems is prohibited from using these code point values (unless the sending and receiving parties have a private agreement stating otherwise), as there's no guarantee that a receiving process would know what meaning to apply to them.

The remaining characters with code point values beginning with F form the Compatibility Area. This catch-all area contains characters that are included in Unicode simply to maintain backward compatibility with the source encodings. It includes various ideographs that would be unified with ideographs in the CJK Unicode Ideographs except that the source encodings draw a distinction, presentation forms for various writing systems, especially Arabic, and half-width and full-width variants of various Latin and Japanese characters, among other things. This section isn't the only area of the encoding space containing compatibility characters; the Symbols Area includes many blocks of compatibility characters, and some others are scattered throughout the rest of the encoding space. This area also contains a number of special-purpose characters and noncharacter code points. Figure 3.5 shows the Compatibility Area.

Figure 3.5Figure 3.5 The Compatibility Area

The Supplementary Planes

Planes 1 through 16 are collectively known as the Supplementary Planes. They include rarer or more specialized characters.

Figure 3.6 depicts plane 1. The area marked "Letters" includes a number of obsolete writing systems and will expand to include more. The area marked "Music" includes a large collection of musical symbols, and the area marked "Math" includes a special set of alphanumeric characters intended to be used as symbols in mathematical formulas.

Figure 3.4Figure 3.6 Plane 1: The Supplementary Multilingual Plane

Figure 3.7 depicts plane 2. It's given over entirely to Chinese ideographic char

Figure 3.7Figure 3.7 Plane 2: The Supplementary Ideographic Plane

Figure 3.8 shows plane 14. It currently contains only a small collection of

Figure 3.8Figure 3.8 Plane 14: The Supplementary Special-Purpose Plane

Although few unassigned code point values are left in the BMP, there are thousands and thousands in the other planes. Except for the Private Use Areas, Unicode implementations are not permitted to use the unassigned code point values for anything. All of them are reserved for future expansion, and they may be assigned to characters in future versions of Unicode. Conforming Unicode implementations can't use these values for any purpose or emit text purporting to be Unicode that uses them. This restriction also applies to the planes above plane 16, even though they may never be used to encode characters. It's also illegal to use the unused bits in a UTF-32 code unit to store other data.

Noncharacter Code Point Values

The code point values U+FFFE and U+FFFF, plus the corresponding code point values from all the other planes, are also illegal. They're not to be used in Unicode text at all. U+FFFE can be used in conjunction with the Unicode byte-order mark (U+FEFF) to detect byte-ordering problems (for example, if a Unicode text file produced on a Wintel PC starts with the byte-order mark, a Macintosh program reading it will read the byte-order mark as the illegal value U+FFFE and know that it has to byte-swap the file to read it properly).

U+FFFF is illegal for two main reasons. First, it provided a non-Unicode value that can be used as a sentinel value by Unicode-conformant processes. For example, the getc() function in C has to have a return type of int even though it generally returns only character values, which fit into a char. Because all char values are legal character codes, no values that are available to serve as the end-of-file signal. The int value -1 is the end-of-file signal—you can't use the char value -1 as end-of-file because it's the same as 0xFF, which is a legal character. The Unicode version of getc(), on the other hand, could return unsigned short (or wchar_t on many systems) and still have a noncharacter value of that type—U+FFFF—available to use as the end-of-file signal.

Second, U+FFFF isn't a legal Unicode code point value for the reason given in the following example: Say you want to iterate over all of the Unicode code point values. You write the following (in C):

unsigned short c;
for (c = 0; c <= 0xFFFF; ++c) {
       // etc...

The loop will never terminate, because the next value after 0xFFFF is 0. Designating U+FFFF as a non-Unicode value enables you to write loops that iterate over the entire Unicode range in a straightforward manner without having to resort to a larger type (and a lot of casting) for the loop variable or other funny business to make sure the loop terminates.

The corresponding code points in the other planes were reserved for the same reasons, although this is mostly a historical curiosity now. In the original design of ISO 10646, each plane was expected to function as a more or less independent encoding space. If you dealt with characters from only one plane, you might have had to represent them with 16-bit units (effectively chopping off the plane and group numbers) and encountered the same problem as described above.

Unicode 3.1 sets aside 32 additional code point values, U+FDD0 to U+FDEF, as noncharacter code points. This change makes these values available to implementations for their internal use as markers or sentinel values without the implementations having to worry about their being assigned to characters in the future. These values are not private-use code points and therefore aren't supposed to be used to represent characters. Like the other noncharacter code points, they're never legal in serialized Unicode text.

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