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The Brahmi-derived writing systems are mostly rather similar in structure, but have different letter shapes. So ISCII encodes letters with the same phonetic value at the same codepoint, overlaying the various scripts. For example, the ISCII codes 0xB3 0xDB represent [ki]. This will be rendered as कि in Devanagari, as ਕਿ in Gurmukhi, and as கி in Tamil. The writing system can be selected in rich text by markup or in plain text by means of the ATR code described below.
One motivation for the use of a single encoding is the idea that it will allow easy transliteration from one writing system to another. However, there are enough incompatibilities that this is not really a practical idea. See About ISCII.
ISCII is a stateful 8-bit encoding. The lower 128 codepoints are plain ASCII, the upper 128 codepoints are ISCII-specific. In addition to the codepoints representing characters, ISCII makes use of a codepoint with mnemonic ATR that indicates that the following byte contains one of two kinds of information. One set of values changes the writing system until the next writing system indicator or end-of-line. Another set of values select display modes, such as bold and italic. ISCII does not provide a means of indicating the default writing system.
ISCII has not been widely used outside of certain government institutions and has now been rendered largely obsolete by Unicode. Unicode uses a separate block for each Indic writing system, and largely preserves the ISCII layout within each block.
Codepage layout
The following table shows the character set for Devanagari. The code sets for Assamese, Bengali, Gujarati, Gurmukhi, Kannada, Malayalam, Oriya, Tamil, and Telugu are similar, with each Devanagari form replaced by the equivalent form in each writing system. Each character is shown with its decimal code and its Unicode equivalent.
The nukta is used to create a number of characters which have precomposed forms in Unicode, as well as a number of rarer characters which don't exist in the main ISCII set, such as the Sanskrit character ॠ.
