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Unicode Demystified by Richard Gillam

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The Unicode Character–Glyph Model

The first and most important thing to understand about Unicode is what is known as the character–glyph model. Until the introduction of the Macintosh in 1984, text was usually displayed on computer screens in a fairly simple fashion. The screen would be divided up into a number of equally sized display cells. The most common video mode on the old IBM PCs, for example, had 25 rows of 80 display cells each. A video buffer in memory consisted of 2,000 bytes, one for each display cell. The video hardware contained a character generator chip that contained a bitmap for each possible byte value, and this chip was used to map from the character codes in memory to a particular set of lit pixels on the screen.

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