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Special codes in the first line of a Big Font file specify how to read two-byte hexidecimal codes.
A font with hundreds or thousands of characters must be handled differently from a font containing the ASCII set of up to 256 characters. In addition to using more complicated techniques for searching the file, AutoCAD needs a way to represent characters with two-byte codes as well as one-byte codes. Both situations are addressed by the use of special codes at the beginning of a Big Font file.
The first line of a Big Font shape definition file must be as follows:
*BIGFONT nchars,nranges,b1,e1,b2,e2,...
where nchars is the approximate number of character definitions in this set; if it is off by more than about 10 percent, either speed or file size suffers. You can use the rest of the line to name special character codes (escape codes) that signify the start of a two-byte code. For example, on Japanese computers, Kanji characters start with hexadecimal codes in the range 90-AF or E0-FF. When the operating system sees one of these codes, it reads the next byte and combines the two bytes into a code for one Kanji character. In the *BIGFONT line, nranges tells how many contiguous ranges of numbers are used as escape codes; b1, e1, b2, e2, and so on, define the beginning and ending codes in each range. Therefore, the header for a Japanese Big Font file might look like this:
*BIGFONT 4000,2,090,0AF,0E0,0FF
After the *BIGFONT line, the font definition is just like a regular AutoCAD text font, except that character codes (shape numbers) can have values up to 65535.