先从ASCII说起，ASCII是用来表示英文字符的一种编码规范。每个ASCII字符占用1个字节，因此，ASCII编码可以表示的最大字符数是255（00H—FFH）。其实，英文字符并没有那么多，一般只用前128个（00H—7FH，最高位为0），其中包括了控制字符、数字、大小写字母和其它一些符号。而最高位为1的另128个字符（80H—FFH）被称为“扩展ASCII”，一般用来存放英文的制表符、部分音标字符等等的一些其它符号。

-start with the ASCII, ASCII is used to express the English characters a coding standard. Each ASCII characters occupy a byte, therefore, ASCII coding can indicate the maximum number of characters is 255 (00H - FFH). In fact, the English characters and less than that, just before the general 128 (00H - 7FH, the highest-0), including control characters, numbers, upper and lower case letters and other symbols. And the highest position for a another 128 characters (80H - FFH) known as "extended ASCII" generally used to store the English tabs, some phonetic transcr iption of the characters, and so on some other symbols.