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。

-ASCII is used to express a coding standard English characters. Each ASCII characters take up a byte, therefore, ASCII codes can be expressed as the maximum number of characters is 255 (00H-FFH). In fact, the English characters are not so many, it is generally used before the 128 (00H-7FH, the highest bit is 0), including control characters, numbers, uppercase and lowercase letters and other symbols. The highest bit is 1 the other 128 characters (80H-FFH) is called " extended ASCII" , usually used to store the English tabs, part of the phonetic characters, and so some of the other symbols.