补充一下，这个数字叫 raw bytes ，属于一个特殊字符集 eight-bit。
On rare occasions, Emacs encounters raw bytes : single bytes whose values are in the range 128 (0200 octal) through 255 (0377 octal), which Emacs cannot interpret as part of a known encoding of some non-ASCII character. Such raw bytes are treated as if they belonged to a special character set
eight-bit; Emacs displays them as escaped octal codes (this can be customized; see Display Custom). In this case, C-x = shows ‘raw-byte’ instead of ‘file’. In addition, C-x = shows the character codes of raw bytes as if they were in the range#x3FFF80..#x3FFFFF, which is where Emacs maps them to distinguish them from Unicode characters in the range#x0080..#x00FF.