Operation

TEX uses two array internally, |xord[]| and |xchr[]| for transformation on input resp. on output. When reading, every character |s| is replaced by the character |t:=xord[s]|, when writing, the character |t| is subsequently again replaced by |s:=xchr[t]|. Eventually, the character of the computer is represented in TEX by the character |t|.

Special characters (i.e. control characters and 8 bit characters) are normally represented by |8x|, (|xx| is thereby the characters position in the TEXcharacter set in hex notation), this has the advantage that this character can be again read correctly machine independant. For the user though it is making more sense to present displayable special characters as such, that is why TEX uses a further array, in which the display mode is recorded.

In the Codepage definition you now have the possibility, to change the default content of this array.