Sorry for not providing a Minimal Working/Demonstrating Example.
The question is not about code but about understanding what is written in the TeXbook.
TeXbook, Appendix C: Character Codes presents three tables.
The first table shows the (unextended) ASCII (which provides code-points in range from 0(dec) to 127(dec) and bijectively maps specific characters to these code-points -- the resulting set of mappings represents the specific by which ASCII is defined).
The second table shows how to access all characters of (unextended) ASCII using only the ninety-five characters that usually were typeable on the terminals/keyboards in use in the IT-era when the TeXbook was written.
^^-notation requires only a strict subset of the ninety-five typeable standard characters, that second table shows how to access via
^^-notation the thirty-three characters of (unextended) ASCII located in the ASCII-code-points 0(dec) to 31(dec) and 127(dec) which you could probably not type on a terminal/keyboard in use in the time when the TeXbook was written and which with most nowadays' keyboards you still cannot type.
But I don't have a clue what the third table shows:
It deviates from (unextended) ASCII but like with (unextended) ASCII the range of code-points provided is from 0(dec) to 127(dec).
Wikipedia etc tell that "extended ASCII" provides code-points in range from 0(dec) to 255(dec).
Nonetheless the paragraph of the TeXbook which introduces that third table refers to
- some character-encoding-scheme "which was developed at MIT and which is slightly better than" something "developed at Stanford",
- an "extended ASCII code intended for text editing and interactive computing" which "was developed at several universities about 1965".
That paragraph also is about the fact that
- "for many years there have been terminals in use at Stanford, MIT, Carnegie-Mellon, and elsewhere that have 120 or 121 symbols, not just 95."
Can somebody please bring order to confusion/chaos and explain precisely the way in which (unextended) ASCII and extended ASCII and the mentioned terminals that have 120 or 121 symbols, not just 95 correlate with that third table which deviates from (unextended) ASCII but like ASCII provides code-points only in range from 0(dec) to 127(dec).
In TeXbook, Appendix C you find the phrase "extended ASCII code".
If you spell out the acronym, you get:
"extended American Standard Code for Information Interchange code".
That's weird and confusing. Isn't "extended ASCII" sufficient as the "C" in "ASCII" stands for "Code"?
\message) of symbols not covered by (unextended) ascii. And he indicates that this may decrease interchangeability of TeX-code between different computers and suggests designers of TeX macro packages that are intended to be widely used to stick to the standard ascii characters.
\otimeswas, as I remember, somewhere near the "O" or "P" and accessed with the "supershift". It was colloquially referred to as "splat", and was used in TeX78 as the "tab" marker in tables, which is now (in TeX82) indicated by the ampersand "&".