Finding INITEX?

Question

In the file plain.tex, in many places there are references like

% When INITEX (the TeX initializer) starts up,
% it has defined the following \catcode values:
% \catcode`\^^@=9 % ascii null is ignored
...

and

% INITEX sets up \mathcode x=x, for x=0..255, except that
% \mathcode x=x+"7100, for x = `A to `Z and `a to `z;
% \mathcode x=x+"7000, for x = `0 to `9.

, for instance.

Where are these INITEX definitions located at?

Answer 1

INITEX is a slightly modified version of tex suitable for creating format definitions; as such, it starts with a very blank slate. Its starting state is described in the TeXbook, p.39:

There's a program called INITEX that is used to install TeX, starting from scratch; INITEX is like TeX except that it can do even more things....INITEX needs extra space to carry out such tasks [hyphenation tables, formats], so it generally has less memory available for typesetting....

When INITEX begins, it knows nothing but TeX's primitives. All 256 characters are initially of category 12, except that <return> has category 5, <space> has category 10, <null> has category 9, <delete> has category 15, the 52 letters A...Z and a...z have category 11, % and \ have the respective categories 14 and 0.

So the answer to your question is that these definitions are made in the TeX source code; I do not have TeX: the program at hand right now, so I can't give you a line number.

These days, there is may be no actual program called initex on your computer; instead, as the TeX FAQ explains, it is incorporated into tex itself via command-line options.