[tex-k] Does TeX's Input Processor Tokenize The Entire Input File First?

[Joseph Wright]

On 17/04/2019 04:25, Jon Forrest wrote:
> I'm trying to understand TeX internals.
> 
> One thing I haven't found a definitive explanation of is whether the
> input processor tokenizes the entire input file before the execution
> processor gets the token list. TexbyTopic says "... all [processor]
> levels are simultaneously  active, and there is interaction between
> them.". Given the primitive  computing environment TeX was developed in,
> it's hard to imagine how the  4 processors could run simultaneously. As
> far as I can see, TeX has no multi-threading ability so it would be up
> to TeX to "process" switch.
> 
> So, how and why do the processors run simultaneously? How much of the
> input file gets tokenized before the token list is handed to the
> expansion processor?
> 
> I wonder if Knuth were writing TeX today would he use multi-threading?
> 
> Cordially,
> Jon Forrest
> 

TeX tokenizes one item at a time. That's seen of course in the fact that 
one can change catcodes during a document

     \catcode`~=13 %
     \def~{Hello}%
     \catcode`~=12 %
     ~ world
     \bye

As such, the different stages of TeX processing cannot be run in parallel.

Joseph