Is PCL still portable?

[macgreg@vaxa.isi.edu]

Once upon a time PCL was intended to run on any version
of Common Lisp without modification, and ".LOW" files
were intended to be optional, used to speed up the implementation.
It appears that both the ".LOW" files and "FIN.LISP" have reached
a point where one can't run PCL on a new Common Lisp without
installing machine- or compiler-specific code.  Is this true?
If so, is it possible that some compiler-independent code could
be added to restore the portability?  If not, maybe the start-up
documentation could be augmented to indicate how to bring
up the portable version.

Having a portable version of the PCL code would make it
easier to bring PCL up on a new Lisp, and would provide a guide
indicating what functionality PCL expects of a compiler, and
where optimizations should be made.  Right now, the various
implementations collectively define an implicit guide, but
there appears to be no centralized place in the code indicating
what customizations are expected.