% -*-LaTeX-*-
% <BEEBE.EPSILON>TECOQREG.LTX.8, 28-May-86 16:33:25, Edit by BEEBE
% This is part of TECO.LTX

  In order to support local \index{local variables} and \X{global
variables}, \TECO{} provides named objects called {\em
Q-registers}.  These have very simple names which allow the
interpreter to find them by fast table lookup, rather than by
having to collect a name string and then search a symbol table
for it.  The names follow one of the functions
 \begin{flushleft}
  \begin{tabular}{ll}
\TX{G} & get Q-register contents into buffer\\
\TX{:I} & store string value into Q-register\\
\TX{M} & execute \TECO{} code in Q-register\\
\TX{Q} & get numeric value from Q-register\\
\TX{U} & store numeric value into Q-register\\
\TX{X} & store buffer text into Q-register
  \end{tabular}
 \end{flushleft}
and are a letter or digit preceded by zero, one,
or two periods.  Letter case does not matter; \T{QA} is the same
as \T{qa}.   Examples of legal Q-register references
are \T{Q0}, \T{Q.0}, \T{Q..0}, and \T{QZ}.  Some {\em illegal}
references  are \T{Q+}, \T{Q...}, and \T{Q*}.

The \X{dotted Q-registers} were not present in older \TECO{}'s, but
were added to \ETECO{} to provide for commonly-used \X{global
variables} which must be rapidly accessible.  Their values are
normally not changed by \TECO{} programmers if disaster is to be
avoided.  For example, \ETECO{}'s initialization function is
stored in \TX{..L}, and executing it re-initializes the
interpreter; if it does not contain valid code, \EMACS{} may
crash.  In this \TECO{}, the following Q-registers are currently
predefined:
    \begin{flushleft}
      \begin{tabular}{ll}
	\TX{..L} & length of last string inserted, or found by search\\
	\TX{..R} & last regular-expression search string\\
	\TX{..S} & last search string\\
	\TX{..T} & temporary storage for coercions\\
	\TX{..W} & argument flags\\
	\TX{..X} & pre-comma argument\\
	\TX{..Y} & post-comma argument\\
      \end{tabular}
    \end{flushleft}
They may be read, but should not be modified.
\typeout{Remove N from TECO, implement F^XF^Y}

Only the undotted Q-registers should be used for local variables.
Any which are used by a callable function must be preserved by
that function, which is easily achieved by pushing them onto the
Q-register stack, and restoring them on return.  The function
return mechanism will restore them automatically.

A Q-register can contain either a string, or a number, and a
type code stored with it tells which of the two it is.  All
Q-registers are initialized to numeric values of 0 when \TECO{}
starts up.  To allow a program to distinguish between the two
possibilities, the function \T{FQq} returns -1 if Q-register
\T{q} is a number, and otherwise, it returns the \X{string length}.
Thus, if we execute \T{164Uq}, then \T{FQq} returns -1, while
\T{:Iqfoo\A{ESC}} followed by \T{FQq} returns 3.