The Multics preprocessor was much more limited than other PL/I preprocessors, more similar to C's preprocessor.
Major features lacking were preprocessor labels, %GOTO, %DO, and preprocessor procedures.
The preprocessor was a standalone program not integhrated with the compiler.
Statements:
%INCLUDE <identifier>;
provides an expansion time include file feature.
%INCLUDE <quoted-string>;
provides an expansion time include file feature.
%abort <char_string>;
allows you to send messages to user_output at macro time and sets
the minimum value of pl1_macro_severity_ to four.
%default <identifier> to <constant-expression>;
is ignored if the identifier has been used in a parameter statement;
otherwise, causes the same substitution behavior as %replace.
%error <char_string>;
allows you to send messages to user_output at macro time and sets
the minimum value of pl1_macro_severity_ to three.
%if <constant-expression> %then <token-string>
where <token-string> is a possibly null string of tokens and the
<constant-expression>'s must evaluate to a bit_string constant.
[%else <token-string>] %endif
where <token-string> is a possibly null string of tokens.
[%elseif <constant-expression> %then <token-string>]...
where <token-string> is a possibly null string of tokens and the
<constant-expression>'s must evaluate to a bit_string constant.
%print <char_string>;
allows you to send messages to user_output at macro time and sets
the minimum value of pl1_macro_severity_ to zero.
%replace <identifier> by <constant-expression>
where <constant-expression> is an expression whose operands are
either constants or identifiers previously defined in other
statements or on the command line.
%set <identifier> to <constant-expression>
is like %replace in the way it deals with replacement activity and
constant expression evaluation, conflicts with parameters, etc.
%warn <char_string>;
allows you to send messages to user_output at macro time and sets
the minimum value of pl1_macro_severity_ to one.
Notes on skip and page macros:
These are features that the pl1 compiler accepts. The macro processor
checks them for syntactic correctness and passes the statement through.
Notes on expansion time include files:
%INCLUDE <identifier>;
%INCLUDE <quoted-string>;
provides an expansion time include file feature. Include files are
found through the translator search rules and have the same naming
conventions as compile time include files. You are permitted a maximum
of 255 include files in one expansion, and you can nest them 64 deep.
This differs from %include in that the macro processor merely checks to
see that the %include statement is syntactically correct and outputs
the statement.
Builtins:
%isarg (<char_string>)
is a macro builtin function that returns a value of data type bit(1).
%isarg (<pl1-token>)
is a macro builtin function that returns a value of data type bit(1).
%isdef (<identifier>)
is a macro builtin function that returns a value of data type bit(1).
%target (<identifier>)
is a replacement identifier of data type bit (1) whose value is
true only if the value of the <identifier> is equal to the value of
the identifier given as the argument of -target on the command
line.