12/24/82 -- New features of the PL/I Compiler

This info segment contains information on new PL/I features not documented in
the current PL/I Language Manual AG94-2.  The new features are:

(1)	wordno builtin function
	charno builtin function
	bitno builtin function
	segno builtin function
(2)	addbitno builtin function
	setbitno builtin function
	addcharno builtin function
	setcharno builtin function
	setwordno builtin function
	addwordno builtin function


New Pointer Manipulation Builtins:
The following paragraphs describe new builtins for manipulating pointers that
effectively replace the char_offset_ family of subroutines. These builtins,
like the subroutines, reflect a model of pointer values that is neither that
of the PL/I standard nor that of the Multics hardware. Users should read the
following section, which describes the model, before using the new facilities.


Multics PL/I model of pointers:
In PL/I, a pointer may point to an arbitrary element of a structure or
array. In the case of unaligned quantities, then, a pointer may point to a
location that is not aligned on a 36 bit word boundary.


In standard PL/I, no provisions are made for addressing some subportion of a
value. In particular, the only way standard PL/I allows a pointer to point to
a substring of a string is to use a defined string or array to overlay the
desired location.


The Multics PL/I pointer manipulation builtins allow the user to consider any
segment to be considered as a vector of either bits, characters, or words,
and to construct pointers that point to arbitrary elements of the vector.
There are three primitive operations: set, add, and extract. Set constructs a
pointer to an arbitrary element of a segment. Add takes a pointer and moves
it some distance up or down in a segment. Extract returns the position of the
pointer in the segment.


Note that these are relative to a segment, NOT to some pl1 datum within the
segment. The examples below should make this clear.


The setwordno, setcharno, and setbitno builtins:
A reference to one of these builtins takes the form:

    R = setcharno (P, O);

where P is a scalar pointer value, O is a value convertable to a fixed binary
quantity, and R is a pointer value. The result R is a pointer that points to 
char or bit O if the segment specified by P, where the first character is
numbered 0. Note that the offset of P is ignored; an expression like

   declare 1 s aligned,
           2 header aligned like p,
	 2 chars char (12);

     s_ptr = setcharno (addr (s.chars), 4);

Will NOT result in a pointer to the the 5th character in s.chars. The
expression should be:

   s_ptr = addcharno (addr (s.chars), 4);


Builtin functions addwordno, addcharno, and addbitno:
A reference to one of these functions takes the form:

   R = addcharno (P, O);

Where P is a scalar pointer value, O is convertable to a fixed binary
quantity, and R is a pointer. The result R is the pointer that results when P
is moved O characters, words, or bits. O may be negative. The result of the
addcharno and addwordno functions is undefined if P does not address a
character or word boundary, respectively.


The charno, bitno, wordno, and segno functions:
These builtins extract information from pointers, and are defined as follows:

     C = charno (P);

P must be a scalar pointer value. The result, C, is a real fixed binary of
precision (21, 0), containing the zero origined character address of P.

     B = bitno (P);

P must be a scalar pointer value. The result, B, is a real fixed binary of
precision (24, 0), containing the zero origined bit address of P.

     W = wordno (P);

P must be a scalar pointer value. The result, W, is a real fixed binary of
precision (18, 0), containing the zero origined word address of P.

     S = segno (P);

P must be a scalar pointer value. The result, S, is a real fixed binary of
precision (15, 0), containing the segment number of P.


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Historical Background

This edition of the Multics software materials and documentation is provided and donated
to Massachusetts Institute of Technology by Group BULL including BULL HN Information Systems Inc. 
as a contribution to computer science knowledge.  
This donation is made also to give evidence of the common contributions of Massachusetts Institute of Technology,
Bell Laboratories, General Electric, Honeywell Information Systems Inc., Honeywell BULL Inc., Groupe BULL
and BULL HN Information Systems Inc. to the development of this operating system. 
Multics development was initiated by Massachusetts Institute of Technology Project MAC (1963-1970),
renamed the MIT Laboratory for Computer Science and Artificial Intelligence in the mid 1970s, under the leadership
of Professor Fernando Jose Corbato. Users consider that Multics provided the best software architecture 
for managing computer hardware properly and for executing programs. Many subsequent operating systems 
incorporated Multics principles.
Multics was distributed in 1975 to 2000 by Group Bull in Europe , and in the U.S. by Bull HN Information Systems Inc., 
as successor in interest by change in name only to Honeywell Bull Inc. and Honeywell Information Systems Inc. .

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Permission to use, copy, modify, and distribute these programs and their documentation for any purpose and without
fee is hereby granted,provided that the below copyright notice and historical background appear in all copies
and that both the copyright notice and historical background and this permission notice appear in supporting
documentation, and that the names of MIT, HIS, BULL or BULL HN not be used in advertising or publicity pertaining
to distribution of the programs without specific prior written permission.
    Copyright 1972 by Massachusetts Institute of Technology and Honeywell Information Systems Inc.
    Copyright 2006 by BULL HN Information Systems Inc.
    Copyright 2006 by Bull SAS
    All Rights Reserved