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XPL -- an Early Example of Literary Programming

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XPL is documented in the book A Compiler Generator by McKeeman, Horning and Wortman, published by Prentice-Hall, 1970, ISBN 13-155077-2. The original compiler was designed for the IBM S/360. The language is XPL, a dialect of PL/I. The compiler is written in XPL.

XPL was one of the first high level languages that implemented automatic garbage collection (for strings only).

The book and the authors approach can be considered as the early example of literary programming but the term was coined much later by Donald Knuth.


Old News ;-)






95060 94305 94305


The Optimizing version of XPL was delevoped by:

R. A. Vowels, formerly of
Department of Computer Science
Royal Melbourne Institute of Technology,
124 La Trobe Street,

email address is now: [email protected]

This optimizing XCOM compiler is a standard implementation of XPL and is
based on McKeeman, Horning, and Wortman's improved XCOM (which employs
hashed symbol table generation). It includes the extra built-in

The following areas have been optimized: procedure calls when the
argument and corresponding parameter are of the same type, and when the
argument is a constant; constant subscripts; use of CORELHALFWORD and
COREWORD; string constants of length one; iterative DO statements by
transferring code to the end of the loop.

String constants of length one do not require a descriptor, hence more
descriptors are available for string variables. Comparison operations
are treated as commutative, and an improved Commute algorithm is used.
Halfword instructions are generated for BIT(16) variables.

These areas have been improved or re-written: calls on OUTPUT,
catenation, integer-to-string conversion, multiply, divide, and MOD. An
emitter for SS-type instructions has been added.

The compiler achieves an 11% reduction in object code compiling itself,
an 11% increase in compilation rate, a 55% increase in compilation speed
when the $ E toggle is set. Special treatment for catenating a string to
an integer substantially decreases consumption of the free string area,
and decreases string moves. The latter improvement is most noticeable
on small core machines.

Core requirements: less than the improved XCOM on which it is based
(approx. 98000 bytes). Symbol table size is 468.

ports: IBM System 370
portability: The compiler is written in XPL. The code generators are machine-specific.

This version distributed by Robin Vowels. Date: 11 November 1993. 11 December 1998.


Re: XPL Language

From comp.compilers

From: [email protected] (Steve Meyer)
Newsgroups: comp.compilers
Date: 27 Aug 2000 22:27:57 -0400
Organization: Compilers Central
References: 00-06-118 00-07-016 00-07-075 00-08-018 00-08-028 00-08-055 00-08-083
Keywords: history

I am not sure it makes sense to continue this historial discussion,
but I think there is a lot more to story. The roots of modern
computing lie in this story. For example, although both PL360 and XPL
were available why did Professor Knuth use assembler in his Art of
Programming books? Also, these original languages (and the Bell Labs
counter-parts) arose in Academic (School of Literate and Science)
computer science departments, but now computing is studied in EE

On 13 Aug 2000 19:10:55 -0400, Duane Sand <[email protected]> wrote:
>Steve Meyer wrote in message 00-08-055...
>>>>>: Peter Flass <[email protected]> wrote:
>>>>>: > XPL, developed in the 1970's was one of the earliest "compiler
>>>>>: > compilers", was widely ported, and was the basis for a number of
>>>>>: > languages such as the PL/M family.
>>I think PL/M and XPL came from different worlds that did not
>>communicate. I think people saw XPL as too high level. I think PL/M
>>came from other system level languages such as PL/360 (?). My
>>recollection may not be right.
>Niklaus Wirth developed PL360 as an alternative to writing IBM360
>assembly code directly. It was a quick one-person project. The
>parser used "operator precedence' techniques which predated practical
>LR methods. The tables could be worked out by hand in no time but the
>method couldn't handle BNFs of most languages. It was quite low
>level, mapping infix syntactic forms directly to single 360

Parsing may have gotten tenure for lots of professors but the most
advanced programming language areas such as HDLs (hardware
descriptions languages) now use the "predated" operator precedence
methods. Also Professor Wirth's languages have remained at the
for-front of academic programming languages.

>instructions without any optimizations. The PL360 paper inspired lots
>of people to develop their own small languages.

I think PL360 was very popular within IBM and among the back then
"modernist" movement away from assembly language. I know it was very
popular at SLAC.

>McKeeman etc developed XPL on 360 as a tidy subset of PL/I that could
>be implemented by a few people and be useful in coding biggish things,
>including the compiler and parser generator. The parser was initially
>based on their extensions to operator precedence, which relaxed BNF
>restrictions but required use of a parser generator tool and was still
>limited compared to LR. XPL was "high level" only in having built-in
>a varying-length string data type supported by a garbage collector.
>There were no struct types.

I think it was hard back then to differentiate XPL from Mckeeman's
advocacy of Burroughs B5500 style stack machine research program.

>Univ of Washington ported XPL onto SDS/Xerox systems that were like
>360 but with one instruction format.
>UW graduate Gary Kildall developed Intel's first programming tools for
>the 8008 and 8080, in trade for a very early portable computer: an
>8008 without keyboard or monitor, installed in a briefcase. Kildall
>used these (plus a floppy drive adapted by UW grad John Torode) to
>develop CP/M, the precursor to MS DOS. The Intel tools included an
>assembler and PL/M, both coded in Fortran. PL/M was inspired by the
>example of PL360 and the implementation methods of XPL. Kildall left
>before UW's XPL project but was likely very aware of it.
>PL/M's level was limited by the 8008's near inability to support proc
>calls. The first micro language to see significant use was Basic,
>implemented by assembler-coded interpreters. Implementing real
>applications in real compiled languages required later chips with
>nicer instruction sets, eg 8088 (gag) and M6800.

As the Z80 showed, 8088 was only one index register away from being
real computer.

Historical question I think is why there was so little communciation
between the current most popular BCPL, B, C, C++ research program and
the Stanford/Silicon Valley research program.

Just my two cents.
Steve Meyer Phone: (415) 296-7017
Pragmatic C Software Corp. Fax: (415) 296-0946
220 Montgomery St., Suite 925 email: [email protected]
San Francisco, CA 94104

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The XPL Programming Language

Dave Bodenstab's Home PageThe XPL compiler and supporting programs, ported to FreeBSD on an Intel 486

XPL is documented in the book A Compiler Generator by McKeeman, Horning and Wortman, published by Prentice-Hall, 1970, ISBN 13-155077-2. The original compiler was designed for the IBM S/360. The language is XPL, a dialect of PL/I. The compiler is written in XPL.

I picked up this package from the SHARE organization back in the mid 70's. It was squirreled away on 9-track tape until 1984 when I dug it out and wrote the S/360 emulator. Then it was stored on floppies until I had some time and decided to finally port it to the Intel 486. This was the first compiler source that I was exposed to and fueled my interest in compilers and operating systems. I guess that's why I dug it out and finished this little porting effort.

The latest version of XPL for FreeBSD is 4.2 -- click here to download.

This package builds the XPL compiler by bootstrapping from the original IBM S/370 binary. The build process requires GNU make and the RCS source control tools. If you have trouble building the Intel binary for the compiler, you can get a pre-built copy here. You will still need to compile the sub-monitor, xplsm, for your operating system.



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