Personal Computer World July 1983 page 198.
IBM COMES FORTH
An exclusive review of PolyForth as presented by Dick Olney
As the popularity of Forth among micro users increases so, too, does the number
of compilers available. The problem is that either they are cassette-based or
they run under one of the standard operating systems, both of which situations
disguise the language's full potential.
PolyForth, from Forth Inc of California,
is unique in providing a standalone multi-tasking operating system and an
overwhelming selection of extensions and utilities for a mere £250. It was
designed to run on an IBM PC with at least one single-sided disk drive. A
conversion utility is included which allowed me quickly to reconfigure the
software and make full use of the dual 320k drives installed in the machine I
The PC automatically boots up
the PolyForth when it's switched on,
whereupon a title and date appear in the top left-hand corner of the screen
together with the word 'HI'. At this point the Forth 'nucleus', that is, those
words which are pre compiled on the system disk, is already in place and most of
the major features have been loaded (like any good Forth all the non-standard
extensions are supplied as source). Typing 'HI' at this stage loads a further
selection of utilities and displays a help screen containing brief details of a
selection of useful commands.
MS-DOS users might be rather disturbed at this
point to find the cursor and edit controls on the numeric pad completely
disenabled (the keys always echo their number value). This is because these
functions - like the rest of the keyboard - are dealt with in software. In fact
the Forth has a keyboard interpretation chart residing in block 0 of the system
disk. The version I used was still configured to a US keyboard. The "
symbols were reversed and the £ returned a #. Comsol, the European distributor
of this product, is working on a UK conversion utility to be included in the
package. It is possible to use all of the special keys but you will need to
develop your own routines to handle their functions. The ten keys labelled F1
to F10 can be attached to a word simply by naming it with the legend on the key
as in: F1 ." This is Function F1"; which will cause the embedded text to print
out whenever the F1 key is depressed. For other keys, however, such as CTRL and
ALT you will need to test for ASCII values in a special input routine.
system includes most standard Forth words and much more besides. One omission
which might surprise Fig Forth users is VLIST (giving a listing of the
dictionary). It's easy to test for the existence of a word and the command LOCATE will automatically display the source
of any word loaded from disk not contained in the object nucleus - an invaluable
debugging aid. A full selection of 32-bit math's operators is included as well as
an 8088 assembler - written in Forth - and a standard line editor.
As is usual the assembler and editor words are held in discrete vocabularies which must be
declared before they are used. A further 5 user vocabularies can be added. The
system keeps track of these using an array called GOLDEN which contains the
start (or end, depending on how you look at it) of the dictionary lists. At any
one time up to four vocabularies can be current, with the compiler following a
pre-specified search sequence determined by the contents of the variable
CURRENT. Used in conjunction with the turnkey compiler (see below) this enables
the programmer to create a self-loading application where the user can only
access an appropriate selection of high level commands.
A copy of the GOLDEN
array, called CONTEXT, has its link addresses updated as new words are added to
the various vocabularies. The word EMPTY copies the contents of GOLDEN into
CONTEXT, thereby erasing from the dictionary any entries compiled since its
previous execution. The source for this reads GOLDEN CONTEXT 20 MOVE. The
reversed phrase CONTEXT GOLDEN 20 MOVE re-initialises GOLDEN thus protecting all
currently compiled words from the use of EMPTY. In this way applications can be
efficiently re-compiled during development and protected after successful
debugging using a technique which is much more controlled than the traditional
All dates in the system, including today's, are held as 16-bit numbers
described as 'modified Julian dates' giving the number of days which have
elapsed since 1 January, 1900. They use a compact representation which allows
arithmetic to be performed on the date directly. Two formats are provided for
input and output of which examples are 14 MAY 1983 or 5/ 14/83. Little effort
has been put into conversion for the British market though it would be no
problem to develop your own routines using the existing low level words.
Multi-tasking is probably the most exciting feature of PolyForth not least
because of its ease of use. It relies upon a tiny (25 bytes) core routine
called PAUSE. When executed in a program this word passes control to an idle
loop which runs around a circular chain of addresses called a 'round robin'
searching for the next task ready to be activated. New background tasks are
created using three programmer-supplied values. The first of these represents
the 'user area' in which key variables may be protected from other tasks. Such
variables are defined in the format:
offset USER name
where the offset is added to the task's base address to give the address
of the variable. Each task maintains its own parameter and return stack, and
the other two values represent the byte sizes of these. To define a
background task the command is:
user parameter return BACKGROUND name
which constructs a table to be used when the task is initialised with
the word BUILD, whereupon it is linked into the round robin.
Once a task has been defined and created it can be activated by any
program using the following convention: word task-name ACTIVATE (words
executed by task);
There are, however a number of vital
restrictions of which the
programmer must be aware if this is to be successful. First a background
task cannot perform terminal I/O and is thus unable to issue error
messages, so if anything goes wrong the whole system will crash. A
similar effect occurs if the task is allowed to reach the semi-colon,
and it must therefore be explicitly terminated with the word STOP.
The reason that a background task cannot
perform terminal I/O is that
such a potential demands the ability to vector to unique,routines when
executing words like TYPE and CR. It is possible to overcome this by
moving text directly into screen memory, but you will need to develop your
own routines in assembler to access memory above 64k. The two
predefined tasks called OPERATOR and TYPIST are, of course, capable of
terminal I/O since they control screen and printer respectively. This
means that screen activity remains relatively unaffected when the printer is
Although the multi-tasking facility is simple and effective much of its
operation depends on the programmer. Remember that in reality only one task
is actually active at any one time, and it is therefore essential in both
foreground and background tasks
Personal Computer World July 1983 page 201.
IBM COMES FORTH. . .
ground tasks to use PAUSE in a fashion most
appropriate to the application. In
addition the use of any shared resources, such as the disk buffers, must be
carefully monitored and, if necessary, controlled using special routines. Note
that, once built into the round robin task, definitions cannot be erased from
the memory by EMPTY or FORGET and any attempt to do so has dire
Despite the vulnerability of
the PolyForth multi-tasking, it does give this
system an immediate lead over most of the more conventional operating systems
currently available. Used properly with careful and extensive debugging it
could provide the basis for many impressive applications.
The initialisation of diskettes and the manipulation of data on them is
fundamental to any disk-based operating system.
In this system these functions
are grouped in a utility dubbed 'Disking'. All
the standard commands are provided including initialisation, backup, and block
copying as well as disk diagnostics. One command that is particularly useful
is MATCHES, which is not used to activate an optional cigarette lighter, but
to compare the contents of two specified ranges of blocks and report any
The most common criticism of Forth is that the source is
unreadable to anyone but the originator. Forth Inc has attempted to
counter this with a facility called 'shadow documentation'. Two disks are
supplied with the package, the system disk which resides in drive 0, and a
documentation disk for use in drive 1. The documentation or shadow disk
contains brief descriptions of all the words existing as source on the
system disk. To access the shadow blocks you must first LIST the relevant
source: at this point the command Q will toggle the display between the
source and its associated documentation. This idea is that shadow
documentation is written at the same time as the source to assist in any future
modification or debugging.
The printing utility makes use of the shadow documentation by allowing
double-sided printing. With careful positioning of the paper the source and
shadow blocks can be printed so as to appear opposite each other when filed
in an A4 binder. Alternatively, source without shadow documentation can
be laid out with alternate pages on different sides of the paper to give a
similar effect. Printing is initialised by preceding any valid words with the
command PRINT. This literally sends the rest of the input stream to the TYPIST
terminal task. Because of its mode of operation, however, this word cannot be
used inside colon definitions, where TYPIST ACTIVATE performs a similar
function. The system gives full support for the standard IBM (Epson)
parallel printer, though you need to write your own
control functions using EMIT
and the ASCII values given in the IBM Guide to Operations
To complete the image of this
package as a powerful software development tool, a
number of additional support features have been included. One of the most
exciting of these, especially bearing in mind the multi-tasking capability, is
the colour graphics option. Unfortunately, I do not have access to a colour
display so I was unable to use this facility, though I did see a demonstration
on one of IBM's machines which drew some fairly convincing square spirals. The
option allows basic point plotting in medium-resolution (200 x 320) colour mode.
From the manual I would guess it's fairly primitive but, like the rest of this
system, I'm sure it could be extended into a powerful vocabulary.
Another option is the screen
editor, which uses all the IBM function keys, and
which I used to write this review. It's better than using my old typewriter but
lines do not run on, neither do blocks, so it's hardly a word processor.
Nevertheless it serves its purpose and perusal of the source reveals an
interesting definition of a word named :K which reads: :K (k) : LAST @ @ CFA
2+ SWAP 'FUNCTION ;
within this definition means that upon execution :K will itself cause a new word
to be compiled from the input stream, and it is at this time that the colon is
presented with a matching semi-colon. When :K encounters the latter compilation
ceases and the rest of the code is executed. The technique is used here to place
the execution address of a colon definition into a function table at the time
it is compiled, thereby facilitating vectored execution. The word :K expects the
offset in the table for the new word to be on the stack. An example of its use
is: 71 :K HOME 0 C# ! 0 L# !:
where HOME places the cursor at the first position on a block by zeroing the
column and line counts.
Although IBM apparently does not support it, there is a spare socket on the
system board for an 8087 math's processor, and the PolyForth manual gives full
instructions for installation and details of an extensive supporting
vocabulary. The conditions of sale from IBM, however, suggest that such a
modification would render the warranty invalid, so I resisted the temptation
to test out this option.
If applications are to be written in Forth, then sooner or later they are
bound to be presented with files created under PC DOS, and the next option on
the list is PC DOS file handler. All this really does is to take care of the
DOS sectoring and provide for basic file manipulation. Actually to access data
in applications you'd need to extend considerably the vocabulary. Still, the
basis of it is there and it would be fairly easy to impose your own data
structures over the DOS format.
Last, but not least, is the
turnkey compiler which allows
application disks to be created. This is achieved by first EMPTYing
the dictionary then loading up your applications, followed by the turnkey utility. At this point a controlling application word to be executed on
entry can be specified together with any blocks to be loaded from disk at that
time. The word PROGRAM then transfers a custom bootstrap dictionary to an
initialised diskette in drive 1 (instructions are also provided for generating
turnkey diskettes on single drive machines). Applications, using a startup
program in conjunction with sealed vocabularies or closed program loops, can
effectively deny a user access to the Forth itself, even though the core code is
resident in the application. If Forth Inc is convinced that you have done this
properly then the company won't claim for any copyright payments.
I timed the PolyForth using the Forth benchmark programs published in the
January issue of PCW. To do this I had to write an assembler routine to reset
the stack pointer, which luckily turned out to be very simple (my assembler is
limited to say the least!) and my version of SP! reads:
CODE (SP!) S POP NEXT :
SP! SO @ 2 - (SP!)
Having checked the reliability of the system clock I decided to let the
machine time itself and printed out the contents of the system variable
TICKS at the beginning and end of the multiplier routine. Remember that the
timing for this word is subtracted from the other values to give the true
figure so it doesn't matter that my version takes slightly longer.
The results are the fastest recorded Forth benchmarks, particularly the
arithmetic program which this system runs four times faster than its
nearest competitor. An additional test of my own, using double-number and
mixed operators, ran almost as fast as the original arithmetic program. There's
no doubt that if it's speed you're after then this is the package for you.
(For 'mixed maths' benchmarks, see below.)
Mixed math's benchmarks
2: mixed-maths .TICKS 10001 1 DO
3 900 900 M* 9 M/ 0 90000 D+
4 SP! LOOP .TICKS ;
Personal Computer World July 1983 page 204.
IBM COMES FORTH...
Very sensibly Forth Inc has used the superb Starting Forth as one the manuals
for its PolyForth. This book has become the Forth programmer's bible, providing
the beginner with a detailed overview of the language and many useful examples.
The other manual supplied with the package, held in a solid foam-backed binder,
contains general details of specific features, though really to understand how
everything works you will need to read the shadow documentation as well. Everything
is covered and the manual includes an excellent index and glossary, as well as
full source and shadow listings.
Level 2 PolyForth is at the bottom end of a range
of three products. Level 3, which costs £490, in-
cludes a database system and support
for multi-terminal tasking and networking. It also includes a much more detailed manual. The most
expensive system, Level 4, adds a target compiler and carries full source listings of all words.
It costs £2750, but this buys you certain rights of reproduction as well. Unfortunately, you
get no discount on Level 3 if you have already got a Level 2, though full discount is offered
when upgrading from Level 3 to Level 4. It is, however, possible to write most of the features
included in Level 3 with the facilities available in Level 2.
As you may have guessed, I was extremely impressed with this package. Forth remains (in this
country at least) relatively unexploited as a serious software development tool,
partly because many of the compilers available
many of the compilers available are simply not suitable for this purpose. Level 2 PolyForth
is a quality product aimed at the professional or semi-professional programmer (at a price
that should present no problem to anyone who can afford an IBM PC!). How successful it is in
competing with the mainstream languages and operating systems will depend to some extent on
the marketing, an area about which Comsol is still hazy, though it has been submitted to IBM
for approval. I do know that the company plans to go all out in the autumn, and is looking
for big sales to make the venture worthwhile.
Enquiries should be directed to: Computer Solutions Ltd,
Treway House, Han worth Lane, Chertsey, Surrey KT16 9LA.
Tel: Chertsey (00000) 000000.