TweetFollow Us on Twitter

Forth Structures
Volume Number:2
Issue Number:9
Column Tag:Threaded Code

Adding Record Structures to Forth

By Jörg Langowski, EMBL, c/o I.L.L., Grenoble, Cedex, France, MacTutor Editorial Board

Records with local field names

Data representation is a field that is neglected by many Forth dialects. Basic Forth-83 doesn't even provide for simple one and two dimensional matrices, neither are more complex types of data supported, such as Pascal records or C structs. These latter forms of data representation play a most important role in Toolbox programming, since very many traps expect pointers to records as parameters.

A letter received through BITNET from a reader who was wondering how to install a way to handle such data structures in Forth got me started on this month's column:

"I posted the following article to the USENET, but got little in the way of a response. Any help you can give will be much appreciated. By the way, I know that the rectangle definitions given below are inaccurate for the Mac, but I was trying to be machine independent in posting to the Forth language newsgroup.

From postnews Thu Jun 12 15:23:34 1986

Subject: Defining a structure in FORTH?

Newsgroups: net.lang.forth

Distribution: net

I am very much a novice FORTH programmer, and I don't even have a good textbook to go by. I recently purchased a FORTH for my Macintosh at home (MACH1, distributed by the Palo Alto Shipping Co.), and would like some advice. Professionally I do a lot of work with LISP, and I would like to implement something similar to a `DEFSTRUCT' package in FORTH. In other words, I'd like to be able to do something like:

    DEFSTRUCT[ RECTANGLE
           TOP    2
           LEFT   2
           BOTTOM 2
           RIGHT  2 ]ENDSTRUCT

Which would automatically define the following:

    8 CONSTANT RECTANGLE-SIZE
    : RECTANGLE-TOP@ ( a - n ) @ ;
    : RECTANGLE-TOP! ( n a - ) ! ;
    : RECTANGLE-LEFT@ ( a - n ) 2 + @ ;
    : RECTANGLE-LEFT! ( n a - ) 2 + ! ;
    : RECTANGLE-BOTTOM@ ( a - n ) 4 + @ ;
    : RECTANGLE-BOTTOM! ( n a - ) 4 + ! ;
    : RECTANGLE-RIGHT@ ( a - n ) 6 + @ ;
    : RECTANGLE-RIGHT! ( n a - ) 6 + ! ;
    : MAKE-RECTANGLE ( whatever code
 necessary to allocate 8 bytes of variable storage and assign a dictionary 
entry to the word which follows.This I guess would be implementation 
specific. ) ;

While I'm sure that this could be done by defining 'DEFSTRUCT[' so that it constructs all of the necessary dictionary headers etc. at the bit and byte level, this would doubtless be complicated and not very portable. I wonder then, if there is a higher level method of defining such a beast? Any help (even "no that can't be done") would be appreciated."

--Bruce Florman florman@rand-unix.ARPA

Since I think the question put forward by Bruce Florman is of very general interest to Macintosh Forth programmers, I'll try to show a way how such data structures may be implemented in MacForth or Mach2.

Structures in MacForth (CSI method)

MacForth (Kernel 2.4) provides a simple and effective way to implement structure definitions. A structure definition is a way to assemble information about a data structure (the lengths of the various fields and the total length of the structure). Example:

structure testrec
 long: ^date
 long: ^time
 byte: ^flag
   20 string: ^description
structure.end 

defines the data structure testrec with four fields, date, time, flag, and description. testrec is not a defining word. When executed, it merely leaves on the stack the length of the structure that is going to be defined; this number can then be used to allot an appropriate number of bytes in the dictionary. So, creation of a testrec would be done like:

create myrec testrec allot

The words that are used to access the field, ^date, ^time, ^flag, and ^description, simply add an offset to the number on top of stack. If this number is the address of a valid structure, like myrec,

myrec  ^flag

would indeed yield the address of the flag field in myrec. [Note that the circumflex in front of the field names is purely a MacForth convention, you could name the fields as you like].

This solution is beautifully simple and helps very much improving the readability of your program text if you are working with lots of structured data types. There is one drawback, however, that the field name definitions are global to the program and therefore violate the conventional definition of a Pascal record, in which field names are always local to the structure.

This means you have to exercise a lot of discipline when you work with structures defined in this way. On executing a field operator, it is not checked whether the address on top of stack is really the address of a structure, so bugs that leave unexpected values on the stack would be harder to detect. Furthermore, since all the field names are global, they may not occur in several different structure definitions in different contexts.

Therefore, I'd like to present an alternative to CSI's implementation of structures which uses local field names. This is slower during compilation, since every structure definition will have its own local dictionary that has to be searched, but in most cases has the same speed during execution. It offers the additional advantage that by a very simple modification, a rudimentary NEON-like class behavior may be built in.

Record definition with local field names

From now on, we'll call the type of data structure dealt with a record, to emphasize the similarity with Pascal records. A record definition will be a template from which an arbitrary number of instances of this record can be built (note that this already strongly resembles NEON's terminology). Each instance will consist of a reference to its template and the data fields as defined in the template (Fig. 1).

A record definition (Listing 1) then consists of:

- the word :record, which sets up a defining word for the instances and initializes the stack for the field name definitions following;

- field name definitions (>long, >word, etc.), which add names to the record template and store (after the name) the length of the data field and its position within the record;

- ;record, which closes the definition, stores a 16-bit zero and the total length of the record at the end of the template, and checks for completeness of the definition.

An example definition is given at the end of Listing 1.

Run-time behavior of records

The run-time behavior of a record template defined through :record is given by the word do.record. This word scans the list of field names in the record template and creates a new instance of the record with a pointer to the template in its first four bytes and space for the data fields following it.

The run-time behavior of the record instance is just to place its base address (the pointer to the template) on the stack. Access to the record fields is provided through ^field, which expects an address of a record instance and a string address on the stack. ^field will search the record template for the field name and leave the (absolute) field address on the stack or abort with an error message if the string does not match any field name in that particular record.

The operator ^ is provided for readability; executing

r1 ^ date

will give the same result as executing

r1 " date" ^field.

So far, we have only talked about execution time behavior of records. However, most of the times one would want to compile references to record fields into Forth definitions rather than execute them directly. For inclusion into Forth definitions, one way is to write

: test1 [ r1 ^ date ] literal ....... ;

which compiles the address of the date field of r1 into the definition as a literal. If a run-time reference to an arbitrary record is to be made, one can either write

: test2 ( record addr -- addr of date field )
  " date" ^field ;

which also checks at runtime for the validity of the date reference (something like 'late binding'), or, for faster execution, one writes

: test3 (record addr -- addr of date field )
  [ r1 dup ^ date - ] literal + ;

which assumes that the record address passed at run time refers to a record of the same type as r1. But in that case, CSI's structure definition is, of course, equivalent and easier to read.

From record to class definitions - using record fields as vectors

A simple, again very rudimentary, implementation of a NEON class like structure can be obtained using the record definition given here. If the data contained in a >long field (lets say with the field name print) is the cfa of a Forth word, writing

r1 ^ print @ execute (Mach2) or

r1 ^ print @ make.token execute
  (MacForth)

will execute the word that the print field of r1 points to. (In MacForth, one might also reserve a >word field and store a token there, then say r1 ^ print @ execute).

Vectors within records are very similar to methods associated with objects. Of course, method inheritance from superclasses has not been implemented here, so the resemblance to 'real' object oriented languages is not very strong.

Some extensions to Mach2 for MacForth compatibility

At the beginning of Listing 1 I have included some definitions for MacForth words that are not included in Mach2. Those are the words =cells, needed, and -string. The latter, a string comparison operator, has been implemented in two different ways; in both cases, the top two stack items are string addresses, and the flag returned is 0 if the strings are equal and 1 if not. =string uses the IUMagIDString routine from the international utilities package, which does a better job in comparing name strings that contain umlauts, diacritical marks etc., but is slower. -string uses the _Cmpstring trap, which is much faster and the recommended one to use for applications like this one.

MacForth Plus - no more Level 1,2,3

Readers of the CSI newsletter might have received an announcement of their latest update to MacForth by the time this is in print. Anyway, I'll tell you a few things about it that I was told in a letter at the time I wrote this (June).

• MacForth Plus, to be released at the end of August, will supersede all previous versions and levels of MacForth. Its version of the kernel will "...execute considerably faster than K2.4 It will execute faster than Mach1."

• Normal text file editing will be supported, as well as block file editing.

• Multitasking, which was undocumented, but in principle possible with MacForth, will be fully supported in MacForth Plus.

• The documentation will contain in one single manual the Level 1,2 and 3 informations, as well as the new features.

• Stand-alone applications can be produced by a built-in turnkey mechanism.

• The upgrade from Level 2 will be available for (scheduled) $49 upgrade fee, which includes the manual. Level 3 users will receive a free upgrade.

This sounds very interesting. I hope I'll soon have a test copy to write about.

Listing 1: record structures in Forth
( Structures, Mach-2 version                  
----------
Adding a structure compiler to Forth. JL 26.6.86.
This file defines a Pascal-like 'record' structure;
a record is a template for instances of the structure.
Example
:record a
    >long field1
    >word field2
;record

myrec r1 \this creates an instance r1 of myrec whose fields
          may be accessed through myrec ^ field1 etc..

for 'late binding' usage, the word ^field is provided)

only forth also assembler
decimal
( some MacForth definitions that Mach1 is missing )
: =cells dup 2 mod + ;
: needed depth 1- > abort" NEEDED- not enough stack items" ;

CODE =string 
      count rot count rot swap  
      MOVE.W    #0,-(A7)
      MOVE.L    $C(A6),-(A7)
      MOVE.L    $8(A6),-(A7)
      MOVE.W    $6(A6),-(A7)
      MOVE.W    $2(A6),-(A7)
      MOVE.W    #12,-(A7)
      _pack6 
      ADDQ.L    #8,A6
      ADDQ.L    #8,A6
      MOVE.W    (A7)+,-(A6)
      MOVE.W    #0,-(A6)
      RTS
END-CODE

CODE -string 
      count rot count swap  
      MOVE.L    (A6)+,A0
      MOVE.L    (A6)+,D0
      SWAP.W    D0
      MOVE.L    (A6)+,D1
      MOVE.W    D1,D0   
      MOVE.L    (A6)+,A1
      _cmpstring
      MOVE.L    D0,-(A6)
      RTS
END-CODE

( do.record, creating one instance of a record )  ( 062686 jl )
: do.record  ( addr of master | -- )
    create  dup ,
        begin dup c@ dup while ( not zero, i.e. end)
            1+ =cells 4 + + ( next field in template )
        repeat
    drop 2+ w@ ( length stored here ) allot
    does>  ( nothing special )
;

( :record ;record and friends)                    ( 062686 jl )

:  :record  create 13579 4 does> do.record ;

:  ;record  2 needed
    0 w, ( end of list) w, ( total length )
    13579 = 0= abort" ;record without :record"
;

:  put.fieldname
    32 word here over c@ 1+ dup =cells allot cmove ;


( field defining words )                          ( 062686 jl )
: >long ( addr | addr+4)
        put.fieldname dup w, 4 w, 4 + ;
: >word ( addr | addr+2)
        put.fieldname dup w, 2 w, 2+ ;
: >byte ( addr | addr+1)
        put.fieldname dup w, 1 w, 1+ ;
: >bytes ( addr \ n | addr+n )
        put.fieldname over w, dup w, + ;
 
( ^field, addressing a field within a record )    ( 062686 jl )
: ^field ( addr name | address of field )
    over @ ( addr name master )
        begin 2dup -string while ( no match )
            dup c@ 6 + =cells +
            dup c@ 0= ( end of list )
                abort" RECORD- specified field does not exist"
        repeat
    ( match found )
    dup c@ 1+ =cells + w@ ( start within record )
    swap drop   +  ( address of field )
;

( ^ )                                   ( 062686 jl )

: ^   32 word ^field ;

( example of a record structure )                 ( 062686 jl )
:record testrec
    >long date
    >long time
    >byte flag
    >word counts
 30 >bytes description
;record

testrec r1
testrec r2
 
AAPL
$100.96
Apple Inc.
-0.83
MSFT
$47.52
Microsoft Corpora
+0.84
GOOG
$596.08
Google Inc.
+6.81

MacTech Search:
Community Search:

Software Updates via MacUpdate

WhatRoute 1.13.0 - Geographically trace...
WhatRoute is designed to find the names of all the routers an IP packet passes through on its way from your Mac to a destination host. It also measures the round-trip time from your Mac to the... Read more
Chromium 37.0.2062.122 - Fast and stable...
Chromium is an open-source browser project that aims to build a safer, faster, and more stable way for all Internet users to experience the web. FreeSMUG-Free OpenSource Mac User Group build is... Read more
Attachment Tamer 3.1.14b9 - Take control...
Attachment Tamer gives you control over attachment handling in Apple Mail. It fixes the most annoying Apple Mail flaws, ensures compatibility with other email software, and allows you to set up how... Read more
Duplicate Annihilator 5.0 - Find and del...
Duplicate Annihilator takes on the time-consuming task of comparing the images in your iPhoto library using effective algorithms to make sure that no duplicate escapes. Duplicate Annihilator detects... Read more
jAlbum Pro 12.2 - Organize your digital...
jAlbum Pro has all the features you love in jAlbum, but comes with a commercial license. With jAlbum, you can create gorgeous custom photo galleries for the Web without writing a line of code!... Read more
jAlbum 12.2 - Create custom photo galler...
With jAlbum, you can create gorgeous custom photo galleries for the Web without writing a line of code! Beginner-friendly, with pro results Simply drag and drop photos into groups, choose a design... Read more
Quicken 2015 2.0.4 - Complete personal f...
Quicken 2015 helps you manage all your personal finances in one place, so you can see where you're spending and where you can save. Quicken automatically categorizes your financial transactions,... Read more
iMazing 1.0 - Complete iOS device manage...
iMazing (formerly DiskAid) is the ultimate iOS device manager with capabilities far beyond what iTunes offers. With iMazing and your iOS device (iPhone, iPad, or iPod), you can: Copy music to and... Read more
Xcode 6.0.1 - Integrated development env...
Apple Xcode is Apple Computer's integrated development environment (IDE) for OS X. The full Xcode package is free to ADC members and includes all the tools you need to create, debug, and optimize... Read more
Apple Safari 7.1 - Apple's Web brow...
Apple Safari in OS X Mavericks brings you all-new ways to find and enjoy the best of the web. It works with iCloud to give you a seamless browsing experience across all your devices. It looks out for... Read more

Latest Forum Discussions

See All

View Source – HTML, JavaScript and CSS...
View Source – HTML, JavaScript and CSS 1.0 Device: iOS Universal Category: Utilities Price: $.99, Version: 1.0 (iTunes) Description: View Source is an app plus an iOS 8 Safari extension that makes it easy to do one key web developer... | Read more »
Avenged Sevenfold’s Hail To The King: De...
Avenged Sevenfold’s Hail To The King: Deathbat is Coming to iOS on October 16th Posted by Jessica Fisher on September 19th, 2014 [ permalink ] Just in time for Halloween, on October 16 Avenged Sevenfold will be launching | Read more »
Talisman Has Gone Universal – Can Now be...
Talisman Has Gone Universal – Can Now be Played on the iPhone Posted by Jessica Fisher on September 19th, 2014 [ permalink ] | Read more »
Tap Army Review
Tap Army Review By Jennifer Allen on September 19th, 2014 Our Rating: :: SHOOT EM ALLUniversal App - Designed for iPhone and iPad Mindless but fun, Tap Army is a lane-based shooter that should help you relieve some stress.   | Read more »
Monsters! Volcanoes! Loot! Epic Island f...
Monsters! Volcanoes! Loot! | Read more »
Plunder Pirates: Tips, Tricks, Strategie...
Ahoy There, Seadogs: Interested in knowing our thoughts on all this plundering and pirating? Check out our Plunder Pirates Review! Have you just downloaded the rather enjoyable pirate-em-up Plunder Pirates and are in need of some assistance? Never... | Read more »
Goat Simulator Review
Goat Simulator Review By Lee Hamlet on September 19th, 2014 Our Rating: :: THE GRUFFEST OF BILLY GOATSUniversal App - Designed for iPhone and iPad Unleash chaos as a grumpy goat in this humorous but short-lived casual game.   | Read more »
A New and Improved Wunderlist is Here fo...
A New and Improved Wunderlist is Here for iOS 8 Posted by Jessica Fisher on September 19th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Evernote Update for iOS 8 Adds Web Clipp...
Evernote Update for iOS 8 Adds Web Clipping, Quick Notes, and More Posted by Ellis Spice on September 19th, 2014 [ permalink ] | Read more »
Apple Names Ultimate Productivity Bundl...
Apple Names Ultimate Productivity Bundle by Readdle as the Essential Bundle on the App Store Posted by Jessica Fisher on September 19th, 2014 [ permalink | Read more »

Price Scanner via MacPrices.net

Mac Pros available for up to $260 off MSRP
Adorama has Mac Pros on sale for up to $260 off MSRP. Shipping is free, and Adorama charges sales tax in NY & NJ only: - 4-core Mac Pro: $2839.99, $160 off MSRP - 6-core Mac Pro: $3739.99, $260... Read more
13-inch 2.6GHz/256GB Retina MacBook Pros avai...
B&H Photo has the 13″ 2.6GHz/256GB Retina MacBook Pro on sale for $1379 including free shipping plus NY sales tax only. Their price is $120 off MSRP. Read more
Previous-generation 15-inch 2.0GHz Retina Mac...
B&H Photo has leftover previous-generation 15″ 2.0GHz Retina MacBook Pros now available for $1599 including free shipping plus NY sales tax only. Their price is $400 off original MSRP. B&H... Read more
21″ 2.7GHz iMac available for $1179, save $12...
Adorama has 21″ 2.7GHz Hawell iMacs on sale for $1179.99 including free shipping. Their price is $120 off MSRP. NY and NJ sales tax only. Read more
iOS 8 Adoption Rate Slower than iOS 7, 6, Hit...
Apple began pushing out iOS 8 updates to eligible devices around 1pm ET on September 17, 2014. However, unlike with iOS 7, which boasted a wide variety of differences from its predecessor iOS 6, in... Read more
LIkely Final Definitive OS X 10.9.5 Mavericks...
Apple has released what will almost certainly be the last incremental version number update of OS X 10.9 Mavericks (save for futire security updates) before OS X 10.10 Yosemite is released next month... Read more
Fingerprints, Apple Pay and Identity Theft Wa...
On Sep 9th, CEO Tim Cook unveiled Apple Pay, along with the new iPhone 6 and iWatch. Apple Pay is a newly developed technology that utilizes a near field communication (NFC) to enable customer... Read more
Amazon Introduces Two All-New Kindles
Amazon on Thursday introduced the 7th generation of its Kindle dedicated e-reader device: Kindle Voyage, its top-of-the-line e-reader, and the new $79 Kindle, with a 20% faster processor, twice the... Read more
Save up to $300 on the price of a new Mac wit...
Purchase a new Mac or iPad at The Apple Store for Education and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free,... Read more
13-inch 2.8GHz Retina MacBook Pro available f...
B&H Photo has the new 2014 13″ 2.8GHz Retina MacBook Pro on sale for $1699.99 including free shipping plus NY sales tax only. They’ll also include free copies of Parallels Desktop and LoJack for... Read more

Jobs Board

Project Manager, *Apple* Financial Services...
**Job Summary** Apple Financial Services (AFS) offers consumers, businesses and educational institutions ways to finance Apple purchases. We work with national and Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.