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
$99.00
Apple Inc.
+1.33
MSFT
$44.07
Microsoft Corpora
-0.43
GOOG
$589.79
Google Inc.
+0.77

MacTech Search:
Community Search:

Software Updates via MacUpdate

Acorn 4.4 - Bitmap image editor. (Demo)
Acorn is a new image editor built with one goal in mind - simplicity. Fast, easy, and fluid, Acorn provides the options you'll need without any overhead. Acorn feels right, and won't drain your bank... Read more
Bartender 1.2.20 - Organize your menu ba...
Bartender lets you organize your menu bar apps. Features: Lets you tidy your menu bar apps how you want. See your menu bar apps when you want. Hide the apps you need to run, but do not need to... Read more
TotalFinder 1.6.2 - Adds tabs, hotkeys,...
TotalFinder is a universally acclaimed navigational companion for your Mac. Enhance your Mac's Finder with features so smart and convenient, you won't believe you ever lived without them. Tab-based... Read more
Vienna 3.0.0 RC 2 :be5265e: - RSS and At...
Vienna is a freeware and Open-Source RSS/Atom newsreader with article storage and management via a SQLite database, written in Objective-C and Cocoa, for the OS X operating system. It provides... Read more
VLC Media Player 2.1.5 - Popular multime...
VLC Media Player is a highly portable multimedia player for various audio and video formats (MPEG-1, MPEG-2, MPEG-4, DivX, MP3, OGG, ...) as well as DVDs, VCDs, and various streaming protocols. It... Read more
Default Folder X 4.6.7 - Enhances Open a...
Default Folder X attaches a toolbar to the right side of the Open and Save dialogs in any OS X-native application. The toolbar gives you fast access to various folders and commands. You just click... Read more
TinkerTool 5.3 - Expanded preference set...
TinkerTool is an application that gives you access to additional preference settings Apple has built into Mac OS X. This allows to activate hidden features in the operating system and in some of the... Read more
Audio Hijack Pro 2.11.0 - Record and enh...
Audio Hijack Pro drastically changes the way you use audio on your computer, giving you the freedom to listen to audio when you want and how you want. Record and enhance any audio with Audio Hijack... Read more
Intermission 1.1.1 - Pause and rewind li...
Intermission allows you to pause and rewind live audio from any application on your Mac. Intermission will buffer up to 3 hours of audio, allowing users to skip through any assortment of audio... Read more
Autopano Giga 3.6 - Stitch multiple imag...
Autopano Giga allows you to stitch 2, 20, or 2,000 images. Version 3.0 integrates impressive new features that will definitely make you adopt Autopano Pro or Autopano Giga: Choose between 9... Read more

Latest Forum Discussions

See All

Traps n’ Gemstones Review
Traps n’ Gemstones Review By Campbell Bird on July 28th, 2014 Our Rating: :: CASTLEVANIA JONESUniversal App - Designed for iPhone and iPad Fight mummies, dig tunnels, and ride a runaway minecart to discover ancient secrets in this... | Read more »
The Phantom PI Mission Apparition Review
The Phantom PI Mission Apparition Review By Jordan Minor on July 28th, 2014 Our Rating: :: GHOSTS BUSTEDUniversal App - Designed for iPhone and iPad The Phantom PI is an exceedingly clever and well-crafted adventure game.   | Read more »
More Stubies Are Coming Your Way in a Ne...
More Stubies Are Coming Your Way in a New Update Posted by Jessica Fisher on July 28th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
The Great Prank War Review
The Great Prank War Review By Nadia Oxford on July 28th, 2014 Our Rating: :: PRANKING IS SERIOUS BUSINESSUniversal App - Designed for iPhone and iPad Though short, The Great Prank War offers an interesting and fun mix of action and... | Read more »
Marvel Contest of Champions Announced at...
Marvel Contest of Champions Announced at Comic-Con Posted by Jennifer Allen on July 28th, 2014 [ permalink ] Announced over the weekend at San Diego Comic-Con was the fairly exciting looking Marvel Contest of Champions. | Read more »
Teenage Mutant Ninja Turtles Review
Teenage Mutant Ninja Turtles Review By Jennifer Allen on July 28th, 2014 Our Rating: :: DULL SWIPINGUniversal App - Designed for iPhone and iPad The pizza power is weak when it comes to this Teenage Mutant Ninja Turtles game.   | Read more »
Exploration Focused Puzzle Game Beatbudd...
Exploration Focused Puzzle Game Beatbuddy Set to Make Transition from PC to iOS this September Posted by Jennifer Allen on July 28th, 2014 [ permalink ] | Read more »
PlanetHD
PlanetHD By Nadia Oxford on July 28th, 2014 Our Rating: :: SPACE MADNESSUniversal App - Designed for iPhone and iPad PlanetHD will keep players busy for a while, though its unpredictable physics are a handful to deal with.   | Read more »
This Week at 148Apps: July 21-25, 2014
Another Week of Expert App Reviews   At 148Apps, we help you sort through the great ocean of apps to find the ones we think you’ll like and the ones you’ll need. Our top picks become Editor’s Choice, our stamp of approval for apps with that little... | Read more »
Reddme for iPhone - The Reddit Client (...
Reddme for iPhone - The Reddit Client 1.0 Device: iOS iPhone Category: News Price: $.99, Version: 1.0 (iTunes) Description: Reddme for iPhone is an iOS 7-optimized Reddit client that offers a refreshing new way to experience Reddit... | Read more »

Price Scanner via MacPrices.net

13-inch 2.5GHz MacBook Pro on sale for $1099,...
Best Buy has the 13″ 2.5GHz MacBook Pro available for $1099.99 on their online store. Choose free shipping or free instant local store pickup (if available). Their price is $100 off MSRP. Price is... Read more
Roundup of Apple refurbished MacBook Pros, th...
The Apple Store has Apple Certified Refurbished 13″ and 15″ MacBook Pros available for up to $400 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free. Their prices... Read more
Record Mac Shipments In Q2/14 Confound Analys...
A Seeking Alpha Trefis commentary notes that Apple’s fiscal Q3 2014 results released July 22, beat market predictions on earnings, although revenues were slightly lower than anticipated. Apple’s Mac’... Read more
Intel To Launch Core M Silicon For Use In Not...
Digitimes’ Monica Chen and Joseph Tsai, report that Intel will launch 14nm-based Core M series processors specifically for use in fanless notebook/tablet 2-in-1 models in Q4 2014, with many models to... Read more
Apple’s 2014 Back to School promotion: $100 g...
 Apple’s 2014 Back to School promotion includes a free $100 App Store Gift Card with the purchase of any new Mac (Mac mini excluded), or a $50 Gift Card with the purchase of an iPad or iPhone,... Read more
iMacs on sale for $150 off MSRP, $250 off for...
Best Buy has iMacs on sale for up to $160 off MSRP for a limited time. Choose free home shipping or free instant local store pickup (if available). Prices are valid for online orders only, in-store... Read more
Mac minis on sale for $100 off MSRP, starting...
Best Buy has Mac minis on sale for $100 off MSRP. Choose free shipping or free instant local store pickup. Prices are for online orders only, in-store prices may vary: 2.5GHz Mac mini: $499.99 2.3GHz... Read more
Global Tablet Market Grows 11% in Q2/14 Notwi...
Worldwide tablet sales grew 11.0 percent year over year in the second quarter of 2014, with shipments reaching 49.3 million units according to preliminary data from the International Data Corporation... Read more
New iPhone 6 Models to Have Staggered Release...
Digitimes’ Cage Chao and Steve Shen report that according to unnamed sources in Apple’s upstream iPhone supply chain, the new 5.5-inch iPhone will be released several months later than the new 4.7-... Read more
New iOS App Helps People Feel Good About thei...
Mobile shoppers looking for big savings at their favorite stores can turn to the Goodshop app, a new iOS app with the latest coupons and deals at more than 5,000 online stores. In addition to being a... Read more

Jobs Board

*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
Sr. Product Leader, *Apple* Store Apps - Ap...
**Job Summary** Imagine what you could do here. At Apple , great ideas have a way of becoming great products, services, and customer experiences very quickly. Bring Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
WW Sales Program Manager, *Apple* Online St...
**Job Summary** Imagine what you could do here. At Apple , great ideas have a way of becoming great products, services, and customer experiences very quickly. Bring Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.