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:

           TOP    2
           LEFT   2
           BOTTOM 2
           RIGHT  2 ]ENDSTRUCT

Which would automatically define the following:

    : 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

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

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.
:record a
    >long field1
    >word field2

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
( 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)
      ADDQ.L    #8,A6
      ADDQ.L    #8,A6
      MOVE.W    (A7)+,-(A6)
      MOVE.W    #0,-(A6)

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
      MOVE.L    D0,-(A6)

( 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 )
    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"
    ( 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

testrec r1
testrec r2

Community Search:
MacTech Search:

Software Updates via MacUpdate

Garmin Express - Manage your Gar...
Garmin Express is your essential tool for managing your Garmin devices. Update maps, golf courses and device software. You can even register your device. Update maps Update software Register your... Read more
Duet - Use your iPad as an exter...
Duet is the first app that allows you to use your iDevice as an extra display for your Mac using the Lightning or 30-pin cable. Note: This app requires a $14.99 iOS companion app. Version Read more
Apple iTunes 12.5.2 - Play Apple Music a...
Apple iTunes lets you organize and stream Apple Music, download and watch video and listen to Podcasts. It can automatically download new music, app, and book purchases across all your devices and... Read more
Pinegrow Web Designer 2.95 - Mockup and...
Pinegrow Web Designer is desktop app that lets you mockup and design webpages faster with multi-page editing, CSS and LESS styling, and smart components for Bootstrap, Foundation, Angular JS, and... Read more
jAlbum Pro 13.6 - Organize your digital...
jAlbum Pro has all the features you love in jAlbum, but comes with a commercial license. You can create gorgeous custom photo galleries for the Web without writing a line of code! Beginner-friendly... Read more
jAlbum 13.6 - 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
TextMate 2.0-beta.12.26 - Code/markup ed...
TextMate is a versatile plain text editor with a unique and innovative feature set which caused it to win an Apple Design Award for Best Mac OS X Developer Tool in August 2006. A rapidly growing... Read more
VMware Fusion 8.5.1 - Run Windows apps a...
VMware Fusion 8 and Fusion 8 Pro--the latest versions of its virtualization software for running Windows on a Mac without rebooting--include full support for Windows 10, OS X El Capitan, and the... Read more
Apple Final Cut Pro X 10.3 - Professiona...
Apple Final Cut Pro X is a professional video editing solution.Completely redesigned from the ground up, Final Cut Pro adds extraordinary speed, quality, and flexibility to every part of the post-... Read more
Civilization VI 1.0.0 - Next iteration o...
Sid Meier’s Civilization VI is the next entry in the popular Civilization franchise. Originally created by legendary game designer Sid Meier, Civilization is a strategy game in which you attempt to... Read more

Latest Forum Discussions

See All

Plants vs. Zombies Heroes guide: How to...
Plants vs. Zombies Heroes might look cute and cuddly, but it's actually a very complex deck builder that requires a lot of time to master. While there's a bit of a learning curve, it's easy for people of any skill level to have fun with the game. | Read more »
Roofbot: Puzzler On The Roof (Games)
Roofbot: Puzzler On The Roof 1.0.2 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.2 (iTunes) Description: Guide Roofie through gorgeous, meditative rooftops and try to get the right color energy balls into the... | Read more »
The 4 best food delivery apps
As the temperatures continue to drop, so does the motivation to venture outside. Sometimes you still want to eat a nice meal from that sushi place down the road though. Thankfully in these trying times, there are a number of fine food delivery... | Read more »
Toca Life: Farm (Education)
Toca Life: Farm 1.0 Device: iOS Universal Category: Education Price: $2.99, Version: 1.0 (iTunes) Description: Work and play the farmer's way! Milk your cow, gather eggs from your hens and raise your crops. Have a picnic, play the... | Read more »
The Lost Shield (Games)
The Lost Shield 1.0.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0.0 (iTunes) Description: The Lost shield is a brick break/adventure game. You play as a hero who must return a powerful but dangerous magic shield... | Read more »
The Forgotten Room (Games)
The Forgotten Room 1.0.1 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0.1 (iTunes) Description: Play as paranormal investigator John “Buster of Ghosts” Murr as he explores yet another mysteriously creepy house. This... | Read more »
5 Halloween mobile games for wimps
If you're anything like me, horror games are a great way to have nightly nightmares for the next decade or three. They're off limits, but perhaps you want to get in on the Halloween celebrations in some way. Fortunately not all Halloween themed... | Read more »
The 5 scariest mobile games
It's the most wonderful time of the year for people who enjoy scaring themselves silly with haunted houses, movies, video games, and what have you. Mobile might not be the first platform you'd turn to for quality scares, but rest assured there are... | Read more »
Lifeline: Flatline (Games)
Lifeline: Flatline 1.0.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.0 (iTunes) Description: The Lifeline series takes a terrifying turn in this interactive horror experience. Every decision you make could help... | Read more »
Game of Dice is now available on Faceboo...
After celebrating its anniversary in style with a brand new update, there’s even more excitement in store for Game of Dice has after just being launched on Facebook Gameroom. A relatively new platform, Facebook Gameroom has been designed for PC... | Read more »

Price Scanner via

WaterField Unveils Syde MacBook Pro Touch Bar...
The new WaterField Designs Syde Case for Apple’s new MacBook Pro Touch Bar combines the top-notch laptop protection of a sleeve with the functionality and handsome look of an over-the-shoulder or... Read more
Apple Unveils Redesigned MacBook Pro With Tou...
October 27, 2016 – Apple today introduced the thinnest and lightest MacBook Pro yet, along with a new interface innovation that replaces the traditional row of function keys with a Retina-quality... Read more
Apple Unveils New TV App for Apple TV, iPhone...
October 27, 2016 – Apple today introduced a new TV app, offering a unified experience for discovering and accessing TV shows and movies from multiple apps on Apple TV, iPhone and iPad. The TV app... Read more
Price drops on select refurbished 2015 13″ Re...
Apple dropped prices on select Certified Refurbished 2015 13″ Retina MacBook Pros by as much as $90. An Apple one-year warranty is included with each model, and shipping is free: - 13″ 2.7GHz/256GB... Read more
Apple reveals new next-generation 15″ and 13″...
Apple today revealed their next-generation 15″ and 13″ MacBook Pros. The new models are thinner and lighter than before with a new aluminum design featuring an enhanced keyboard with retina, multi-... Read more
Worldwide Smartphone Shipments Up 1.0% Year o...
According to preliminary results from the International Data Corporation (IDC) Worldwide Quarterly Mobile Phone Tracker, vendors shipped a total of 362.9 million smartphones worldwide in the third... Read more
TuneBand Arm Band For iPhone 7 and 7 Plus Rel...
Grantwood Technology has added the TuneBand for iPhone 7 and 7 Plus to its smartphone armband series. The TuneBand provides a lightweight and comfortable way to wear the iPhone while running,... Read more
1.4GHz Mac mini on sale for $449, save $50
Adorama has the 1.4GHz Mac mini on sale for $50 off MSRP including free shipping plus NY & NJ sales tax only: - 1.4GHz Mac mini (Apple sku# MGEM2LL/A): $449 $50 off MSRP To purchase a mini at... Read more
21-inch 1.6GHz iMac on sale for $999, save $1...
B&H has the 21″ 1.6GHz Apple iMac on sale for $999 including free shipping plus NY sales tax only. Their price is $100 off MSRP. Read more
Macs’ Superior Enterprise Deployment Cost Eco...
IBM’s debunking of conventional wisdom and popular mythology about the relative cost of using Apple Mac computers as opposed to PCs running Microsoft Windows at the sixth annual Jamf Nation User... Read more

Jobs Board

*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Software Engineering Intern: UI Applications...
Job Summary Apple is currently seeking enthusiastic interns who can work full-time for a minimum of 12-weeks between Fall 2015 and Summer 2016. Our software Read more
Security Data Analyst - *Apple* Information...
…data sources need to be collected to allow Information Security to better protect Apple employees and customers from a wide range of threats.Act as the subject Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Solutions Consultant - Apple (United...
# Apple Solutions Consultant Job Number: 52812872 Houston, Texas, United States Posted: Oct. 18, 2016 Weekly Hours: 40.00 **Job Summary** As an Apple Solutions Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.