TweetFollow Us on Twitter

Pocket Forth
Volume Number:5
Issue Number:4
Column Tag:Forth Forum

Pocket Forth

By örg Langowski, MacTutor Editorial Board

Note: Source code files accompanying article are located on MacTech CD-ROM or source code disks.

“Public Domain Pocket Forth”

Imagine: a compiler that creates applications or desk accessories from the same source code, with only a one or two line change. Impossible? Read on. That compiler will work interactively, so that you can create code as you go, typing in routine after routine and checking them out on the fly. The applications will be very small, they can be run in a 32K partition. And, of course, the code will be fast.

I’m not joking, such a development system does exist. Even more unbelievable, it’s free. It’s now been several months since I downloaded PocketForth from the GEnie Forth Roundtable, and had a lot of fun with it since then. Lately, we even received a letter requesting a review of PocketForth, so I thought this a good opportunity to introduce you to the public domain Forth for the Macintosh.

PocketForth has been written by Chris Heilman, and unfortunately all the author leaves in the documentation is his Compuserve address (70566,1474); no mail address, no phone number. Since I am not on Compuserve (can’t access from here), I wasn’t able to contact him. Therefore, Chris, when you read this: my apologies that we couldn’t warn you. I hope you’ll appreciate this review, and please contact us if you have any comments.

Although PocketForth is completely public domain - even the sources are available upon request - we’d like to have the author’s authorization before putting his system on the source code disk. We are working on it, but the Forth compiler might come on a later disk. Meanwhile, you can download the system from GEnie or Compuserve; the Stuffit file is about 150K long and contains ample documentation and examples.

PocketForth implementation

PocketForth is based on FIG-Forth and a Forth for the 68000 described in Dr. Dobb’s Journal (G. Y. Fletcher, DDJ no. 123, January 1987). It uses a 16-bit stack and base-relative addressing with 32K offset, therefore the total code size is restricted to 32K bytes. The implementation uses subroutine threading with JSRs relative to the base pointer, which is kept in A3. An example illustrates this. Our test routine simply adds 3 and 4, outputs the sum and a space:

: test 3 4 + . 32 emit ;

this compiles to

 move.w #3,-(a6) ; literal 3
 move.w #4,-(a6) ; literal 4
 jsr  $E94(a3) ; +
 jsr  $BF0(a3) ; .
 move.w #32,-(a6); literal 32 
 jsr  $9FA(a3) ; emit
 rts

As you see, the code is dependent on the correct setup of A3. PocketForth must therefore execute in a locked block of memory, which is allocated at startup. The initialization code makes all the standard calls (_MoreMasters, _InitGraf, _InitFonts, _InitWindows, _InitMenus, _InitDialogs, _TEInit, _FlushEvents, _InitCursor), gets the PocketForth main code from the resource DICT ID=257 and jumps to its beginning. DICT 257 is the PocketForth dictionary and contains the names and executable code of all the known Forth words (this is in contrast to Mach2, which creates headerless code and the names are kept in a separate vocabulary). The DICT resource is locked, so that the block won’t move while PocketForth is executing. The startup sequence sets A3 to point to the beginning of the DICT block, initializes stack pointers and other things, and enters the Forth interpreter.

The PocketForth Dictionary

PocketForth dictionary entries have a header consisting of a name field and a link field. The name field is 4 bytes long, the first byte containing the name length, and the next three bytes the first three characters of the name. This means that the words compile and compute will have the same dictionary entry (caution!). The upper bit of the name field’s first byte is the immediate bit; when set, it indicates an immediate execution word. The link field, after the name field, is 2 bytes long and points to the previous dictionary entry. The link field is followed by the definition’s executable code.

Applications vs. desk accessories

You might have already guessed why PocketForth separates the setup code and the DICT resource. This way, application and DA ‘shells’ can be made that set up the environment so that the Forth code in the dictionary can be executed without making big changes between the two versions.

The shell is a dumb terminal window with an Apple, File and Edit menu. The window will accept keyboard input, which is interpreted by the Forth system. Files can be loaded with the word -->, and they will be normal text files, no block file business here. Text pasted from the clipboard will be interpreted just like keyboard input.

The application and the DA look exactly the same, and behave almost exactly the same. Forth code that creates a turnkey application will, if done correctly, create a ‘turnkey DA’ with only minor changes. This is achieved by accessing PocketForth’s system variables through a table using the word +md, which adds the offset of a ‘Mac Data’ block to the top of stack. This block is located at different positions in the application and the DA, and using +md lets you access the system variables transparently.

Examples of the variables pointed to by +md are the main window pointer, vectors to activate, update and mousedown handlers, a vector to an idle routine which is run once on each pass through the event loop (for the APPL) or when the accRun message is received (for the DA). The +md data block also contains an event table, which is a jump table to the event handlers for event types 0 to 15 (APPL) or 0 to 8 (DA). To change default event handling one installs new vectors in this table.

The Example

I rewrote one example from Palo Alto Shipping’s source code disk in PocketForth (Listing 1) to show you some of the techniques used in this Forth implementation. First, we have to redefine a couple of useful Mach2 words which are not present in PocketForth. pick and roll, 16-bit versions of the corresponding Mach2 words, are implemented in 68000 code. PocketForth has no assembler, but can compile 16-bit hex constants inline using the word ,$.

Toolbox access is also done using inline code. Before calling the trap, we must set up the A7 stack; like Mach2, PocketForth uses A6 for the parameter stack and A7 for the return stack. The words >r, 2>r, r> and 2r> are provided for moving 16- and 32- bit quantities to and from the A7 stack. Addresses of PocketForth variables and words are always 16-bit relative to the start of the dictionary, before calling a trap they must be converted to 32-bit absolute addresses with >abs.

The central part of the example is pretty standard Forth; PocketForth has no local variables, so we have to dup swap drip flip flop a little more than usual.

The last part of the example sets up the PocketForth system to start up automatically with the example program, saves the changes to the dictionary and quits. Make sure you have made a backup before you execute the example, the changes are irreversible. The way we make PocketForth run our program on startup is through the activate handler. We install a new activate vector in the event table which will execute our program’s start sequence on the first activate event; thereafter activate events will be ignored. The start sequence calls the word reflect which installs a vector to an idle routine that does the graphical display, and disables keyboard input by storing the null event vector at the keydown position of the event table. In order to execute the idle routine, the DA has to have the accRun flag set in its header. The correct value for the drvrFlags is $6400; change with ResEdit if necessary.

Chris Heilman gives another method to patch the Forth system with an autostart vector. He patches a JMP instruction into the initialization code in the dictionary. However, I was not able to find the correct patch position for the desk accessory, so I used the method I just described, which works for APPL and DA in the same way.

Speed

No review is complete without the results of the Sieve benchmark (Listing 2), so I’ll give them to you: 3.3 seconds for ten iterations of the standard benchmark (1899 primes). MacForth Plus takes the same time, 3.3 seconds, while Mach2 takes 1.9 seconds; therefore PocketForth compares very well with the two major Macintosh Forth systems. Note in the code that the word to access the loop index is r, not i as in the other Forths.

Summary

PocketForth comes in a 150K Stuffit file that contains: the application, the desk accessory, a demo application that has been created under PocketForth, source code for that demo and various other examples, including a floating point package, a mini-paint program and the Sieve benchmark. A manual and a glossary of Forth words is also contained in the package.

PocketForth has been designed to create compact applications and DAs; the maximum code size is restricted to 32K, anyway. However, it is amazing what can be done in so little space, given the compactness of Forth code; each routine call requires only 4 bytes. The example application is only 9K long, including bundle, menu and window resources, and the corresponding desk accessory takes only 8K. You can decrease the application’s partition size in Multifinder down to 32k without any problems.

PocketForth has its limitations, of course: restricted maximum size, few utilities, no built-in editor (I used McSink when I wrote this). There is no assembler, and I used the Mach2 assembler to write the machine code words. Well, there must be something that makes it worth paying for Mach2 or MacForth, I guess if you have a major project in Forth, you have to get a full development system, of course. But for creating ‘instant’ desk accessories, or small applications, or for just fumbling around with the machine and producing interesting hacks (or bombs, for that matter), PocketForth is just the ideal system.

Listing 1: ‘Reflections’ demo rewritten for Pocket Forth

( Reflections demo from Mach2 demo disk; rewritten )
( for PocketForth v.3 )
( J. Langowski / MacTutor Feb. 1988 )

( Compile this demo with a COPY of Pocket Forth or the )
( Pocket Forth DA; the dictionary will be irreversibly )
( changed to create a turnkey application / DA. )

( Note that the change required to compile this example ) 
( with the DA version consists only of a 1 line deletion; )
( see at the bottom of the listing. )

forget task
: task ;

: pick ( n -- dup stack item n levels down )
        ,$ 301E ( move.w [a6]+,d0)
        ,$ E380 ( asl.l  #1,d0 )
        ,$ 3D36 ,$ 0 ( move.w [a6,d0.w],-[a6] )
;

: roll ( n -- move up stack item n levels down )
        ,$ 2F02  (     move.l d2,-[a7] )
        ,$ 301E  (     move.w [a6]+,d0 )
        ,$ 6F16  (     ble.s   @1 )
        ,$ 5380  (     subq.l  #1,d0 )
        ,$ 3200  (     move.w  d0,d1 )
        ,$ 3F1E  ( @2  move.w [a6]+,-[a7] )
        ,$ 51C8
        ,$ FFFC  (     dbf     d0,@2 )
        ,$ 341E  (     move.w  [a6]+,d2 )
        ,$ 3D1F  ( @3  move.w  [a7]+,-[a6] )
        ,$ 51C9
        ,$ FFFC  (     dbf     d1,@3 )
        ,$ 3D02  (     move.w  d2,-[a6] )
        ,$ 241F  (     move.l  [a7]+,d2 )
;                ( @1  rts )

: range ( value lo hi -- flag ) 
        2 pick <  rot rot < or 0=
;         

: 4dup ( n1 n2 n3 n4 - n1 n2 n3 n4 n1 n2 n3 n4 )
 3 pick 3 pick 3 pick 3 pick 
;

4 +md constant wrect ( Pocket Forth main window )

2variable myport
: getport >abs 2>r ,$ A874 ; ( _GetPort )
: setport 2@ 2>r ,$ A873 ; ( _SetPort )
: cls wrect >abs 2>r ,$ A8A3 ; ( _EraseRect )

( QuickDraw Equates )
hex
8      constant PatCopy
B      constant PatBic
10     constant PortRect
decimal

( Window Size Variables )
variable        WTop
variable        WLeft
variable        WBottom
variable        WRight
variable        WWidth
variable        WHeight

( Positions     Velocities )
variable xx1    variable xx1dot
variable yy1    variable yy1dot
variable xx2    variable xx2dot
variable yy2    variable yy2dot

: GetWCoords ( -- )
        wrect       @  WTop    !
        wrect 2+    @  WLeft   !
        wrect 4 +   @  WBottom !
        wrect 6 +   @  WRight  !

        ( Calculate the current window width and height. )
        WBottom @ WTop  @ - WHeight !
        WRight  @ WLeft @ - WWidth  !  
;

( Erase the window and set the initial pen positions and velocities. 
)
: SetupReflect (  -  )
        cls
        GetWCoords
        WWidth  @ 3 /    xx1 !   3 xx1dot !
        WHeight @        yy1 !  -4 yy1dot !
        WWidth  @ 3 / 2* xx2 !   4 xx2dot !
        WHeight @        yy2 !  -3 yy2dot ! ;

( Draws a newline and leaves coords on stack. )
: NewCoords ( -- xx1 yy1 xx2 yy2 )
        ( Increment the line position. )
        xx1dot @ xx1 +!
        yy1dot @ yy1 +!
        xx2dot @ xx2 +!
        yy2dot @ yy2 +!

        xx1 @ 1 WWidth @ range 0=
        if xx1dot @ negate xx1dot ! then

        yy1 @ 1 WHeight @ range 0=
        if yy1dot @ negate yy1dot ! then

        xx2 @ 1 WWidth @ range 0=
        if xx2dot @ negate xx2dot ! then

        yy2 @ 1 WHeight @ range 0=
        if yy2dot @ negate yy2dot ! then 

        xx1 @ yy1 @ xx2 @ yy2 @
;

( Leaves 40 coordinate pairs on the stack and draws the 1st ten lines. 
)
: First20Lines (  -  )
        20 0 do
                PatCopy >r ,$ A89C ( _PenMode )
                NewCoords 4dup
                !pen -to
        loop ;

20 +md constant idlevector
: LinesAdvance (  -  )
                PatCopy >r ,$ A89C ( _PenMode )
                NewCoords 4dup
                !pen -to

                83 roll 83 roll 83 roll 83 roll
                PatBic >r ,$ A89C ( _PenMode ) 
                        ( and white out the n-21st line)
                !pen -to
;

‘ LinesAdvance constant LAdv

12 +md constant actVect
actVect @ constant actDefault

24 +md constant nullevent
nullevent 6 + constant keyvector
nullevent @ constant rien

: Reflect (  -  )
        SetUpReflect
        First20Lines cls
        LAdv idlevector !
        rien keyvector !  ;

variable flag  1 flag !
: start drop ( act/deact flag) 
        cls
        flag @ if 
                reflect 0 flag !
                begin ?terminal drop again
                ( leave out in DA version )
        then  ;
‘ start actvect !   
 save   ( CAUTION: changes dictionary irreversibly )

: bye ,$ A9F4 ( _ExitToShell )  ; bye

Listing 2: Sieve benchmark for PocketForth

( © Chris Heilman )
( Sleeve of Erastothanes )
( optomized for Pocket Forth with inline machine code )
9000 room - grow  ( provide for 9000 dictionary bytes )
forget task : TASK ;  decimal

( timer )
: START ( -- d ) 362 0 dl@ ;  ( get ‘ticks’ )
: T. ( sec -- ) s>d <# # 46 hold #S #> type ;  ( print sec.tenths )
: STOP ( d -- ) start 2swap dnegate d+ drop  6 / t. .” Seconds” ;

8190 constant SIZE
variable FLAGS size allot

( compile these 2 byte words inline )
: [DUP] ( n -- n n ) [ ‘ dup @ literal ] , ; 
 IMMEDIATE  ( equal to:  dup )
: [DROP] ( n -- ) [ ‘ drop @ literal ] , ; 
 IMMEDIATE  ( equal to:  drop )
: [1+] ( n -- n+1 ) [ ‘ 1+ @ literal ] , ; 
 IMMEDIATE  ( equal to:  1+ )

( compile machine code inline routines )
: R+ ( n -- n+r ) 12311 , 53590 , ; 
 IMMEDIATE  ( equal to:  r + )
: 0RC! ( -- ) 12311 , 16947 , 0 , ; 
 IMMEDIATE  ( equal to:  0 r c! )

: PRIME  flags size 1 fill
    0 size 0 DO
      flags r+ c@ IF
        3 r+ r+ [dup] r+ size < IF
          size flags + over r+ flags +
          DO  0rc! [dup]  +LOOP
        THEN [drop] [1+]
      THEN
    LOOP . .” primes” cr ;

: SIEVE  page  .”        The Sieve of Erastothanes” decimal
    cr  start  10 0 DO prime LOOP  beep
    cr  stop  cr .” Not too shabby, eh?”  cr ;

sieve

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Mac DVDRipper Pro 7.1 - Copy, backup, an...
Mac DVDRipper Pro is the DVD backup solution that lets you protect your DVDs from scratches, save your batteries by reading your movies from your hard disk, manage your collection with just a few... Read more
VOX 3.0.1 - Music player that supports m...
VOX just sounds better! The beauty is in its simplicity, yet behind the minimal exterior lies a powerful music player with a ton of features and support for all audio formats you should ever need.... Read more
Pinegrow 4 - Mockup and design webpages...
Pinegrow (was 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,... Read more
iExplorer 4.1.11 - View and transfer fil...
iExplorer is an iPhone browser for Mac lets you view the files on your iOS device. By using a drag and drop interface, you can quickly copy files and folders between your Mac and your iPhone or... Read more
Evernote 6.13.1 - Create searchable note...
Evernote allows you to easily capture information in any environment using whatever device or platform you find most convenient, and makes this information accessible and searchable at anytime, from... Read more
Myriad 4.2.1 - Audio batch processor.
Myriad is, simply put, one of the best audio batch processors. Totally redesigned, it looks beautiful and delivers incredible performance. Let Myriad do the heavy lifting while you get back to doing... Read more
Garmin Express 5.8.0.0 - 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
Arq 5.10 - Online backup to Google Drive...
Arq is super-easy online backup for Mac and Windows computers. Back up to your own cloud account (Amazon Cloud Drive, Google Drive, Dropbox, OneDrive, Google Cloud Storage, any S3-compatible server... Read more
Garmin Express 5.8.0.0 - 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
Myriad 4.2.1 - Audio batch processor.
Myriad is, simply put, one of the best audio batch processors. Totally redesigned, it looks beautiful and delivers incredible performance. Let Myriad do the heavy lifting while you get back to doing... Read more

Latest Forum Discussions

See All

warbot.io wants you for the robot wars
Fans of epic gundam-style battles will find a lot to love in warbot.io, the first game for up and coming developer Wondersquad. The game saw a lot of success when it first launched for browsers and Facebook, and now even more people are getting the... | Read more »
Uncover alien mysteries in cross-genre s...
If the Alien franchise taught us anything, it’s that landing on a strange planet at the behest of a faceless corporation is probably asking for trouble. And Eldritch Game’s Deliria doesn’t prove otherwise. In 2107, Dimension LG7 is rich with... | Read more »
The best mobile games to play during dre...
| Read more »
The mobile gamer's guide to Black F...
We're starting to catch wind of some exciting deals in the mobile gaming space for Black Friday. There are big discounts on mobile phones and accessories cropping up already, so you might want to get a move on things ahead of the big day. It's... | Read more »
The best pre-Black Friday deals - Novemb...
Black Friday will soon be upon us, but online retailers are already getting a headstart on the steep discounts. Don't wait until Friday—you'll find some pretty good deals all over the internet without waiting in lines or competing with other... | Read more »
Mighty Battles guide - how to build a so...
Mighty Battles, the latest title from Hothead Games, is set to take the App Store by storm. The game puts a welcome twist on lane battlers, adding FPS elements to spice things up a bit. You'll collect cards to put your own military unit to gether,... | Read more »
Rules of Survival guide - how to be the...
The PUBG craze makes its way to mobile, with more and more battle royale games debuting on iOS and Android. Rules of Survival joins the ranks of mobile PUBG-likes, offering a classic battle royale experiences that doesn't vary too much from its... | Read more »
The best new games we played this week -...
The weekend is upon us friends, and it's time to take a look back and reflect on all of the wonderful games we've played over the past few days. This week was jam packed with new releases. There were some big, long awaited launches, some fun... | Read more »
Lineage II: Revolution guide - tips and...
At long last, Lineage II: Revolution has now come to western shores, bring Netmarble's sweeping MMORPG to mobile devices. It's an addictive, epic experience, but some of the systems in the game can be a bit overwhelming. Here are a few tips to help... | Read more »
A Boy and His Blob (Games)
A Boy and His Blob 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: | Read more »

Price Scanner via MacPrices.net

Lowest Black Friday prices on Apple MacBooks:...
Save $150-$420 on the purchase of a MacBook Pro, MacBook, or MacBook Air this Black Friday and Holiday weekend with Certified Refurbished models at Apple. In many cases, Apple’s refurbished prices... Read more
Black Friday: Apple Watch Series 1 for $70 of...
Macy’s has discounted Series 1 Apple Watches by $70 on their online store as part of their Black Friday sale: – 38mm Series 1 Apple Watch: $179, $70 off – 42mm Series 1 Apple Watch: $209, $70 off... Read more
Apple offers 2016 13-inch MacBook Airs, certi...
Apple has Certified Refurbished 2016 13″ MacBook Airs available starting at $809. An Apple one-year warranty is included with each MacBook, and shipping is free: – 13″ 1.6GHz/8GB/128GB MacBook Air: $... Read more
Black Friday sale: Mac minis for $100 off MSR...
B&H Photo has Mac minis on sale for up to $100 off MSRP as part of their Black Friday sale, each including free shipping plus NY & NJ sales tax only: – 1.4GHz Mac mini: $399 $100 off MSRP – 2... Read more
Use your Apple Education discount to save up...
Purchase a new Mac using Apple’s Education discount, and take up to $300 off MSRP. All teachers, students, and staff of any educational institution with a .edu email address qualify for the discount... Read more
Adorama posts Black Friday deals on Apple Mac...
Adorama has posted Black Friday sale prices on many Macs, with MacBooks and iMacs available for up to $200 off MSRP. Shipping is free, and Adorama charges sales tax in NJ and NY only: MacBook Pros... Read more
Save up to $300 on 15″ 2.2GHz MacBook Pros
B&H Photo has the 15″ 2.2GHz MacBook Pro available for $200 off MSRP including free shipping plus NY & NJ sales tax only: – 15″ 2.2GHz MacBook Pro (MJLQ2LL/A): $1799 $200 off MSRP Amazon.com... Read more
Save up to $180 with Apple Certified Refurbis...
Apple has Certified Refurbished 2017 13″ MacBook Airs available starting at $849. An Apple one-year warranty is included with each MacBook, and shipping is free: – 13″ 1.8GHz/8GB/128GB MacBook Air (... Read more
Black Friday deals on Apple Macs now live at...
Amazon has MacBook Pros, MacBook Airs, MacBooks, and iMacs on sale for up to $200 off MSRP for Black Friday week. Shipping is free. Note that stock of some Macs may come and go during the week, so... Read more
Black Friday pricing on Macs and iPads now av...
B&H Photo has lowered prices on many Macs, iPads, and iPad Pros as part of their Black Friday week sale. Save up to $200 on MacBooks and iMacs and up to $150 on iPads. B&H charges sales tax... Read more

Jobs Board

*Apple* Professional Learning Specialist - A...
# Apple Professional Learning Specialist Job Number: 112953711 Houston, Texas, United States Posted: 07-Sep-2017 Weekly Hours: 40.00 **Job Summary** The Apple 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
Product Manager - *Apple* Pay on the *Appl...
Job Summary Apple is looking for a talented product manager to drive the expansion of Apple Pay on the Apple Online Store. This position includes a unique Read more
*Apple* Pro/Consumer Apps Support Engineer -...
…exemplify AppleCare's expert technical support paired with exceptional customer service for Apple 's software apps. This person is a problem solver, who understands Read more
Partner Marketing Manager, *Apple* Pay - Ap...
Job Summary The Apple Pay partner marketing team is looking for a Marketing Manager to develop and drive US programs. The right candidate will be passionate about Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.