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

BlueStacks 4.140.13 - Run Android applic...
BlueStacks App Player lets you run your Android apps fast and fullscreen on your Mac. Feature comparison chart Version 4.140.13: Highlights/Bug Fixes: Feel free to use BlueStacks as your go to... Read more
Adobe Premiere Pro CC 2020 14.0.1 - Digi...
Premiere Pro CC 2020 is available as part of Adobe Creative Cloud for as little as $52.99/month. The price on display is a price for annual by-monthly plan for Adobe Premiere Pro only Adobe Premiere... Read more
VirtualBox 6.1.2 - x86 virtualization so...
VirtualBox is a family of powerful x86 virtualization products for enterprise as well as home use. Not only is VirtualBox an extremely feature rich, high performance product for enterprise customers... Read more
RoboForm 8.6.8 - Password manager; syncs...
RoboForm is a password manager that offers one-click login, mobile syncing, easy form filling, and reliable security. Password Manager. RoboForm remembers your passwords so you don't have to! Just... Read more
Postbox 7.0.11 - Powerful and flexible e...
Postbox is a new email application that helps you organize your work life and get stuff done. It has all the elegance and simplicity of Apple Mail, but with more power and flexibility to manage even... Read more
calibre 4.9.0 - Complete e-book library...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital librarian... Read more
Notability 4.2 - Note-taking and annotat...
Notability is a powerful note-taker to annotate documents, sketch ideas, record lectures, take notes and more. It combines, typing, handwriting, audio recording, and photos so you can create notes... Read more
FoldersSynchronizer 5.0.1 - Synchronize...
FoldersSynchronizer is a popular and useful utility that synchronizes and backs-up files, folders, disks and boot disks. On each session you can apply special options like Timers, Multiple Folders,... Read more
Sketch 62 - Design app for UX/UI for iOS...
Sketch is an innovative and fresh look at vector drawing. Its intentionally minimalist design is based upon a drawing space of unlimited size and layers, free of palettes, panels, menus, windows, and... Read more
ScreenFlow 9.0.2 - Create screen recordi...
ScreenFlow is powerful, easy-to-use screencasting software for the Mac. With ScreenFlow you can record the contents of your entire monitor while also capturing your video camera, microphone and your... Read more

Latest Forum Discussions

See All

Slingsters is a physics-based puzzler fo...
Slingsters is a physics-based puzzle game where the aim is to collect various different monsters by flinging them from one side of a level to the other and into a box. It's also the first game from Nappy Cat and is available now for iOS and... | Read more »
Spiritwish's latest update sees the...
A sizeable update has hit Nexon's MMORPG Spiritwish today. It brings a new game mode, characters and there will also be a special event to celebrate the update with a firework display. [Read more] | Read more »
Maze Machina, a turn-based puzzler from...
The latest game from Arnold Rauers also known as Tiny Touch Tales is now available. You may be familiar with one of his many excellent titles such as Card Crawl, Enyo and Card Thief. His latest endeavour is called Maze Machina and you can grab it... | Read more »
Mario Kart Tour's Ice Tour races to...
Can you believe Mario Kart Tour is already on its 9th tour? The game only launched back in September, and since then it's become increasingly tricky to keep on top of the amount of new content Nintendo is pumping out. [Read more] | Read more »
Apple Arcade: Ranked - Top 50 [Updated 1...
In case you missed it, I am on a quest to rank every Apple Arcade game there is. [Read more] | Read more »
Marvel Future Fight's latest update...
Marvel Future Fight's latest update has added an all-new team of heroes to recruit and do battle with. The 'Warriors of the Sky' include Blue Dragon, War Tiger, Sun Bird, and Shadow Shell. As is the norm, each character comes with their own unique... | Read more »
Klee: Spacetime Cleaners is a fast-paced...
Klee: Spacetime Cleaners is a fast-paced auto-shooter that sports a cute retro aesthetic thathad racked up an impressive 100,000 pre-registers prior to its release. It's available now for both iOS and Android. [Read more] | Read more »
Hearthstone's latest solo adventure...
Blizzard has unveiled a plethora of details about its upcoming expansion for its popular card game Hearthstone. It's called Galakrond's Awakening and will bring the Year of the Dragon storyline to a close. It's available now for pre-purchase ahead... | Read more »
Cultist Simulator's Dancer DLC laun...
Cultist Simulator's Dancer DLC is set to launch for iOS and Android on January 22nd. It revolves around the occult cabaret known as the 'Ecdysis Club', where "the distinction between pleasure and pain is as delicate and essential as the human skin... | Read more »
Death Comes True releases a new characte...
Death Comes True is an FMV murder mystery from Kazutaka Kodaka, the creator of Danganronpa. It sees you playing as an amnesiac young man, called Makoto Karaki, who wakes up in a mysterious hotel to find that he's being hunted for a series of... | Read more »

Price Scanner via MacPrices.net

New Verizon deal: Apple iPhone XR for $300 of...
Switch to Verizon and sign up with one of their Unlimited plans, and Verizon will take $300 off the price of an Apple iPhone XR (regularly $749), plus get a free $200 prepaid Mastercard. This is an... Read more
Amazon’s popular AirPods sale is back with mo...
Amazon has new 2019 Apple AirPods on sale today ranging up to $40 off MSRP, starting at $129, as part of their popular Apple AirPods sale. Shipping is free: – AirPods Pro: $234.98 $15 off MSRP –... Read more
Apple’s top of the line 10.5″ 256GB WiFi + Ce...
B&H Photo has the top of the line 10.5″ 256GB WiFi + Cellular iPad Air on sale for $599 shipped. That’s $180 off Apple’s MSRP for this model and the cheapest price available. Overnight shipping... Read more
Apple’s refurbished iPad Pros are the cheapes...
Apple has Certified Refurbished 11″ iPad Pros available on their online store for up to $220 off the cost of new models. Prices start at $679. Each iPad comes with a standard Apple one-year warranty... Read more
Just in: Take $100 off the price of the 3.0GH...
Apple resellers are offering new 2018 6-Core Mac minis for $100 off Apple’s MSRP today, only $999. B&H Photo has 6-Core Mac minis on sale for $100 off Apple’s standard MSRP. Overnight shipping is... Read more
Apple has 4-core and 6-core 2018 Mac minis av...
Apple has Certified Refurbished 2018 Mac minis available on their online store for $120-$170 off the cost of new models. Each mini comes with a new outer case plus a standard Apple one-year warranty... Read more
Amazon offers $200 discount on 13″ MacBook Ai...
Amazon has new 2019 13″ MacBook Airs with 256GB SSDs on sale for $200 off Apple’s MSRP, now only $1099, each including free shipping. Be sure to select Amazon as the seller during checkout, rather... Read more
Apple’s AirPods drop to $129 again at these r...
Apple resellers are offering $30 discounts on Apple AirPods with Charging Case, with prices at $129. These prices are the cheapest new AirPods prices currently available from any Apple reseller: (1)... Read more
Save up to $250 on an iPhone XS with these Ce...
Apple is now offering Certified Refurbished iPhone XS models for up to $350 off MSRP with prices starting at $699. Each iPhone is unlocked and comes with Apple’s standard one-year warranty and a new... Read more
B&H offers 13″ MacBook Airs for $150 off...
B&H Photo has 13″ 128GB MacBook Airs on sale for $150 off Apple’s MSRP, only $949. Overnight shipping is free to many locations in the US: – 13″ 1.6GHz/128GB MacBook Air Space Gray: $949.99 $150... Read more

Jobs Board

Best Buy *Apple* Computing Master - Best Bu...
**745058BR** **Job Title:** Best Buy Apple Computing Master **Job Category:** Store Associates **Store NUmber or Department:** 001080-Lake Charles-Store **Job Read more
Geek Squad *Apple* Consultation Professiona...
**756640BR** **Job Title:** Geek Squad Apple Consultation Professional **Job Category:** Store Associates **Store NUmber or Department:** 000484-Manchester-Store Read more
*Apple* Phenology Coordinator - Montana Stat...
…Rachel Leisso at ### or ###@montana.edu Classification Title Working Title Apple Phenology Coordinator Brief Position Overview This position provides support for Read more
Business Development Specialist - *Apple* -...
Responsible for driving sales growth specifically for the Apple product line within CDW's account base. Primarily working with CDW and Apple sales teams and Read more
Geek Squad *Apple* Consultation Professiona...
**757640BR** **Job Title:** Geek Squad Apple Consultation Professional **Job Category:** Store Associates **Store NUmber or Department:** 000802-Las Cruces-Store Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.