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

FontExplorer X Pro 7.1.3 - Font manageme...
FontExplorer X Pro is optimized for professional use; it's the solution that gives you the power you need to manage all your fonts. Now you can more easily manage, activate and organize your... Read more
DiskCatalogMaker 8.2.5 - Catalog your di...
DiskCatalogMaker is a simple disk management tool which catalogs disks. Simple, light-weight, and fast Finder-like intuitive look and feel Super-fast search algorithm Can compress catalog data for... Read more
Skim 1.5.12 - PDF reader and note-taker...
Skim is a PDF reader and note-taker for OS X. It is designed to help you read and annotate scientific papers in PDF, but is also great for viewing any PDF file. Skim includes many features and has a... Read more
rekordbox 6.1.0.0030 - Professional DJ m...
rekordbox is the best way of preparing and managing your tracks, be it at home, in the studio, or even on the plane! It allows you to import music from other music-management software using the... Read more
iExplorer 4.4.0 - View and transfer file...
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
OmniGraffle 7.17.5 - Create diagrams, fl...
OmniGraffle helps you draw beautiful diagrams, family trees, flow charts, org charts, layouts, and (mathematically speaking) any other directed or non-directed graphs. We've had people use Graffle to... Read more
Apple Configurator 2.13.1 - Configure an...
Apple Configurator makes it easy to deploy iPad, iPhone, iPod touch, and Apple TV devices in your school or business. Use Apple Configurator to quickly configure large numbers of devices connected to... Read more
OmniGraffle Pro 7.17.5 - Create diagrams...
OmniGraffle Pro helps you draw beautiful diagrams, family trees, flow charts, org charts, layouts, and (mathematically speaking) any other directed or non-directed graphs. We've had people use... Read more
FoldersSynchronizer 5.1.6 - 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
Adobe After Effects 17.1.4 - Create prof...
After Effects is available as part of Adobe Creative Cloud for $52.99/month (or $20.99/month for a single app license). The new, more connected After Effects can make the impossible possible. Get... Read more

Latest Forum Discussions

See All

Infinity Mechs is an upcoming idle game...
Indie developer SkullStar studio has announced an upcoming idle mech game called Infinity Mechs. It draws inspiration from the mobile game Iron Saga and has been officially licensed by Game Duchy. It's set to launch for both iOS and Android on... | Read more »
PUBG Mobile Lite's latest update se...
PUBG Mobile Lite, the streamlined version of the popular battle royale that's designed to work on less powerful devices, sees the return of a popular game variant today, Survive Till Dawn mode. It arrives as part of the 0.19.0 content update. [... | Read more »
Matchy Catch, Jyamma Games’ new hyper-ca...
Matchy Catch is a new hyper-casual puzzler from Jyamma Games, the Italian studio behind the Pong-inspired puzzle-adventure Hi-Ball Rush. It’s only the developer’s second game for iOS and Android devices, but it promises to be every bit as fun and... | Read more »
Among Us! Imposter Guide - How to be a s...
Among Us! continues to be getting a lot of play in these parts, and since our first guide we've learned a thing or two about the game. This is especially true regarding the imposter role, as its a relatively rare opportunity that we've now put... | Read more »
Paladin's Story is an upcoming fant...
Paladin's Story is an upcoming fantasy RPG with an off-kilter sense of humour that's heading for iOS and Android. It will officially launch for both on September 16th though the game is already available on Google Play in Early Access. [Read more... | Read more »
Among Us! Guide - Tips for the uninitiat...
| Read more »
A Pretty Odd Bunny is a stealth-platform...
A Pretty Odd Bunny is a stealth-platformer from two-man team AJ Ordaz and René Rivera. It follows the story of a red-eyed rabbit who is allergic to carrots and instead has a penchant for devouring pigs. It's available now for Android devices. [... | Read more »
Apple Arcade: Ranked - Top 25 [Updated 9...
In case you missed it, I am on a quest to rank every Apple Arcade game there is. [Read more] | Read more »
The 5 Best Mobile Games Like Tony Hawk...
Tony Hawk’s Pro Skater 1+2 dropped last week, meaning you can get remastered versions of two of the most iconic “sports” games ever made and experience some of the highest watermarks set in games of generations’ past. [Read more] | Read more »
Bouncing Box is a challenging platformer...
In platformers, we often spend a large amount of time mindless destroying boxes by jumping onto or into them, whatever makes them splinter apart. Bouncing Box from developer The K Brothers aims to give those poor destructible cubes a chance to be... | Read more »

Price Scanner via MacPrices.net

Price drop! Get a 44mm Apple Watch Series 5 G...
Amazon has dropped their price on the 44mm Apple Watch Series 5 GPS + Cellular by $100 to $429 shipped. That’s $100 off Apple’s original MSRP for this model. For the latest prices and sales, see our... Read more
Verizon offers $200 discount on new Apple Wat...
Verizon will take up to $200 off the purchase of a new GPS + Cellular Apple Watch Series 6 or Apple Watch SE with select trade-in and the purchase of a new iPhone with service. The fine print: “Get... Read more
Verizon offers $250 discount on new 8th gener...
Verizon will take up to $250 off the price of an 8th generation 2020 Apple Cellular iPad with select trade-ins and a new iPhone purchase. Plus get Apple News+ free for 6 months. The fine print: “Save... Read more
Apple’s Implementation Of COVID-19 Exposure...
NEWS: 09.18.20 – The latest effort by Apple to embed exposure notifications for COVID-19 contact tracing right into its mobile operating system has some iPhone users weary of being exposed to... Read more
Here’s how to get a 16″ MacBook Pro for $300...
B&H Photo has new 16″ MacBook Pros on sale today for $250-$300 off Apple’s MSRP, starting at $2099. Expedited shipping is free to many addresses in the US: – 2019 16″ 2.6GHz 6-Core MacBook Pro... Read more
Apple has Certified Refurbished 16″ MacBook P...
Apple has Certified Refurbished 2019 16″ MacBook Pros available for up to $420 off the cost of new models, starting at $2039. Each model features a new outer case, shipping is free, and an Apple 1-... Read more
Price drops! Apple reseller B&H drops App...
B&H Photo has dropped prices on Apple Watch Series 5 models by $50-$70 off Apple’s original MSRP. Shipping is free. These are the same Apple Watch models sold by Apple in their retail and online... Read more
Apple extends their 2020 Back-to-School promo...
As part of their Back to School promotion for 2020, Apple will include one free pair Apple AirPods (with charging case) with the purchase of a MacBook Air, MacBook Pro, iMac, iMac Pro (Mac Pro and... Read more
Apple 7.9″ iPad minis are on sale today for $...
Amazon has new 7.9″ 64GB WiFi iPad minis on sale today for $50 off Apple’s MSRP, each including free shipping. Prices start at $349. These are the same iPad minis sold by Apple in their retail and... Read more
Lowest price anywhere! New 16″ 6-core MacBook...
Expercom has the Silver 16″ 6-core MacBook Pro back on sale for a limited time for $2079 shipped. Their price is $320 off Apple’s MSRP for this model, and it’s the cheapest price currently available... Read more

Jobs Board

Security Officer ($23.00/Hourly) - *Apple*...
**Security Officer \($23\.00/Hourly\) \- Apple Store** **Description** About NMS Built on a culture of safety and integrity, NMSdelivers award\-winning, integrated Read more
Platform - Workplace Eng - *Apple* Enterpri...
MORE ABOUT THIS JOB We are looking for an Apple Platform Engineer who will bring a unique engineering skill set, support, clarity, organization and above all else, Read more
*Apple* Certified Repair Technician - Utah S...
…selected candidate will work in the USU Campus Store Tech Department as an Apple Certified Repair Technician and floor associate. This position is for both summer Read more
Senior Data Engineer - *Apple* - Theorem, L...
Job Summary Apple is seeking an experienced, detail-minded data engineeringconsultant to join our worldwide business development and strategy team. If you are Read more
Desktop Support Technician - A+, *Apple* -...
Desktop Support Technician - A+, Apple **Ref No.:** 20-01604 **Location:** Miami, Florida **e** at http://www.excell.com/ **X** at http://www.excell.com/ **cell** at Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.