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

Latest Forum Discussions

See All

Embark into the frozen tundra of certain...
Chucklefish, developers of hit action-adventure sandbox game Starbound and owner of one of the cutest logos in gaming, has released their roguelike deck-builder Wildfrost. Created alongside developers Gaziter and Deadpan Games, Wildfrost will... | Read more »
MoreFun Studios has announced Season 4,...
Tension has escalated in the ever-volatile world of Arena Breakout, as your old pal Randall Fisher and bosses Fred and Perrero continue to lob insults and explosives at each other, bringing us to a new phase of warfare. Season 4, Into The Fog of... | Read more »
Marvel Future Fight celebrates nine year...
Announced alongside an advertising image I can only assume was aimed squarely at myself with the prominent Deadpool and Odin featured on it, Netmarble has revealed their celebrations for the 9th anniversary of Marvel Future Fight. The Countdown... | Read more »
HoYoFair 2024 prepares to showcase over...
To say Genshin Impact took the world by storm when it was released would be an understatement. However, I think the most surprising part of the launch was just how much further it went than gaming. There have been concerts, art shows, massive... | Read more »
Explore some of BBCs' most iconic s...
Despite your personal opinion on the BBC at a managerial level, it is undeniable that it has overseen some fantastic British shows in the past, and now thanks to a partnership with Roblox, players will be able to interact with some of these... | Read more »
Play Together teams up with Sanrio to br...
I was quite surprised to learn that the massive social network game Play Together had never collaborated with the globally popular Sanrio IP, it seems like the perfect team. Well, this glaring omission has now been rectified, as that instantly... | Read more »
Dark and Darker Mobile gets a new teaser...
Bluehole Studio and KRAFTON have released a new teaser trailer for their upcoming loot extravaganza Dark and Darker Mobile. Alongside this look into the underside of treasure hunting, we have received a few pieces of information about gameplay... | Read more »
DOFUS Touch relaunches on the global sta...
After being a big part of a lot of gamers - or at the very least my - school years with Dofus and Wakfu, Ankama sort of shied away from the global stage a bit before staging a big comeback with Waven last year. Now, the France-based developers are... | Read more »
Howl lets you plan up to six steps ahead...
Rage against the howling plague as a deaf prophetess in Howl, a visually stunning turn-based title from Astragon Entertainment and Mi'pu'mi. In this folktale world where to hear means to be cursed, you'll have to battle wolfish monstrosities coming... | Read more »
Netmarble celebrates Easter with events...
One of the many good things about holding a suitcase full of titles is the ability to truly embrace each holiday, and Netmarble is doing just this with four of its top games. To celebrate Easter, players will be able to jump into their favourite... | Read more »

Price Scanner via MacPrices.net

Sunday Sale: 13-inch M3 MacBook Air for $999,...
Several Apple retailers have the new 13″ MacBook Air with an M3 CPU in stock and on sale today for only $999 in Midnight. These are the lowest prices currently available for new 13″ M3 MacBook Airs... Read more
Multiple Apple retailers are offering 13-inch...
Several Apple retailers have 13″ MacBook Airs with M2 CPUs in stock and on sale this weekend starting at only $849 in Space Gray, Silver, Starlight, and Midnight colors. These are the lowest prices... Read more
Roundup of Verizon’s April Apple iPhone Promo...
Verizon is offering a number of iPhone deals for the month of April. Switch, and open a new of service, and you can qualify for a free iPhone 15 or heavy monthly discounts on other models: – 128GB... Read more
B&H has 16-inch MacBook Pros on sale for...
Apple 16″ MacBook Pros with M3 Pro and M3 Max CPUs are in stock and on sale today for $200-$300 off MSRP at B&H Photo. Their prices are among the lowest currently available for these models. B... Read more
Updated Mac Desktop Price Trackers
Our Apple award-winning Mac desktop price trackers are the best place to look for the lowest prices and latest sales on all the latest computers. Scan our price trackers for the latest information on... Read more
9th-generation iPads on sale for $80 off MSRP...
Best Buy has Apple’s 9th generation 10.2″ WiFi iPads on sale for $80 off MSRP on their online store for a limited time. Prices start at only $249. Sale prices for online orders only, in-store prices... Read more
15-inch M3 MacBook Airs on sale for $100 off...
Best Buy has Apple 15″ MacBook Airs with M3 CPUs on sale for $100 off MSRP on their online store. Prices valid for online orders only, in-store prices may vary. Order online and choose free shipping... Read more
24-inch M3 iMacs now on sale for $150 off MSR...
Amazon is now offering a $150 discount on Apple’s new M3-powered 24″ iMacs. Prices start at $1149 for models with 8GB of RAM and 256GB of storage: – 24″ M3 iMac/8-core GPU/8GB/256GB: $1149.99, $150... Read more
15-inch M3 MacBook Airs now on sale for $150...
Amazon is now offering a $150 discount on Apple’s new M3-powered 15″ MacBook Airs. Prices start at $1149 for models with 8GB of RAM and 256GB of storage: – 15″ M3 MacBook Air/8GB/256GB: $1149.99, $... Read more
The latest Apple Education discounts on MacBo...
If you’re a student, teacher, or staff member at any educational institution, you can use your .edu email address when ordering at Apple Education to take up to $300 off the purchase of a new MacBook... Read more

Jobs Board

Retail Assistant Manager- *Apple* Blossom Ma...
Retail Assistant Manager- APPLE BLOSSOM MALL Brand: Bath & Body Works Location: Winchester, VA, US Location Type: On-site Job ID: 04225 Job Area: Store: Management Read more
Housekeeper, *Apple* Valley Village - Cassi...
Apple Valley Village Health Care Center, a senior care campus, is hiring a Part-Time Housekeeper to join our team! We will train you for this position! In this role, Read more
Sonographer - *Apple* Hill Imaging Center -...
Sonographer - Apple Hill Imaging Center - Evenings Location: York Hospital, York, PA Schedule: Full Time Sign-On Bonus Eligible Remote/Hybrid Regular Apply Now See Read more
Senior Software Engineer - *Apple* Fundamen...
…center of Microsoft's efforts to empower our users to do more. The Apple Fundamentals team focused on defining and improving the end-to-end developer experience in Read more
New RN Case Manager *Apple* Valley ,CA 40 h...
$50-55/hr Apple Valley, CA Amergis HEALTHCARE STAFFING NEW RN Case Manager is responsible for coordinating continuum of care activities for assigned patients and Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.