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

 
AAPL
$101.58
Apple Inc.
+0.72
MSFT
$46.52
Microsoft Corpora
-0.24
GOOG
$584.77
Google Inc.
+4.82

MacTech Search:
Community Search:

Software Updates via MacUpdate

BusyCal 2.6 - Powerful calendar app with...
BusyCal is an award-winning desktop calendar that combines personal productivity features for individuals with powerful calendar sharing capabilities for families and workgroups. BusyCal's unique... Read more
Apple iOS 8.0 - The latest version of Ap...
The latest version of iOS can be downloaded through iTunes. Apple iOS 8 comes with big updates to apps you use every day, like Messages and Photos. A whole new way to share content with your family.... Read more
Apple Digital Camera RAW Compatibility 5...
Apple Digital Camera RAW Compatibility update adds RAW image compatibility to Aperture 3 and iPhoto '11. For more information on supported RAW formats, see here.Version 5.07: Adds RAW camera... Read more
Transmit 4.4.7 - Excellent FTP/SFTP clie...
Transmit is an excellent FTP (file transfer protocol), SFTP, S3 (Amazon.com file hosting) and iDisk/WebDAV client that allows you to upload, download, and delete files over the internet. With the... Read more
Macgo Blu-ray Player 2.10.8.1715 - Blu-r...
Macgo Mac Blu-ray Player can bring you the most unforgettable Blu-ray experience on your Mac. Overview Macgo Mac Blu-ray Player can satisfy just about every need you could possibly have in a Blu-ray... Read more
Capture One Pro 8.0.0.433 - RAW workflow...
Capture One Pro 8 is a professional RAW converter offering you ultimate image quality with accurate colors and incredible detail from more than 300 high-end cameras -- straight out of the box. It... Read more
Adobe Acrobat Pro 11.0.09 - Powerful PDF...
Adobe Acrobat allows users to communicate and collaborate more effectively and securely. Unify a wide range of content in a single organized PDF Portfolio. Collaborate through electronic document... Read more
Adobe Reader 11.0.09 - View PDF document...
Adobe Reader allows users to view PDF documents. You may not know what a PDF file is, but you've probably come across one at some point. PDF files are used by companies and even the IRS to... Read more
iFFmpeg 4.6.1 - Convert multimedia files...
iFFmpeg is a graphical front-end for FFmpeg, a command-line tool used to convert multimedia files between formats. The command line instructions can be very hard to master/understand, so iFFmpeg does... Read more
NTFS 11.3.62 - Provides full read and wr...
Paragon NTFS breaks down the barriers between Windows and OS X. Paragon NTFS effectively solves the communication problems between the Mac system and NTFS, providing full read and write access to... Read more

Latest Forum Discussions

See All

Minuum - The Little Keyboard for Big Fin...
Minuum - The Little Keyboard for Big Fingers 1.0 Device: iOS iPhone Category: Utilities Price: $1.99, Version: 1.0 (iTunes) Description: Type faster, see more of your screen, and take control of autocorrect with Minuum: the little... | Read more »
TextExpander 3 + custom keyboard (Utili...
TextExpander 3 + custom keyboard 3.0 Device: iOS Universal Category: Utilities Price: $4.99, Version: 3.0 (iTunes) Description: | Read more »
Swype - Keyboard (Utilities)
Swype - Keyboard 1.0.1411 Device: iOS Universal Category: Utilities Price: $.99, Version: 1.0.1411 (iTunes) Description: Why type when you can Swype? One of the most requested apps for iPhone is finally here. "If Swype itself ever... | Read more »
Joinz (Games)
Joinz 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Joinz is a clever block sliding puzzle game that is easy to play, nice to look at but tricky to master. You will be coming back again... | Read more »
Little World Escape Review
Little World Escape Review By Jordan Minor on September 17th, 2014 Our Rating: :: EARTHBOUNDUniversal App - Designed for iPhone and iPad Little World Escape draws players in with captivating concepts before pushing them away with... | Read more »
Light in the Dark Review
Light in the Dark Review By Nadia Oxford on September 17th, 2014 Our Rating: :: LIGHT 'EM UP UP UPUniversal App - Designed for iPhone and iPad Light in the Dark is an interesting and challenging puzzle game with some amusing bits... | Read more »
ShareFile for iPad is iOS 8-Ready with N...
ShareFile for iPad is iOS 8-Ready with New File-Editi​ng Feature Posted by Jessica Fisher on September 17th, 2014 [ permalink ] | Read more »
Pizo Animals
Pizo Animals By Amy Solomon on September 17th, 2014 Our Rating: :: DELIGHTFUL PUZZLESUniversal App - Designed for iPhone and iPad Pizo Animals is a puzzle app that includes many options adults can choose to customize the experience... | Read more »
ETA for iOS 8 Introduces Today View Exte...
ETA for iOS 8 Introduces Today View Extension and is Having a 50% Off Sale Posted by Jessica Fisher on September 17th, 2014 [ permalink ] | Read more »
Get Ready to Hit the Ice – NHL 2K is Com...
Get Ready to Hit the Ice – NHL 2K is Coming to the App Store Soon Posted by Jessica Fisher on September 17th, 2014 [ permalink ] 2K announced today that they are bringing NHL 2K< | Read more »

Price Scanner via MacPrices.net

Logitech Bluetooth Multi-Device Cross-Platfor...
Logitech has an enviable track record of making some of the best computer keyboards and mice. At least in my estimation, the best freestanding keyboards I’ve ever used have been Logitech units,... Read more
Roundup of Apple refurbished iPad Airs and iP...
Apple is offering Certified Refurbished iPad Airs for up to $140 off MSRP. Apple’s one-year warranty is included with each model, and shipping is free. Stock tends to come and go with some of these... Read more
Sprint offers 16GB iPad mini for $199.99 with...
Sprint is offering 1st generation 16GB iPad minis for $199.99 with a 2-year service agreement. Standard MSRP for this iPad is $429. Their price is the lowest available for this model. Read more
2.5GHz Mac mini remains on sale for $549, sav...
B&H Photo has the 2.5GHz Mac mini on sale for $549.99 including free shipping. That’s $50 off MSRP, and B&H will also include a free copy of Parallels Desktop software. NY sales tax only. Read more
Apple refurbished iMacs available for up to $...
The Apple Store has Apple Certified Refurbished iMacs available for up to $300 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free. These are the best prices on... Read more
13″ 2.5GHz MacBook Pro offered for $100 off M...
B&H Photo has the 13″ 2.5GHz MacBook Pro on sale for $999.99 including free shipping plus NY sales tax only. Their price is $100 off MSRP. Read more
Free GIMP Professional Grade Graphics App Ver...
The latest 2.8.14 version of the oddly-named GIMP (acronym for: GNU Image Manipulation Program) open source, high-end image editing and creation alternative to Adobe’s Photoshop and refuge from... Read more
Apple Announces Record Pre-orders for iPhone...
Apple has released metrics showing a record number of first day pre-orders of iPhone 6 and iPhone 6 Plus, with over four million sold in the first 24 hours. Demand for the new iPhones exceeds the... Read more
10% off iPhone 6 and 6 Plus Otterbox cases
Get 10% off on popular Otterbox iPhone 6 and iPhone 6 Plus cases at MacMall through September 19th. Use code OTTERBOX10 to see the discount. Read more
15-inch MacBook Pros on sale for up to $125 o...
Amazon has the new 2014 15″ Retina MacBook Pros on sale for up to $125 off MSRP including free shipping: - 15″ 2.2GHz Retina MacBook Pro: $1899.99 save $100 - 15″ 2.5GHz Retina MacBook Pro: $2374... Read more

Jobs Board

*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.