TweetFollow Us on Twitter

Forth Decompiler
Volume Number:1
Issue Number:2
Column Tag:Forth Forum

“A Forth Decompiler”

By Joerg Langowski

“A Forth Decompiler”

Everyone of you Mac Forth users is familiar enough with Forth to know that it is a ‘threaded interpretive’ language. A Forth definition (as you type it into your machine) consists of a string of other, previously defined Forth words, and is compiled as a string of addresses that point to the definitions of these other words.

This makes for a rather fast interpretation of the resulting code. However, some of the very primitive words and those words whose execution is time-critical may also be defined in machine language. MacForth has devised a very elegant way to distinguish between Forth words defined from within Forth and machine code.

Structure of a Forth Definition

The object code of any Forth word, as it is compiled into the object dictionary, starts with at least one 16-bit word that is a meaningful executable 68000 machine language instruction. When the Forth word is executed, the interpreter simply jumps to this address. Forth definitions (colon definitions, constants, variables) now start with one of the 68000 TRAP instructions ($4E4X, where X can be anything from $0 to $F). The corresponding trap vector points to a routine which e.g. in the case of a colon definition gets the next 16-bit word and interprets it as a Forth token (converts it to an address and executes it), or - in the case of a variable - puts the address of the variable on the stack.

If the word is defined completely in machine language, the code is executed until a special JMP instruction transfers control to the next higher level (I’ll describe that later).

At this point I have to confess that I would not have come even this far if it had not been for two excellent routines that I found on a CALL-A.P.P.L.E. public domain disk. One of those - a Forth decompiler - is included below so that you can enjoy hacking into the Forth engine, the other one - a disassembler - was too long to be printed here.

An Example

Admittedly, part of the above sounds a bit dry and theoretical. Lets look at a simple example.

Assume you had defined the word TEST as follows:

: TEST DUP 2* SWAP DROP . ; 

The Forth compiler will then create a list of 16-bit words that looks like:

 $4E4F  (trap for colon definition)
 $0498  (Forth token for DUP    )
 $074E  (   “          “     “   2*     )
 $049C  (   “          “     “   SWAP)
 $00EC  (   “          “     “   DROP )
 $0EBE  (   “          “     “    .     )
 $0060      (   “          “     “  EXIT  )

Interpretation always ends at the EXIT token.

TOKENS

What are those ‘tokens’? They are the starting addresses of the Forth definitions that are offset by a constant that is contained in register A4 (probably to make the object code relocatable). There is a word in MacForth that converts a token to an address, TOKEN>ADDR. The token of a word is extracted from the vocabulary by the Forth word, FIND. Therefore, you will get the starting address of the example above by executing

FIND TEST TOKEN>ADDR .

The address that you’ll see displayed, of course, depends on how much object code you have already in your system. Let’s call this number TESTADDR. Then define the following word:

 :  TEST.DISP  7 0 DO I 2* TESTADDR ( insert your # here) + W@ . LOOP 
;

and execute TEST.DISP; you will see the list of words above.

This way you can decompile any Forth word that you find in the system. The decompiler is somewhat more convenient, of course; if you use the procedure above, you still have to convert the tokens into Forth words. This is done (for one token on the stack) by executing

 NFA ID.

This converts the token (not the pfa, as the Forth 1.1 manual says) into the name field address (NFA) and then displays the name of the word (ID.).

Machine language definitions

What if the definition is direct machine language code? Again, let us look at an example, the word SWAP. FIND SWAP TOKEN>ADDR gives (in my system) $5B60. At this address, however, we find code that does not start with a trap statement; it is a routine that does what we expect:

 202F 0004MOVE.L 4(A7),D0    /move item below top -> D0
 2F57 0004MOVE.L (A7),4(A7) /move top item one down
 2E80   MOVE.L D0,(A7)       /move D0 -> top of stack
 4ED4   JMP (A4)                  
/get next token

We see that indeed the two top stack items are exchanged. The last statement is the end of any machine language Forth definition. This jump to the address in A4 is what I briefly mentioned above. A4 contains the address of a routine that gets and executes the next Forth token from the object code (which A3 points to):

 MOVE (A3)+,D0 
/next object token -> D0
 BMI L1 
/is it neg. get address from token table
 JMP (A4,D0.W) 
/jump to start of definition (token + A4)
L1 MOVE(A4,D0.W),D1
/get address from table
 JMP (A4,D1.W) 
/and jump to start of definition

Hidden definitions

When you decompile (with the program below) the word SELECT.WINDOW, you’ll see something funny. It seems to be a regular Forth colon definition; however, the tokens displayed seem to have no name. Only ??? and the token numbers are displayed. These are tokens whose names have been deleted from the vocabulary, but their corresponding addresses (A4+token) point to valid definitions. The reason why CSI did this is probably to keep the vocabulary short and to make words inaccessible to users whose misuse could have a disasterous effect on the system. Anyway, the word SUBLEVEL in the definitions below will decompile and display any such ‘hidden’ code, if it is a colon definition. It will display nothing for machine code definitions, you have to disassemble them.

SELECT.WINDOW, with this tool, then becomes very clear. Its first level definition looks like:

: SELECT.WINDOW {2142} {1B0E}  ;

where the braces indicate those ‘no-name’ tokens. {2142} merely checks if the pointer on the stack is a valid window pointer, to keep the toolbox routine from crashing; decompile it with SUBLEVEL to see what it does exactly. {1B0E} is a 2-word machine code routine:

 $A91F  
/toolbox trap for SELECT.WINDOW
 JMP (A4) 
/and get next word. That’s all!

Listing 1: Forth decompiler
( DECOMPILER Blocks File -- Version 1.00 )        ( ADG - modif. 110384 
jl ) 
 DECOMP  ( -- )                                                 
Decompiles the definition of the next word in the input stream.  A line 
is displayed for each word in the definition. Each line begins with its 
relative code address in hex.  Next is the name of the word.  Finally, 
if the word has an in-line parameter, it is shown.  If the word is a 
branching word, the value is the target address.  If the parameter is 
a token, its name is shown.  If it is a string, the string is shown in 
double-quotes.  If it is a word or double-word, its hex value is followed 
by its decimal value.                            
#DECOMP in block 8 can also be loaded by those who wish to write a routine 
to pass tokens on the stack to be decompiled. For valid tokens, its output 
is identical to that of DECOMP .
 Written: 07/21/84   By: Alan D. Galumbeck  [70220,200]         
 NO RIGHTS RESERVED    NO RIGHTS RESERVED    NO RIGHTS RESERVED 

BASE @ DECIMAL   VARIABLE HIGH.PFA  16384 MINIMUM.OBJECT  2048 MINIMUM.VOCAB 
        : .DIGITS  ( n1\n2 --  | Types the low-order n2 digits of n1 
)  0 <# DO # LOOP #> TYPE ;          : SPACE.TO  ( n -- | Spaces to column 
n or 2 spaces if past n ) COL @ - 2 MAX SPACES ;               : DISP.WORD 
   ( pfa -- pfa+2 | Display a 16-bit parameter)     
  DUP W@ 4 .DIGITS 31 SPACE.TO DECIMAL DUP <W@ . HEX  DUP W@    
  NFA ?DUP IF 42 SPACE.TO ID. THEN  2+ ;
: DISP.DBL     ( pfa -- pfa+4 | Display a 32-bit parameter )    
  DUP @ DUP 8 .DIGITS 31 SPACE.TO DECIMAL . HEX 4+ ;  
: DISP.STRING  ( pfa -- pfa+len | Display a string parameter )  
  34 EMIT COUNT 2DUP TYPE 34 EMIT + =CELLS ;                    
: DISP.TARGET  ( base.pfa\pfa -- base.pfa\pfa+2 )               
  ( Display a branch target and save if it’s the highest )      
  DUP <W@ OVER + DUP HIGH.PFA @ > IF HIGH.PFA ! ELSE DROP THEN  
  2DUP SWAP - OVER <W@ + . 2+ ;                                 
: DISP.TOKEN   ( pfa -- pfa+2 | Display a token parameter)      
  DUP W@ NFA ?DUP  IF ID.  ELSE DUP W@ 4 .DIGITS  THEN  2+ ;    
: DISP.ADDR  ( pfa -- pfa+4 | Display an address parameter )    
  DUP @ NFA ?DUP  IF ID.  ELSE DUP @ NEXT.PTR + 8 .DIGITS THEN 4+ ;  
  
: SPECIAL.TOKENS ( base.pfa\pfa\token -- [base.pfa\next.pfa] or 
                                         [base.pfa\next.pfa\0] )
  ( Handle in-line parameters and terminating words )           
  CASE  TOKEN.FOR  EXIT       OF        0                  ENDOF
        TOKEN.FOR  (;CODE@)   OF        DISP.TOKEN 0       ENDOF
        TOKEN.FOR  COMPILE    OF        DISP.TOKEN         ENDOF
        TOKEN.FOR  0BRANCH    OF        DISP.TARGET        ENDOF
        TOKEN.FOR  BRANCH     OF        DISP.TARGET        ENDOF
        TOKEN.FOR  (OF)       OF        DISP.TARGET        ENDOF
        TOKEN.FOR  (LOOP)     OF        DISP.TARGET        ENDOF
        TOKEN.FOR  (+LOOP)    OF        DISP.TARGET        ENDOF
        TOKEN.FOR (MENU.SELECTION:) OF  DISP.TARGET        ENDOF
        TOKEN.FOR  ALIT       OF        DISP.ADDR          ENDOF
        TOKEN.FOR  WLIT       OF        DISP.WORD          ENDOF
        TOKEN.FOR  LIT        OF        DISP.DBL           ENDOF
        TOKEN.FOR  (.”)       OF        DISP.STRING        ENDOF
        TOKEN.FOR  ($LIT)     OF        DISP.STRING        ENDOF
        TOKEN.FOR  (ERROR”)   OF        DISP.STRING        ENDOF
        TOKEN.FOR  (ABORT”)   OF        DISP.STRING        ENDOF
        TOKEN.FOR  $ADDR      OF        DISP.STRING        ENDOF
        ( Insert the ones I’ve missed here. )                   
        0                     OF        2 - DISP.TOKEN     ENDOF
  ENDCASE ;  

: DECODE.TOKENS  ( pfa --  | Display the words starting at pfa )
  DUP HIGH.PFA ! DUP                                            
  BEGIN                                                         
    HEX 2DUP SWAP - CR 4 .R 2 SPACES DUP 2+ SWAP W@ DUP NFA ?DUP     
                            
      IF    ID.    ELSE .” ???” drop 0  THEN                         
        
      20 SPACE.TO SPECIAL.TOKENS ?DUP                           
      IF    FALSE  ELSE  DUP HIGH.PFA @ > THEN                       
        
  UNTIL                                                         
  2DROP ;                                                       

: .VALUE  ( n1\n2 --  | Display constants and UA variables )    
  HEX .DIGITS .”  hex  “ DECIMAL . .” decimal )”  ;             
                                                                
: DECODE.VECTOR  ( pfa\vector -- | Display definition type )    
  CASE                                                          
    11 OF .” User Area variable ( Offset = “ W@ DUP 4  .VALUE  ENDOF
    12 OF .” 16 bit constant ( Value = “ <W@ DUP 4 .VALUE  ENDOF
    13 OF .” 32 bit constant ( Value = “ @ DUP 8 .VALUE   ENDOF
    14 OF .” Variable, array, or string” DROP              ENDOF
    15 OF .” Colon definition” DECODE.TOKENS               ENDOF
    .” Unknown code type ( Vector = “ 2 .VALUE .” )”            
  ENDCASE ;                                                     
                                                                 
: CHK.CODE.TYPE  ( token -- [pfa\vector\true] or [false] |      
   Returns false for machine code definitions, true for others )
   TOKEN>ADDR DUP 2+ SWAP W@ DUP 16/ 1252 = 
   IF 15 AND TRUE ELSE 2DROP FALSE THEN ;                     
 ( Note: 1252 is the machine code for a 68000 TRAP instruction  
         divided by 16.  Vector is the low-order four bits of   
         the TRAP instruction. )                                
: sublevel chk.code.type if drop decode.tokens then ;           
                                                               
: DECOMP  ( --  | Decompile the next word in the input stream ) 
  GET.LINE.HEIGHT  GET.TEXTSIZE  BASE @  9 TEXTSIZE  10 LINE.HEIGHT  
                                  
  +FIND CR POCKET COUNT TYPE .”  -- “                           
     IF 
        IF .” IMMEDIATE “ THEN                                  
        CHK.CODE.TYPE IF DECODE.VECTOR ELSE .” Machine code definition” 
THEN                   
     ELSE .” Not in dictionary” THEN                            
  BASE !  TEXTSIZE  LINE.HEIGHT  CR ;                           
                                                                
: #DECOMP ( token --  | Decompile word whose token is supplied )
  BASE @  GET.LINE.HEIGHT  GET.TEXTSIZE  4 PICK DUP             
  9 TEXTSIZE  10 LINE.HEIGHT NFA ?DUP CR                        
     IF DUP ID.  .” -- “ C@ 128 AND                              
        IF .” IMMEDIATE “ THEN                                  
        CHK.CODE.TYPE  IF DECODE.VECTOR  ELSE .” Machine code definition” 
THEN                   
     ELSE HEX 4 .DIGITS .” -- Not a valid token” THEN           
  TEXTSIZE  LINE.HEIGHT  BASE !  DROP  CR ;                     


 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Tor Browser Bundle 7.0.7 - Anonymize Web...
The Tor Browser Bundle is an easy-to-use portable package of Tor, Vidalia, Torbutton, and a Firefox fork preconfigured to work together out of the box. It contains a modified copy of Firefox that... Read more
Data Rescue 5.0.1 - Powerful hard drive...
Data Rescue’s new and improved features let you scan, search, and recover your files faster than ever before. We have modernized the file-preview capabilities, added new files types to the recovery... Read more
Alfred 3.5.1 - Quick launcher for apps a...
Alfred is an award-winning productivity application for OS X. Alfred saves you time when you search for files online or on your Mac. Be more productive with hotkeys, keywords, and file actions at... Read more
Tunnelblick 3.7.3 - GUI for OpenVPN.
Tunnelblick is a free, open source graphic user interface for OpenVPN on OS X. It provides easy control of OpenVPN client and/or server connections. It comes as a ready-to-use application with all... Read more
DEVONthink Pro 2.9.16 - Knowledge base,...
Save 10% with our exclusive coupon code: MACUPDATE10 DEVONthink Pro is your essential assistant for today's world, where almost everything is digital. From shopping receipts to important research... Read more
AirRadar 4.0 - $9.95
With AirRadar, scanning for wireless networks is now easier and more personalized! It allows you to scan for open networks and tag them as favourites or filter them out. View detailed network... Read more
ForkLift 3.0.8 Beta - Powerful file mana...
ForkLift is a powerful file manager and ferociously fast FTP client clothed in a clean and versatile UI that offers the combination of absolute simplicity and raw power expected from a well-executed... Read more
Opera 48.0.2685.50 - High-performance We...
Opera is a fast and secure browser trusted by millions of users. With the intuitive interface, Speed Dial and visual bookmarks for organizing favorite sites, news feature with fresh, relevant content... Read more
FotoMagico 5.5 - Powerful slideshow crea...
FotoMagico lets you create professional slideshows from your photos and music with just a few, simple mouse clicks. It sports a very clean and intuitive yet powerful user interface. High image... Read more
Adobe Audition CC 2018 11.0.0 - Professi...
Audition CC 2018 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous Audition customer). Adobe Audition CC 2018 empowers you to create and... Read more

Wheels of Aurelia (Games)
Wheels of Aurelia 1.0.1 Device: iOS Universal Category: Games Price: $3.99, Version: 1.0.1 (iTunes) Description: | Read more »
Halcyon 6: Starbase Commander guide - ti...
Halcyon 6 is a well-loved indie RPG with stellar tactical combat and some pretty good writing, too. It's now landed on the App Store, so mobile fans, if you're itching for a good intergalactic adventure, here's your game. Being a strategy RPG, the... | Read more »
Game of Thrones: Conquest guide - how to...
Fans of base building games might be excited to know that yet another entry in the genre has materialized - Game of Thrones: Conquest. Yes, you can now join the many kingdoms of the famed book series, or create your own, as you try to conquer... | Read more »
Halcyon 6: Starbase Commander (Games)
Halcyon 6: Starbase Commander 1.4.2.0 Device: iOS Universal Category: Games Price: $6.99, Version: 1.4.2.0 (iTunes) Description: An epic space strategy RPG with base building, deep tactical combat, crew management, alien diplomacy,... | Read more »
Legacy of Discord celebrates its 1 year...
It’s been a thrilling first year for fans of Legacy of Discord, the stunning PvP dungeon-crawling ARPG from YOOZOO Games, and now it’s time to celebrate the game’s first anniversary. The developers are amping up the festivities with some exciting... | Read more »
3 reasons to play Thunder Armada - the n...
The bygone days of the Battleship board game might have past, but naval combat simulators still find an audience on mobile. Thunder Armada is Chinese developer Chyogames latest entry into the genre, drawing inspiration from the explosive exchanges... | Read more »
Experience a full 3D fantasy MMORPG, as...
Those hoping to sink their teeth into a meaty hack and slash RPG that encourages you to fight with others might want to check out EZFun’s new Eternity Guardians. Available to download for iOS and Android, Eternity Guardians is an MMORPG that lets... | Read more »
Warhammer Quest 2 (Games)
Warhammer Quest 2 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Dungeon adventures in the Warhammer World are back! | Read more »
4 of the best Halloween updates for mobi...
Halloween is certainly one of our favorite times for mobile game updates. Many popular titles celebrate this spooky season with fun festivities that can stretch from one week to even the whole month. As we draw closer and closer to Halloween, we'... | Read more »
Fire Rides guide - how to swing to succe...
It's another day, which means another Voodoo game has come to glue our hands to our mobile phones. Yes, it's been an especially prolific month for this particular mobile publisher, but we're certainly not complaining. Fire Rides is yet another... | Read more »

Price Scanner via MacPrices.net

Apple restocks full line of refurbished 13″ M...
Apple has restocked a full line of Apple Certified Refurbished 2017 13″ MacBook Pros for $200-$300 off MSRP. A standard Apple one-year warranty is included with each MacBook, and shipping is free.... Read more
13″ 3.1GHz/256GB MacBook Pro on sale for $167...
Amazon has the 2017 13″ 3.1GHz/256GB Space Gray MacBook Pro on sale today for $121 off MSRP including free shipping: – 13″ 3.1GHz/256GB Space Gray MacBook Pro (MPXV2LL/A): $1678 $121 off MSRP Keep an... Read more
13″ MacBook Pros on sale for up to $120 off M...
B&H Photo has 2017 13″ MacBook Pros in stock today and on sale for up to $120 off MSRP, each including free shipping plus NY & NJ sales tax only: – 13-inch 2.3GHz/128GB Space Gray MacBook... Read more
15″ MacBook Pros on sale for up to $200 off M...
B&H Photo has 15″ MacBook Pros on sale for up to $200 off MSRP. Shipping is free, and B&H charges sales tax in NY & NJ only: – 15″ 2.8GHz MacBook Pro Space Gray (MPTR2LL/A): $2249, $150... Read more
Roundup of Apple Certified Refurbished iMacs,...
Apple has a full line of Certified Refurbished 2017 21″ and 27″ iMacs available starting at $1019 and ranging up to $350 off original MSRP. Apple’s one-year warranty is standard, and shipping is free... Read more
Sale! 27″ 3.8GHz 5K iMac for $2098, save $201...
Amazon has the 27″ 3.8GHz 5K iMac (MNED2LL/A) on sale today for $2098 including free shipping. Their price is $201 off MSRP, and it’s the lowest price available for this model (Apple’s $1949... Read more
Sale! 10″ Apple WiFi iPad Pros for up to $100...
B&H Photo has 10.5″ WiFi iPad Pros in stock today and on sale for $50-$100 off MSRP. Each iPad includes free shipping, and B&H charges sales tax in NY & NJ only: – 10.5″ 64GB iPad Pro: $... Read more
Apple iMacs on sale for up to $130 off MSRP w...
B&H Photo has 21-inch and 27-inch iMacs in stock and on sale for up to $130 off MSRP including free shipping. B&H charges sales tax in NY & NJ only: – 27″ 3.8GHz iMac (MNED2LL/A): $2179 $... Read more
2017 3.5GHz 6-Core Mac Pro on sale for $2799,...
B&H Photo has the 2017 3.5GHz 6-Core Mac Pro (MD878LL/A) on sale today for $2799 including free shipping plus NY & NJ sales tax only . Their price is $200 off MSRP. Read more
12″ 1.2GHz Space Gray MacBook on sale for $11...
Amazon has the 2017 12″ 1.2GHz Space Gray Retina MacBook on sale for $100 off MSRP. Shipping is free: 12″ 1.2GHz Space Gray MacBook: $1199.99 $100 off MSRP Read more

Jobs Board

Product Manager - *Apple* Pay on the *Appl...
Job Summary Apple is looking for a talented product manager to drive the expansion of Apple Pay on the Apple Online Store. This position includes a unique Read more
*Apple* Retail - Multiple Positions - Farmin...
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
Frameworks Engineer, *Apple* Watch - Apple...
Job Summary Join the team that is shaping the future of software development for Apple Watch! As a software engineer on the Apple Watch Frameworks team you will Read more
*Apple* News Product Marketing Mgr., Publish...
Job Summary The Apple News Product Marketing Manager will work closely with a cross-functional group to assist in defining and marketing new features and services. Read more
Fraud Analyst, *Apple* Advertising Platform...
Job Summary Apple Ad Platforms has an opportunity to redefine advertising on mobile devices. Apple reaches hundreds of millions of iPhone, iPod touch, and iPad Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.