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 ;                     


 
AAPL
$101.32
Apple Inc.
+0.74
MSFT
$45.15
Microsoft Corpora
-0.07
GOOG
$582.56
Google Inc.
-0.81

MacTech Search:
Community Search:

Software Updates via MacUpdate

Garmin Express 3.2.17.0 - Manage your Ga...
Garmin Express is your essential tool for managing your Garmin devices. Update maps, golf courses and device software. You can even register your device. Update maps Update software Register your... Read more
Audio Hijack Pro 2.11.1 - Record and enh...
Audio Hijack Pro drastically changes the way you use audio on your computer, giving you the freedom to listen to audio when you want and how you want. Record and enhance any audio with Audio Hijack... Read more
calibre 2.0.0 - Complete e-library manag...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital... Read more
Apple iMovie 10.0.5 - Edit personal vide...
With an all-new design, Apple iMovie lets you enjoy your videos like never before. Browse your clips more easily, instantly share your favorite moments, and create beautiful HD movies and Hollywood-... Read more
Apple Keynote 6.2.2 - Apple's prese...
Apple Keynote makes it simple to create and deliver beautiful presentations. Powerful tools and dazzling effects bring your ideas to life. You can work seamlessly between Mac and iOS devices. And... Read more
Apple Numbers 3.2.2 - Apple's sprea...
With Apple Numbers, sophisticated spreadsheets are just the start. The whole sheet is your canvas. Just add dramatic interactive charts, tables, and images that paint a revealing picture of your data... Read more
OpenOffice 4.1.1 - Free and open-source...
OpenOffice.org is both an Open Source product and a project. The product is a multi-platform office productivity suite. It includes the key desktop applications, such as a word processor,... Read more
Pages 5.2.2 - Apple's word processo...
Apple Pages is a powerful word processor that gives you everything you need to create documents that look beautiful. And read beautifully. It lets you work seamlessly between Mac and iOS devices. And... Read more
Quicken 2015 2.0.1 - Complete personal f...
The new Quicken 2015 helps you manage all your personal finances in one place, so you can see where you're spending and where you can save. Quicken automatically categorizes your financial... Read more
CleanMyMac 2.2.7 - Delete files that was...
CleanMyMac makes space for the things you love. Sporting a range of ingenious new features, CleanMyMac 2 lets you safely and intelligently scan and clean your entire system, delete large, unused... Read more

Latest Forum Discussions

See All

Trolls vs Vikings Update Adds Over One H...
Trolls vs Vikings Update Adds Over One Hundred Levels, Reduces Item Cost, and More Posted by Ellis Spice on August 22nd, 2014 [ permalink ] | Read more »
SNK Celebrates the 20th Anniversary of T...
SNK Celebrates the 20th Anniversary of The King of Fighters With a Big Sale Posted by Ellis Spice on August 22nd, 2014 [ permalink ] | Read more »
It Came From Canada: Star Wars: Commande...
With a brand new Star Wars trilogy on the horizon, prepare yourselves for Disney and George Lucas’s space fantasy throwback to be more omnipresent than ever before. So it should come as no surprise that new adventures in that galaxy far, far away... | Read more »
Swing Copters Review
Swing Copters Review By Jordan Minor on August 22nd, 2014 Our Rating: :: DIE TRYINGUniversal App - Designed for iPhone and iPad The creator of Flappy Bird is back with a vengeance.   | Read more »
Beam Me an Update Scotty – Star Trek Tre...
Beam Me an Update Scotty – Star Trek Trexels Receives its Biggest Update Yet Posted by Jessica Fisher on August 22nd, 2014 [ permalink ] | Read more »
The Outcast Review
The Outcast Review By Nadia Oxford on August 22nd, 2014 Our Rating: :: HANDS OFF. WAY OFF.Universal App - Designed for iPhone and iPad It’s easy to see what The Outcast is trying for, but its execution needs a lot of work.   | Read more »
HeroCraft Unveils New iOS Game, Marble D...
HeroCraft Unveils New iOS Game, Marble Duel Posted by Jessica Fisher on August 22nd, 2014 [ permalink ] HeroCraft is developing a new chain popper game called Marble Duel, wh | Read more »
Brain+ Review
Brain+ Review By Nadia Oxford on August 22nd, 2014 Our Rating: :: DIM BULBUniversal App - Designed for iPhone and iPad Brain+ is just another entry in an over-saturated brain-training marketplace – and not a particularly fun entry... | Read more »
The Witcher Battle Arena – New Gameplay...
The Witcher Battle Arena – New Gameplay Trailer Revealed Posted by Jessica Fisher on August 22nd, 2014 [ permalink ] Based in the Witcher universe, | Read more »
Max Gentlemen Review
Max Gentlemen Review By Jennifer Allen on August 22nd, 2014 Our Rating: :: OUTSTAYING ITS WELCOMEiPhone App - Designed for the iPhone, compatible with the iPad Max Gentlemen seems pretty quirky initially but that appeal wears thin... | Read more »

Price Scanner via MacPrices.net

Updated Mac Price Trackers
We’ve updated our Mac Price Trackers with the latest information on prices, bundles, and availability on systems from Apple’s authorized internet/catalog resellers: - 15″ MacBook Pros - 13″ MacBook... Read more
Leftover 15-inch 2.0GHz Retina MacBook Pros a...
B&H Photo has leftover previous-generation 15″ 2.0GHz Retina MacBook Pros now available for $1599 including free shipping plus NY sales tax only. Their price is $400 off original MSRP. B&H... Read more
Pro.Calendar – New Productivity App for iPad...
Austin, Texas based mobile business and productivity app developer LightArrow, Inc. has announced Pro.Calendar, a powerful and intuitive calendar app with eight versatile calendar options including... Read more
SanDisk Ultra II SSD — Supercharge Your Syste...
SanDisk Corporation has announced the new SanDisk Ultra II SSD with enhanced SSD Dashboard. The new drive is designed to deliver a cost-effective and easy upgrade solution for PC owners looking to... Read more
Samsung and Barnes & Noble Introduce New...
Samsung Electronics America and NOOK Media, a subsidiary of Barnes & Noble, Inc. have announced the introduction of the new Samsung Galaxy Tab 4 NOOK, a 7-inch tablet combining Samsung’s leading... Read more
21-inch iMacs on sale for up to $150 off MSRP
B&H Photo has 21″ iMacs on sale for up to $150 off MSRP including free shipping plus NY sales tax only. B&H will also include a free copy of Parallels Desktop software: - 21″ 2.7GHz iMac: $... Read more
27-inch 3.2GHz iMac on sale for $1698, save $...
Abt has the 27″ 3.2GHz iMac on sale for $1698 including free shipping. Their price is $101 off MSRP. Read more
Mac Backup Guru 2.0 Drive Backup/Cloneing Uti...
Mac Backup Guru developer MacDaddy has released Mac Backup Guru 2.0, offering new and enhanced advanced features, such as bootable backups, synchronised volumes and folders, and a Snapshot mode that... Read more
Operate GE’s New Free-Standing KItchen Range...
Think you accidentally left the oven on? Switch it off while on the go. The new free-standing Profile™ Series gas and electric ranges are GE’s second cooking appliances, following their wall oven, to... Read more
Parallels Announces Parallels Desktop 10 for...
The no. 1-selling software for running Windows applications on a Mac becomes an even easier choice for millions of consumers and IT professionals worldwide with the launch of the most powerful... Read more

Jobs Board

Position Opening at *Apple* - Apple (United...
…customers purchase our products, you're the one who helps them get more out of their new Apple technology. Your day in the Apple Store is filled with a range of Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** As businesses discover the power of Apple computers and mobile devices, it's your job - as a Solutions Engineer - to show them how to introduce these Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** At the Apple Store, you connect business professionals and entrepreneurs with the tools they need in order to put Apple solutions to work in their Read more
Project Manager / Business Analyst, WW *Appl...
…a senior project manager / business analyst to work within our Worldwide Apple Fulfillment Operations and the Business Process Re-engineering team. This role will work 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.