















                           -----------------------------



                           INSIDE TURBO PASCAL 6.0 UNITS



                           -----------------------------























                                         by

                                  William L. Peavy

                                 -----------------

                              Revised: April 16, 1991






                                      ABSTRACT

            If you  want to  know what is in a .TPU (unit) file produced
            by Version  6.0 of  Turbo Pascal from Borland International,
            then this  paper is  for you.    It  doesn't  explain  quite
            everything since the I don't have access to secret documents
            or anything  like that  and since  some of  the data in .TPU
            files just doesn't have enough auxiliary information to make
            its role  clear.   However, it  is possible to learn a great
            deal about  how Turbo  Pascal organizes  the information  it
            needs to  refer to,  and it  is also  possible to learn just
            what kind of code the compiler produces.

            This is  the third  in a series of reports on the subject of
            Turbo Pascal  Units, the  first treating  with Turbo  Pascal
            Version 5.0  and the  second with  Turbo Pascal  5.5.    The
            evolution  of   these  files   in  the   face  of   changing
            requirements has  been fascinating to behold and deciphering
            their contents has been challenging to say the least.

            The programs supplied with this report have been reorganized
            from their  5.5 style in some ways and many identifiers have
            been changed.   These  changes were  more for style than for
            substance.   Other changes  were dictated  by the changes in
            the organization  of the  TPU file itself and certain errors
            in the 5.5 programs have been corrected.  In addition, other
            errors of  interpretation have  been fixed  which has led to
            some enhanced descriptive capability.

            Since I  have a "real" job which requires my full attention,
            and since  it doesn't  involve use  of these products in any
            direct way,  I am  usually hard-pressed to find the personal
            time to  conduct this  research.    Consequently,  I  always
            refuse to  commit to follow-up or even error correction.  It
            would  be  irresponsible  of  me  to  pretend  it  could  be
            otherwise.  Even so, this is a revised report which contains
            a few  error fixes  and discusses the newly enhanced program
            which incorporates  these fixes  and  sports  some  enhanced
            capabilities.



                                      Contents



           Introduction ................................................. 5

           1. Gross File Structure ...................................... 5
               1.1 User Units ........................................... 6

           2. Locators .................................................. 7
               2.1 Local Links .......................................... 7
               2.2 Global Links ......................................... 7
               2.3 Table Offsets ........................................ 7
               2.4 Basic Relationships .................................. 8

           3. Unit Header .............................................. 11
               3.1 Description ......................................... 11
               3.2 UNIT Size ........................................... 14

           4. Symbol Dictionaries ...................................... 14
               4.1 Organization ........................................ 14
               4.2 Interface Dictionary ................................ 14
               4.3 Debug Dictionary .................................... 15
               4.4 Dictionary Elements ................................. 15

                   4.4.1 Hash Tables ................................... 15
                       4.4.1.1 Size .................................... 16
                       4.4.1.2 Scope ................................... 16
                       4.4.1.3 Special Cases ........................... 17

                   4.4.2 Dictionary Headers ............................ 17

                   4.4.3 Dictionary Stubs .............................. 18
                       4.4.3.1 Label Declaratives ("O") ................ 18
                       4.4.3.2 Un-Typed Constants ("P") ................ 18
                       4.4.3.3 Named Types ("Q") ....................... 18
                       4.4.3.4 Variables, Fields, Typed Cons ("R") ..... 19
                       4.4.3.5 Subprograms & Methods ("S") ............. 20
                       4.4.3.6 Turbo Std Procedures ("T") .............. 21
                       4.4.3.7 Turbo Std Functions ("U") ............... 21
                       4.4.3.8 Turbo Std "NEW" Routine ("V") ........... 21
                       4.4.3.9 Turbo Std Port Arrays ("W") ............. 21
                       4.4.3.10 Turbo Std External Variables ("X") ..... 21
                       4.4.3.11 Units ("Y") ............................ 22

                   4.4.4 Type Descriptors .............................. 22
                       4.4.4.1 Scope ................................... 23
                       4.4.4.2 Prefix Part ............................. 23
                       4.4.4.3 Suffix Parts ............................ 24
                           4.4.4.3.1 Un-Typed .......................... 25
                           4.4.4.3.2 Structured Types .................. 25
                               4.4.4.3.2.1 ARRAY Types ................. 25
                               4.4.4.3.2.2 RECORD Types ................ 25
                               4.4.4.3.2.3 OBJECT Types ................ 26
                               4.4.4.3.2.4 FILE (non-TEXT) Types ....... 27
                               4.4.4.3.2.5 TEXT File Types ............. 27
                               4.4.4.3.2.6 SET Types ................... 27



                                       - iii -



                                      Contents


                               4.4.4.3.2.7 POINTER Types ............... 27
                               4.4.4.3.2.8 STRING Types ................ 27
                           4.4.4.3.3 Floating-Point Types .............. 27
                           4.4.4.3.4 Ordinal Types ..................... 28
                               4.4.4.3.4.1 "Integers" .................. 28
                               4.4.4.3.4.2 BOOLEANs .................... 28
                               4.4.4.3.4.3 CHARs ....................... 28
                               4.4.4.3.4.4 ENUMERATions ................ 29
                           4.4.4.3.5 SUBPROGRAM Types .................. 29

           5. Maps and Lists ........................................... 30
               5.1 PROC Map ............................................ 30
               5.2 CSeg Map ............................................ 31
               5.3 Typed CONST DSeg Map ................................ 31
               5.4 Global VAR DSeg Map ................................. 32
               5.5 Donor Unit List ..................................... 32
               5.6 Source File List .................................... 33
               5.7 DEBUG Trace Table ................................... 34

           6. Code, Data, Fix-Up Info .................................. 35
               6.1 Object CSegs ........................................ 35
               6.2 CONST DSegs ......................................... 35
               6.3 Fix-Up Data Table ................................... 36

           7. Supplied Program ......................................... 37

               7.1 TPU6 ................................................ 37
                   7.1.1 UNIT TPU6AMS .................................. 37
                   7.1.2 UNIT TPU6EQU .................................. 38
                   7.1.3 UNIT TPU6UTL .................................. 38
                   7.1.4 UNIT TPU6RPT .................................. 38
                   7.1.5 UNIT TPU6UNA .................................. 38

               7.2 Modifications ....................................... 39

               7.3 Notes on Program Logic .............................. 39
                   7.3.1 Formatting the Dictionary ..................... 39
                   7.3.2 The Disassembler .............................. 41

           8. Unit Libraries ........................................... 43
               8.1 Library Structure ................................... 43

           9. Application Notes ........................................ 44

           10. Acknowledgements ........................................ 45

           11. References .............................................. 46

           INDEX ....................................................... 47








                                       - iv -



                           Inside TURBO Pascal 6.0 Units              
       ----------------------------------------------------------------------

       INTRODUCTION


       This document  is  the  outcome  of  an  inquiry  conducted  into  the
       structure and  content of  Borland Turbo  Pascal  (Version  6.0)  Unit
       files.   The original  purpose of the inquiry was to provide a body of
       theory enabling  Cross-Reference programs  to  resolve  references  to
       symbols defined  in .TPU  files where qualification was not explicitly
       provided.   As is  so often the case, one thing led to another and the
       scope of  the inquiry  was expanded dramatically.  While this document
       should not be regarded as definitive, the author feels that the entire
       Turbo Pascal  User community might gain from the information extracted
       from these files at the cost of so much time and effort.

       The  material   contained   herein   represents   the   findings   and
       interpretations of  the author.    A  great  deal  of  guess-work  was
       required and  no assurances are given as to the accuracy of either the
       findings of fact or the inferences contained herein which are the sole
       work-product of the author.  In particular, the author had access only
       to materials  or information  that any  normal  Borland  customer  has
       access to.   Further,  no Borland  source-codes were  available as the
       Library Routine source is not licensed to the author.  In short, there
       was nothing irregular about how these findings were achieved.

       The material  contained herein  is placed in the public domain free of
       copyright for  use of  the general public at its own risk.  The author
       assumes no  liability for  any damages  arising from  the use  of this
       material by  others.   If you make use of this information and you get
       burned, TOUGH!   The  author accepts no obligation to correct any such
       errors as  may exist  in the  supplied programs  or in the findings of
       fact or  opinion contained  herein.   On the other hand, this is not a
       "complete" work in that a great many questions remain open, especially
       as regards fine details.  (The author is not highly-qualified in Intel
       80xxx Assembly  Language and  several open  questions  might  best  be
       addressed by persons competent in this area.)  The author welcomes the
       input of  interested readers  who might be able to "flesh-out" some of
       these open questions with "hard" answers.


       1. GROSS FILE STRUCTURE


       A Turbo  Pascal Unit  file consists  of an array of bytes that is some
       exact multiple  of sixteen  (16).   "Signature" information allows the
       compiler to  verify that  the .TPU  file was compiled with the correct
       compiler version  and to  verify that the file is of the correct size.
       The fine  structure of the file will be addressed in later sections at
       ever increasing levels of detail.








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                           Inside TURBO Pascal 6.0 Units              
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       Graphically, the  file may be regarded as having the following general
       layout:

             +-------------------+
             | Unit Header       |        Main Index to Unit File
             |-------------------|
             | Dictionaries:     |
             |   a) Interface    |
             |   b) Debug      * |        For Local Symbol Access
             |-------------------|
             | PROC Map          |
             |-------------------|
             | CSeg Map        * |        May be Empty
             |-------------------|
             | CONST DSeg Map  * |        May be Empty
             |-------------------|
             | VAR DSeg Map    * |        May be Empty
             |-------------------|
             | Donor Units     * |        May be Empty
             |-------------------|
             | Source Files      |
             |-------------------|
             | Trace Table     * |        May be Empty
             |-------------------|
             | CODE Segment(s) * |        May be Empty
             |-------------------|
             | DATA Segment(s) * |        May be Empty
             |-------------------|
             | FIX-UP Data     * |        May be Empty
             +-------------------+


       1.1 USER UNITS


       Units prepared by the compiler available to ordinary users have a very
       straight-forward appearance  and content.   There may even be a little
       "wasted" space  that might  be removed  if the  compiler were  just  a
       little cleverer.  The SYSTEM.TPU file is quite another thing however.

       The SYSTEM.TPU  file (found  in TURBO.TPL)  is extraordinary  in  that
       great pains  seem to  have been  taken to  compact it.    Further,  it
       contains a  great many  types of  entries that  just don't  seem to be
       achievable by  ordinary users  and I suspect that much (if not all) of
       it was "hand-coded" in Assembler Language.

       In the  following sections, the details of these optimizations will be
       explained  in  the  context  of  the  structural  element  then  under
       discussion.







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                           Inside TURBO Pascal 6.0 Units              
       ----------------------------------------------------------------------

       2. LOCATORS


       The data  in these  files has  need of  structure and  organization to
       support efficient access by the various programs such as the compiler,
       the linker  and the  debugger.   This organization is built on a solid
       foundation of locators employed in the unit's data structures.



       2.1 LOCAL LINKS


       Local Links  (LL's) are  items of type WORD (2 bytes) which contain an
       offset which  is relative to the origin of the unit file itself.  This
       implies that  a unit must be somewhat less than 64K bytes in size.  If
       the .TPU  file is  loaded into  the heap,  then an  LL can  be used to
       locate any  byte in  the segment  beginning with the load point of the
       file.



       2.2 GLOBAL LINKS


       Global Links  (LG's) are used to locate type descriptors and to locate
       allocation data  for variables  with the  ABSOLUTE attribute which may
       reside in  other Units  (i.e., units  external to  the present  unit).
       LG's are  structured items  consisting of two (2) words.  The first of
       these is  an LL  that is  relative to  the origin  of  the  (possibly)
       external unit.  It locates either a Type Descriptor or the stub of the
       Dictionary entry  which establishes  storage allocation.   The  second
       word is  an LL which locates the stub of the unit entry in the current
       unit dictionary  for the  (possibly) external  unit.   This dictionary
       entry provides  the name  of the  unit that  contains the  item the LG
       points to.

       This provides  a handy  mechanism for  locating type  descriptors  and
       allocation information  which  may  be  defined  in  other  separately
       compiled units.



       2.3 TABLE OFFSETS


       Finally, various  data-structures within  a .TPU file are organized as
       arrays of fixed-length records or as lists of variable-length records.
       Efficient access  to such  records is  achieved by  means  of  offsets
       rather than subscripts (an addressing technique denied Pascal).  These
       offsets are  relative to  the  origin  of  the  array  or  list  being
       referenced rather than the origin of the unit.




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                           Inside TURBO Pascal 6.0 Units              
       ----------------------------------------------------------------------

       2.4 BASIC RELATIONSHIPS


          +-------------+                            +----------------------+
          |    Unit     |                            | INTERFACE Dictionary |
          |   Header    |                            |                      |
          +-------------+                            | Public and Private   |
                 |                                   | Names, Nested Hash   |
                 |   LL    +----------------+  LL's  | Tables, INLINE code, |
                 |-------->| INTERFACE Hash |------->| Type Descriptors.    |
                 |         +----------------+        +----------------------+
                 |                                        (LL's ^ & LG's)
                 |                                   +----------------------+
                 |   LL    +----------------+  LL's  |   DEBUG Dictionary   |
                 |-------->|   DEBUG Hash   |------->| IMPLEMENTATION and   |
                 |         +----------------+        | nested scope names,  |
                 |                                  ?| stored for DEBUG.    |
                 |   LL    +----------------+        | Same structure as in |
                 |-------->| PROC Map Table |        | INTERFACE.  Linked   |
                 |         +----------------+        | to INTERFACE part by |
                 |   LL    +----------------+        | LL's.  BUILT ONLY IF |
                 |-------->| CSeg Map Table |?       | LOCAL SYMBOLS ARE    |
                 |         +----------------+        | ENABLED AT COMPILE.  |
                 |   LL    +----------------+        +----------------------+
                 |-------->| DSeg Map CONST |?
                 |         +----------------+
                 |   LL    +----------------+
                 |-------->| DSeg Map VAR's |?
                 |         +----------------+             IMPORTANT NOTES
                 |   LL    +----------------+         ----------------------
                 |-------->| Donor Unit List|?        Some of the structures
                 |         +----------------+         shown in this figure
                 |   LL    +------------------+       are built only if they
                 |-------->| Source File List |       are needed.  These are
                 |         +------------------+       marked by a "?" next
                 |   LL    +------------------+       to the box.
                 |-------->| Debug Step Ctls  |?
                 |         +------------------+       If the DEBUG Dictionary
                 |   **    +---------------+          is missing, its LL
                 |-------->| CODE Segments |?         leads directly to the
                 |         +---------------+          INTERFACE Dictionary.
                 |   **    +-----------------+        ----------------------
                 |-------->| CONST DATA Segs |?
                 |         +-----------------+
                 |   **    +----------------+
                 +-------->| Fix-Up Lists   |?
                           +----------------+



       This figure  illustrates the role of the Unit Header in tying together
       the various  data structures  in the  Unit.  The type of link is shown
       next to a flow-line by "LL", "LG" or "**".  "LL" and "LG" are explicit
       pointers while  "**" shows  a locator  whose value  is computed  using
       other data in the Unit Header and that no explicit pointer exists.

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       Rev: April 16, 1991                                             Page 8



                           Inside TURBO Pascal 6.0 Units              
       ----------------------------------------------------------------------


        +----(from hash tables,other Dictionary Entries)
        |
        |    +------------------------------------------------+
        |    | Header Part | Stub Part -- many formats        |
        +--->| - - - - - - | - - - +------------------------- |
             |             | data, | Some stubs have embedded | Dictionary
             | Name, Class | links | Type Descriptors         |   Entry
             | and link to | (see  |     +------------------- |
             | entries who | below)|     | INLINE Declarative |
             | have same   |   *   |     | code bytes for a   |
             | hash        |   |   |     | "macro" type PROC  |
             +-----------------|------------------------------+
                    +----------+
                    |
                    | FAR pntr   +----------------------------+
                    |----------->| Absolute Memory Locations  |
                    |            +----------------------------+
                    |            +-----------------------------+
                    |   LG's     | Type Descriptors and stubs  |
                    |----------->| of Dictionary Entries used  |
                    |            | for absolute equivalences   |
                    |            +-----------------------------+
                    |            +---------------------------------+
                    |   LL's     | Nested Scope Hash Tables        |
                    |----------->| Parent Scope Dictionary Entries |
                    |            | Record Fields                   |
                    |            | Object Fields/Methods           |
                    |            +---------------------------------+
                    |            +----------------------+
                    | Offsets    | CONST DSeg Map Table |
                    +----------->| PROC Map Table       |
                                 | VAR DSeg Map Table   |
                                 +----------------------+



       This figure illustrates the many types of entities that associate with
       Dictionary Entries and particularly with their Stub Parts.  Not all of
       the links shown occur in a single Stub format, but all of the links in
       the figure can and do exist in selected cases.  The purpose here is to
       show the  flexibility of  the system  of links in associating required
       data with the Dictionary Entry and its identifying symbol.

       While it may not be apparent from the figure, the dictionary structure
       as a whole may be viewed as a cyclic directed graph which is rooted in
       the DEBUG  Hash Table.  The recursive properties exhibited by the node
       relationships permit direct support of the scope rules of Turbo Pascal
       with simplicity and elegance.  As one might expect, the representation
       of the  required information  lends itself to efficient use of storage
       since the  representations are compact and there is very little in the
       way of  redundancy.  The small amount of redundancy that does exist is
       apparently aimed at speeding access to certain structures by the Turbo
       components (compiler, linker and debugger).


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                           Inside TURBO Pascal 6.0 Units              
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        +----(implied links, explicit LG's from other structures)
        |
        |    +---------------------------------------------+
        |    | Flags and codes, allocation widths for data |  Type
        +--->| and VMT's, subrange constraints, formal     |  Descriptor
             | parameter descriptors, implicit associated  |  Contents &
             | type descriptors, LL's, LG's and Offsets.   |  Linkages
             +---------------------------------------------+
                    |
                    |
                    |    LG's       +------------------+
                    |-------------->| Type Descriptors |
                    |               +------------------+
                    |
                    |               +-------------------------------+
                    |    LL's       | Method Dictionary Entries     |
                    |-------------->| Nested Scope Hash Tables      |
                    |               | Nested Scope Field Chains     |
                    |               | Parent Scope Dictionary Entry |
                    |               +-------------------------------+
                    |
                    |  Offsets      +----------------------------------+
                    +-------------->| VMT pointers in Object Instances |
                                    | CONST DSeg Map Table Entries     |
                                    +----------------------------------+


       This figure illustrates the relationships between Type Descriptors and
       other structures in the dictionary.  Not all the links shown can exist
       with a single Type Descriptor since there are several variant forms of
       these descriptors  (depending on  base type) but in combination, these
       linkages are  feasible.   In addition to links, a great amount of data
       is stored  which is peculiar to a given type declaration.  Descriptors
       can be  -- and are -- shared.  Indeed, they were designed with that in
       mind.   Once a  named type is declared, all entities that reference it
       are linked to it in some way (usually by an LG).

       Almost every  form of  type descriptor is found in the SYSTEM unit and
       this fact is used to advantage.  When un-typed constants are declared,
       a built-in  type descriptor  is referenced  (via an LG) which provides
       necessary information for maintenance of orderly dictionary structure.
       When a  named-type is declared, it is almost always decomposed into an
       expression based on the built-in types of Turbo Pascal which are found
       in the SYSTEM unit with the aid of an LG.

       The semantics  underlying the  idea of  the  Unit  mandate  this  very
       approach since  program modules  of any class which make references to
       units for  definitions use  the definitions as implemented by the unit
       which contains them.  Re-defining the unit or any of its defined types
       leads to  a natural  requirement to  re-compile those  program modules
       which rely  on the  unit for  definitions.   The impact is fundamental
       since the  storage representation  of a  unit-defined named  type  can
       change in quite radical ways.


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                           Inside TURBO Pascal 6.0 Units              
       ----------------------------------------------------------------------



       3. UNIT HEADER


       The Unit  Header comprises  the first  64 bytes  of the .TPU file.  It
       contains LL's  that effectively  locate all other sections of the .TPU
       file plus  statistics  that  enable  a  little  cross-checking  to  be
       performed.   Some parts  of the  Unit Header appear to be reserved for
       future use  since no  unit examined  by this author has ever contained
       non-zero data in these apparently reserved fields.



       3.1 DESCRIPTION


       The Unit Header provides a high-level locator table whereby each major
       structure in the unit file can be addressed.  The following provides a
       Pascal-like explanation  of the  layout  of  the  header  followed  by
       further narrative  discussion of the contents of the individual fields
       in the Unit Header.

             Type HdrAry = Array[0..3] of Char; LL = Word;

             UnitHeader = Record

              UHEYE : HdrAry;   { +00 : = 'TPU9'                      }
              UHxxx : HdrAry;   { +04 : = $00000000                   }
              UHUDH : LL;       { +08 : to Dictionary Head-This Unit  }
              UGIHT : LL;       { +0A : to Hash Table (INTERFACE)     }
              UHPMT : LL;       { +0C : to PROC Map                   }
              UHCMT : LL;       { +0E : to CSeg Map                   }
              UHTMT : LL;       { +10 : to DSeg Map-Typed CONST's     }
              UHDMT : LL;       { +12 : to DSeg Map-GLOBAL Variables  }
              UHxxy : LL;       { +14 : Purpose Unknown               }
              UHLDU : LL;       { +16 : to Donor Unit List            }
              UHLSF : LL;       { +18 : to Source file List           }
              UHDBT : LL;       { +1A : to Debug Trace Step Controls  }
              UHENC : LL;       { +1C : to end non-code part of Unit  }
              UHZCS : Word;     { +1E : Size of CSEGs (aggregate)     }
              UHZDT : Word;     { +20 : Size of Typed Constant Data   }
              UHZFA : Word;     { +22 : Fix-Up Bytes (CSegs)          }
              UHZFT : Word;     { +24 : Fix-Up Bytes (Typed CONST's)  }
              UHZFV : Word;     { +26 : Size of GLOBAL VAR Data       }
              UHDHT : LL;       { +28 : to Hash Table (DEBUG)         }
              UHSOV : Word;     { +2A : Overlay Involved if non-zero  }
              UHPad : Array[0..9]
                      of Word;  { +2C : Reserved for Future Expansion }

             End; { UnitHeader }

         UHEYE   contains the  characters "TPU9"  in that  order.   This is
                 clear evidence that this unit was compiled by Turbo Pascal
                 Version 6.0.

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                           Inside TURBO Pascal 6.0 Units              
       ----------------------------------------------------------------------

         UHxxx   is apparently reserved and contains binary zeros.

         UHUDH   contains an  LL (WORD)  which  points  to  the  Dictionary
                 Header in which the name of this unit is found.

         UHIHT   contains an LL (WORD) which points to a Hash table that is
                 the root of the Interface Dictionary graph.

         UHPMT   contains an  LL (WORD)  which points  to the  PROC Map for
                 this unit.   The  PROC Map  contains  an  entry  for  each
                 Procedure or  Function declared  in the  unit (except  for
                 INLINE types),  plus an  entry for the Unit Initialization
                 section.   The length  of  the  PROC  Map  (in  bytes)  is
                 determined by subtracting this UHPMT from UHCMT.

         UHCMT   contains an  LL (WORD)  which points  to  the  CSeg  (CODE
                 Segment) Map  for this  unit.   The CSeg  Map contains  an
                 entry for  each CODE Segment produced by the compiler plus
                 an entry  for each  of the  CODE Segments included via the
                 {$L filename.OBJ}  compiler directive.  The length of this
                 Map (in  bytes) is  obtained  by  subtracting  UNCMT  from
                 UHTMT.   The result may be zero in which case the CSeg Map
                 is empty.

         UHTMT   contains an  LL (WORD)  which points  to  the  DSeg  (DATA
                 Segment) Map  that maps  the initializing  data for  Typed
                 CONST items  plus  templates  for  VMT's  (Virtual  Method
                 Tables) that  are associated  with  OBJECTS  which  employ
                 Virtual Methods.   The  length of  this Map  (in bytes) is
                 obtained by  subtracting UHTMT from UHDMT.  The result may
                 be zero in which case this DSeg Map is empty.

         UHDMT   contains an  LL (WORD)  which points  to  the  DSeg  (DATA
                 Segment) Map  that contains  the specifications  for  DSeg
                 storage required  by VARiables whose scope is GLOBAL.  The
                 length of  this Map  (in bytes) is obtained by subtracting
                 UHDMT from  UHxxy.   The result  may be zero in which case
                 this DSeg Map is empty.

         UHxxy   Purpose of  this word is unknown.  No non-zero values have
                 ever been observed here.  (May be for TP-Windows?)

         UHLDU   contains an  LL (WORD)  which points  to a  table of units
                 which contribute  either CODE or DATA Segments to the .EXE
                 file for  a program  using this  Unit.  This is called the
                 "Donor Unit  Table".   The length of this table (in bytes)
                 is obtained by subtracting UHLDU from the word UHLSF.  The
                 result may be zero in which case this table is empty.








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                           Inside TURBO Pascal 6.0 Units              
       ----------------------------------------------------------------------

         UHLSF   contains an  LL (WORD)  which points to a list of "source"
                 files.   These are  the files  whose CODE or DATA Segments
                 are included  in this  Unit by the compiler.  Examples are
                 the Pascal Source for the Unit itself, plus the .OBJ files
                 included via  the {$L  filename.OBJ}  compiler  directive.
                 The length  of  this  table  (in  bytes)  is  obtained  by
                 subtracting UHLSF  from the word UHDBT.  The result may be
                 zero in which case this table is empty.

         UHDBT   contains an  LL (WORD)  which points to a Trace Table used
                 by the  DEBUGGER for  "stepping"  through  a  Function  or
                 Procedure contained  in this  Unit.   The length  of  this
                 table (in bytes) is obtained by subtracting UHDBT from the
                 word UHENC.   The  result may  be zero  in which case this
                 table is empty.

         UHENC   contains an  LL (WORD) which points to the first free byte
                 which follows  the Trace  Table (if  any).  It serves as a
                 delimiter for  determining the  size of  the Trace  Table.
                 This LL  (when rounded up to the next integral multiple of
                 16) serves to locate the start of the code/data segments.

         UHZCS   is a  WORD that  contains the total byte count of all CODE
                 Segments compiled into this Unit.

         UHZDT   is a  WORD that contains the total byte count of all Typed
                 CONST and VMT DATA Segments compiled into this unit.

         UHZFA   is a WORD that contains the total byte count of the Fix-Up
                 Data Table for this unit for CODE (CSegs).

         UHZFT   is a WORD that contains the total byte count of the Fix-Up
                 Data Table for Typed CONST's.  This usually implies that a
                 VMT is getting its pointers relocated.

         UHZFV   is a WORD that contains the total byte count of all GLOBAL
                 VAR DATA Segments compiled into this unit.

         UHDHT   contains an  LL (WORD)  which points to a Hash Table which
                 is the  root of the DEBUGGER Dictionary.  If Local Symbols
                 were generated  by the compiler (directive {$L+}) then ALL
                 symbols declared  in the  unit can  be accessed  from this
                 Hash Table.   If Local Symbols were suppressed there is no
                 such Dictionary  and the  LL stored  here  points  to  the
                 INTERFACE Dictionary.

         UHSOV   Purpose of  this word is unknown.  It has been observed to
                 be non-zero  when overlay  directives are  used.   So  far
                 however, this  hasn't enabled  me to  come up  with a good
                 guess as to just what the observed values actually mean.

         UHPad   begins a  series of  ten (10)  words that  are  apparently
                 reserved for future use.  Nothing but zeros have ever been
                 seen here by this author.


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                           Inside TURBO Pascal 6.0 Units              
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       3.2 UNIT SIZE


       An independent  check on  the size of the .TPU file is available using
       information contained  in the Unit Header.  This is also important for
       .TPL (Unit Library) organization.  To compute the file :size, refer to
       the five  (5) words  -- UHENC,  UHZCS, UHZDT, UHZFA, and UHZFT.  Round
       the contents  of each of these words to the lowest multiple of 16 that
       is greater  than or  equal to the content of that word.  Then form the
       sum of the rounded words.  This is the .TPU file size in bytes.


       4. SYMBOL DICTIONARIES


       This area contains all available documentation of declared symbols and
       procedure blocks  defined within  the unit.    Depending  on  compiler
       options in  effect when  the unit  was  compiled,  this  section  will
       contain at  a minimum,  the INTERFACE  declarations, and at a maximum,
       ALL declarations.   The information stored in the dictionary is highly
       dependent on  the context  of the  symbol declared.   We defer further
       explanation to the appropriate section which follows.


       4.1 ORGANIZATION


       A dictionary  is organized  with a  Hash Table  as its root.  The hash
       table is used to provide rapid access to identifiers.

       A dictionary  may be thought of as a directed graph.  Each subgraph is
       rooted in  a hash  table.   There may be a great many hash tables in a
       given unit  and their number depends on unit complexity as well as the
       options chosen when the unit was compiled.  Use of the {$L+} directive
       produces the  largest dictionaries.   The hash tables are explained in
       detail a few sections further on.

       Hash tables  point to  Dictionary Headers.   When  two or more symbols
       produce the  same hash  function result, a collision is said to occur.
       Collisions  are  resolved  by  the  time-honored  method  of  chaining
       together the  Dictionary Headers of those symbols having the same hash
       function result.   Dictionary supersetting is accomplished using these
       chains.


       4.2 INTERFACE DICTIONARY


       The INTERFACE  dictionary  contains  all  symbols  and  the  necessary
       explanatory data  for the  INTERFACE section  of a  Unit.  Symbols get
       added to  the  Unit  using  increasing  storage  addresses  until  the
       IMPLEMENTATION section is encountered.



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       4.3 DEBUG DICTIONARY


       The Debug  dictionary (if  present) is  a superset  of  the  INTERFACE
       dictionary.   It is  used by  the Turbo  Debugger to  support its many
       features when  tracing through a unit.  If present, this dictionary is
       rooted in  its  own  hash  table.    The  hash  table  is  effectively
       initialized when  the  IMPLEMENTATION  keyword  is  processed  by  the
       compiler.   This takes  the form  (initially) of an unmodified copy of
       the INTERFACE  hash table,  to which  symbols are  added in  the usual
       fashion.   Thus, the  hash chains constructed or extended at this time
       lead naturally to the INTERFACE chains and this is how the superset is
       effectively implemented.


       4.4 DICTIONARY ELEMENTS


       The dictionary contains four major elements.  These are:  hash tables,
       Dictionary Headers,  Dictionary  Stubs  and  Type  Descriptors.    The
       distinction between  Dictionary Headers  and Stubs  might appear to be
       rather arbitrary.   They  might just as easily be regarded as a single
       element (such  as symbol  entry).   However, the case for the separate
       entity approach  is strong since Stubs are DIRECTLY addressed via LG's
       and --  more to  the point -- ONLY by LG's.  Thus, it seems reasonable
       that this  is a  separate and  very important structure -- at least in
       the minds of the architects at Borland.


       4.4.1 HASH TABLES


       As has  been intimated,  Hash Tables  are  the  glue  that  binds  the
       dictionary entries  together and  gives the  dictionary  its  "shape".
       They effectively  implement the  scope rules of the language and speed
       access to essential information.

       Each Hash table begins with a 2-byte size descriptor.  This descriptor
       contains the  number of bytes in the table proper (less 2).  Thus, the
       descriptor directly  points to the last bucket in the hash table.  For
       a hash  table of  128 bytes,  the size  descriptor contains  126.  The
       first bucket in the table immediately follows the size descriptor.














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       4.4.1.1 SIZE


       So far,  three different  hash table  sizes have  been observed.   The
       INTERFACE and  DEBUG hash tables are usually 128 bytes (64 entries) in
       size plus  2 bytes  of size  description, but the SYSTEM.TPU unit is a
       special case,  containing only  16 entries.   Hash tables which anchor
       subgraphs whose  scope is  relatively local  usually contain  four (4)
       entries (8 bytes).

       Graphically, a Hash Table with four slots has the following layout:

             +--------------------+
             |       0006h        |      Size Descriptor
             |--------------------|
             |       slot 0       |      an LL or zero
             |--------------------|
             |       slot 1       |      an LL or zero
             |--------------------|
             |       slot 2       |      an LL or zero
             |--------------------|
             |       slot 3       |      an LL or zero
             +--------------------+

       It should  be noted  that the Size Descriptor furnishes an upper bound
       for the  hash function  itself.  Thus, it seems possible that a single
       hash function is used for all hash tables and that its result is ANDed
       with the  Size Descriptor  to get the final result.  Because the sizes
       are chosen  as they  are (powers of 2) this is feasible.  Note that in
       the above  example, 6  = 2 * (n - 1) where n = 4 {slot count}.  All of
       the hash tables observed so far have this property.

       One final  note on this subject.  Given these properties, "Folding" of
       sparse hash  tables is  a rather  trivial exercise  so long as the new
       hash table also contains a number of slots that is a power of 2.  This
       point is  intriguing when  one recalls  that the SYSTEM.TPU hash table
       has only 16 slots rather than the usual 64.



       4.4.1.2 SCOPE


       The INTERFACE  and Debug  dictionary hash  tables are  Global in Scope
       even though the symbols accessed directly via either hash table may be
       private.   On the  other hand,  other hash  tables are purely local in
       scope.   For example,  the fields declared within a record are reached
       via a small local hash table, as are the arguments and local variables
       declared within  procedures and  functions.   Even  OBJECTS  use  this
       technique to provide access to Methods and Object Fields.

       Access to  such local  scope fields/methods  requires use of qualified
       names which  ensures conformity  to Pascal scope rules.  The method is
       truly simple and elegant.


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       4.4.1.3 SPECIAL CASES


       The SYSTEM.TPU  Unit is  a special case.  Its INTERFACE hash table has
       apparently been  "hand-tuned" for  small size  and  it  contains  only
       sixteen (16)  entries.   In addition,  the Debug  hash table is absent
       since there  is no  local symbol  generation in this unit.  Therefore,
       the Debug hash table does not exist as a separate entity, its function
       being served  by the  INTERFACE hash  table.  The pointer to the Debug
       hash table  (in the  Unit Header) has the same value as the pointer to
       the INTERFACE hash table.


       4.4.2 DICTIONARY HEADERS


       This is  the structure  that anchors  all  information  known  by  the
       compiler about any symbol.  The format is as follows:

         +00:    An LL  which points  to the  next (previous) symbol in the
                 same unit which had the same hash function value.

         +02:    A character  that defines  the category the symbol belongs
                 to and  defines the  format of  the Dictionary  Stub which
                 follows the  Dictionary Header.  If the symbol is declared
                 in the  component list  of the "private" part of an Object
                 declaration, then this character is modified by adding $80
                 to  its  ordinal  value.    Thus,  an  ordinary  Function,
                 Procedure or  Method is  of category  "S" while  a private
                 Method is of category Chr(Ord('S')+$80).

         +03:    A String  (in the  Pascal sense)  of  variable  size  that
                 contains the  text of  the symbol  (in UPPER-CASE  letters
                 only).   The SizeOf  function is  not  defined  for  these
                 strings since they are truncated to match the symbol size.
                 The "value"  of the  SizeOf function  can be determined by
                 adding  1  to  the  first  byte  in  the  string.    Thus,
                 Ord(Symbol[0])+1 is  the expression  that defines the Size
                 of the  symbol string.  Turbo Pascal defines a symbol as a
                 string of  relatively arbitrary size, the most significant
                 63 characters  of which  will be stored in the dictionary.
                 Thus, we  conclude that  the maximum size of such a string
                 is 64 bytes.












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                           Inside TURBO Pascal 6.0 Units              
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       4.4.3 DICTIONARY STUBS


       Dictionary Stubs immediately follow their respective headers and their
       format is  determined by  the category  character  in  the  Dictionary
       Header.   The function  of the  stub is  to organize  the  information
       appropriate to  the symbol and provide a means of accessing additional
       information such as type descriptors, constant values, parameter lists
       and nested  scopes.   The format  of each  Stub is  presented  in  the
       following sub-sections.


       4.4.3.1 LABEL DECLARATIVES ("O")


       This Stub consists of a WORD whose function is (as yet) unknown.


       4.4.3.2 UN-TYPED CONSTANTS ("P")


       This Stub consists of (2) two fields:

         +00:    An LG  which points  to  a  Type  Descriptor  (usually  in
                 SYSTEM.TPU).     This  establishes   the  minimum  storage
                 requirement for  the constant.   The  rules vary  with the
                 type, but  the size  of the  constant  data  field  (which
                 follows) is defined using the Type Descriptor(s).

         +04:    The value  of the constant.  For ordinal types, this value
                 is stored as a LONGINT (size=4 bytes).  For Floating-Point
                 types, the  size is  implicit in  the type  itself.    For
                 String types,  the size  is determined  from the length of
                 the string  which is  stored in  the initial  byte of  the
                 constant.


       4.4.3.3 NAMED TYPES ("Q")


       This Stub  consists of  an  LG  (4-bytes)  that  points  to  the  Type
       Descriptor for this symbol.













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       4.4.3.4 VARIABLES, FIELDS, TYPED CONS ("R")


       This Stub contains information required to allocate and describe these
       types of entities.  The format and content is as follows:

         +00:    A one-byte flag that precisely identifies the class of the
                 item being described.  The known values and their apparent
                 meanings follow:

                 $00 -> Global Variables (Allocated in DS);
                 $01 -> Typed Constants (Allocated in DS);
                 $02 -> Procedure LOCAL Variables on STACK;
                 $03 -> Variables at Absolute Addresses;
                 $06 -> ADDRESS Arguments allocated on STACK; (This is now
                        used only for SELF in Method calls;)
                 $08 -> Fields sub-allocated in RECORDS and OBJECTS, plus
                        METHODS declared for OBJECTS.
                 $10 -> Variable Equivalenced to another via the
                        Absolute Clause;
                 $22 -> Arguments whose VALUEs are passed on the stack;
                 $26 -> Arguments whose ADDRESSes are passed on the stack.

         +01     Two words  whose content vary with the codes above.  Their
                 content is explained following the last item in the stub.

         +05:    An LG  that locates  the proper  Type Descriptor  for this
                 symbol.

       When the  code byte  at +00 is $02,$06,$22 or $26 (arguments), the two
       words at +01 are used as follows:

         +01     Word -- Offset relative to either DS or BP.
         +03     Word -- LL to Dict Header of Parent Scope, or zero.

       If the code byte is $00 or $01 (VAR's or typed CONSTs), then we have:

         +01     Word -- Offset relative to allocation area origin;
         +03     Word -- Offset to entry in VAR/CONST Map for item
                         allocation;

       When the code byte is $03 (Absolute Address Variable), then we have:

         +01     DWord -- FAR Pointer to Absolute Memory Address.

       When the  code byte  is $08  (Record/Object Fields/Methods),  then  we
       have:

         +01     Word -- Allocation Offset within Record/Object;
         +03     Word -- LL to next Field/Method.

       When the code byte is $10 (Absolute Equivalences), then we have:

         +01     DWord -- LG to STUB of variable/parameter declaration that
                          actually establishes the allocation;

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       4.4.3.5 SUBPROGRAMS & METHODS ("S")


       Subprograms (PROC's),  especially since  Object Methods are supported,
       have a rather involved stub.  Its format is as follows:

         +00:    A byte  that contains  bit-switches that  seem to describe
                 the Call  Model and  imply the  size of  this stub.  These
                 switches determine what kind of code (if any) is generated
                 when the  PROC is  referenced.  The observed values are as
                 follows:

                 xxxxx001 -> PROC uses FAR Call Model;
                 xxxx0010 -> PROC uses INLINE Model (no Call);
                 xxxx0100 -> PROC uses INTERRUPT Model (no Call);
                 xxxx100x -> PROC has EXTERNAL attribute;
                 xxx1xxxx -> PROC uses METHOD Call Model;
                 x011xxxx -> PROC is a CONSTRUCTOR Method;
                 x101xxxx -> PROC is a DESTRUCTOR Method;
                 1xxxxxxx -> PROC has ASSEMBLER directive.

         +01     A byte whose function is not yet known. (TP Windows?)

         +02:    A Word  whose interpretation  depends on whether or not we
                 have an  INLINE Declarative  Subprogram.   If this  is  an
                 INLINE Declarative Subprogram, then this word contains the
                 byte-count of  the INLINE  code text  at the  end of  this
                 stub.   Otherwise, this word is the offset within the PROC
                 Map that locates the object code for this Subprogram.

         +04:    A Word  that contains  an LL  which locates the containing
                 scope in the dictionary, or zero if none.

         +06:    A Word  that contains  an LL  which locates the local Hash
                 Table for  this scope.  A local hash table provides access
                 to all  formal parameters of the Subprogram as well as all
                 Symbols whose  declarations are local to the scope of this
                 Subprogram.

         +08:    A Word that is zero unless the symbol is a Virtual Method.
                 In this  case, then  the content  is the offset within the
                 VMT for  the owning  object that  defines  where  the  FAR
                 POINTER to this Virtual Method is stored.

         +0A:    A complete  Type-Descriptor  for  this  Subprogram.    The
                 length is  variable and  depends upon the number of Formal
                 Parameters declared in the header.  (See 4.4.4.3.5).

         +??:    If  this   Symbol   represents   an   INLINE   Declarative
                 Subprogram, then  the object-code  text begins  here.  The
                 byte-count of  the text  occurs at  offset 0002h  in  this
                 stub.



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                           Inside TURBO Pascal 6.0 Units              
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       4.4.3.6 TURBO STD PROCEDURES ("T")


       This Stub consists of two bytes, the first of which is unique for each
       procedure and  increments by  4.   I have  found nothing in the SYSTEM
       unit (which  is where  this entry  appears) that  this seems  directly
       related to.  The second byte is always zero.


       4.4.3.7 TURBO STD FUNCTIONS ("U")


       This Stub consists of two bytes, the first of which is unique for each
       function and increments by 4.  I have found nothing in the SYSTEM unit
       (which is  where this  entry appears) that this seems directly related
       to.   I wouldn't  be surprised if this byte were an index into a TURBO
       compiler table that points to specialized parse tables/action routines
       for handling these functions and their non-standard parameter lists.

       The second byte seems to be a flag having the values $00, $40 and $C0.
       I strongly  suspect that  the flag  $C0 marks  exactly those functions
       which may  be evaluated at compile-time.  The meaning behind the other
       values is not known to me.


       4.4.3.8 TURBO STD "NEW" ROUTINE ("V")


       This Stub consists of a WORD whose function is (as yet) unknown.  This
       is the  only Standard  Turbo routine that can behave as a procedure as
       well as a function (returning a pointer value).


       4.4.3.9 TURBO STD PORT ARRAYS ("W")


       This Stub  consists of  a byte whose value is 0 for byte arrays, and 1
       for word arrays.


       4.4.3.10 TURBO STD EXTERNAL VARIABLES ("X")


       This Stub  consists of  an  LG  (4-bytes)  that  points  to  the  Type
       Descriptor for this symbol.











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                           Inside TURBO Pascal 6.0 Units              
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       4.4.3.11 UNITS ("Y")


       Unit Stubs have the following content:

         +00:    A Word  whose apparently  reserved for use by the Compiler
                 or Linker.

         +02:    A Word that seems to contain some kind of "signature" used
                 to detect  inconsistent Unit Versions.  Borland calls this
                 a "unit  version number,  which is basically a checksum of
                 the interface  part."   I have  seen a thread in CIS which
                 says that it is a CRC value.  Food for thought?

         +04:    A Word  that contains  an LL  which locates  the Successor
                 Unit in  the "Uses"  list.   In fact,  the "Uses" lists of
                 both the INTERFACE and IMPLEMENTATION sections of the Unit
                 are merged  by this  Word into  a single list.  A value of
                 zero is used to indicate no successor.

         +06:    A Word  that contains  an LL which locates the Predecessor
                 Unit in  the "Uses" list.  For the SYSTEM unit entry, this
                 value is  always zero to indicate no predecessor.  For the
                 Unit being compiled, this LL locates the final Unit in the
                 combined "Uses" list.

       In effect,  the two  LL's at  offsets 0004 and 0006 organize the units
       into both  forward and backward linked chains.  The entry for the unit
       being compiled  is effectively  the head  of both  the forward and the
       backward chains.  The final unit in the merged "Uses" list is the tail
       of the  forward chain, and the SYSTEM unit is the tail of the backward
       chain.


       4.4.4 TYPE DESCRIPTORS


       Type Descriptors  store much  of the semantic information that applies
       to the  symbols declared  in the  unit.  Implementation details can be
       managed using  high-level abstractions  and these  abstractions can be
       shared.















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       4.4.4.1 SCOPE


       Type Descriptor  sharing can  occur across  the boundaries  which  are
       implicit in  unit modules.   Thus,  a type  defined in one unit may be
       "imported" by  some other  module.  Also, the pre-defined Pascal Types
       (plus the  Turbo Pascal extensions) are defined in the SYSTEM.TPU unit
       and there  needs to  be a  means of  "importing" such Type Descriptors
       during compilation.  This is precisely the objective of the LG locator
       which was  described in  section 2.2  (above).   Type Descriptors  are
       NEVER copied between units.  The binding always occurs by reference at
       compile time  and this helps support the technique of modifying a unit
       and compiling  it to a .TPU file, then re-compiling all units/programs
       that "USE" it.

       Type Descriptors  have many  roles so  their format  varies.   We have
       divided these  structures into  two parts:   The PREFIX Part (which is
       always present  and) whose  format is  fairly constant  and the SUFFIX
       Part whose  content and format depends on the attributes that are part
       of the type definition.


       4.4.4.2 PREFIX PART


       The Prefix  Part of  every Type  Descriptor consists of six (6) bytes.
       The usage  is consistent for all types observed by this author and the
       format is as follows:

         +00:    A Byte  that identifies  the format  of the  Suffix  part.
                 This is essentially based on several high-level categories
                 which the Suffix Parts support directly.  The observed set
                 of values is as follows:

                 00h -> an un-typed entity;
                 01h -> an ARRAY type;
                 02h -> a RECORD type;
                 03h -> an OBJECT type;
                 04h -> a FILE type (other than TEXT);
                 05h -> a TEXT File type;
                 06h -> a SUBPROGRAM type;
                 07h -> a SET type;
                 08h -> a POINTER type;
                 09h -> a STRING type;
                 0Ah -> an 8087 Floating-Point type;
                 0Bh -> a REAL type;
                 0Ch -> a Fixed-Point ordinal type;
                 0Dh -> a BOOLEAN type;
                 0Eh -> a CHAR type;
                 0Fh -> an Enumerated ordinal type.






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                           Inside TURBO Pascal 6.0 Units              
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         +01:    A Byte  used as a modifier.  Since the above scheme is too
                 general for  machine-dependent  details  such  as  storage
                 width  and  sign  control,  this  modifier  byte  supplies
                 additional data.   The author has identified several cases
                 in which  this information is vital but has not spent very
                 much time  on the  subject.  The chief areas of importance
                 seem to  be in  the 8087  Floating-Point  types,  and  the
                 Fixed-Point ordinal  types.   The semantics  seem to be as
                 follows:

                 0A 00 -> The type "SINGLE"
                 0A 02 -> The type "EXTENDED"
                 0A 04 -> The type "DOUBLE"
                 0A 06 -> The type "COMP"

                 0C 00 -> an un-named BYTE integer
                 0C 01 -> The type "SHORTINT"
                 0C 02 -> The type "BYTE"
                 0C 04 -> an un-named WORD integer
                 0C 05 -> The type "INTEGER"
                 0C 06 -> The type "WORD"
                 0C 0C -> an un-named double-word integer
                 0C 0D -> The type "LONGINT"

                 One important  feature of  the above semantics is the fact
                 that an un-typed CONST declaration refers to the above two
                 bytes  to  determine  the  storage  space  needed  in  the
                 dictionary for  the data  value of the constant.  This can
                 be a  little involved  however as the constant may contain
                 its own  length descriptor (as in a string)  in which case
                 it may  be sufficient  to  identify  the  high-level  type
                 category without any modifier byte.

         +02:    A Word  that contains  the number of bytes of storage that
                 are required  to contain  an object/entity  of this  type.
                 For types  that represent variable-length objects/entities
                 such as  strings, this  word may define the value returned
                 by the SIZEOF function as applied to the type.

         +04     A Word that is zero unless the descriptor is for an Object
                 Method.   In this  case, the  content  is  an  LL  to  the
                 Dictionary Header of the SUCCEEDING Method for the Object,
                 in order of declaration, or zero if none.


       4.4.4.3 SUFFIX PARTS


       Suffix Parts further refine the implementation details of the type and
       also provide  subrange constraints  where appropriate.   In some cases
       the Suffix  part is  empty since  all semantic  data for  the type  is
       contained in the Prefix part.




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       4.4.4.3.1 UN-TYPED


       This Suffix Part is empty.  Nothing is known about an un-typed entity.


       4.4.4.3.2 STRUCTURED TYPES


       The structured  types represent  aggregates of  lower-level types.  We
       include ARRAY,  RECORD, OBJECT,  FILE, TEXT,  SET, POINTER  and STRING
       types in this category.


       4.4.4.3.2.1 ARRAY TYPES


       The Suffix  Part of  the ARRAY type is so constructed as to be able to
       support recursive  or nested  definition of arrays.  The suffix format
       is as follows:

         +00:    An LG that locates the Type Descriptor for the "base-type"
                 of the  array.   This is  the type  of  the  entity  being
                 arrayed (which may itself be an array).

         +04:    An LG  that locates  the Type  Descriptor  for  the  array
                 bounds which is a constrained ordinal type or subrange.


       4.4.4.3.2.2 RECORD TYPES


       RECORD types  have nested  scopes.   The Suffix  part provides  a base
       structure by  which to  locate the  fields local  to the  scope of the
       Record type itself.  The format is as follows:

         +00:    A Word containing an LL which locates the local Hash Table
                 that provides access to the fields in the nested scope.

         +02:    A Word  containing an  LL  which  locates  the  Dictionary
                 Header of  the initial  field in  the nested  scope.  This
                 supports a  "left-to-right" traversal  of the  fields in a
                 record.













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       4.4.4.3.2.3 OBJECT TYPES


       OBJECT types also have nested scopes.  The Suffix part provides a base
       structure by which to locate the fields and METHODS local to the scope
       of  the  OBJECT  type  itself.    In  addition,  inheritance  and  VMT
       particulars are stored.  The format is as follows:

         +00:    A Word containing an LL which locates the local Hash Table
                 that provides  access to  the fields  and METHODS local to
                 the nested scope.

         +02:    A Word  containing an  LL  which  locates  the  Dictionary
                 Header of the initial field or METHOD in the nested scope.
                 This supports  a "left-to-right"  traversal of  the fields
                 and METHODS in an OBJECT.

         +04:    An LG  which locates  the Type  Descriptor of  the  Parent
                 Object.  This field is zero if there is no such Parent.

         +08:    A Word  which contains  the size  in bytes  of the VMT for
                 this Object.   This field is zero if the object employs no
                 Virtual Methods, Constructors or Destructors.

         +0A:    A Word which contains the offset within the CONST DSeg Map
                 that locates  the VMT  skeleton or template segment.  This
                 field equals  FFFFh  if  the  object  employs  no  Virtual
                 Methods, Constructors or Destructors.

         +0C:    A Word which contains the offset within an Object instance
                 where the NEAR POINTER to the VMT for the object is stored
                 (within the DATA SEGMENT).  This field equals FFFFh if the
                 object  employs   no  Virtual   Methods,  Constructors  or
                 Destructors.

         +0E:    A Word  which contains  an LL which locates the Dictionary
                 Header for the name of the OBJECT itself.

         +10:    A Word (not yet understood) containing $FFFF.

         +12:    Three Words (not yet understood) containing zeroes.















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       4.4.4.3.2.4 FILE (NON-TEXT) TYPES


       This Suffix  consists of an LG that locates the Type Descriptor of the
       base type  of the  file.  Note that the Type Descriptor may be that of
       an un-typed entity (for un-typed files).


       4.4.4.3.2.5 TEXT FILE TYPES


       This Suffix  consists of an LG that locates the Type Descriptor of the
       base type of the file -- in this case SYSTEM.CHAR.


       4.4.4.3.2.6 SET TYPES


       This Suffix  consists of  an LG  that locates the base-type of the set
       itself.    Pascal  limits  such  entities  to  simple  ordinals  whose
       cardinality is limited to 256.


       4.4.4.3.2.7 POINTER TYPES


       This Suffix consists of an LG that locates the base-type of the entity
       pointed at.


       4.4.4.3.2.8 STRING TYPES


       This is a special case of an ARRAY type.  The format is as follows:

         +00:    An LG to the Type Descriptor SYSTEM.CHAR which is the base
                 type of all Turbo Pascal Strings.

         +04:    An  LG  to  the  Type  Descriptor  for  the  array  bounds
                 constraints for the string.  When the unconstrained STRING
                 type is  used, this points to SYSTEM.BYTE which is defined
                 as a subrange 0..255.


       4.4.4.3.3 FLOATING-POINT TYPES


       The Suffix  part for  all Floating-Point  types is  EMPTY.   All  data
       needed to  specify these  approximate number types is contained in the
       Prefix part.   The  Types included  in this  class are SINGLE, DOUBLE,
       EXTENDED, COMP and REAL.





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       4.4.4.3.4 ORDINAL TYPES


       The Ordinal  Types consist  of the  various "integer"  types plus  the
       BOOLEAN, CHAR and Enumerated types.


       4.4.4.3.4.1 "INTEGERS"


       These types  include BYTE, SMALLINT, WORD, INTEGER and LONGINT.  Their
       Suffix parts are identical in format:

         +00:    A double-word  containing the  LOWER bound of the subrange
                 constraint on the type;

         +04:    A double-word  containing the  UPPER bound of the subrange
                 constraint on the type;

         +08:    An LG  that locates  the Type  Descriptor of  the  largest
                 upward compatible  type.  This is the Type Descriptor that
                 is used  to control  the width  of an un-typed constant in
                 the dictionary  stub.  For the "integer" types, this is an
                 LG to SYSTEM.LONGINT.


       4.4.4.3.4.2 BOOLEANS


       This type Suffix has the following format:

         +00:    A double-word  containing the  LOWER bound of the subrange
                 constraint on the type;

         +04:    A double-word  containing the  UPPER bound of the subrange
                 constraint on the type;

         +08:    An LG  that locates  the Type  Descriptor  SYSTEM.BOOLEAN.
                 There is no "upward compatible" type.


       4.4.4.3.4.3 CHARS


       This type Suffix has the following format:

         +00:    A double-word  containing the  LOWER bound of the subrange
                 constraint on the type;

         +04:    A double-word  containing the  UPPER bound of the subrange
                 constraint on the type;

         +08:    An LG that locates the Type Descriptor SYSTEM.CHAR.  There
                 is no "upward compatible" type.


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       4.4.4.3.4.4 ENUMERATIONS


       This type Suffix is unusual and has the following format:

         +00:    A double-word  containing the  LOWER bound of the subrange
                 constraint on the type;

         +04:    A double-word  containing the  UPPER bound of the subrange
                 constraint on the type;

         +08:    An  LG  that  locates  the  Prefix  of  the  current  Type
                 Descriptor.  There is no upward compatible type.

       What follows  is a full-fledged SET Type Descriptor whose base type is
       the Type Descriptor of the Enumerated Type itself.  The author has not
       yet discovered the reason for this.

       At least  one case  has been  observed where  a set type descriptor is
       followed by  a word  containing zero  but I  know of  no  explanation.
       Could this be a (shudder) BUG in Turbo?


       4.4.4.3.5 SUBPROGRAM TYPES


       The length of this Suffix is variable.  The format is as follows:

         +00:    An LG  that locates  the Type  Descriptor of  the FUNCTION
                 result returned  by the Subprogram.  This field is zero if
                 the Subprogram is a PROCEDURE.

         +04:    A Word  that contains  the number  of Formal Parameters in
                 the Function/Procedure  header.   If non-zero,  then  this
                 word is  followed by the parameter list itself as a simple
                 array of parameter descriptors.

                 The format of a parameter descriptor is as follows:

                 0000: An LG that locates the Type Descriptor of the   
                       corresponding parameter;

                 0004: A Byte that identifies the parameter passing    
                       mechanism used for this entry as follows:

                       02h -> VALUE of parameter is passed on STACK,   
                       06h -> ADDRESS of parameter is passed on STACK.








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                           Inside TURBO Pascal 6.0 Units              
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       5. MAPS AND LISTS


       The "MAPS  and LISTS"  are not part of the symbol dictionary.  Rather,
       these structures provide access to the Code and Data Segments produced
       by the  compiler or  included via  the {$L  name.OBJ} directive.   The
       format and  purpose (as  understood by  this author)  of each of these
       tables is explained in the following sections.


       5.1 PROC MAP


       The PROC  Map provides a means of associating the various Function and
       Procedure declarations with the Code Segments.  There is some evidence
       that the  Compiler produces  CODE (and  DATA) Segments for EACH of the
       Subprograms defined  in the  Unit as  well as  for the  un-named  Unit
       Initialization code block.  There is also evidence that EXTERNAL PROCs
       must be  assembled separately  in order  to exploit  fully  the  Turbo
       "Smart Linker" since Turbo Pascal places some significant restrictions
       on  EXTERNAL  routines  in  the  area  of  Segment  Names  and  Types.
       Specifically, only  code segments named "CODE" and data segments named
       "DATA" or  "CONST" will  be used  by the  "Smart Linker" as sources of
       code and  data for  inclusion in a Turbo Pascal .EXE file.  (Turbo 6.0
       relaxed Name  constraints but only one code segment per .OBJ remains a
       limitation).

       The first  entry in  the PROC  Map is reserved for Unit Initialization
       block.   If there  is no Unit Initialization block, this entry will be
       filled with  $FF.  In addition, each and every PROC in the Unit has an
       entry in this table.

       If an  EXTERNAL routine  is included, then ALL PUBLIC PROC definitions
       in that  routine must  be declared  in the  Unit Source  Code with the
       EXTERNAL attribute.

       The size  of the  PROC Map  Table (in  Bytes) is  implied in  the Unit
       Header by the LL's that occur at offsets +0C and +0E.

       The Format of a single PROC Map Entry is as follows:

         +00:    A Word presumably reserved as a work area; always zero.

         +02:    A Word presumably reserved as a work area; always zero.

         +04:    A Word  that contains an offset within the CSeg Map.  This
                 is used to locate the code segment containing the PROC.

         +06:    A Word  that contains  an offset  within the  CODE Segment
                 that defines  the PROC  entry point  relative to  the load
                 point of the referenced CODE Segment.




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                           Inside TURBO Pascal 6.0 Units              
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       5.2 CSEG MAP


       The CSeg  Map provides  a convenient  descriptor table  for each  CODE
       Segment present  in the  Unit and serves to relate these segments with
       the Segment  Relocation Data  and the  Segment Trace  Table.  It seems
       reasonable to infer that the "Smart Linker" is able to include/exclude
       code/data at the SEGMENT level only.

       The CSeg  Map is  an array  of fixed-length records whose format is as
       follows:

         +00:    A Word apparently reserved for use by TURBO.

         +02:    A Word that contains the Segment Length (in bytes).

         +04:    A Word  that contains  the Length of the Fix-Up Data Table
                 for this Code Segment (in bytes).

         +06:    A Word  that contains  the offset of the Trace Table Entry
                 for this  Segment (if it was compiled with DEBUG Support).
                 If there  is no  Trace Table  for this  segment, then this
                 Word contains FFFFh.


       5.3 TYPED CONST DSEG MAP


       The CONST  DSeg Map  provides a  convenient descriptor  table for each
       DATA Segment  which was  spawned by the presence of Typed Constants or
       VMT's in the Pascal Code.  It serves to relate these segments with the
       Segment Fix-Up (relocation) Data and with the Code Segments that refer
       to these  DATA  elements.    One  entry  is  present  for  each  CONST
       declaration part containing typed constants and for each CONST segment
       linked from  an ".OBJ" file.  The CONST DSeg Map is an array of fixed-
       length records whose format is as follows:

         +00:    A Word apparently reserved for use by TURBO.

         +02:    A Word that contains the Segment Length (in bytes).

         +04:    A Word  that contains  the Length of the Fix-Up Data Table
                 for this DATA Segment (in bytes).

         +06:    A Word  that contains  an LL which locates the OBJECT that
                 owns this VMT template or zero if the segment is not a VMT
                 template.

       One can  determine the defining block for a Typed Constant declaration
       and our  program attempts  to do  just that.    A  by-product  of  the
       dictionary mapping  algorithm allows  the declaring  block to be found
       and its  qualified name  printed.   This information  is also  used to
       explain fix-up  data as  to its  source.   Results will  be incomplete
       unless a really comprehensive dictionary is present in the unit.


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                           Inside TURBO Pascal 6.0 Units              
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       5.4 GLOBAL VAR DSEG MAP


       The VAR  DSeg Map provides a convenient descriptor table for each DATA
       Segment present in the Unit.

       One entry  exists for  each CODE  segment which refers to GLOBAL VAR's
       allocated in  the DATA  Segment.   These references may be seen in the
       Fix-Up Data  Table.   Each EXTERNAL  CSeg having  a segment named DATA
       also spawns  an entry in this table.  Only the Code Segments that meet
       these criteria cause entries to be generated in the VAR Dseg Map.

       The VAR  DSeg Map  is an array of fixed-length records whose format is
       as follows:

         +00:    A Word apparently reserved for use by TURBO.

         +02:    A Word  that contains the Segment Length (in bytes).  This
                 may be zero, especially if the EXTERNAL routine contains a
                 DATA segment  whose sole purpose is to declare one or more
                 EXTRN symbols  that  are  defined  in  some  DATA  segment
                 external to the Assembly.

         +04:    A Word apparently reserved for use by TURBO.

         +06:    A Word apparently reserved for use by TURBO.

       One can determine the defining block for a Global VARiable declaration
       and our  program attempts  to do  just that.    A  by-product  of  the
       dictionary mapping  algorithm allows  the declaring  block to be found
       and its  qualified name  printed.   This information  is also  used to
       explain fix-up  data as  to its  source.   Results will  be incomplete
       unless a really comprehensive dictionary is present in the unit.  Such
       DSegs can  be referenced  by many  CSegs and  we only locate the first
       one.   This is  okay for  Pascal code but it's ambiguous for assembler
       since the names may be PUBLIC and referenced by more than one module.


       5.5 DONOR UNIT LIST


       This list contains an entry for each Unit (taken from the "USES" list)
       which MAY  contribute either CODE or DATA to the executable file.  Not
       all units do make such a contribution as some exist merely to define a
       collection of  Types, etc.  A Unit gets into this list if there exists
       a single Fix-Up Data Entry that references CODE or DATA in that Unit.

       The list  is comprised  of elements  whose SIZE  is variable and whose
       format is as follows:

         +00:    A WORD apparently reserved for use by TURBO.

         +02:    A variable-length String containing the unit name.


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                           Inside TURBO Pascal 6.0 Units              
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       5.6 SOURCE FILE LIST


       This list contains an entry for each "source" file used to compile the
       Unit.   This includes the Primary Pascal file, files containing Pascal
       code included  by means  of the  {$I filename.xxx} compiler directive,
       and .OBJ files included by the {$L filename.OBJ} compiler directive.

       The order  of entries  in this list is critical since it maps the CODE
       segments stored in the unit.  The order of the entries is as follows:

         1)      The Primary Pascal file;

         2)      All Included Pascal files;

         3)      All Included .OBJ files.

       Mapping of CSegs to files is done as follows:

         a)      Each .OBJ file contributes a SINGLE Code Segment (if any).
                 Note that this author has not observed an .OBJ module that
                 contains only  a DATA  Segment (but  that seems a distinct
                 possibility).

         b)      The Primary  Pascal file (augmented by all included Pascal
                 Files) contributes zero or more CODE Segments.

       Therefore, there  are at least as many CSeg entries as .OBJ files.  If
       more, then  the excess entries (those at the front of the list) belong
       to the Pascal files that make up the Pascal source for the unit.

       The format of an entry in this list is as follows:

         +00:    A flag byte that indicates the type of file represented;      

                 04h -> the Primary Pascal Source File,        
                 03h -> an Included Pascal Source File,        
                 05h -> an .OBJ file that contains a CODE segment.

         +01:    A Word apparently reserved for use by the Compiler/Linker.

         +03:    A Word  that is zero for .OBJ files and which contains the
                 file directory time-stamp for Pascal Files.

         +05:    A Word  that is zero for .OBJ files and which contains the
                 file directory date-stamp for Pascal Files.

         +07:    A  variable-sized   string  containing  the  filename  and
                 extension of the file used during compilation.






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                           Inside TURBO Pascal 6.0 Units              
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       5.7 DEBUG TRACE TABLE


       If Debug  support was  selected at  compile time, then all Pascal code
       which supports  Debugging produces  an entry in this table.  The table
       entries themselves are variable in size and have the following format:

         +00:    A Word  which contains  an LL  that locates  the Directory
                 Header of the Symbol (a PROC name) this entry represents.

         +02:    A Word  which contains  the offset (within the Source File
                 List) of  the entry that names the file that generated the
                 CSeg being traced.  This allows the file included by means
                 of the  {$I filename} directive to be identified for DEBUG
                 purposes, as well as code produced from the Primary File.

         +04:    A Word containing the number of bytes of data that precede
                 the BEGIN statement code in the segment.  For Pascal PROCS
                 these  bytes   consist  of   literal  constants,  un-typed
                 constants, and  other data  such as range-checking limits,
                 etc.

         +06:    A Word  containing the  Line Number of the BEGIN statement
                 for the PROC.

         +08:    A Word  containing the  number of  lines of Source Code to
                 Trace in this Segment.

         +0A:    An array  of bytes  whose size   is at least the number of
                 source code  lines in  the PROC.   Each  byte contains the
                 number of bytes of object code in the corresponding source
                 line.   This appears to be an array of SHORTINT since if a
                 "line" contains more than 127 bytes, then a single byte of
                 $80 precedes  the actual  byte count as a sort of "escape"
                 and the  next byte  records the  up to  255 bytes  for the
                 line.  This situation has not yet been fully explored.  We
                 do not  yet know  what happens  in the  event  a  line  is
                 credited with spawning more than 255 bytes of code.


















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       6. CODE, DATA, FIX-UP INFO


       This area  begins at the start of the next free PARAGRAPH.  This means
       that its  offset from  the beginning  of the  Unit ALWAYS  ends in the
       digit zero.

       This area  contains the  CODE segments,  CONST DATA  segments, and the
       Relocation (Fix-Up) Data required for linking.


       6.1 OBJECT CSEGS


       Each CODE  segment included  in the  unit appears here as specified by
       the CSeg  Map Table.  Depending on usage, these segments may appear in
       the executable file.  There are no filler bytes between segments.


       6.2 CONST DSEGS


       This section begins at the start of the first free PARAGRAPH following
       the end  of the  Object CSegs.   This  means that  its offset from the
       beginning of the Unit ALWAYS ends in the digit zero.

       A DATA  segment fragment  appears here  for each  CSeg that declares a
       typed constant,  and for  each OBJECT  which employs  Virtual Methods,
       Constructors or  Destructors.   There  are  no  filler  bytes  between
       segments.

       If local symbols were generated, there is always enough information to
       allow documenting the scope of the declaration as well as interpreting
       the data  in the display since the needed type declarations would also
       be available.  Our program merely identifies the defining block.





















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       6.3 FIX-UP DATA TABLE


       This table  begins at  the start of the first free PARAGRAPH following
       the end  of the  CONST DSegs.   This  means that  its offset  from the
       beginning of  the Unit  ALWAYS ends  in the digit zero.  There are two
       sections in  this table:   one  for code,  and one  for  data.    Both
       sections are  aligned on  paragraph boundaries.   This may result in a
       "slack" entry  between the  code and data sub-sections, but this entry
       is included  in the  byte tally  for the  section stored  in the  Unit
       Header Table at UHZFA (offset +22).

       The table  begins with  entries for the CSeg Map and ends with entries
       for the  CONST DSeg  Map.   The appropriate  Map entry  specifies  the
       number of  bytes of  Relocation Data  for the  corresponding  segment.
       This number  may be zero in which case there is no Relocation Data for
       the given segment.

       The Table  consists of an array of eight (8) byte entries whose format
       is as follows:

         +00:    A Byte containing the offset within the Donor Unit List of
                 the Unit  name that this entry refers to.  This can be the
                 compiled Unit or some previously compiled external unit.

         +01:    A Byte of BIT switches that identify the type of reference
                 and the  size of the needed fix-up (WORD or DWORD).  A lot
                 of guess-work led to the following interpretation:

                 7654  (bits 3-0 don't seem to be used)

                 00--  Locate item via a PROC Map,
                 01--  Locate item via a CSeg Map,
                 10--  Locate item via a Global VAR DSeg Map,
                 11--  Locate item via a Const DSeg Map,
                 --00  WORD offset has NO effective address adjustment,
                 --01  WORD offset HAS an effective address adjustment,
                 --10  WORD SEGMENT-Only fix-up (address of some PUBLIC
                       segment),
                 --11  DWORD (FAR) pointer; possible effective address
                       adjustment.

         +02:    A  Word   containing  the  offset  within  the  Map  table
                 referenced according to the above code scheme.

         +04:    A Word  containing an  offset within  the  target  segment
                 which will  be  added  to  the  effective  address.    For
                 example, a  reference to  the VAR  DSeg Map will require a
                 final offset to locate the item (variable) within the DATA
                 SEGMENT being  referenced here.   This  may also be needed
                 for references to LITERAL DATA embedded in a CODE SEGMENT.

         +06:    A Word  containing the  offset within  the  CODE  or  DATA
                 segment owning  this entry  that contains  the area  to be
                 patched with the value of the final effective address.

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       7. SUPPLIED PROGRAM


       In order that the above information be made constructively useful, the
       author has designed a program that automates the process of discovery.
       It is  not a  "handsome" program and it is not a work of art.  It does
       give useful results provided your PC has enough available memory.

       It should  be obvious  that the  program was  not designed "top-down".
       Rather, it  just evolved as each new discovery was made.  Later on, it
       seemed reasonable to try to document some of the relations between the
       various lists  and tables  and the program tries to make some of these
       relations clear, albeit with varying degrees of success.


       7.1 TPU6


       This is  the main program.  It will ask for the name of the unit to be
       documented.   Reply with  the unit name only.  The program will append
       the ".TPU"  extension and  will search  for the  proper file.  It will
       also search TURBO.TPL if necessary.

       The program  will then ask if Dis-Assembly is desired and will require
       a "y" or "n" answer.  If "y", it also asks about the CPU.

       The  current  directory  will  be  searched  first,  followed  by  all
       directories in  the current  PATH.  If the .TPU file is not found, the
       program will  search for  it in the "TURBO.TPL" (Turbo Pascal Library)
       file.   Units in  the "USES"  list(s) will  also be  loaded to  enable
       resolution of LG items.

       If the  desired unit  is found, the program will write a report to the
       current directory  named "unitname.lst"  which contains  its analysis.
       The format of the report is such that it may be copied to a printer if
       that printer supports TTY control codes with form-feeds.  Be judicious
       in doing  this however  since there  can be a lot of information.  The
       Turbo SYSTEM.TPU  unit file  produces almost ninety (90) pages without
       the disassembly  option.  When disassembly is requested for the SYSTEM
       unit, the size of the output file exceeds 700K bytes.


       7.1.1 UNIT TPU6AMS


       This Unit  contains all  Type Definitions,  Structures, and  primitive
       Functions and  Procedures required  by the  program.   All  structures
       documented in  this report  are also documented in TPU6AMS by means of
       the TYPE  mechanism.   Some of  the structures  are difficult  if  not
       impossible to  handle using  ISO Pascal  but Turbo Pascal provides the
       means for getting the job done.




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                           Inside TURBO Pascal 6.0 Units              
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       7.1.2 UNIT TPU6EQU


       This Unit  is new  and contains constants and types of general utility
       that are  not strictly  unit related.   It  also constains the pointer
       manipulation routines  that are sensitive to the particular version of
       Turbo Pascal Version 6.0.  It also contains a Heap Error Function that
       keeps track  of the high-water mark of Heap Utilization of any program
       that uses it.  This function gets installed automatically.


       7.1.3 UNIT TPU6UTL


       This Unit  is new.   It  contains the higher-level analysis algorithms
       formerly located  in the  main program and in TPU6AMS.  The algorithms
       have been  re-cast with  object-orientation in mind and have potential
       for re-use  in other  contexts.   The unit  computes a  cover for  the
       dictionary and  deduces relationships  between dictionary,  code, data
       and the  CSeg, PROC,  CONST and VAR Maps discussed in section 5.  This
       information is  retrieved by  the main  program to  drive the printing
       process.

       This Unit also loads all units specified in the USES list of the prime
       unit to allow the names of externally defined types to be recovered on
       the report.   Array  bounds are  also retrieved in this way.  The code
       will search for needed units in TURBO.TPL without intervention.  Close
       attention is  paid to  Heap Management and minimal utilization of Heap
       storage.   The dictionary  areas of the Units located in the USES list
       get loaded  into the  Heap at  no  extra  charge.    Nothing  but  the
       dictionary area  is of  any use  at this  point.   The name and fully-
       qualified file  name of  each unit  successfully loaded are printed at
       the top  of the  listing.  Unit version numbers must agree or the unit
       will not  be loaded.   Dictionary  covers are computed for each loaded
       unit to aid in rapid LG-resolution.


       7.1.4 UNIT TPU6RPT


       This is  a Unit that contains the text-file output primitives required
       by the main program.  It's not very pretty but it does work.


       7.1.5 UNIT TPU6UNA


       This unit is a rudimentary disassembler.  The output will not assemble
       and may  look strange  to a "real" assembler programmer since I am not
       well-qualified in this area.  However, the basis for support of 80286,
       80386 etc.  processors is  present as well as coprocessor support.  Of
       perhaps the  greatest interest  is that  it does  appear to decode the
       emulated coprocessor instructions that are implemented via INT 34-3D.



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                           Inside TURBO Pascal 6.0 Units              
       ----------------------------------------------------------------------

       Be warned  however.  The output is not guaranteed since this was coded
       by myself  and I  am perhaps  the rankest amateur that ever approached
       this quite  awful assembler  language.   For convenience,  the operand
       coding mimics TASM "Ideal" mode.

       As is  usual with programs of this type, error-recovery is minimal and
       no context  checking is  performed.  If the operation code is found to
       be valid,  then a  valid instruction  is assumed  -- even  if  invalid
       operands are present.

       The only positives that apply to this program are that it doesn't slow
       the cpu down (although a lot more output is produced), and it does let
       one "tune" code for compactness by letting one view the results of the
       coding directly.   Also,  incomplete instructions  are handled as data
       rather than overrunning into the next proc.


       7.2 MODIFICATIONS


       It was intended from the beginning that this program should be able to
       be enhanced  to permit  external units  to be  referenced  during  the
       analysis of  any given  unit, even  if they  were library  components.
       Since the  original release of this document, the program has been so-
       enhanced.

       This program  was NOT intended as a pilot for some future product.  It
       WAS intended as a rather "ersatz" tool for myself.


       7.3 NOTES ON PROGRAM LOGIC


       The following  sections discuss  a few  of the methods employed by the
       supplied program.


       7.3.1 FORMATTING THE DICTIONARY


       Printing the unit dictionary area in a way that exposes its underlying
       semantics is  no small  task.   The unit  dictionary area  itself is a
       rather  amorphous-looking  mass  of  data  composed  of  hash  tables,
       dictionary headers  and stubs,  type descriptors,  etc.   In order  to
       present all  this information  in a  meaningful way, we have to reveal
       its structure  and this  cannot be  done  by  means  of  a  sequential
       "browse" technique.   Rather,  we have  to  visit  all  nodes  in  the
       dictionary area  so that  each may  be formatted in a way that exposes
       their function  and meaning.   This is made necessary by the fact that
       items are  added to  the dictionary  as encountered  and no convenient
       ordering of  entry types  exists.  What we have here is the problem of
       finding a  minimal "cover"  for  the  dictionary  area  that  properly
       exposes the content and structure of the dictionary area.



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                           Inside TURBO Pascal 6.0 Units              
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       To do  this, we  construct (in  the heap) a stack and a queue, both of
       which are  initially empty.   The entries we put in the stack identify
       the class  of entry (Hash Table, Dictionary Header, Type Descriptor or
       In-Line Code  group), the  location of the structure, and the location
       of its  immediate "owner"  or "parent"  dictionary entry (which allows
       some limited information about scope to be printed).

       To the  empty stack,  we add  an entry  for the  unit name  dictionary
       entry, the  INTERFACE hash table, and the Debug hash table.  All these
       are located  via direct  pointers (LL's) in the Unit Header Table.  We
       then pop one entry off the stack and begin our analysis.

       a)    If the  entry we  popped off  the stack  is not  present in  the
             queue, we add it and call a routine that can interpret the entry
             (aka, "cover")  for a  Dictionary Header,  Hash Table,  or  Type
             Descriptor.  (This may lead to additional entries being added to
             the stack  such as nested-scope hash tables, Dictionary Headers,
             Type Descriptors or In-Line Code group entries.)

       b)    While the  stack is  not empty,  we pop another entry and repeat
             step "a" (above) until no more entries are available.

       The result  is a  queue containing one entry for each structure in the
       unit  dictionary  area  that  is  identifiable  via  traversal.    (In
       practice, the  method we use is similar to a "breadth-first" traversal
       of an  n-way tree that is implemented in non-recursive fashion.)  Each
       entry in  the queue  contains the  information described above and the
       queue itself thus forms a set of descriptors that drive the process of
       formatting the  dictionary area  for display.    The  process  may  be
       likened to  "painting by  the numbers" or to finding a way to lay tile
       on a flat surface using tiles of four different irregular shapes until
       the floor is exactly covered.

       There is  one significant limitation that needs to be pointed out.  It
       is not  always possible to determine the "parent" or "owner" of a node
       with certainty.   The  following discussion illustrates the problem of
       finding the "real" parent of a Type Descriptor.

       Almost every "type" in Turbo Pascal is actually derived from the basic
       types that  are defined  in the  SYSTEM.TPU unit  --  e.g.  "INTEGER",
       "BYTE", etc.   In  addition, several  of the  Type Descriptors  in the
       SYSTEM unit  are referenced  by more than one Dictionary Entry.  Thus,
       we find that a "many-to-one" relationship may exist between Dictionary
       Entries and  Type Descriptors.   How  does one  find out  which is the
       entry that actually gave rise to the Type Descriptor?











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                           Inside TURBO Pascal 6.0 Units              
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       The Dictionary  Area of  a unit  has some  special properties,  one of
       which is  the fact  that the  Dictionary Entries  for named  Types are
       often  located  quite  near  their  primary  type  descriptors.    The
       Dictionary Area seems to be treated as an upward growing heap with the
       various structures being added by Turbo as encountered.  This makes it
       likely that  the Type "Q" header which gives rise to a type descriptor
       is quite likely to occur earlier in the Dictionary Area than any other
       header which refers to the same descriptor.  We take advantage of this
       property to allocate "ownership" but it may not be "fool-proof".  Some
       type descriptors are spawned by other type descriptors, especially for
       structured types.   We  don't attempt to allocate "ownership" to these
       "lower-level" descriptors  but we  do  try  to  keep  track  of  scope
       information.

       A useful  by-product of  the above  process is the ability to discover
       many of  the associations  between Global  Variables,  Typed  CONST's,
       VMT's and the blocks in which they are declared or defined.


       7.3.2 THE DISASSEMBLER


       To start with, I apologize up front for mistakes which are bound to be
       present in  this routine.   I  am not really a MASM or TASM programmer
       and I will not pretend otherwise.  This being the case, the formatting
       I have  chosen for  the operands  may be  erroneous or  misleading and
       might (if  submitted to  one of  the "real" assemblers) produce object
       code quite different from what is expected.  I hope not, but I have to
       admit it's possible.

       My intention  in adding this unit was to support hand-tuning of object
       code.   With practice  and some  effort, one can observe the effect on
       the object  module caused  by specific  Pascal coding.    Thus,  where
       compactness or  speed is an issue of paramount importance, TPU6UNA can
       be of  help.   In some  cases, a  simple re-arrangement  of the  local
       variable declarations  in a procedure can have a significant effect on
       the size  of the  code if it means the difference between 1 and 2-byte
       displacements for  each instruction  that references  a specific local
       variable.   Potential  applications  along  these  lines  seem  almost
       unlimited.

       I adopted an operand format not unlike that of TASM "Ideal" mode since
       it was  more convenient  to do  so and  looked more readable to me.  I
       relied on  several reference books for guidance in decoding the entire
       mess and  I found  that there were several flaws (read ERRORS) in some
       of them  which made  the  job  that  much  more  difficult.    I  then
       compounded my  problems by  attempting to  handle 80386  specific code
       even though  Turbo Pascal does not yet generate code specific to these
       processors.   I simply  felt that  the effort  involved in writing any
       sort of Dis-Assembly program for Turbo Pascal units was an effort best
       experienced not  more than  once.  With all this self-flagellation out
       of my  system once  and for all, I will try to show the basic strategy
       of the  program and  to  explain  the  limitations  and  some  of  the
       discoveries I made.


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                           Inside TURBO Pascal 6.0 Units              
       ----------------------------------------------------------------------

       The routine  is intended  to be  idiotically simple - i.e., no smarter
       than the  DEBUG command  in principle.   The basic idea is:  pass some
       text to  the routine and get back ONE line derived from some prefix of
       that text.   Repeat  as necessary until all text is gone.  Thus, there
       is no attempt to check the context of the text being processed.  Also,
       some configurations  of the  "modR/M" byte  may invalid  for  selected
       instructions.  I don't try to screen these out since the intent was to
       look at  the presumably  correct code  produced by TURBO Pascal -- not
       devious assembly language.  Also, this program regards WAIT operations
       as "stand-alone"  -- i.e.,  it doesn't  check to  see if a coprocessor
       operation follows for which the WAIT might be regarded as a prefix.

       One area  of real  difficulty  was  figuring  out  the  Floating-Point
       emulations used  by Turbo  Pascal that  are implemented  by  means  of
       interrupts $34  through $3D.   I don't know if I got it right, but the
       results seem reasonable and consistent.  In the listing, the Interrupt
       is produced  on one line, followed by its parameters on the next line.
       The parameter line is given the op-code "EMU_xxxx" where "xxxx" is the
       coprocessor op-code  I felt  was being  emulated.  Interrupt $3C was a
       real puzzler  but after  seeing a lot of code in context, I think that
       the segment  override is  communicated to the emulator by means of the
       first byte after the $3C.

       Normally, in  a non-emulator  environment, all  coprocessor operations
       (ignoring any  WAIT prefixes)  begin with  $D8-$DF.  What Borland (and
       maybe Microsoft)  seem to  have done  here is to change the $D8-$DF so
       that bits  7 and 6 of this byte are replaced with the one's complement
       of  the   2-bit  segment   register  number   found  in  various  8086
       instructions.  This seems to be how an override for the DS register is
       passed to  the emulator.    I  don't  KNOW  this  to  be  the  correct
       interpretation, but  the code I have examined in context seems to work
       under this  scheme, so  TPU6UNA  uses  it  to  interpret  the  operand
       accordingly.

       For 80x86 machines, the problem was somewhat simpler.  TPU6UNA takes a
       quick look at the first byte of the text.  Almost any byte is valid as
       the initial byte of an instruction, but some instructions require more
       than one  byte to  hold the  complete operation  code.   Thus, step  1
       classifies bytes in several ways that lead to efficient recognition of
       valid operation codes.

       Once the  instruction has  been identified  in this way, it is more or
       less easy  to link  to supplemental  information that provides operand
       editing guidance, etc.

       The tables  that embody  the recognition scheme were constructed using
       PARADOX 3.0  (another fine Borland product) and suitably coded queries
       were used to generate the actual Turbo Pascal code for compilation.

       For those  that are  interested, TPU6UNA supports the address-size and
       operand-size prefixes  of the  80386 as  well as  32-bit operands  and
       addresses but  remember that  Turbo Pascal  doesn't generate these.  A
       trivial change  is provided for which allows segments which default to
       32-bit mode to be handled as well.


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                           Inside TURBO Pascal 6.0 Units              
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       There is  a simple  mode variable  that gets  passed to TPU6UNA by its
       caller which  specifies the most-capable processor whose code is to be
       handled.   Codes are  provided for  the 8086 (8088 is the same), 80186
       (same as 80286 without protected mode instructions), 80286 (80186 plus
       protected mode), and 80386.  You now get asked which one to use.

       No such  specifier is provided for coprocessor support.  What is there
       is what  I think an 80387 supports.  I don't think that this is really
       a problem  if you  don't try  to use  TPU6UNA for  anything but  Turbo
       Pascal code.

       Error recovery is predictably simple.  The initial text byte is output
       as the  operand of a DB pseudo-op and provision is made to resume work
       at the next byte of text.

       I hope  this program  is found  to be useful in spite of the errors it
       must surely  contain.   I have yet to make much sense of the rules for
       MASM or  TASM operand  coding and I found very little of value in many
       of the  so-called "texts"  on the  subject.   I found  myself  in  the
       position of  that legendary  American in  England watching  a  Cricket
       match for the first time ("You mean it has RULES?").


       8. UNIT LIBRARIES


       I have examined .TPL files in passing and feel that their structure is
       trivial.   It's so  easy to handle them that the program now routinely
       examines TURBO.TPL to resolve named types.


       8.1 LIBRARY STRUCTURE


       A Turbo  Pascal Library  (.TPL) file  is a  simple catenation of Turbo
       Pascal Unit  (.TPU) files.    Since  the  length  of  a  Unit  may  be
       determined from the Unit Header (see section 3.1), it is simple to see
       that one may "browse" through a .TPL file looking for an external unit
       such as  SYSTEM.TPU.   The supplied program does just that in its unit
       retrieval process  so the  TPUMOVER utility  is no longer required for
       processing of units in TURBO.TPL















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                           Inside TURBO Pascal 6.0 Units              
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       9. APPLICATION NOTES


       One of the more obvious applications of this information would seem to
       be in the area of a Cross-Reference Generator.

       There is  a very  fine example  of such a program in the public domain
       that was  written by  Mr. R. N. Wisan called  "PXL".  This program has
       been around since the days of Turbo Pascal Version 1.  The program has
       been continually enhanced by the author in the way of features and for
       support of the newer Turbo Pascal versions.  It does not however solve
       the problem  of telling  one which  unit contains  the definition of a
       given symbol.   In fairness to "PXL" however, this is no small problem
       since the  format of  .TPU files  keeps changing  (Turbo 6.0 Units are
       not object-code  compatible with  Turbo 5.x  Units, and  so on...) and
       Mr. Wisan probably has more than enough other projects to keep himself
       occupied.

       However, for  the user who is willing to work a little (maybe a lot?),
       this document would seem to provide the information needed to add such
       a function to his own pet cross-reference generator.

       Further, with  SIGNIFICANTLY more  effort, it should be possible to do
       much of  the job of de-compilation -- provided the DEBUG dictionary is
       present.   At the very least, most declarations should be recoverable.
       It's another  thing entirely  to try  to reconstruct  plausable  TURBO
       Pascal code  from the CSegs.  This would be a formidable task and lots
       of knowledge  about TURBO's code generators would have to be acquired.
       At present, the only way I know to get this information is to have the
       run-time library  source codes and then work-work-work at testing code
       produced by  the compiler  for a  huge number of test case units.  You
       have to  want to  do this really badly in order to invest the time.  I
       am not that tired of living.























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                           Inside TURBO Pascal 6.0 Units              
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       10. ACKNOWLEDGEMENTS


       This project  would have  been totally  infeasible without  the aid of
       some very  fine tools.  As it was, several hundred man hours have been
       expended on  it and  as you can see, there are a few unresolved issues
       that have  been (graciously)  left for  others to  address.  The tools
       used by this author consisted of:

       1)    Turbo Pascal 6.0 Professional by Borland International

       2)    Microsoft WORD (version 5.0)

       3)    LIST (version 7.5) by Vernon D. Buerg

       4)    the DEBUG utility in MS-DOS Version 3.3.

       5)    PARADOX 3.0 by Borland International

       6)    QUATTRO PRO by Borland International

       7)    TURBO ASSEMBLER 1.1 by Borland International

       (PARADOX and QUATTRO PRO were used for data collection and analysis in
       the course of coding the recognizer tables for the disassembler unit.)

       The references  listed were of great value in this project.  [Intel85]
       was a valuable source of information about coprocessor instructions as
       well as offering hints about the differences between the 8086/8088 and
       the 80286.   The  [Borland] TASM  manuals offered  further info on the
       80186.     [Nelson]  provided  presentations  of  well-organized  data
       directed at the problem of disassembly but the tables were flawed by a
       number of  errors which  crept into my databases and which caused much
       of the extra debugging effort.  [Intel89] offered valuable insights on
       the 80386  addressing schemes  as well  as the 32-bit data extensions.
       Finally,  [Brown]   provided  valuable  clues  on  the  Floating-Point
       emulators used  by Borland  (and Microsoft?).   As  you can  see,  the
       amount of  hard information  available to me on this project was quite
       limited since I am unaware of any other existing body of literature on
       this subject.

       That's it  folks.   Does anyone wonder why it took several hundred man
       hours to  get to  this point?   It  took a  lot of  hard (and at times
       tedious) work  coupled with a great many lucky guesses to achieve what
       you see here.











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                           Inside TURBO Pascal 6.0 Units              
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                                   11. REFERENCES


       [Borland], TURBO ASSEMBLER  REFERENCE  GUIDE,  Borland  International,
                  1988.

       [Borland], TURBO ASSEMBLER USER'S GUIDE, Borland International, 1988.

       [Borland]  TURBO PASCAL  6.0 PROGRAMMING GUIDE, Borland International,
                  1990.

       [Borland]  TURBO  PASCAL   LIBRARY  REFERENCE   Version  6.0,  Borland
                  International, 1990.

       [Borland]  TURBO   PASCAL    USER'S   GUIDE   Version   6.0,   Borland
                  International, 1990.

       [Brown],   INTER191.ARC, Ralf Brown, 1991

       [Intel85], iAPX 286  PROGRAMMER'S REFERENCE  MANUAL INCLUDING THE iAPX
                  286 NUMERIC  SUPPLEMENT, Intel  Corporation,  1985,  (order
                  number 210498-003).

       [Intel89], 386  SX MICROPROCESSOR PROGRAMMER'S REFERENCE MANUAL, Intel
                  Corporation, 1989, (order number 240331-001).

       [Nelson]   THE 80386  BOOK:   ASSEMBLY LANGUAGE PROGRAMMER'S GUIDE FOR
                  THE 80386, Ross P. Nelson, Microsoft Press, 1988.

       [Scanlon], 80286  ASSEMBLY   LANGUAGE  ON  MS-DOS  COMPUTERS,  Leo  J.
                  Scanlon, Brady 1986.

























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                           Inside TURBO Pascal 6.0 Units              
       ----------------------------------------------------------------------

                                       INDEX


       .OBJ file  12, 13, 30, 31, 33
       .TPL file  6, 14, 37, 38, 43
       .TPU
         file  5, 7, 11, 14, 23, 37, 43, 44
           size  14
         SYSTEM  6, 16, 17, 18, 23, 37, 40, 43
       Assembler  6
       Attribute
         ABSOLUTE  7
         EXTERNAL  20, 30
       Call Model
         ASSEMBLER  20
         FAR  20
         INLINE  20
         INTERRUPT  20
       CONST  6, 11, 12, 13, 19, 24, 26, 31, 35, 36, 38
       Constraint  28, 29
       CSeg  6, 11, 12, 30, 31, 32, 33, 34, 35, 36, 38
       Defining block  31, 32
       Directive  12, 13, 14, 20, 30, 33, 34
       External  7, 30, 32, 36, 39, 43
       Hash  11, 12, 13, 14, 15, 16, 17, 20, 25, 26, 39, 40
       Include  33, 34
       Interface  6, 11, 12, 13, 14, 15, 16, 17, 22, 40
       Locator
         LG  7, 10, 18, 19, 21, 23, 25, 26, 27, 28, 29
         LL  7, 11, 16, 22, 30, 40
         offset  7, 9, 10, 19, 20, 26, 30, 31, 34, 35, 36
       Method  20
         CONSTRUCTOR  20
         DESTRUCTOR  20
         Self  19
       Operand offset  36
       Parameter  18, 19, 20, 21, 29
       PROC  6, 11, 12, 20, 30, 34, 36, 38, 39
       SEGMENT  36
       Signature  5, 22
       Stub  7, 17, 18, 19
       Type Descriptor  18, 19, 21, 23, 25, 26, 27, 28, 29, 40, 41
       VAR  32, 38
       VMT  12, 13, 20, 26, 31












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