Power Programming/powerprogramming1994.iso/progtool/turbopas/tutorpas.arc/TUTOR10.DOC

Text File | 1989-05-21 | 113KB | 3,986 lines

OPAA



























                            LET'S BUILD A COMPILER!

                                       By

                            Jack W. Crenshaw, Ph.D.

                                  21 May 1989


                           Part X: INTRODUCING "TINY"




























PAA





       *****************************************************************
       *                                                               *
       *                        COPYRIGHT NOTICE                       *
       *                                                               *
       *   Copyright (C) 1989 Jack W. Crenshaw. All rights reserved.   *
       *                                                               *
       *****************************************************************


       INTRODUCTION

       In the last installment, I showed you the general  idea  for  the
       top-down development of  a  compiler.    I gave you the first few
       steps  of  the process for compilers for  Pascal  and  C,  but  I
       stopped  far  short  of  pushing  it through to completion.   The
       reason was simple: if we're going to produce  a  real, functional
       compiler  for  any  language, I'd rather  do  it  for  KISS,  the
       language that I've been defining in this tutorial series.

       In this installment, we're going to do just that, for a subset of
       KISS which I've chosen to call TINY.

       The process  will be essentially that outlined in Installment IX,
       except  for  one  notable  difference.   In that  installment,  I
       suggested  that  you  begin  with  a full BNF description of  the
       language.  That's fine for something like Pascal or C,  for which
       the language definition is  firm.   In the case of TINY, however,
       we don't yet have a full  description  ... we seem to be defining
       the language as we go.  That's OK.    In  fact,  it's preferable,
       since we can tailor the language  slightly  as we go, to keep the
       parsing easy.

       So in the development  that  follows,  we'll  actually be doing a
       top-down development of BOTH the  language and its compiler.  The
       BNF description will grow along with the compiler.

       In this process, there will be a number of decisions to  be made,
       each of which will influence the BNF and therefore the  nature of
       the language.   At  each  decision  point I'll try to remember to
       explain  the  decision  and the rationale behind my choice.  That
       way, if you happen to hold a different opinion and would prefer a
       different option, you can choose it instead.  You  now  have  the
       background  to  do  that.  I guess the important thing to note is
       that  nothing  we  do  here  is  cast  in  concrete.  When YOU'RE
       designing YOUR language, you should feel free to do it YOUR way.

       Many of you may be asking at this point: Why bother starting over
       from  scratch?  We had a working subset of KISS as the outcome of
       Installment  VII  (lexical  scanning).  Why not just extend it as
       needed?  The  answer  is  threefold.    First of all, I have been
       making  a  number  of changes to further simplify the program ...
       changes  like  encapsulating  the  code generation procedures, so
       that  we  can  convert to a different target machine more easily.
       Second, I want you to see how the development can indeed  be doneA*A*
                                     - 2 -
PAA





       from the top down as outlined in the last installment.   Finally,
       we both need the practice.  Each time I go through this exercise,
       I get a little better at it, and you will, also.


       GETTING STARTED

       Many  years  ago  there were languages called  Tiny  BASIC,  Tiny
       Pascal, and Tiny C, each of which was a subset of its parent full
       language.  Tiny BASIC,  for  example,  had  only single-character
       variable names and global variables.   It supported only a single
       data type.  Sound familiar?  At this point we have almost all the
       tools we need to build a compiler like that.

       Yet a language called Tiny-anything  still  carries  some baggage
       inherited from its parent language.   I've often wondered if this
       is a  good  idea.    Granted,  a  language based upon some parent
       language will have the  advantage  of  familiarity, but there may
       also  be  some  peculiar syntax carried over from the parent that
       may tend  to add unnecessary complexity to the compiler. (Nowhere
       is this more true than in Small C.)

       I've wondered just how small and simple a compiler could  be made
       and  still  be  useful, if it were designed from the outset to be
       both easy to use and to  parse.    Let's find out.  This language
       will just be called "TINY," period.  It's a subset of KISS, which
       I  also  haven't  fully  defined,  so  that  at  least  makes  us
       consistent (!).  I suppose you could call it TINY KISS.  But that
       opens  up a whole can of worms involving  cuter  and  cuter  (and
       perhaps more risque) names, so let's just stick with TINY.

       The main limitations  of  TINY  will  be because of the things we
       haven't yet covered, such as data types.  Like its cousins Tiny C
       and Tiny BASIC,  TINY  will  have  only one data type, the 16-bit
       integer.    The  first  version  we  develop  will also  have  no
       procedure  calls  and  will  use single-character variable names,
       although as you will see we can remove these restrictions without
       much effort.

       The language I have in mind will share some of the  good features
       of  Pascal,  C,  and Ada.  Taking a lesson from the comparison of
       the Pascal and  C  compilers in the previous installment, though,
       TINY will have a decided Pascal flavor.  Wherever  feasible,    a
       language structure will  be  bracketed by keywords or symbols, so
       that  the parser will know where it's  going  without  having  to
       guess.

       One other ground rule:  As we go, I'd like  to  keep the compiler
       producing real, executable code.  Even though it may not  DO much
       at the beginning, it will at least do it correctly.

       Finally,  I'll  use  a couple of Pascal  restrictions  that  make
       sense:  All data and procedures must be declared before  they are
       used.  That makes good sense,  even  though for now the only dataA*A*
                                     - 3 -
PAA





       type we'll use  is a word.  This rule in turn means that the only
       reasonable place to put the  executable code for the main program
       is at the end of the listing.

       The top-level definition will be similar to Pascal:


            <program> ::= PROGRAM <top-level decl> <main> '.'


       Already, we've reached a decision point.  My first thought was to
       make the main block optional.   It  doesn't seem to make sense to
       write a "program" with no main program, but it does make sense if
       we're  allowing  for  multiple modules, linked together.    As  a
       matter of fact,  I intend to allow for this in KISS.  But then we
       begin  to open up a can of worms that I'd rather leave closed for
       now.  For example, the  term "PROGRAM" really becomes a misnomer.
       The MODULE of Modula-2 or the Unit of Turbo Pascal would  be more
       appropriate.  Second,  what  about  scope  rules?    We'd  need a
       convention for  dealing  with  name  visibility  across  modules.
       Better  for  now  to  just  keep  it  simple  and ignore the idea
       altogether.

       There's also a decision in choosing to require  the  main program
       to  be  last.    I  toyed  with  the idea of making its  position
       optional,  as  in  C.  The nature of SK*DOS, the OS I'm compiling
       for, make this very easy to do.   But  this  doesn't  really make
       much sense in view of the Pascal-like requirement  that  all data
       and procedures  be declared before they're referenced.  Since the
       main  program can only call procedures  that  have  already  been
       declared, the only position that makes sense is at the end,  a la
       Pascal.

       Given  the  BNF  above, let's write a parser that just recognizes
       the brackets:


       {--------------------------------------------------------------}
       {  Parse and Translate a Program }

       procedure Prog;
       begin
          Match('p');
          Header;
          Prolog;
          Match('.');
          Epilog;
       end;
       {--------------------------------------------------------------}


       The procedure Header just emits  the startup code required by the
       assembler:A6A6
                                     - 4 -A*A*
PAA





       {--------------------------------------------------------------}
       { Write Header Info }

       procedure Header;
       begin
          WriteLn('WARMST', TAB, 'EQU $A01E');
       end;
       {--------------------------------------------------------------}


       The procedures Prolog and  Epilog  emit  the code for identifying
       the main program, and for returning to the OS:


       {--------------------------------------------------------------}
       { Write the Prolog }

       procedure Prolog;
       begin
          PostLabel('MAIN');
       end;


       {--------------------------------------------------------------}
       { Write the Epilog }

       procedure Epilog;
       begin
          EmitLn('DC WARMST');
          EmitLn('END MAIN');
       end;
       {--------------------------------------------------------------}


       The  main program just calls Prog, and then  looks  for  a  clean
       ending:


       {--------------------------------------------------------------}
       { Main Program }

       begin
          Init;
          Prog;
          if Look <> CR then Abort('Unexpected data after ''.''');
       end.
       {--------------------------------------------------------------}


       At this point, TINY  will  accept  only  one input "program," the
       null program:


            PROGRAM .   (or 'p.' in our shorthand.)A*A*
                                     - 5 -
PAA





       Note, though, that the  compiler  DOES  generate correct code for
       this program.  It will run, and do  what  you'd  expect  the null
       program to do, that is, nothing but return gracefully to the OS.

       As a matter of interest, one of my  favorite  compiler benchmarks
       is to compile, link,  and  execute  the  null program in whatever
       language   is   involved.     You  can  learn  a  lot  about  the
       implementation by measuring  the  overhead  in  time  required to
       compile what should be a trivial case.  It's also  interesting to
       measure the amount of code produced.  In many compilers, the code
       can be fairly large, because they always include  the  whole run-
       time  library whether they need it or not.    Early  versions  of
       Turbo Pascal produced a 12K object file for  this  case.    VAX C
       generates 50K!

       The  smallest  null  programs  I've  seen are those  produced  by
       Modula-2 compilers, and they run about 200-800 bytes.

       In the case of TINY, we HAVE no run-time library  as  yet, so the
       object code is indeed tiny:  two  bytes.    That's  got  to  be a
       record, and it's  likely  to  remain  one since it is the minimum
       size required by the OS.

       The  next step is to process the code for the main program.  I'll
       use the Pascal BEGIN-block:


            <main> ::= BEGIN <block> END


       Here,  again,  we  have made a decision.  We could have chosen to
       require a "PROCEDURE MAIN" sort of declaration, similar to C.   I
       must  admit  that  this  is  not  a bad idea at all ...  I  don't
       particularly  like  the  Pascal  approach  since I tend  to  have
       trouble locating the main  program  in a Pascal listing.  But the
       alternative is a little awkward, too, since you have to deal with
       the  error condition where the user omits  the  main  program  or
       misspells its name.  Here I'm taking the easy way out.

       Another solution to the "where is the main program" problem might
       be to require a name for  the  program, and then bracket the main
       by


            BEGIN <name>
            END <name>


       similar to the convention of  Modula  2.    This  adds  a  bit of
       "syntactic sugar" to the language.  Things like this are  easy to
       add or change to your liking, if the language is your own design.

       To parse this definition of a main block,  change  procedure Prog
       to read:A*A*
                                     - 6 -
PAA





       {--------------------------------------------------------------}
       {  Parse and Translate a Program }

       procedure Prog;
       begin
          Match('p');
          Header;
          Main;
          Match('.');
       end;
       {--------------------------------------------------------------}


       and add the new procedure:


       {--------------------------------------------------------------}
       { Parse and Translate a Main Program }

       procedure Main;
       begin
          Match('b');
          Prolog;
          Match('e');
          Epilog;
       end;
       {--------------------------------------------------------------}


       Now, the only legal program is:


            PROGRAM BEGIN END . (or 'pbe.')


       Aren't we making progress???  Well, as usual it gets better.  You
       might try some deliberate errors here, like omitting  the  'b' or
       the 'e', and see what happens.  As always,  the  compiler  should
       flag all illegal inputs.


       DECLARATIONS

       The obvious next step is to decide what we mean by a declaration.
       My  intent  here  is to have two kinds of declarations: variables
       and  procedures/functions.    At  the  top  level,   only  global
       declarations are allowed, just as in C.

       For now, there  can  only be variable declarations, identified by
       the keyword VAR (abbreviated 'v'):


            <top-level decls> ::= ( <data declaration> )*A6A6
                                     - 7 -A*A*
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            <data declaration> ::= VAR <var-list>


       Note that since there is only one variable type, there is no need
       to  declare the type.  Later on, for full KISS, we can easily add
       a type description.

       The procedure Prog becomes:


       {--------------------------------------------------------------}
       {  Parse and Translate a Program }

       procedure Prog;
       begin
          Match('p');
          Header;
          TopDecls;
          Main;
          Match('.');
       end;
       {--------------------------------------------------------------}


       Now, add the two new procedures:


       {--------------------------------------------------------------}
       { Process a Data Declaration }

       procedure Decl;
       begin
          Match('v');
          GetChar;
       end;


       {--------------------------------------------------------------}
       { Parse and Translate Global Declarations }

       procedure TopDecls;
       begin
          while Look <> 'b' do
             case Look of
               'v': Decl;
             else Abort('Unrecognized Keyword ''' + Look + '''');
             end;
       end;
       {--------------------------------------------------------------}


       Note that at this point, Decl  is  just  a stub.  It generates no
       code, and it doesn't process a list ... every variable must occur
       in a separate VAR statement.A*A*
                                     - 8 -
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       OK,  now  we  can have any  number  of  data  declarations,  each
       starting with a 'v' for VAR,  before  the BEGIN-block.  Try a few
       cases and see what happens.


       DECLARATIONS AND SYMBOLS

       That looks pretty good, but  we're still only generating the null
       program  for  output.    A  real compiler would  issue  assembler
       directives to allocate storage for  the  variables.    It's about
       time we actually produced some code.

       With  a  little  extra  code,  that's  an  easy  thing to do from
       procedure Decl.  Modify it as follows:


       {--------------------------------------------------------------}
       { Parse and Translate a Data Declaration }

       procedure Decl;
       var Name: char;
       begin
          Match('v');
          Alloc(GetName);
       end;
       {--------------------------------------------------------------}


       The procedure Alloc just  issues  a  command  to the assembler to
       allocate storage:


       {--------------------------------------------------------------}
       { Allocate Storage for a Variable }

       procedure Alloc(N: char);
       begin
          WriteLn(N, ':', TAB, 'DC 0');
       end;
       {--------------------------------------------------------------}


       Give  this  one  a  whirl.    Try  an  input  that declares  some
       variables, such as:

            pvxvyvzbe.

       See how the storage is allocated?    Simple, huh?  Note also that
       the entry point, "MAIN," comes out in the right place.

       For the record, a "real" compiler would also have a  symbol table
       to record the variables being used.  Normally,  the  symbol table
       is necessary to record the type  of  each variable.  But since in
       this case  all  variables  have  the  same  type, we don't need aA*A*
                                     - 9 -
PAA





       symbol  table  for  that reason.  As it turns out, we're going to
       find a symbol  necessary  even without different types, but let's
       postpone that need until it arises.

       Of course, we haven't really parsed the correct syntax for a data
       declaration, since it involves a variable list.  Our version only
       permits a single variable.  That's easy to fix, too.

       The BNF for <var-list> is


            <var-list> ::= <ident> (, <ident>)*


       Adding this syntax to Decl gives this new version:


       {--------------------------------------------------------------}
       { Parse and Translate a Data Declaration }

       procedure Decl;
       var Name: char;
       begin
          Match('v');
          Alloc(GetName);
          while Look = ',' do begin
             GetChar;
             Alloc(GetName);
          end;
       end;
       {--------------------------------------------------------------}


       OK, now compile this code and give it  a  try.    Try a number of
       lines of VAR declarations, try a list of several variables on one
       line, and try combinations of the two.  Does it work?


       INITIALIZERS

       As long as we're dealing with data declarations, one thing that's
       always  bothered  me  about  Pascal  is  that  it  doesn't  allow
       initializing  data items in the declaration.    That  feature  is
       admittedly sort of a frill, and it  may  be  out  of  place  in a
       language that purports to  be  a minimal language.  But it's also
       SO easy to add that it seems a shame not  to  do  so.    The  BNF
       becomes:


            <var-list> ::= <var> ( <var> )*

            <var> ::= <ident> [ = <integer> ]ABAB
                                    - 10 -A*A*
PAA





       Change Alloc as follows:


       {--------------------------------------------------------------}
       { Allocate Storage for a Variable }

       procedure Alloc(N: char);
       begin
          Write(N, ':', TAB, 'DC ');
          if Look = '=' then begin
             Match('=');
             WriteLn(GetNum);
             end
          else
             WriteLn('0');
       end;
       {--------------------------------------------------------------}


       There you are: an initializer with six added lines of Pascal.

       OK, try this  version  of  TINY  and verify that you can, indeed,
       give the variables initial values.

       By golly, this thing is starting to look  real!    Of  course, it
       still doesn't DO anything, but it looks good, doesn't it?

       Before leaving this section, I should point out  that  we've used
       two versions of function GetNum.  One, the earlier one, returns a
       character value, a single digit.  The other accepts a multi-digit
       integer and returns an integer value.  Either one will work here,
       since WriteLn will handle either type.  But there's no  reason to
       limit ourselves  to  single-digit  values  here,  so  the correct
       version to use is the one that returns an integer.  Here it is:


       {--------------------------------------------------------------}
       { Get a Number }

       function GetNum: integer;
       var Val: integer;
       begin
          Val := 0;
          if not IsDigit(Look) then Expected('Integer');
          while IsDigit(Look) do begin
             Val := 10 * Val + Ord(Look) - Ord('0');
             GetChar;
          end;
          GetNum := Val;
       end;
       {--------------------------------------------------------------}ANAN
                                    - 11 -A*A*
PAA





       As a matter  of  fact,  strictly  speaking  we  should  allow for
       expressions in the data field of the initializer, or at  the very
       least  for  negative  values.  For  now,  let's  just  allow  for
       negative values by changing the code for Alloc as follows:


       {--------------------------------------------------------------}
       { Allocate Storage for a Variable }

       procedure Alloc(N: char);
       begin
          if InTable(N) then Abort('Duplicate Variable Name ' + N);
          ST[N] := 'v';
          Write(N, ':', TAB, 'DC ');
          if Look = '=' then begin
             Match('=');
             If Look = '-' then begin
                Write(Look);
                Match('-');
             end;
             WriteLn(GetNum);
             end
          else
             WriteLn('0');
       end;
       {--------------------------------------------------------------}


       Now  you should be able to  initialize  variables  with  negative
       and/or multi-digit values.


       THE SYMBOL TABLE

       There's one problem  with  the  compiler  as it stands so far: it
       doesn't do anything to record a variable when we declare it.   So
       the compiler is perfectly content to allocate storage for several
       variables with the same name.  You can easily verify this with an
       input like


            pvavavabe.


       Here we've declared the variable A three times.  As you  can see,
       the compiler will  cheerfully  accept  that,  and  generate three
       identical labels.  Not good.

       Later on,  when we start referencing variables, the compiler will
       also let us reference variables  that don't exist.  The assembler
       will  catch  both  of these error conditions, but it doesn't seem
       friendly at all to pass such errors along to the assembler.   The
       compiler should catch such things at the source language level.A6A6
                                    - 12 -A*A*
PAA





       So even though we don't need a symbol table to record data types,
       we ought to install  one  just to check for these two conditions.
       Since at this  point  we are still restricted to single-character
       variable names, the symbol table can be trivial.  To  provide for
       it, first add the following  declaration at the beginning of your
       program:


            var ST: array['A'..'Z'] of char;


       and insert the following function:


       {--------------------------------------------------------------}
       { Look for Symbol in Table }

       function InTable(n: char): Boolean;
       begin
          InTable := ST[n] <> ' ';
       end;
       {--------------------------------------------------------------}


       We  also  need  to initialize the  table  to  all  blanks.    The
       following lines in Init will do the job:


       var i: char;
       begin
          for i := 'A' to 'Z' do
             ST[i] := ' ';
          ...


       Finally,  insert  the  following two lines at  the  beginning  of
       Alloc:


          if InTable(N) then Abort('Duplicate Variable Name ' + N);
          ST[N] := 'v';


       That  should  do  it.  The  compiler  will  now  catch  duplicate
       declarations.  Later, we can  also  use  InTable  when generating
       references to the variables.


       EXECUTABLE STATEMENTS

       At this point, we can generate a null program that has  some data
       variables  declared  and  possibly initialized.  But  so  far  we
       haven't arranged to generate the first line of executable code.A6A6
                                    - 13 -A*A*
PAA





       Believe  it or not, though, we almost  have  a  usable  language!
       What's missing is the executable code that must go into  the main
       program.  But that code is just assignment statements and control
       statements ... all stuff we have done before.   So  it  shouldn't
       take us long to provide for them, as well.

       The BNF definition given earlier  for the main program included a
       statement block, which we have so far ignored:


            <main> ::= BEGIN <block> END


       For now,  we  can  just  consider  a  block  to  be  a  series of
       assignment statements:


            <block> ::= (Assignment)*


       Let's start things off by adding  a  parser for the block.  We'll
       begin with a stub for the assignment statement:


       {--------------------------------------------------------------}
       { Parse and Translate an Assignment Statement }

       procedure Assignment;
       begin
          GetChar;
       end;


       {--------------------------------------------------------------}
       { Parse and Translate a Block of Statements }

       procedure Block;
       begin
          while Look <> 'e' do
             Assignment;
       end;
       {--------------------------------------------------------------}


       Modify procedure Main to call Block as shown below:


       {--------------------------------------------------------------}
       { Parse and Translate a Main Program }

       procedure Main;
       begin
          Match('b');
          Prolog;A*A*
                                    - 14 -
PAA





          Block;
          Match('e');
          Epilog;
       end;
       {--------------------------------------------------------------}


       This version still won't generate any code for  the   "assignment
       statements" ... all it does is to eat characters  until  it  sees
       the 'e' for 'END.'  But it sets the stage for what is to follow.

       The  next  step,  of  course,  is  to  flesh out the code for  an
       assignment statement.  This  is  something  we've done many times
       before,  so  I  won't belabor it.  This time, though, I'd like to
       deal with the code generation a little differently.  Up till now,
       we've always just inserted the Emits that generate output code in
       line with  the parsing routines.  A little unstructured, perhaps,
       but it seemed the most straightforward approach, and made it easy
       to see what kind of code would be emitted for each construct.

       However, I realize that most of you are using an  80x86 computer,
       so  the 68000 code generated is of little use to you.  Several of
       you have asked me if the CPU-dependent code couldn't be collected
       into one spot  where  it  would  be easier to retarget to another
       CPU.  The answer, of course, is yes.

       To  accomplish  this,  insert  the  following  "code  generation"
       routines:


       {---------------------------------------------------------------}
       { Clear the Primary Register }

       procedure Clear;
       begin
          EmitLn('CLR D0');
       end;


       {---------------------------------------------------------------}
       { Negate the Primary Register }

       procedure Negate;
       begin
          EmitLn('NEG D0');
       end;


       {---------------------------------------------------------------}
       { Load a Constant Value to Primary Register }

       procedure LoadConst(n: integer);
       begin
          Emit('MOVE #');A*A*
                                    - 15 -
PAA





          WriteLn(n, ',D0');
       end;


       {---------------------------------------------------------------}
       { Load a Variable to Primary Register }

       procedure LoadVar(Name: char);
       begin
          if not InTable(Name) then Undefined(Name);
          EmitLn('MOVE ' + Name + '(PC),D0');
       end;


       {---------------------------------------------------------------}
       { Push Primary onto Stack }

       procedure Push;
       begin
          EmitLn('MOVE D0,-(SP)');
       end;


       {---------------------------------------------------------------}
       { Add Top of Stack to Primary }

       procedure PopAdd;
       begin
          EmitLn('ADD (SP)+,D0');
       end;


       {---------------------------------------------------------------}
       { Subtract Primary from Top of Stack }

       procedure PopSub;
       begin
          EmitLn('SUB (SP)+,D0');
          EmitLn('NEG D0');
       end;


       {---------------------------------------------------------------}
       { Multiply Top of Stack by Primary }

       procedure PopMul;
       begin
          EmitLn('MULS (SP)+,D0');
       end;


       {---------------------------------------------------------------}
       { Divide Top of Stack by Primary }A6A6
                                    - 16 -A*A*
PAA





       procedure PopDiv;
       begin
          EmitLn('MOVE (SP)+,D7');
          EmitLn('EXT.L D7');
          EmitLn('DIVS D0,D7');
          EmitLn('MOVE D7,D0');
       end;


       {---------------------------------------------------------------}
       { Store Primary to Variable }

       procedure Store(Name: char);
       begin
          if not InTable(Name) then Undefined(Name);
          EmitLn('LEA ' + Name + '(PC),A0');
          EmitLn('MOVE D0,(A0)')
       end;
       {---------------------------------------------------------------}


       The  nice  part  of  this  approach,  of  course,  is that we can
       retarget  the compiler to a new CPU  simply  by  rewriting  these
       "code generator" procedures.  In  addition,  we  will  find later
       that we can improve the code quality by tweaking these routines a
       bit, without having to modify the compiler proper.

       Note that both LoadVar  and  Store check the symbol table to make
       sure that the variable is defined.  The  error  handler Undefined
       simply calls Abort:


       {--------------------------------------------------------------}
       { Report an Undefined Identifier }

       procedure Undefined(n: string);
       begin
          Abort('Undefined Identifier ' + n);
       end;
       {--------------------------------------------------------------}


       OK, we are now finally ready to begin processing executable code.
       We'll  do  that  by  replacing  the  stub  version  of  procedure
       Assignment.

       We've been down this  road  many times before, so this should all
       be familiar to you.    In fact, except for the changes associated
       with the code generation, we  could just copy the procedures from
       Part  VII.    Since we are making some changes, I won't just copy
       them, but we will go a little faster than usual.

       The BNF for the assignment statement is:A6A6
                                    - 17 -A*A*
PAA





            <assignment> ::= <ident> = <expression>

            <expression> ::= <first term> ( <addop> <term> )*

            <first term> ::= <first factor> <rest>

            <term> ::= <factor> <rest>

            <rest> ::= ( <mulop> <factor> )*

            <first factor> ::= [ <addop> ] <factor>

            <factor> ::= <var> | <number> | ( <expression> )


       This version of the BNF is  also  a bit different than we've used
       before ... yet another "variation on the theme of an expression."
       This particular version  has  what  I  consider  to  be  the best
       treatment  of  the  unary minus.  As you'll see later, it lets us
       handle   negative  constant  values  efficiently.    It's   worth
       mentioning  here  that  we  have  often  seen  the advantages  of
       "tweaking"  the  BNF  as we go, to help make the language easy to
       parse.    What  you're looking at here is a bit different:  we've
       tweaked  the  BNF  to make the CODE  GENERATION  more  efficient!
       That's a first for this series.

       Anyhow, the following code implements the BNF:


       {---------------------------------------------------------------}
       { Parse and Translate a Math Factor }

       procedure Expression; Forward;

       procedure Factor;
       begin
          if Look = '(' then begin
             Match('(');
             Expression;
             Match(')');
             end
          else if IsAlpha(Look) then
             LoadVar(GetName)
          else
             LoadConst(GetNum);
       end;


       {--------------------------------------------------------------}
       { Parse and Translate a Negative Factor }

       procedure NegFactor;
       begin
          Match('-');A*A*
                                    - 18 -
PAA





          if IsDigit(Look) then
             LoadConst(-GetNum)
          else begin
             Factor;
             Negate;
          end;
       end;


       {--------------------------------------------------------------}
       { Parse and Translate a Leading Factor }

       procedure FirstFactor;
       begin
          case Look of
            '+': begin
                    Match('+');
                    Factor;
                 end;
            '-': NegFactor;
          else  Factor;
          end;
       end;


       {--------------------------------------------------------------}
       { Recognize and Translate a Multiply }

       procedure Multiply;
       begin
          Match('*');
          Factor;
          PopMul;
       end;


       {-------------------------------------------------------------}
       { Recognize and Translate a Divide }

       procedure Divide;
       begin
          Match('/');
          Factor;
          PopDiv;
       end;


       {---------------------------------------------------------------}
       { Common Code Used by Term and FirstTerm }

       procedure Term1;
       begin
          while IsMulop(Look) do begin
             Push;A*A*
                                    - 19 -
PAA





             case Look of
              '*': Multiply;
              '/': Divide;
             end;
          end;
       end;


       {---------------------------------------------------------------}
       { Parse and Translate a Math Term }

       procedure Term;
       begin
          Factor;
          Term1;
       end;


       {---------------------------------------------------------------}
       { Parse and Translate a Leading Term }

       procedure FirstTerm;
       begin
          FirstFactor;
          Term1;
       end;


       {--------------------------------------------------------------}
       { Recognize and Translate an Add }

       procedure Add;
       begin
          Match('+');
          Term;
          PopAdd;
       end;


       {-------------------------------------------------------------}
       { Recognize and Translate a Subtract }

       procedure Subtract;
       begin
          Match('-');
          Term;
          PopSub;
       end;


       {---------------------------------------------------------------}
       { Parse and Translate an Expression }

       procedure Expression;A*A*
                                    - 20 -
PAA





       begin
          FirstTerm;
          while IsAddop(Look) do begin
             Push;
             case Look of
              '+': Add;
              '-': Subtract;
             end;
          end;
       end;


       {--------------------------------------------------------------}
       { Parse and Translate an Assignment Statement }

       procedure Assignment;
       var Name: char;
       begin
          Name := GetName;
          Match('=');
          Expression;
          Store(Name);
       end;
       {--------------------------------------------------------------}


       OK, if you've  got  all  this  code inserted, then compile it and
       check  it out.  You should  be  seeing  reasonable-looking  code,
       representing a complete program that will  assemble  and execute.
       We have a compiler!


       BOOLEANS

       The next step should also  be  familiar  to  you.    We  must add
       Boolean  expressions  and relational operations.    Again,  since
       we've already dealt with them more than once,  I  won't elaborate
       much on them, except  where  they  are  different from what we've
       done before.  Again, we won't just copy from other  files because
       I've changed a few things just a bit.  Most  of  the changes just
       involve encapsulating the machine-dependent parts as  we  did for
       the   arithmetic  operations.    I've  also  modified   procedure
       NotFactor  somewhat,  to  parallel  the structure of FirstFactor.
       Finally,  I  corrected  an  error  in  the  object code  for  the
       relational operators:  The Scc instruction I used  only  sets the
       low 8 bits of D0.  We want all 16 bits set for a logical true, so
       I've added an instruction to sign-extend the low byte.

       To begin, we're going to need some more recognizers:


       {--------------------------------------------------------------}
       { Recognize a Boolean Orop }A6A6
                                    - 21 -A*A*
PAA





       function IsOrop(c: char): boolean;
       begin
          IsOrop := c in ['|', '~'];
       end;


       {--------------------------------------------------------------}
       { Recognize a Relop }

       function IsRelop(c: char): boolean;
       begin
          IsRelop := c in ['=', '#', '<', '>'];
       end;
       {--------------------------------------------------------------}


       Also, we're going to need some more code generation routines:


       {---------------------------------------------------------------}
       { Complement the Primary Register }

       procedure NotIt;
       begin
          EmitLn('NOT D0');
       end;
       {---------------------------------------------------------------}
       .
       .
       .
       {---------------------------------------------------------------}
       { AND Top of Stack with Primary }

       procedure PopAnd;
       begin
          EmitLn('AND (SP)+,D0');
       end;


       {---------------------------------------------------------------}
       { OR Top of Stack with Primary }

       procedure PopOr;
       begin
          EmitLn('OR (SP)+,D0');
       end;


       {---------------------------------------------------------------}
       { XOR Top of Stack with Primary }

       procedure PopXor;
       begin
          EmitLn('EOR (SP)+,D0');A*A*
                                    - 22 -
PAA





       end;


       {---------------------------------------------------------------}
       { Compare Top of Stack with Primary }

       procedure PopCompare;
       begin
          EmitLn('CMP (SP)+,D0');
       end;


       {---------------------------------------------------------------}
       { Set D0 If Compare was = }

       procedure SetEqual;
       begin
          EmitLn('SEQ D0');
          EmitLn('EXT D0');
       end;


       {---------------------------------------------------------------}
       { Set D0 If Compare was != }

       procedure SetNEqual;
       begin
          EmitLn('SNE D0');
          EmitLn('EXT D0');
       end;


       {---------------------------------------------------------------}
       { Set D0 If Compare was > }

       procedure SetGreater;
       begin
          EmitLn('SLT D0');
          EmitLn('EXT D0');
       end;


       {---------------------------------------------------------------}
       { Set D0 If Compare was < }

       procedure SetLess;
       begin
          EmitLn('SGT D0');
          EmitLn('EXT D0');
       end;
       {---------------------------------------------------------------}ANAN
                                    - 23 -A*A*
PAA





       All of this  gives us the tools we need.  The BNF for the Boolean
       expressions is:


            <bool-expr> ::= <bool-term> ( <orop> <bool-term> )*

            <bool-term> ::= <not-factor> ( <andop> <not-factor> )*

            <not-factor> ::= [ '!' ] <relation>

            <relation> ::= <expression> [ <relop> <expression> ]


       Sharp-eyed readers might  note  that this syntax does not include
       the non-terminal  "bool-factor" used in earlier versions.  It was
       needed then because I also allowed for the Boolean constants TRUE
       and FALSE.   But  remember  that  in TINY there is no distinction
       made between Boolean and arithmetic  types ... they can be freely
       intermixed.   So there is really no  need  for  these  predefined
       values ... we can just use -1 and 0, respectively.

       In C terminology, we could always use the defines:


            #define TRUE -1
            #define FALSE 0


       (That is, if TINY had a  preprocessor.)   Later on, when we allow
       for  declarations  of  constants,  these  two   values   will  be
       predefined by the language.

       The reason that I'm harping on this is that  I've  already  tried
       the alternative, which is to  include TRUE and FALSE as keywords.
       The problem with that approach is that it  then  requires lexical
       scanning for EVERY variable name  in every expression.  If you'll
       recall,  I pointed out in Installment VII  that  this  slows  the
       compiler  down considerably.  As long as  keywords  can't  be  in
       expressions, we need to do the scanning only at the  beginning of
       every  new  statement  ...  quite  an improvement.  So using  the
       syntax above not only simplifies the parsing, but  speeds  up the
       scanning as well.

       OK, given that we're  all  satisfied  with  the syntax above, the
       corresponding code is shown below:


       {---------------------------------------------------------------}
       { Recognize and Translate a Relational "Equals" }

       procedure Equals;
       begin
          Match('=');
          Expression;A*A*
                                    - 24 -
PAA





          PopCompare;
          SetEqual;
       end;


       {---------------------------------------------------------------}
       { Recognize and Translate a Relational "Not Equals" }

       procedure NotEquals;
       begin
          Match('#');
          Expression;
          PopCompare;
          SetNEqual;
       end;


       {---------------------------------------------------------------}
       { Recognize and Translate a Relational "Less Than" }

       procedure Less;
       begin
          Match('<');
          Expression;
          PopCompare;
          SetLess;
       end;


       {---------------------------------------------------------------}
       { Recognize and Translate a Relational "Greater Than" }

       procedure Greater;
       begin
          Match('>');
          Expression;
          PopCompare;
          SetGreater;
       end;


       {---------------------------------------------------------------}
       { Parse and Translate a Relation }


       procedure Relation;
       begin
          Expression;
          if IsRelop(Look) then begin
             Push;
             case Look of
              '=': Equals;
              '#': NotEquals;
              '<': Less;A*A*
                                    - 25 -
PAA





              '>': Greater;
             end;
          end;
       end;


       {---------------------------------------------------------------}
       { Parse and Translate a Boolean Factor with Leading NOT }

       procedure NotFactor;
       begin
          if Look = '!' then begin
             Match('!');
             Relation;
             NotIt;
             end
          else
             Relation;
       end;


       {---------------------------------------------------------------}
       { Parse and Translate a Boolean Term }

       procedure BoolTerm;
       begin
          NotFactor;
          while Look = '&' do begin
             Push;
             Match('&');
             NotFactor;
             PopAnd;
          end;
       end;


       {--------------------------------------------------------------}
       { Recognize and Translate a Boolean OR }

       procedure BoolOr;
       begin
          Match('|');
          BoolTerm;
          PopOr;
       end;


       {--------------------------------------------------------------}
       { Recognize and Translate an Exclusive Or }

       procedure BoolXor;
       begin
          Match('~');
          BoolTerm;A*A*
                                    - 26 -
PAA





          PopXor;
       end;


       {---------------------------------------------------------------}
       { Parse and Translate a Boolean Expression }

       procedure BoolExpression;
       begin
          BoolTerm;
          while IsOrOp(Look) do begin
             Push;
             case Look of
              '|': BoolOr;
              '~': BoolXor;
             end;
          end;
       end;
       {--------------------------------------------------------------}


       To tie it all together, don't forget to change the  references to
       Expression in  procedures Factor and Assignment so that they call
       BoolExpression instead.

       OK, if  you've  got  all  that typed in, compile it and give it a
       whirl.    First,  make  sure  you  can  still parse  an  ordinary
       arithmetic expression.  Then, try a Boolean one.    Finally, make
       sure  that you can assign the results of  relations.    Try,  for
       example:

            pvx,y,zbx=z>ye.

       which stands for:

            PROGRAM
            VAR X,Y,Z
            BEGIN
            X = Z > Y
            END.


       See how this assigns a Boolean value to X?

       CONTROL STRUCTURES

       We're almost home.   With  Boolean  expressions  in place, it's a
       simple  matter  to  add control structures.  For TINY, we'll only
       allow two kinds of them, the IF and the WHILE:


            <if> ::= IF <bool-expression> <block> [ ELSE <block>] ENDIF

            <while> ::= WHILE <bool-expression> <block> ENDWHILEA*A*
                                    - 27 -
PAA





       Once  again,  let  me  spell  out the decisions implicit in  this
       syntax, which departs strongly from that of C or Pascal.  In both
       of those languages, the "body" of an IF or WHILE is regarded as a
       single  statement.  If you intend to use a block of more than one
       statement, you have to build a compound statement using BEGIN-END
       (in Pascal) or  '{}' (in C).  In TINY (and KISS) there is no such
       thing as a compound statement  ... single or multiple they're all
       just blocks to these languages.

       In KISS, all the control structures will have explicit and unique
       keywords  bracketing  the  statement block, so there  can  be  no
       confusion as to where things begin  and  end.  This is the modern
       approach, used in such respected languages as Ada  and  Modula 2,
       and it completely eliminates the problem of the "dangling else."

       Note  that I could have chosen to use the same keyword END to end
       all  the constructs, as is done in Pascal.  (The closing '}' in C
       serves the same purpose.)  But this has always led  to confusion,
       which is why Pascal programmers tend to write things like


            end { loop }

       or   end { if }


       As I explained in  Part  V,  using  unique terminal keywords does
       increase  the  size  of the keyword list and therefore slows down
       the  scanning, but in this case it seems a small price to pay for
       the added insurance.   Better  to find the errors at compile time
       rather than run time.

       One last thought:  The two constructs above each  have  the  non-
       terminals


             <bool-expression> and <block>


       juxtaposed with no separating keyword.  In Pascal we would expect
       the keywords THEN and DO in these locations.

       I have no problem with leaving out these keywords, and the parser
       has no trouble either, ON CONDITION that we make no errors in the
       bool-expression part.  On  the  other hand, if we were to include
       these extra keywords we would get yet one more level of insurance
       at very little  cost,  and  I  have no problem with that, either.
       Use your best judgment as to which way to go.

       OK, with that bit of explanation let's proceed.  As  usual, we're
       going to need some new  code generation routines.  These generate
       the code for conditional and unconditional branches:ABAB
                                    - 28 -A*A*
PAA





       {---------------------------------------------------------------}
       { Branch Unconditional  }

       procedure Branch(L: string);
       begin
          EmitLn('BRA ' + L);
       end;


       {---------------------------------------------------------------}
       { Branch False }

       procedure BranchFalse(L: string);
       begin
          EmitLn('TST D0');
          EmitLn('BEQ ' + L);
       end;
       {--------------------------------------------------------------}


       Except for the encapsulation of  the code generation, the code to
       parse the control constructs is the same as you've seen before:


       {---------------------------------------------------------------}
       { Recognize and Translate an IF Construct }

       procedure Block; Forward;


       procedure DoIf;
       var L1, L2: string;
       begin
          Match('i');
          BoolExpression;
          L1 := NewLabel;
          L2 := L1;
          BranchFalse(L1);
          Block;
          if Look = 'l' then begin
             Match('l');
             L2 := NewLabel;
             Branch(L2);
             PostLabel(L1);
             Block;
          end;
          PostLabel(L2);
          Match('e');
       end;


       {--------------------------------------------------------------}
       { Parse and Translate a WHILE Statement }A6A6
                                    - 29 -A*A*
PAA





       procedure DoWhile;
       var L1, L2: string;
       begin
          Match('w');
          L1 := NewLabel;
          L2 := NewLabel;
          PostLabel(L1);
          BoolExpression;
          BranchFalse(L2);
          Block;
          Match('e');
          Branch(L1);
          PostLabel(L2);
       end;
       {--------------------------------------------------------------}


       To tie everything  together,  we need only modify procedure Block
       to recognize the "keywords" for the  IF  and WHILE.  As usual, we
       expand the definition of a block like so:


            <block> ::= ( <statement> )*


       where


            <statement> ::= <if> | <while> | <assignment>


       The corresponding code is:


       {--------------------------------------------------------------}
       { Parse and Translate a Block of Statements }

       procedure Block;
       begin
          while not(Look in ['e', 'l']) do begin
             case Look of
              'i': DoIf;
              'w': DoWhile;
             else Assignment;
             end;
          end;
       end;
       {--------------------------------------------------------------}


       OK,  add the routines I've given, compile and  test  them.    You
       should be able to parse the single-character versions  of  any of
       the control constructs.  It's looking pretty good!A6A6
                                    - 30 -A*A*
PAA





       As a matter  of  fact, except for the single-character limitation
       we've got a virtually complete version of TINY.  I call  it, with
       tongue planted firmly in cheek, TINY Version 0.1.


       LEXICAL SCANNING

       Of course, you know what's next:  We have to convert  the program
       so that  it can deal with multi-character keywords, newlines, and
       whitespace.   We have just gone through all  that  in  Part  VII.
       We'll use the distributed scanner  technique that I showed you in
       that  installment.    The  actual  implementation  is   a  little
       different because the way I'm handling newlines is different.

       To begin with, let's simply  allow for whitespace.  This involves
       only adding calls to SkipWhite at the end of the  three routines,
       GetName, GetNum, and Match.    A call to SkipWhite in Init primes
       the pump in case there are leading spaces.

       Next, we need to deal with  newlines.   This is really a two-step
       process,  since  the  treatment  of  the  newlines  with  single-
       character tokens is different from that for multi-character ones.
       We can eliminate some work by doing both  steps  at  once,  but I
       feel safer taking things one step at a time.

       Insert the new procedure:


       {--------------------------------------------------------------}
       { Skip Over an End-of-Line }

       procedure NewLine;
       begin
          while Look = CR do begin
             GetChar;
             if Look = LF then GetChar;
             SkipWhite;
          end;
       end;
       {--------------------------------------------------------------}


       Note that  we  have  seen  this  procedure  before in the form of
       Procedure Fin.  I've changed the name since this  new  one  seems
       more descriptive of the actual function.  I've  also  changed the
       code  to  allow  for multiple newlines and lines with nothing but
       white space.

       The next step is to insert calls to NewLine wherever we  decide a
       newline is permissible.  As I've pointed out before, this  can be
       very different in different languages.   In TINY, I've decided to
       allow them virtually anywhere.  This means that we need  calls to
       NewLine at the BEGINNING (not the end, as with SkipWhite)  of the
       procedures GetName, GetNum, and Match.A*A*
                                    - 31 -
PAA





       For procedures that have while loops, such as TopDecl, we  need a
       call  to NewLine at the beginning of the  procedure  AND  at  the
       bottom  of  each  loop.  That way, we can be assured that NewLine
       has just been called at the beginning of each  pass  through  the
       loop.

       If you've got all this done, try the program out and  verify that
       it will indeed handle white space and newlines.

       If it does, then we're  ready to deal with multi-character tokens
       and keywords.   To begin, add the additional declarations (copied
       almost verbatim from Part VII):


       {--------------------------------------------------------------}
       { Type Declarations }

       type Symbol = string[8];

            SymTab = array[1..1000] of Symbol;

            TabPtr = ^SymTab;


       {--------------------------------------------------------------}
       { Variable Declarations }

       var Look : char;             { Lookahead Character }
           Token: char;             { Encoded Token       }
           Value: string[16];       { Unencoded Token     }

           ST: Array['A'..'Z'] of char;

       {--------------------------------------------------------------}
       { Definition of Keywords and Token Types }

       const NKW =   9;
             NKW1 = 10;

       const KWlist: array[1..NKW] of Symbol =
                     ('IF', 'ELSE', 'ENDIF', 'WHILE', 'ENDWHILE',
                      'VAR', 'BEGIN', 'END', 'PROGRAM');

       const KWcode: string[NKW1] = 'xilewevbep';
       {--------------------------------------------------------------}


       Next, add the three procedures, also from Part VII:


       {--------------------------------------------------------------}
       { Table Lookup }

       function Lookup(T: TabPtr; s: string; n: integer): integer;A*A*
                                    - 32 -
PAA





       var i: integer;
           found: Boolean;
       begin
          found := false;
          i := n;
          while (i > 0) and not found do
             if s = T^[i] then
                found := true
             else
                dec(i);
          Lookup := i;
       end;
       {--------------------------------------------------------------}
       .
       .
       {--------------------------------------------------------------}
       { Get an Identifier and Scan it for Keywords }

       procedure Scan;
       begin
          GetName;
          Token := KWcode[Lookup(Addr(KWlist), Value, NKW) + 1];
       end;
       {--------------------------------------------------------------}
       .
       .
       {--------------------------------------------------------------}
       { Match a Specific Input String }

       procedure MatchString(x: string);
       begin
          if Value <> x then Expected('''' + x + '''');
       end;
       {--------------------------------------------------------------}


       Now, we have to make a  fairly  large number of subtle changes to
       the remaining procedures.  First,  we  must  change  the function
       GetName to a procedure, again as we did in Part VII:


       {--------------------------------------------------------------}
       { Get an Identifier }

       procedure GetName;
       begin
          NewLine;
          if not IsAlpha(Look) then Expected('Name');
          Value := '';
          while IsAlNum(Look) do begin
             Value := Value + UpCase(Look);
             GetChar;
          end;
          SkipWhite;A*A*
                                    - 33 -
PAA





       end;
       {--------------------------------------------------------------}


       Note that this procedure leaves its result in  the  global string
       Value.

       Next, we have to change every reference to GetName to reflect its
       new form. These occur in Factor, Assignment, and Decl:


       {---------------------------------------------------------------}
       { Parse and Translate a Math Factor }

       procedure BoolExpression; Forward;

       procedure Factor;
       begin
          if Look = '(' then begin
             Match('(');
             BoolExpression;
             Match(')');
             end
          else if IsAlpha(Look) then begin
             GetName;
             LoadVar(Value[1]);
             end
          else
             LoadConst(GetNum);
       end;
       {--------------------------------------------------------------}
       .
       .
       {--------------------------------------------------------------}
       { Parse and Translate an Assignment Statement }

       procedure Assignment;
       var Name: char;
       begin
          Name := Value[1];
          Match('=');
          BoolExpression;
          Store(Name);
       end;
       {---------------------------------------------------------------}
       .
       .
       {--------------------------------------------------------------}
       { Parse and Translate a Data Declaration }

       procedure Decl;
       begin
          GetName;
          Alloc(Value[1]);A*A*
                                    - 34 -
PAA





          while Look = ',' do begin
             Match(',');
             GetName;
             Alloc(Value[1]);
          end;
       end;
       {--------------------------------------------------------------}


       (Note that we're still  only  allowing  single-character variable
       names,  so we take the easy way out here and simply use the first
       character of the string.)

       Finally, we must make the changes to use Token instead of Look as
       the  test  character  and to call Scan at the appropriate places.
       Mostly, this  involves  deleting  calls  to  Match,  occasionally
       replacing calls to  Match  by calls to MatchString, and Replacing
       calls  to  NewLine  by  calls  to  Scan.    Here are the affected
       routines:

       {---------------------------------------------------------------}
       { Recognize and Translate an IF Construct }

       procedure Block; Forward;


       procedure DoIf;
       var L1, L2: string;
       begin
          BoolExpression;
          L1 := NewLabel;
          L2 := L1;
          BranchFalse(L1);
          Block;
          if Token = 'l' then begin
             L2 := NewLabel;
             Branch(L2);
             PostLabel(L1);
             Block;
          end;
          PostLabel(L2);
          MatchString('ENDIF');
       end;


       {--------------------------------------------------------------}
       { Parse and Translate a WHILE Statement }

       procedure DoWhile;
       var L1, L2: string;
       begin
          L1 := NewLabel;
          L2 := NewLabel;
          PostLabel(L1);A*A*
                                    - 35 -
PAA





          BoolExpression;
          BranchFalse(L2);
          Block;
          MatchString('ENDWHILE');
          Branch(L1);
          PostLabel(L2);
       end;


       {--------------------------------------------------------------}
       { Parse and Translate a Block of Statements }

       procedure Block;
       begin
          Scan;
          while not(Token in ['e', 'l']) do begin
             case Token of
              'i': DoIf;
              'w': DoWhile;
             else Assignment;
             end;
             Scan;
          end;
       end;


       {--------------------------------------------------------------}
       { Parse and Translate Global Declarations }

       procedure TopDecls;
       begin
          Scan;
          while Token <> 'b' do begin
             case Token of
               'v': Decl;
             else Abort('Unrecognized Keyword ' + Value);
             end;
             Scan;
          end;
       end;


       {--------------------------------------------------------------}
       { Parse and Translate a Main Program }

       procedure Main;
       begin
          MatchString('BEGIN');
          Prolog;
          Block;
          MatchString('END');
          Epilog;
       end;A6A6
                                    - 36 -A*A*
PAA





       {--------------------------------------------------------------}
       {  Parse and Translate a Program }

       procedure Prog;
       begin
          MatchString('PROGRAM');
          Header;
          TopDecls;
          Main;
          Match('.');
       end;


       {--------------------------------------------------------------}
       { Initialize }

       procedure Init;
       var i: char;
       begin
          for i := 'A' to 'Z' do
             ST[i] := ' ';
          GetChar;
          Scan;
       end;
       {--------------------------------------------------------------}


       That should do  it.    If  all  the changes got in correctly, you
       should now be parsing programs that look like programs.   (If you
       didn't  make  it  through all the  changes,  don't  despair.    A
       complete listing of the final form is given later.)

       Did it work?  If so, then we're just about home.  In fact, with a
       few minor  exceptions we've already got a compiler that's usable.
       There are still a few areas that need improvement.


       MULTI-CHARACTER VARIABLE NAMES

       One of those is  the  restriction  that  we still have, requiring
       single-character variable names.    Now that we can handle multi-
       character keywords, this one  begins  to  look  very much like an
       arbitrary  and  unnecessary  limitation.    And  indeed   it  is.
       Basically, its only virtue is  that it permits a trivially simple
       implementation  of  the   symbol   table.    But  that's  just  a
       convenience to the compiler writers, and needs to be eliminated.

       We've done this step before.  This time, as usual, I'm doing it a
       little differently.  I think  the approach used here keeps things
       just about as simple as possible.

       The natural  way  to  implement  a  symbol  table in Pascal is by
       declaring a record type, and making the symbol table an  array of
       such records.  Here, though, we don't really need  a  type  fieldA*A*
                                    - 37 -
PAA





       yet  (there is only one kind of entry allowed so far), so we only
       need an array of symbols.  This has the advantage that we can use
       the existing procedure Lookup to  search the symbol table as well
       as the  keyword  list.    As it turns out, even when we need more
       fields we can still use the same approach, simply by  storing the
       other fields in separate arrays.

       OK, here are the changes that  need  to  be made.  First, add the
       new typed constant:


             NEntry: integer = 0;


       Then change the definition of the symbol table as follows:


       const MaxEntry = 100;

       var ST   : array[1..MaxEntry] of Symbol;


       (Note that ST is _NOT_ declared as a SymTab.  That declaration is
       a phony one to get Lookup to work.  A SymTab  would  take  up too
       much RAM space, and so one is never actually allocated.)

       Next, we need to replace InTable:


       {--------------------------------------------------------------}
       { Look for Symbol in Table }

       function InTable(n: Symbol): Boolean;
       begin
          InTable := Lookup(@ST, n, MaxEntry) <> 0;
       end;
       {--------------------------------------------------------------}


       We also need a new procedure, AddEntry, that adds a new  entry to
       the table:


       {--------------------------------------------------------------}
       { Add a New Entry to Symbol Table }

       procedure AddEntry(N: Symbol; T: char);
       begin
          if InTable(N) then Abort('Duplicate Identifier ' + N);
          if NEntry = MaxEntry then Abort('Symbol Table Full');
          Inc(NEntry);
          ST[NEntry] := N;
          SType[NEntry] := T;
       end;A*A*
                                    - 38 -
PAA





       {--------------------------------------------------------------}


       This procedure is called by Alloc:


       {--------------------------------------------------------------}
       { Allocate Storage for a Variable }

       procedure Alloc(N: Symbol);
       begin
          if InTable(N) then Abort('Duplicate Variable Name ' + N);
          AddEntry(N, 'v');
       .
       .
       .
       {--------------------------------------------------------------}


       Finally, we must change all the routines that currently treat the
       variable name as a single character.  These include   LoadVar and
       Store (just change the  type  from  char  to string), and Factor,
       Assignment, and Decl (just change Value[1] to Value).

       One  last  thing:  change  procedure  Init to clear the array  as
       shown:


       {--------------------------------------------------------------}
       { Initialize }

       procedure Init;
       var i: integer;
       begin
          for i := 1 to MaxEntry do begin
             ST[i] := '';
             SType[i] := ' ';
          end;
          GetChar;
          Scan;
       end;
       {--------------------------------------------------------------}


       That should do it.  Try it out and verify  that  you can, indeed,
       use multi-character variable names.


       MORE RELOPS

       We still have one remaining single-character restriction: the one
       on relops.  Some of the relops are indeed single  characters, but
       others  require two.  These are '<=' and '>='.  I also prefer the
       Pascal '<>' for "not equals,"  instead of '#'.A*A*
                                    - 39 -
PAA





       If you'll recall, in Part VII I pointed out that the conventional
       way  to  deal  with  relops  is  to  include them in the list  of
       keywords, and let the  lexical  scanner  find  them.  But, again,
       this requires scanning throughout the expression parsing process,
       whereas so far we've been able to limit the use of the scanner to
       the beginning of a statement.

       I mentioned then that we can still get away with this,  since the
       multi-character relops are so few  and so limited in their usage.
       It's easy to just treat them as special cases and handle  them in
       an ad hoc manner.

       The changes required affect only the code generation routines and
       procedures Relation and friends.   First, we're going to need two
       more code generation routines:


       {---------------------------------------------------------------}
       { Set D0 If Compare was <= }

       procedure SetLessOrEqual;
       begin
          EmitLn('SGE D0');
          EmitLn('EXT D0');
       end;


       {---------------------------------------------------------------}
       { Set D0 If Compare was >= }

       procedure SetGreaterOrEqual;
       begin
          EmitLn('SLE D0');
          EmitLn('EXT D0');
       end;
       {---------------------------------------------------------------}


       Then, modify the relation parsing routines as shown below:


       {---------------------------------------------------------------}
       { Recognize and Translate a Relational "Less Than or Equal" }

       procedure LessOrEqual;
       begin
          Match('=');
          Expression;
          PopCompare;
          SetLessOrEqual;
       end;


       {---------------------------------------------------------------}A*A*
                                    - 40 -
PAA





       { Recognize and Translate a Relational "Not Equals" }

       procedure NotEqual;
       begin
          Match('>');
          Expression;
          PopCompare;
          SetNEqual;
       end;


       {---------------------------------------------------------------}
       { Recognize and Translate a Relational "Less Than" }

       procedure Less;
       begin
          Match('<');
          case Look of
            '=': LessOrEqual;
            '>': NotEqual;
          else begin
                  Expression;
                  PopCompare;
                  SetLess;
               end;
          end;
       end;


       {---------------------------------------------------------------}
       { Recognize and Translate a Relational "Greater Than" }

       procedure Greater;
       begin
          Match('>');
          if Look = '=' then begin
             Match('=');
             Expression;
             PopCompare;
             SetGreaterOrEqual;
             end
          else begin
             Expression;
             PopCompare;
             SetGreater;
          end;
       end;
       {---------------------------------------------------------------}


       That's all it takes.  Now  you  can  process all the relops.  Try
       it.ABAB
                                    - 41 -A*A*
PAA





       INPUT/OUTPUT

       We  now  have  a complete, working language, except for one minor
       embarassment: we have no way to get data in or out.  We need some
       I/O.

       Now, the convention these days, established in C and continued in
       Ada and Modula 2, is to leave I/O statements out of  the language
       itself,  and  just  include them in the subroutine library.  That
       would  be  fine, except that so far  we  have  no  provision  for
       subroutines.  Anyhow, with this approach you run into the problem
       of variable-length argument lists.  In Pascal, the I/O statements
       are built into the language because they are the  only  ones  for
       which  the  argument  list can have a variable number of entries.
       In C, we settle for kludges like scanf and printf, and  must pass
       the argument count to the called procedure.  In Ada and  Modula 2
       we must use the  awkward  (and SLOW!) approach of a separate call
       for each argument.

       So I think I prefer the  Pascal  approach of building the I/O in,
       even though we don't need to.

       As  usual,  for  this we need some more code generation routines.
       These turn out  to be the easiest of all, because all we do is to
       call library procedures to do the work:


       {---------------------------------------------------------------}
       { Read Variable to Primary Register }

       procedure ReadVar;
       begin
          EmitLn('BSR READ');
          Store(Value);
       end;


       {---------------------------------------------------------------}
       { Write Variable from Primary Register }

       procedure WriteVar;
       begin
          EmitLn('BSR WRITE');
       end;
       {--------------------------------------------------------------}


       The idea is that READ loads the value from input  to  the D0, and
       WRITE outputs it from there.

       These two procedures represent  our  first  encounter with a need
       for library procedures ... the components of a  Run  Time Library
       (RTL).    Of  course, someone (namely  us)  has  to  write  these
       routines, but they're not  part  of the compiler itself.  I won'tA*A*
                                    - 42 -
PAA





       even bother  showing the routines here, since these are obviously
       very much OS-dependent.   I  _WILL_  simply  say that for SK*DOS,
       they  are  particularly  simple ... almost trivial.  One reason I
       won't show them here is that  you  can add all kinds of fanciness
       to the things, for  example  by prompting in READ for the inputs,
       and by giving the user a chance to reenter a bad input.

       But that is really separate from compiler design, so for now I'll
       just assume that a library call TINYLIB.LIB exists.  Since we now
       need  it  loaded,  we need to add a statement to  include  it  in
       procedure Header:


       {--------------------------------------------------------------}
       { Write Header Info }

       procedure Header;
       begin

          WriteLn('WARMST', TAB, 'EQU $A01E');
          EmitLn('LIB TINYLIB');
       end;
       {--------------------------------------------------------------}

       That takes care of that part.  Now, we also need to recognize the
       read  and  write  commands.  We can do this by  adding  two  more
       keywords to our list:


       {--------------------------------------------------------------}
       { Definition of Keywords and Token Types }

       const NKW =   11;
             NKW1 = 12;

       const KWlist: array[1..NKW] of Symbol =
                     ('IF', 'ELSE', 'ENDIF', 'WHILE', 'ENDWHILE',
                      'READ',    'WRITE',    'VAR',    'BEGIN',   'END',
       'PROGRAM');

       const KWcode: string[NKW1] = 'xileweRWvbep';
       {--------------------------------------------------------------}


       (Note how I'm using upper case codes here to avoid  conflict with
       the 'w' of WHILE.)

       Next, we need procedures for processing the  read/write statement
       and its argument list:


       {--------------------------------------------------------------}
       { Process a Read Statement }A6A6
                                    - 43 -A*A*
PAA





       procedure DoRead;
       begin
          Match('(');
          GetName;
          ReadVar;
          while Look = ',' do begin
             Match(',');
             GetName;
             ReadVar;
          end;
          Match(')');
       end;


       {--------------------------------------------------------------}
       { Process a Write Statement }

       procedure DoWrite;
       begin
          Match('(');
          Expression;
          WriteVar;
          while Look = ',' do begin
             Match(',');
             Expression;
             WriteVar;
          end;
          Match(')');
       end;
       {--------------------------------------------------------------}


       Finally,  we  must  expand  procedure  Block  to  handle the  new
       statement types:


       {--------------------------------------------------------------}
       { Parse and Translate a Block of Statements }

       procedure Block;
       begin
          Scan;
          while not(Token in ['e', 'l']) do begin
             case Token of
              'i': DoIf;
              'w': DoWhile;
              'R': DoRead;
              'W': DoWrite;
             else Assignment;
             end;
             Scan;
          end;
       end;
       {--------------------------------------------------------------}A*A*
                                    - 44 -
PAA





       That's all there is to it.  _NOW_ we have a language!


       CONCLUSION

       At this point we have TINY completely defined.  It's not much ...
       actually a toy  compiler.    TINY  has  only one data type and no
       subroutines  ... but it's a complete,  usable  language.    While
       you're not likely to be able to write another compiler in  it, or
       do anything else very seriously, you could write programs to read
       some input, perform calculations,  and  output  the results.  Not
       too bad for a toy.

       Most importantly, we have a firm base upon which to build further
       extensions.  I know you'll be glad to hear this: this is the last
       time  I'll  start  over in building a parser ... from  now  on  I
       intend to just add features to  TINY  until it becomes KISS.  Oh,
       there'll be other times we will  need  to try things out with new
       copies  of  the  Cradle, but once we've found out how to do those
       things they'll be incorporated into TINY.

       What  will  those  features  be?    Well,  for starters  we  need
       subroutines and functions.    Then  we  need to be able to handle
       different types, including arrays, strings, and other structures.
       Then we need to deal with the idea of pointers.  All this will be
       upcoming in future installments.

       See you then.

       For references purposes, the complete listing of TINY Version 1.0
       is shown below:


       {--------------------------------------------------------------}
       program Tiny10;

       {--------------------------------------------------------------}
       { Constant Declarations }

       const TAB = ^I;
             CR  = ^M;
             LF  = ^J;

             LCount: integer = 0;
             NEntry: integer = 0;


       {--------------------------------------------------------------}
       { Type Declarations }

       type Symbol = string[8];

            SymTab = array[1..1000] of Symbol;A6A6
                                    - 45 -A*A*
PAA





            TabPtr = ^SymTab;


       {--------------------------------------------------------------}
       { Variable Declarations }

       var Look : char;             { Lookahead Character }
           Token: char;             { Encoded Token       }
           Value: string[16];       { Unencoded Token     }


       const MaxEntry = 100;

       var ST   : array[1..MaxEntry] of Symbol;
           SType: array[1..MaxEntry] of char;


       {--------------------------------------------------------------}
       { Definition of Keywords and Token Types }

       const NKW =   11;
             NKW1 = 12;

       const KWlist: array[1..NKW] of Symbol =
                     ('IF', 'ELSE', 'ENDIF', 'WHILE', 'ENDWHILE',
                      'READ',    'WRITE',    'VAR',    'BEGIN',   'END',
       'PROGRAM');

       const KWcode: string[NKW1] = 'xileweRWvbep';


       {--------------------------------------------------------------}
       { Read New Character From Input Stream }

       procedure GetChar;
       begin
          Read(Look);
       end;

       {--------------------------------------------------------------}
       { Report an Error }

       procedure Error(s: string);
       begin
          WriteLn;
          WriteLn(^G, 'Error: ', s, '.');
       end;


       {--------------------------------------------------------------}
       { Report Error and Halt }

       procedure Abort(s: string);
       beginA*A*
                                    - 46 -
PAA





          Error(s);
          Halt;
       end;


       {--------------------------------------------------------------}
       { Report What Was Expected }

       procedure Expected(s: string);
       begin
          Abort(s + ' Expected');
       end;

       {--------------------------------------------------------------}
       { Report an Undefined Identifier }

       procedure Undefined(n: string);
       begin
          Abort('Undefined Identifier ' + n);
       end;


       {--------------------------------------------------------------}
       { Recognize an Alpha Character }

       function IsAlpha(c: char): boolean;
       begin
          IsAlpha := UpCase(c) in ['A'..'Z'];
       end;


       {--------------------------------------------------------------}
       { Recognize a Decimal Digit }

       function IsDigit(c: char): boolean;
       begin
          IsDigit := c in ['0'..'9'];
       end;


       {--------------------------------------------------------------}
       { Recognize an AlphaNumeric Character }

       function IsAlNum(c: char): boolean;
       begin
          IsAlNum := IsAlpha(c) or IsDigit(c);
       end;


       {--------------------------------------------------------------}
       { Recognize an Addop }

       function IsAddop(c: char): boolean;
       beginA*A*
                                    - 47 -
PAA





          IsAddop := c in ['+', '-'];
       end;


       {--------------------------------------------------------------}
       { Recognize a Mulop }

       function IsMulop(c: char): boolean;
       begin
          IsMulop := c in ['*', '/'];
       end;


       {--------------------------------------------------------------}
       { Recognize a Boolean Orop }

       function IsOrop(c: char): boolean;
       begin
          IsOrop := c in ['|', '~'];
       end;


       {--------------------------------------------------------------}
       { Recognize a Relop }

       function IsRelop(c: char): boolean;
       begin
          IsRelop := c in ['=', '#', '<', '>'];
       end;


       {--------------------------------------------------------------}
       { Recognize White Space }

       function IsWhite(c: char): boolean;
       begin
          IsWhite := c in [' ', TAB];
       end;


       {--------------------------------------------------------------}
       { Skip Over Leading White Space }

       procedure SkipWhite;
       begin
          while IsWhite(Look) do
             GetChar;
       end;


       {--------------------------------------------------------------}
       { Skip Over an End-of-Line }

       procedure NewLine;A*A*
                                    - 48 -
PAA





       begin
          while Look = CR do begin
             GetChar;
             if Look = LF then GetChar;
             SkipWhite;
          end;
       end;


       {--------------------------------------------------------------}
       { Match a Specific Input Character }

       procedure Match(x: char);
       begin
          NewLine;
          if Look = x then GetChar
          else Expected('''' + x + '''');
          SkipWhite;
       end;


       {--------------------------------------------------------------}
       { Table Lookup }

       function Lookup(T: TabPtr; s: string; n: integer): integer;
       var i: integer;
           found: Boolean;
       begin
          found := false;
          i := n;
          while (i > 0) and not found do
             if s = T^[i] then
                found := true
             else
                dec(i);
          Lookup := i;
       end;


       {--------------------------------------------------------------}
       { Locate a Symbol in Table }
       { Returns the index of the entry.  Zero if not present. }

       function Locate(N: Symbol): integer;
       begin
          Locate := Lookup(@ST, n, MaxEntry);
       end;


       {--------------------------------------------------------------}
       { Look for Symbol in Table }

       function InTable(n: Symbol): Boolean;
       beginA*A*
                                    - 49 -
PAA





          InTable := Lookup(@ST, n, MaxEntry) <> 0;
       end;


       {--------------------------------------------------------------}
       { Add a New Entry to Symbol Table }

       procedure AddEntry(N: Symbol; T: char);
       begin
          if InTable(N) then Abort('Duplicate Identifier ' + N);
          if NEntry = MaxEntry then Abort('Symbol Table Full');
          Inc(NEntry);
          ST[NEntry] := N;
          SType[NEntry] := T;
       end;


       {--------------------------------------------------------------}
       { Get an Identifier }

       procedure GetName;
       begin
          NewLine;
          if not IsAlpha(Look) then Expected('Name');
          Value := '';
          while IsAlNum(Look) do begin
             Value := Value + UpCase(Look);
             GetChar;
          end;
          SkipWhite;
       end;


       {--------------------------------------------------------------}
       { Get a Number }

       function GetNum: integer;
       var Val: integer;
       begin
          NewLine;
          if not IsDigit(Look) then Expected('Integer');
          Val := 0;
          while IsDigit(Look) do begin
             Val := 10 * Val + Ord(Look) - Ord('0');
             GetChar;
          end;
          GetNum := Val;
          SkipWhite;
       end;


       {--------------------------------------------------------------}
       { Get an Identifier and Scan it for Keywords }A6A6
                                    - 50 -A*A*
PAA





       procedure Scan;
       begin
          GetName;
          Token := KWcode[Lookup(Addr(KWlist), Value, NKW) + 1];
       end;


       {--------------------------------------------------------------}
       { Match a Specific Input String }

       procedure MatchString(x: string);
       begin
          if Value <> x then Expected('''' + x + '''');
       end;


       {--------------------------------------------------------------}
       { Output a String with Tab }

       procedure Emit(s: string);
       begin
          Write(TAB, s);
       end;


       {--------------------------------------------------------------}
       { Output a String with Tab and CRLF }

       procedure EmitLn(s: string);
       begin
          Emit(s);
          WriteLn;
       end;


       {--------------------------------------------------------------}
       { Generate a Unique Label }

       function NewLabel: string;
       var S: string;
       begin
          Str(LCount, S);
          NewLabel := 'L' + S;
          Inc(LCount);
       end;


       {--------------------------------------------------------------}
       { Post a Label To Output }

       procedure PostLabel(L: string);
       begin
          WriteLn(L, ':');
       end;A*A*
                                    - 51 -
PAA





       {---------------------------------------------------------------}
       { Clear the Primary Register }

       procedure Clear;
       begin
          EmitLn('CLR D0');
       end;


       {---------------------------------------------------------------}
       { Negate the Primary Register }

       procedure Negate;
       begin
          EmitLn('NEG D0');
       end;


       {---------------------------------------------------------------}
       { Complement the Primary Register }

       procedure NotIt;
       begin
          EmitLn('NOT D0');
       end;


       {---------------------------------------------------------------}
       { Load a Constant Value to Primary Register }

       procedure LoadConst(n: integer);
       begin
          Emit('MOVE #');
          WriteLn(n, ',D0');
       end;


       {---------------------------------------------------------------}
       { Load a Variable to Primary Register }

       procedure LoadVar(Name: string);
       begin
          if not InTable(Name) then Undefined(Name);
          EmitLn('MOVE ' + Name + '(PC),D0');
       end;


       {---------------------------------------------------------------}
       { Push Primary onto Stack }

       procedure Push;
       begin
          EmitLn('MOVE D0,-(SP)');
       end;A*A*
                                    - 52 -
PAA





       {---------------------------------------------------------------}
       { Add Top of Stack to Primary }

       procedure PopAdd;
       begin
          EmitLn('ADD (SP)+,D0');
       end;


       {---------------------------------------------------------------}
       { Subtract Primary from Top of Stack }

       procedure PopSub;
       begin
          EmitLn('SUB (SP)+,D0');
          EmitLn('NEG D0');
       end;


       {---------------------------------------------------------------}
       { Multiply Top of Stack by Primary }

       procedure PopMul;
       begin
          EmitLn('MULS (SP)+,D0');
       end;


       {---------------------------------------------------------------}
       { Divide Top of Stack by Primary }

       procedure PopDiv;
       begin
          EmitLn('MOVE (SP)+,D7');
          EmitLn('EXT.L D7');
          EmitLn('DIVS D0,D7');
          EmitLn('MOVE D7,D0');
       end;


       {---------------------------------------------------------------}
       { AND Top of Stack with Primary }

       procedure PopAnd;
       begin
          EmitLn('AND (SP)+,D0');
       end;


       {---------------------------------------------------------------}
       { OR Top of Stack with Primary }

       procedure PopOr;
       beginA*A*
                                    - 53 -
PAA





          EmitLn('OR (SP)+,D0');
       end;


       {---------------------------------------------------------------}
       { XOR Top of Stack with Primary }

       procedure PopXor;
       begin
          EmitLn('EOR (SP)+,D0');
       end;


       {---------------------------------------------------------------}
       { Compare Top of Stack with Primary }

       procedure PopCompare;
       begin
          EmitLn('CMP (SP)+,D0');
       end;


       {---------------------------------------------------------------}
       { Set D0 If Compare was = }

       procedure SetEqual;
       begin
          EmitLn('SEQ D0');
          EmitLn('EXT D0');
       end;


       {---------------------------------------------------------------}
       { Set D0 If Compare was != }

       procedure SetNEqual;
       begin
          EmitLn('SNE D0');
          EmitLn('EXT D0');
       end;


       {---------------------------------------------------------------}
       { Set D0 If Compare was > }

       procedure SetGreater;
       begin
          EmitLn('SLT D0');
          EmitLn('EXT D0');
       end;


       {---------------------------------------------------------------}
       { Set D0 If Compare was < }A*A*
                                    - 54 -
PAA





       procedure SetLess;
       begin
          EmitLn('SGT D0');
          EmitLn('EXT D0');
       end;


       {---------------------------------------------------------------}
       { Set D0 If Compare was <= }

       procedure SetLessOrEqual;
       begin
          EmitLn('SGE D0');
          EmitLn('EXT D0');
       end;


       {---------------------------------------------------------------}
       { Set D0 If Compare was >= }

       procedure SetGreaterOrEqual;
       begin
          EmitLn('SLE D0');
          EmitLn('EXT D0');
       end;


       {---------------------------------------------------------------}
       { Store Primary to Variable }

       procedure Store(Name: string);
       begin
          if not InTable(Name) then Undefined(Name);
          EmitLn('LEA ' + Name + '(PC),A0');
          EmitLn('MOVE D0,(A0)')
       end;


       {---------------------------------------------------------------}
       { Branch Unconditional  }

       procedure Branch(L: string);
       begin
          EmitLn('BRA ' + L);
       end;


       {---------------------------------------------------------------}
       { Branch False }

       procedure BranchFalse(L: string);
       begin
          EmitLn('TST D0');
          EmitLn('BEQ ' + L);A*A*
                                    - 55 -
PAA





       end;


       {---------------------------------------------------------------}
       { Read Variable to Primary Register }

       procedure ReadVar;
       begin
          EmitLn('BSR READ');
          Store(Value[1]);
       end;


       { Write Variable from Primary Register }

       procedure WriteVar;
       begin
          EmitLn('BSR WRITE');
       end;


       {--------------------------------------------------------------}
       { Write Header Info }

       procedure Header;
       begin
          WriteLn('WARMST', TAB, 'EQU $A01E');
       end;


       {--------------------------------------------------------------}
       { Write the Prolog }

       procedure Prolog;
       begin
          PostLabel('MAIN');
       end;


       {--------------------------------------------------------------}
       { Write the Epilog }

       procedure Epilog;
       begin
          EmitLn('DC WARMST');
          EmitLn('END MAIN');
       end;


       {---------------------------------------------------------------}
       { Parse and Translate a Math Factor }

       procedure BoolExpression; Forward;A6A6
                                    - 56 -A*A*
PAA





       procedure Factor;
       begin
          if Look = '(' then begin
             Match('(');
             BoolExpression;
             Match(')');
             end
          else if IsAlpha(Look) then begin
             GetName;
             LoadVar(Value);
             end
          else
             LoadConst(GetNum);
       end;


       {--------------------------------------------------------------}
       { Parse and Translate a Negative Factor }

       procedure NegFactor;
       begin
          Match('-');
          if IsDigit(Look) then
             LoadConst(-GetNum)
          else begin
             Factor;
             Negate;
          end;
       end;


       {--------------------------------------------------------------}
       { Parse and Translate a Leading Factor }

       procedure FirstFactor;
       begin
          case Look of
            '+': begin
                    Match('+');
                    Factor;
                 end;
            '-': NegFactor;
          else  Factor;
          end;
       end;


       {--------------------------------------------------------------}
       { Recognize and Translate a Multiply }

       procedure Multiply;
       begin
          Match('*');
          Factor;A*A*
                                    - 57 -
PAA





          PopMul;
       end;


       {-------------------------------------------------------------}
       { Recognize and Translate a Divide }

       procedure Divide;
       begin
          Match('/');
          Factor;
          PopDiv;
       end;


       {---------------------------------------------------------------}
       { Common Code Used by Term and FirstTerm }

       procedure Term1;
       begin
          while IsMulop(Look) do begin
             Push;
             case Look of
              '*': Multiply;
              '/': Divide;
             end;
          end;
       end;


       {---------------------------------------------------------------}
       { Parse and Translate a Math Term }

       procedure Term;
       begin
          Factor;
          Term1;
       end;


       {---------------------------------------------------------------}
       { Parse and Translate a Leading Term }

       procedure FirstTerm;
       begin
          FirstFactor;
          Term1;
       end;


       {--------------------------------------------------------------}
       { Recognize and Translate an Add }

       procedure Add;A*A*
                                    - 58 -
PAA





       begin
          Match('+');
          Term;
          PopAdd;
       end;


       {-------------------------------------------------------------}
       { Recognize and Translate a Subtract }

       procedure Subtract;
       begin
          Match('-');
          Term;
          PopSub;
       end;


       {---------------------------------------------------------------}
       { Parse and Translate an Expression }

       procedure Expression;
       begin
          FirstTerm;
          while IsAddop(Look) do begin
             Push;
             case Look of
              '+': Add;
              '-': Subtract;
             end;
          end;
       end;


       {---------------------------------------------------------------}
       { Recognize and Translate a Relational "Equals" }

       procedure Equal;
       begin
          Match('=');
          Expression;
          PopCompare;
          SetEqual;
       end;


       {---------------------------------------------------------------}
       { Recognize and Translate a Relational "Less Than or Equal" }

       procedure LessOrEqual;
       begin
          Match('=');
          Expression;
          PopCompare;A*A*
                                    - 59 -
PAA





          SetLessOrEqual;
       end;


       {---------------------------------------------------------------}
       { Recognize and Translate a Relational "Not Equals" }

       procedure NotEqual;
       begin
          Match('>');
          Expression;
          PopCompare;
          SetNEqual;
       end;


       {---------------------------------------------------------------}
       { Recognize and Translate a Relational "Less Than" }

       procedure Less;
       begin
          Match('<');
          case Look of
            '=': LessOrEqual;
            '>': NotEqual;
          else begin
                  Expression;
                  PopCompare;
                  SetLess;
               end;
          end;
       end;


       {---------------------------------------------------------------}
       { Recognize and Translate a Relational "Greater Than" }

       procedure Greater;
       begin
          Match('>');
          if Look = '=' then begin
             Match('=');
             Expression;
             PopCompare;
             SetGreaterOrEqual;
             end
          else begin
             Expression;
             PopCompare;
             SetGreater;
          end;
       end;ABAB
                                    - 60 -A*A*
PAA





       {---------------------------------------------------------------}
       { Parse and Translate a Relation }


       procedure Relation;
       begin
          Expression;
          if IsRelop(Look) then begin
             Push;
             case Look of
              '=': Equal;
              '<': Less;
              '>': Greater;
             end;
          end;
       end;


       {---------------------------------------------------------------}
       { Parse and Translate a Boolean Factor with Leading NOT }

       procedure NotFactor;
       begin
          if Look = '!' then begin
             Match('!');
             Relation;
             NotIt;
             end
          else
             Relation;
       end;


       {---------------------------------------------------------------}
       { Parse and Translate a Boolean Term }

       procedure BoolTerm;
       begin
          NotFactor;
          while Look = '&' do begin
             Push;
             Match('&');
             NotFactor;
             PopAnd;
          end;
       end;


       {--------------------------------------------------------------}
       { Recognize and Translate a Boolean OR }

       procedure BoolOr;
       begin
          Match('|');A*A*
                                    - 61 -
PAA





          BoolTerm;
          PopOr;
       end;


       {--------------------------------------------------------------}
       { Recognize and Translate an Exclusive Or }

       procedure BoolXor;
       begin
          Match('~');
          BoolTerm;
          PopXor;
       end;


       {---------------------------------------------------------------}
       { Parse and Translate a Boolean Expression }

       procedure BoolExpression;
       begin
          BoolTerm;
          while IsOrOp(Look) do begin
             Push;
             case Look of
              '|': BoolOr;
              '~': BoolXor;
             end;
          end;
       end;


       {--------------------------------------------------------------}
       { Parse and Translate an Assignment Statement }

       procedure Assignment;
       var Name: string;
       begin
          Name := Value;
          Match('=');
          BoolExpression;
          Store(Name);
       end;


       {---------------------------------------------------------------}
       { Recognize and Translate an IF Construct }

       procedure Block; Forward;


       procedure DoIf;
       var L1, L2: string;
       beginA*A*
                                    - 62 -
PAA





          BoolExpression;
          L1 := NewLabel;
          L2 := L1;
          BranchFalse(L1);
          Block;
          if Token = 'l' then begin
             L2 := NewLabel;
             Branch(L2);
             PostLabel(L1);
             Block;
          end;
          PostLabel(L2);
          MatchString('ENDIF');
       end;


       {--------------------------------------------------------------}
       { Parse and Translate a WHILE Statement }

       procedure DoWhile;
       var L1, L2: string;
       begin
          L1 := NewLabel;
          L2 := NewLabel;
          PostLabel(L1);
          BoolExpression;
          BranchFalse(L2);
          Block;
          MatchString('ENDWHILE');
          Branch(L1);
          PostLabel(L2);
       end;


       {--------------------------------------------------------------}
       { Process a Read Statement }

       procedure DoRead;
       begin
          Match('(');
          GetName;
          ReadVar;
          while Look = ',' do begin
             Match(',');
             GetName;
             ReadVar;
          end;
          Match(')');
       end;


       {--------------------------------------------------------------}
       { Process a Write Statement }A6A6
                                    - 63 -A*A*
PAA





       procedure DoWrite;
       begin
          Match('(');
          Expression;
          WriteVar;
          while Look = ',' do begin
             Match(',');
             Expression;
             WriteVar;
          end;
          Match(')');
       end;


       {--------------------------------------------------------------}
       { Parse and Translate a Block of Statements }

       procedure Block;
       begin
          Scan;
          while not(Token in ['e', 'l']) do begin
             case Token of
              'i': DoIf;
              'w': DoWhile;
              'R': DoRead;
              'W': DoWrite;
             else Assignment;
             end;
             Scan;
          end;
       end;


       {--------------------------------------------------------------}
       { Allocate Storage for a Variable }

       procedure Alloc(N: Symbol);
       begin
          if InTable(N) then Abort('Duplicate Variable Name ' + N);
          AddEntry(N, 'v');
          Write(N, ':', TAB, 'DC ');
          if Look = '=' then begin
             Match('=');
             If Look = '-' then begin
                Write(Look);
                Match('-');
             end;
             WriteLn(GetNum);
             end
          else
             WriteLn('0');
       end;ABAB
                                    - 64 -A*A*
PAA





       {--------------------------------------------------------------}
       { Parse and Translate a Data Declaration }

       procedure Decl;
       begin
          GetName;
          Alloc(Value);
          while Look = ',' do begin
             Match(',');
             GetName;
             Alloc(Value);
          end;
       end;


       {--------------------------------------------------------------}
       { Parse and Translate Global Declarations }

       procedure TopDecls;
       begin
          Scan;
          while Token <> 'b' do begin
             case Token of
               'v': Decl;
             else Abort('Unrecognized Keyword ' + Value);
             end;
             Scan;
          end;
       end;


       {--------------------------------------------------------------}
       { Parse and Translate a Main Program }

       procedure Main;
       begin
          MatchString('BEGIN');
          Prolog;
          Block;
          MatchString('END');
          Epilog;
       end;


       {--------------------------------------------------------------}
       {  Parse and Translate a Program }

       procedure Prog;
       begin
          MatchString('PROGRAM');
          Header;
          TopDecls;
          Main;
          Match('.');A*A*
                                    - 65 -
PAA





       end;


       {--------------------------------------------------------------}
       { Initialize }

       procedure Init;
       var i: integer;
       begin
          for i := 1 to MaxEntry do begin
             ST[i] := '';
             SType[i] := ' ';
          end;
          GetChar;
          Scan;
       end;


       {--------------------------------------------------------------}
       { Main Program }

       begin
          Init;
          Prog;
          if Look <> CR then Abort('Unexpected data after ''.''');
       end.
       {--------------------------------------------------------------}



       *****************************************************************
       *                                                               *
       *                        COPYRIGHT NOTICE                       *
       *                                                               *
       *   Copyright (C) 1989 Jack W. Crenshaw. All rights reserved.   *
       *                                                               *
       *****************************************************************ARAR


                                    - 66 -A*A*
@