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{–}

program Branch;

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{ Constant Declarations }

const TAB = ^I;

CR = ^M;

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{ Variable Declarations }

var Look : char; { Lookahead Character }

Lcount: integer; { Label Counter }

{–}

{ 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;

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{ Report Error and Halt }

procedure Abort(s: string);

begin

Error(s);

Halt;

end;

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{ Report What Was Expected }

procedure Expected(s: string);

begin

Abort(s + ' Expected');

end;

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{ Match a Specific Input Character }

procedure Match(x: char);

begin

if Look = x then GetChar

else Expected('''' + x + '''');

end;

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{ Recognize an Alpha Character }

function IsAlpha(c: char): boolean;

begin

IsAlpha := UpCase(c) in ['A'..'Z'];

end;

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{ Recognize a Decimal Digit }

function IsDigit(c: char): boolean;

begin

IsDigit := c in ['0'..'9'];

end;

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{ Recognize an Addop }

function IsAddop(c: char): boolean;

begin

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

end;

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{ 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;

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{ Get an Identifier }

function GetName: char;

begin

if not IsAlpha(Look) then Expected('Name');

GetName := UpCase(Look);

GetChar;

end;

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{ Get a Number }

function GetNum: char;

begin

if not IsDigit(Look) then Expected('Integer');

GetNum := Look;

GetChar;

end;

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{ Generate a Unique Label }

function NewLabel: string;

var S: string;

begin

Str(LCount, S);

NewLabel := 'L' + S;

Inc(LCount);

end;

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{ Post a Label To Output }

procedure PostLabel(L: string);

begin

WriteLn(L, ':');

end;

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{ Output a String with Tab }

procedure Emit(s: string);

begin

Write(TAB, s);

end;

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{ Output a String with Tab and CRLF }

procedure EmitLn(s: string);

begin

Emit(s);

WriteLn;

end;

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{ Parse and Translate a Boolean Condition }

procedure Condition;

begin

EmitLn('<condition>');

end;

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{ Parse and Translate a Math Expression }

procedure Expression;

begin

EmitLn('<expr>');

end;

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{ Recognize and Translate an IF Construct }

procedure Block(L: string); Forward;

procedure DoIf(L: string);

var L1, L2: string;

begin

Match('i');

Condition;

L1 := NewLabel;

L2 := L1;

EmitLn('BEQ ' + L1);

Block(L);

if Look = 'l' then begin

Match('l');

L2 := NewLabel;

EmitLn('BRA ' + L2);

PostLabel(L1);

Block(L);

end;

Match('e');

PostLabel(L2);

end;

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{ Parse and Translate a WHILE Statement }

procedure DoWhile;

var L1, L2: string;

begin

Match('w');

L1 := NewLabel;

L2 := NewLabel;

PostLabel(L1);

Condition;

EmitLn('BEQ ' + L2);

Block(L2);

Match('e');

EmitLn('BRA ' + L1);

PostLabel(L2);

end;

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{ Parse and Translate a LOOP Statement }

procedure DoLoop;

var L1, L2: string;

begin

Match('p');

L1 := NewLabel;

L2 := NewLabel;

PostLabel(L1);

Block(L2);

Match('e');

EmitLn('BRA ' + L1);

PostLabel(L2);

end;

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{ Parse and Translate a REPEAT Statement }

procedure DoRepeat;

var L1, L2: string;

begin

Match('r');

L1 := NewLabel;

L2 := NewLabel;

PostLabel(L1);

Block(L2);

Match('u');

Condition;

EmitLn('BEQ ' + L1);

PostLabel(L2);

end;

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{ Parse and Translate a FOR Statement }

procedure DoFor;

var L1, L2: string;

Name: char;

begin

Match('f');

L1 := NewLabel;

L2 := NewLabel;

Name := GetName;

Match('=');

Expression;

EmitLn('SUBQ #1,D0');

EmitLn('LEA ' + Name + '(PC),A0');

EmitLn('MOVE D0,(A0)');

Expression;

EmitLn('MOVE D0,-(SP)');

PostLabel(L1);

EmitLn('LEA ' + Name + '(PC),A0');

EmitLn('MOVE (A0),D0');

EmitLn('ADDQ #1,D0');

EmitLn('MOVE D0,(A0)');

EmitLn('CMP (SP),D0');

EmitLn('BGT ' + L2);

Block(L2);

Match('e');

EmitLn('BRA ' + L1);

PostLabel(L2);

EmitLn('ADDQ #2,SP');

end;

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{ Parse and Translate a DO Statement }

procedure Dodo;

var L1, L2: string;

begin

Match('d');

L1 := NewLabel;

L2 := NewLabel;

Expression;

EmitLn('SUBQ #1,D0');

PostLabel(L1);

EmitLn('MOVE D0,-(SP)');

Block(L2);

EmitLn('MOVE (SP)+,D0');

EmitLn('DBRA D0,' + L1);

EmitLn('SUBQ #2,SP');

PostLabel(L2);

EmitLn('ADDQ #2,SP');

end;

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{ Recognize and Translate a BREAK }

procedure DoBreak(L: string);

begin

Match('b');

EmitLn('BRA ' + L);

end;

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{ Recognize and Translate an «Other» }

procedure Other;

begin

EmitLn(GetName);

end;

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{ Recognize and Translate a Statement Block }

procedure Block(L: string);

begin

while not(Look in ['e', 'l', 'u']) do begin

case Look of

'i': DoIf(L);

'w': DoWhile;

'p': DoLoop;

'r': DoRepeat;

'f': DoFor;

'd': DoDo;

'b': DoBreak(L);

else Other;

end;

end;

end;

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{ Parse and Translate a Program }

procedure DoProgram;

begin

Block('');

if Look <> 'e' then Expected('End');

EmitLn('END')

end;

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{ Initialize }

procedure Init;

begin

LCount := 0;

GetChar;

end;

{–}

{ Main Program }

begin

Init;

DoProgram;

end.

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