Compiler - Alpha

Compiles to simplified assembly used by virtual machine with 6 registers a-f.

Tools

• Java 14
• JFlex 1.8.2
• Java Cup 11b

Build

# run from project
mvn -q exec:java -Dexec.args="<infile> <outfile>"

# build .jar
mvn assembly:single
# run from .jar
java -jar Compiler.jar <infile> <outfile>

Grammar

• Program structure
  • With declaration

  DECLARE
      a, b, t(10:20)
  BEGIN
      ...
  END

  • Without declaration

  BEGIN
      ...
  END

• Comments

  [ OK ]
  
  [ Also
  ...
  OK ]
  
  [ so [ wrong ]  ]

• Variables
  • Ordinary - [_a-z]+

  a   [ ok ]
  _a  [ ok ]
  A1  [ Wrong ]

  • Array - t(x:y), where x <= y

  tab(10:20);  [ declaration of array of size=11 indexed from 10 to 20 ]
  tab(15);     [ Ok ]
  tab(21);     [ Index out of bounds]
  tab(a);      [ if 10 <= a <= 20 Ok ]

  • Constants - only integers >= 0

  a := 0;                   [ Ok ]
  a := 1234567890987654321; [ Ok ]
  
  a := -5;                  [ Wrong ]
  a := 123.456;             [ Wrong ]

• IO

  READ a;  		[ Read from stdin to variable 'a' ]
  READ 123; 		[ Wrong ]
  
  WRITE a;		[ Write variable 'a' to stdout ]
  WRITE tab(a);	        [ Write array variable to stdout ]
  WRITE 123;		[ Write constant to stdout ]

• Expressions

  a := 10;
  a := b;
  a := 100 * 200;
  a := b * 10;
  
  a := b + c;     [ addition ]
  a := b - c;     [ subtraction ]
  a := b * c;     [ multiplication ]
  a := b / c;     [ division ]
  a := b % c;     [ modulo ]
  
  a := b * c * d; [ Wrong ]
  a := b * c;     [ Only expressions with single operator are allowed ]
  a := a * d;
  
  a / 0 = 0;      [ Division and modulo by zero always equals zero ]
  a % 0 = 0;

• Conditions

  a = b   [ Equals ]
  a != b  [ Not equal ]
  a < b   [ Less ]
  a > b   [ Greater ]
  a <= b  [ Less equal ]
  a >= b  [ Greater equal ]

• If

  [ If ]
  IF a > b [ condition ] THEN 
      ...
  ENDIF
  
  [ If - Else ]
  IF a > b THEN 
      ...
  ELSE
      ...
  ENDIF

• Loops

  • While

    [ While ]
    WHILE a > b [ condition ] DO
        ...
    ENDWHILE
    
    [ Do - while with negated condition ]
    REPEAT 
        ...
    UNTIL a > b;

  • For

    [ 'i' is local, not declared variable ]
    [ range (a - b or b - a) is calculated at the begining of loop and cannot be changed ]
    [ for variable range use WHILE ]
    [ i++ ]
    FOR i FROM a TO b DO
        i := c;     [ wrong, local variable cannot be overwritten ]
        a := 2 * a; [ range stays the same ] 
        ...
    ENDFOR
    
    [ i-- ]
    FOR i FROM a DOWNTO b DO
        ...
    ENDFOR