Midterm I

Name: ________________________________________________ ID Number: ___________________

Please answer in the space provided! Your name must be legible and in the form used on your University ID card! Illegible and verbose answers will be penalized! This exam is open-book, open-notes, closed neighbor! This exam 10 points; allocate 4-5 minutes per point.

1. Hand assemble the this EAL code, and show the final contents of the symbol table. There are extra blanks! Do not fill in unused symbol table entries or next to lines that assemble nothing into memory. It is OK to cross out and replace values where they are changed. (2 points)

    line     hexadecimal     source
   number  location  value    text
   
     1    ________  _______  .    =    #10          symbol table
                                                   symbol    value
     2    ________  _______       W    A 
                                                 ________  ________
     3    ________  _______  B    =    10
                                                 ________  ________
     4    ________  _______  ; commentary
                                                 ________  ________
     5    ________  _______  A:          
                                                 ________  ________
     6    ________  _______       B    B
   
     7    ________  _______  B    =    .
    
     8    ________  _______       W    B 
   

2. Complete the header comments in this fragment of a parser. (2 points)

   void parse_oddity()
   /* parse one oddity following this EBNF grammar:
   
      _____________________________________________
   
      _____________________________________________
   */
   {
   	if (lex_ispunc( &lex_this, '(' )) {
   		lex_scan();
   		while (!lex_ispunc( &lex_this, ')' )) {
   			parse_oddity;
   		}
   		lex_scan();
   	} else {
   		parse_thingus();
   	}
   }
   

    

   Name: ________________________________________________

    

    

3. Consider this bit of code, from the previous problem:

   void parse_oddity()                                          /*A*/
   {                                                            /*B*/
   	if (lex_ispunc( &lex_this, '(' )) {                  /*C*/
   		lex_scan();                                  /*D*/
   		while (!lex_ispunc( &lex_this, ')' )) {      /*E*/
   			parse_oddity;                        /*F*/
   		}                                            /*G*/
   		lex_scan();                                  /*H*/
   	} else {                                             /*I*/
   		parse_thingus();                             /*J*/
   	}                                                    /*K*/
   }                                                            /*L*/
   

   If the input text has balanced parentheses, it works just fine. If the input text has unbalanced parentheses, it sometimes goes into a loop.

   a) Exactly what kind of unbalanced parens cause this? (1 point)

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   b) Which line should have been augmented with a check for end-of-file to prevent this? (0.5 points)

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   c) Which line should read parse_punc( ')', "end paren expected");? (0.5 points)

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4. Write C code equivalent to that on the left, but without using any macros. (2.0 points)

   #define two(a,b) a b a        ___________________________________
    
   two( i, = ) + 1;              ___________________________________
   
   j = two( 2, + );              ___________________________________
     
   k = two( two( 2, + ), * );    ___________________________________
   
                                 ___________________________________
   

    

    

    

   Name: ________________________________________________

    

5. Consider the following fragment of code from a top-down recursive descent compiler.

   parse_if_statement()
   /* <if statement> ::=
          IF <expression> THEN <block> [ ELSE <block> ] ENDIF
    */
   {
   	int if_number = unique_integer();
   	int else_number = unique_integer();
   	lex_scan();     
   	parse_expression();
   	printf( "  JFALSE .   \n" );
   	printf( "FIX%d = .-2  \n", if_number );
   	parse_keyword( then_handle );
   	parse_block();
   	if (is_keyword( else_handle )) {
   		printf( "  JUMP .     \n" );
   		printf( "FIX%d = .-2  \n", else_number );
   		printf( "HERE  = .    \n" );
   		printf( ". = FIX%d    \n", if_number );
   		printf( "  W HERE     \n" );
   		printf( ". = HERE     \n" );
   		if_number = else_number
   		lex_scan();     
   		parse_block();
   	}
   	parse_keyword( endif_handle );
   	printf( "HERE  = .    \n" );
   	printf( ". = FIX%d    \n", if_number );
   	printf( "  W HERE     \n" );
   	printf( ". = HERE     \n" );
   }
   

   a) Make an educated guess about the format of the operand field of the JUMP and JFALSE instructions. (1 point)

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   a) The output of this compiler is an EAL-like assembly language. How is the forward reference problem solved by the combination of assembler and compiler. (1 point)

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