CS 3721 Programming Languages   Spring 2014
Recitation 3.   Semantic Actions
Week 3: Jan 28-30

Submit following directions at: submissions and rules at: rules. Deadlines are: 2014-02-04  23:59:59 (that's Tue, 04 Feb 2014, 11:59:59 pm) for full credit. 2014-02-07  23:59:59 (that's Fri,    07 Feb 2014, 11:59:59 pm) for 75% credit.

1. Study the the page Semantic Actions. This problem asks you to do the same things as Part 1 of that page, the part that translates to RPN. Use the grammar and example from Problem 2 of Recitation 2.

   Assume the four ids in the sentence from Problem 2 is tagged with a, b, c, and d, in that order, as shown below:

   Input sentence with tags
   $ id + id * id ^ id $ | | | | a b c d

   Here is the grammar from Problem 2, written with one rule per line as shown below. Add semantic actions to the grammar rules, so that the result of parsing the input above (and of carrying out the semantic actions) will be to translate the sentence into RPN.

   Grammar: Arithmetic Expressions
   P −−> E E −−> E + T E −−> E - T E −−> T T −−> T * S T −−> T / S T −−> S S −−> F ^ S S −−> F F −−> ( E ) F −−> id

   Modify add to your answer to Problem 2 of Rec, so that you show the results of the semantic actions, which should be the RPN form of the input expression,

   ---

2. This problem asks you to do the same things as Part 2 of the Semantic Actions page, the part that translates to a simple intermediate code.

   Again add semantic actions the grammar so that it will generate similar intermediate code similar to that on the Semantics Page. You can mostly use the same semantic actions given there, except that you need similar ones for the operator ^. You should end up with complete actions beside each grammar rule:

   Again modify your answer to Problem 2 of Recitation 2 so that you show the results of the semantic actions, which should be the input expression rewritten as a sequence of intermediate code instructions.

3. Translate the following sequence of assignments into MIPS and print the final value stored in e.

   Translate to MIPS
   a = 8 * 2; b = 7 * a; c = b + 1; d = a / c; e = 3 + d; (print value of e)

This is not hard if you follow the pattern of the MIPS program in the third link at the top of this page. Here are two tables from the second link above:

l.d     $f2, 128($s1)
l.d     $f4, 40($s1)
add.d   $f6, $f2, $f4
s.d     $f6, 120($s1)

This shows how to translate f = g + 5 to MIPS assemply. Translation of the earlier 5 assignments is essentially the same.

In order to print the value in the variable e, the third link above gives code to print the value in variable a, with a newline afterwards. (It turns out you have to do it exactly as shown, except for the number 80, which is the offset of "a".)

# Print value in a
        li      $v0, 3
        l.d     $f12, 80($s1)
        syscall
# Print NewL as ASCII char
        li      $v0, 4
        la      $a0, NewL
        syscall

Finally, from the third link above, you need the two instructions at the beginning, and everything after the comment # Stuff at end #. We will be explaining most of these issues.