Description
1. This assignment involves the implementation of an interpreter for a simple low-level
programming language known as three-address code (TAC).
2. Download the file a2.tgz and untar it. This contains some materials you will need for
the assignment. You should work within this directory.
3. Submit your work (as an a2.tgz file) using the Moodle sever on or before the date
indicated above.
Our Language
4. Three-address code is a simple, low-level programming notation that is based on the
following limited “instruction set”. The key instruction is that of simple assignment
statement comprising at most three variables/values (one on the LHS, at most two on
the RHS)– hence the name.
# This is a comment!
x := y op z; # primitive assignment; two vars/values on RHS only
# op in {+, -, *, /, ==, !=, <, <=, >, >=}
x := y; # copy assignment
read x; # prompt user to enter (integer) value, place value in x
write x; # write value of x (integer) to the screen
if ( x == 0 ) goto l; # jump to l if x is false (zero)
if (x != 0 ) goto l; # jump to l if x is true (nonzero)
goto l; # jump unconditionally to l
l: # jump target l
halt; # terminate program execution
Note that: (1) Instruction semantics is as you would expect based on their counterparts in C; (2) Instructions are terminated
by semicolons (apart from the label instruction); (3) Programs manipulate integer quantities only; (4) There are no variable
declarations; (5) Identifiers and labels are alphanumeric strings, beginning with a letter and are case sensitive; (6) The symbol :=
denotes assignment; (7) Comments begin with # and extend to end of line.
5. Appended at the end of this sheet is a sample TAC program (fact.tac) that takes
in a value x and that calculates and prints x! (factorial). The comments on the right
indicate “high-level” pseudocode to indicate the program’s structure.
Our Interpreter
6. A range of classes provided. You will need to modify only TacExecutionEngine.java.
• Various ADT classes: ArrayBasedMap etc. that you may find useful in developing
your code.
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• Various TAC programs: In the a2 directory, you will find a range of .tac programs,
which will be useful when debugging your code.
• TacScanner2 (complete): Reads the contents of a textfile and creates a TACprogram object from the contents.
• TacInstruction2 and TacProgram2 (both complete): Objects of the former class
represent individual TAC instructions, while the latter represents a complete TAC
program.
• TacExecutionEngine: A stub is provided, but you will need to complete this class.
Our Goal
7. Complete TacExecutionEngine.java based on the stub provided.
8. The constructor of TacExecutionEngine takes a TAC program (TacProgram2) as an
argument; method execute simulates the execution of the program instruction by instruction. The main method in this class handles the “translation” (using TacScanner2)
of the source file into the corresponding TacProgram2 object, prior to using the class’s
execute method to run the program.
9. Data structures:
• The program itself will be represented by TacProgram2, which is essentially a list
(ADT List) of TacInstruction2 objects.
• You will need to devise some mechanism to keep track of the values of the various
variables as the program executes.
• You will also need to think about how to maintain a list of the various program
labels and the instruction to which each corresponds.
10. Input and Output: Use java.util.Scanner wrapped around System.in to handle
input and System.out to handle program output.
11. It is assumed that program execution begins with the first instruction and proceeds until
a halt instruction is encountered. Ordinarily execution proceeds from each instruction
to the next, but may be altered by jump (conditional or unconditional). Note that each
TacInstruction2 object encodes the instruction type (assignment, read, goto etc.).
12. The completed application should be able to execute any TAC program specified on the
command line.
java TacExecutionEngine fact.tac
You may assume that the TAC program supplied is syntactically valid.
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Sample TAC Program
read x ; # read x;
t0 = 0; # if 0 < x then
t1 = x;
t2 = t0 < t1;
if (t2 == 0) goto l0;
t3 = 1; # fact := 1;
fact = t3;
l1: # repeat
t4 = fact; # fact := fact * x;
t5 = x;
t4 = t4 * t5;
fact = t4;
t6 = x; # x := x - 1
t7 = 1;
t6 = t6 - t7;
x = t6;
t8 = x; # until x = 0;
t9 = 0;
t10 = t8 = t9;
if (t10 == 0) goto l1;
t11 = fact;
write t11; # write fact
l0: # end
halt;
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