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Three Address Code | Examples

Three Address Code-

Three Address Code is a form of an intermediate code.

The characteristics of Three Address instructions are-
  • They are generated by the compiler for implementing Code Optimization.
  • They use maximum three addresses to represent any statement.
  • They are implemented as a record with the address fields.

General Form-

In general, Three Address instructions are represented as-
a = b op c
Here,

  • a, b and c are the operands.
  • Operands may be constants, names, or compiler generated temporaries.
  • op represents the operator.

Examples-

Examples of Three Address instructions are-
  • a = b + c
  • c = a x b

Common Three Address Instruction Forms-

The common forms of Three Address instructions are-

1. Assignment Statement-

x = y op z and x = op y
Here,
  • x, y and z are the operands.
  • op represents the operator.
It assigns the result obtained after solving the right side expression of the assignment operator to the left side operand.

2. Copy Statement-

x = y
Here,
  • x and y are the operands.
  • = is an assignment operator.
It copies and assigns the value of operand y to operand x.

3. Conditional Jump-

If x relop y goto X
Here,
  • x & y are the operands.
  • X is the tag or label of the target statement.
  • relop is a relational operator.
If the condition “x relop y” gets satisfied, then-
  • The control is sent directly to the location specified by label X.
  • All the statements in between are skipped.
If the condition “x relop y” fails, then-
  • The control is not sent to the location specified by label X.
  • The next statement appearing in the usual sequence is executed.

4. Unconditional Jump-

goto X
Here, X is the tag or label of the target statement.
On executing the statement,
  • The control is sent directly to the location specified by label X.
  • All the statements in between are skipped.

5. Procedure Call-

param x call p return y
Here, p is a function which takes x as a parameter and returns y.

PRACTICE PROBLEMS BASED ON THREE ADDRESS CODE-

To solve the problems, Learn about the Precedence Relations and Associativity of Operators.

Problem-01:

Write Three Address Code for the following expression-
a = b + c + d

Solution-

The given expression will be solved as-



Three Address Code for the given expression is-
(1) T1 = b + c
(2) T2 = T1 + d
(3) a = T2

Problem-02:

Write Three Address Code for the following expression-
-(a x b) + (c + d) – (a + b + c + d)

Solution-

Three Address Code for the given expression is-
(1) T1 = a x b
(2) T2 = uminus T1
(3) T3 = c + d
(4) T4 = T2 + T3
(5) T5 = a + b
(6) T6 = T3 + T5
(7) T7 = T4 – T6

Problem-03:

Write Three Address Code for the following expression-
If A < B then 1 else 0

Solution-

Three Address Code for the given expression is-
(1) If (A < B) goto (4)
(2) T1 = 0
(3) goto (5)
(4) T1 = 1
(5)

Problem-04:

Write Three Address Code for the following expression-
If A < B and C < D then t = 1 else t = 0

Solution-

Three Address Code for the given expression is-
(1) If (A < B) goto (3)
(2) goto (4)
(3) If (C < D) goto (6)
(4) t = 0
(5) goto (7)
(6) t = 1
(7)

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