CS255 Syllabus

The simple if statement

Program control of the if statement:

   if ( condition )   ---->            |
      statement                        |
				+--------------+  FALSE
				|  condition   |--------+
				+--------------+	|
				       |		|
				       | TRUE		|
				       |		|
				       V		|
				   statement		|
				       |		|
				       |		|
				       V		|
				       +<---------------+
				       |
				       V

Assembler construct that realizes the control flow of the if statement is:

      Evaluate "condition" (CMP)
							        FALSE
      Branch on the FALSE outcome of "condition" to here (A:) ----+
		|						  |
		| TRUE						  |
		|						  |
		V						  |
      Translate "statement" into assembler code			  |
		|						  |
		|						  |
		V						  |
    A:		+<------------------------------------------------+
		|
		V

Example 1: find the absolute value

   int x;               Assembler construct for this if-statement:

   if ( x < 0 )		MOVE.L x, D0
      x = -x;		CMP.L  #0, D0	Compares x against 0
			BGE    L1	Skip over "x=-x" when x >= 0

			MOVE.L x, D0	(you can omit this, because D0
					 already has x...)
			NEG.L  D0
			MOVE.L D0, x

		    L1: ...

The flow chart of the program is:

Example assembler program: click here

Example 2: making sure that y is the largest number

   int x;               Assembler construct for this if-statement:
   int y;
   int help;

   if ( x > y )		MOVE.L x, D0
   {  // Swap x and y	CMP.L  y, D0	Compares x against y
      help = x;		BLE    L1	Skip when x <= y
      x = y;
      y = help;		MOVE.L y, x	Note: we don't need a "help"
   }			MOVE.L D0, y	variable to achieve swapping
					The data reg. D0 is 
					the "help" variable...
		    L1: ...

The flow chart of the program is: