CS255 Syllabus

The simple if-else statement

Program control of the if-else statement:

   if ( condition )   ---->            |
      statement1;                      V
   else				+--------------+ FALSE
      statement2;		|  condition   |--------+
				+--------------+	|
				       |		|
				       | TRUE		|
				       |		|
				       V		|
				   statement1		|
				       |		|
			     +---------+		|
			     |	        		|
			     |	       +<---------------+
			     |	       |
			     |	       V
			     |	   statement2
			     |	       |
			     |	       |
			     +-------->+
				       |
				       |
				       V

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

      Evaluate "condition" (CMP)
							FALSE
      Branch on the FALSE outcome of "condition" to A ------------+
		|						  |
		| TRUE						  |
		|						  |
		V						  |
           "statement1" assembler code		  		  |
		|						  |
	Branch always to B					  |
		|						  |
      +---------+						  |
      |		 						  |
      |		 						  |
      |		+<------------------------------------------------+
      |		|
      |		V
  A:  |    "statement2" assembler code
      |         |
      |         V
      +---------+
		|
		V
  B:

Example: make 2 variables equal to the largest of their values

   int x;              
   int y;

   if ( x >= y )  	
      y = x;		
   else   		
      x = y;		

Assembler construct for this if-statement:

        ! get the values into registers to compare.... 

        sethi  %hi(x), %l0
        ld     [%l0 + %lo(x)], %l0  // %l0 = x
        sethi  %hi(y), %l1
        ld     [%l1 + %lo(y)], %l1  // %l1 = y

        ! Now compare them and branch.... 

        cmp    %l0, %l1
	bl     L1		! Jump over the "then" part to the else part
	nop

	sethi  %hi(y), %l2
	st     %l0, [%l2 + %lo(y)]  // y = %l0 (= x)
	ba     L2 		! Jump over the "else" part to the end
	nop

    L1: sethi  %hi(x), %l2
	st     %l1, [%l2 + %lo(x)]  // x = %l1 (= y)

    L2:

Example: find the max of 2 variables

   int x;              
   int y;
   int max;

   if ( x >= y )  	
      max = x;		
   else   		
      max = y;

Assembler construct for this if-statement:

        sethi  %hi(x), %l0
        ld     [%l0 + %lo(x)], %l0    ! l0 = x
        sethi  %hi(y), %l1
        ld     [%l1 + %lo(y)], %l1    ! l1 = y

        cmp    %l0, %l1
        bl     L1
        nop

        sethi  %hi(max), %l2
        st     %l0, [%l2 + %lo(max)]   ! max = l0 (x)

        ba     L2                      ! unconditional branch  
        nop

    L1: sethi  %hi(max), %l2
        st     %l1, [%l2 + %lo(max)]   ! max = l1 (y)

    L2:

Example Program: (Demo above code)
- Prog file: click here