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 here (A:) ----+
		|						  |
		| (TRUE)					  |
		|						  |
		V						  |
           "statement1" assembler code		  		  |
		 						  |
	Branch always to here (B:) ---------+			  |
					    |			  |
					    |			  |
  A:       "statement2" assembler code <--------------------------+
		|			    |
		|			    |
  B:		+<--------------------------+
		|
		V

Example: find the maximum of 2 integers

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

   if ( x >= y )	MOVE.L x, D0
      max = x;		CMP.L  y, D0	Compares x against y
   else   		BLT    L1	Skip when x < y
      max = y;
			MOVE.L x, max
			BRA    L2	Must skip over else part !

		    L1: MOVE.L y, max

		    L2:

The flow chart of the above program is:

Example assembler program: click here

Another example

Program:

Program in Java:

if ( x >= 0 ) max = max + x; else max = max - x;

The flow chart of the above program is:

Program in M68000 assembler code:

move.l x,d0 cmp.l #0,d0 * Computes: d0 - 0 = x - 0 blt else_part * if ( x - 0 < 0 ) branch to else_part * ------------------------ Continue if blt fails to branch * This happens if x-0 >=0, or: x >= 0 then_part: move.l max, d1 add.l d0, d1 move.l d1, max bra IfEnd branch over ElsePart to continue at IfEnd * (Otherwise we execute the ELSE part !!!) * ------------------------ Arrives here if we branched * This happens when x-0 < 0, or: x < 0 else_part: move.l max, d1 sub.l d0, d1 move.l d1, max IfEnd: (program continues here if there are more statements...)

Example Program: (Demo above code)
- Prog file: click here
How to run the program: