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 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 )		sethi  %hi(x), %l0
      x = -x;		ld     [%l0 + %lo(x)], %l0   // reg. l0 = x
			cmp    %l0, 0		     // cmp x against 0
			bge    L1        // Skip over "x=-x" when x >= 0
			nop		// DO NOT FORGET the NOP !!!

			sethi  %hi(x), %l0
			ld     [%l0 + %lo(x)], %l0   (you can omit this, 
						      because reg. l0
					 	      already has x...)

			sub    %g0, %l0, %l0         // reg l0 = -x

			sethi  %hi(x), %l1
			st     %l0, [ [%l1 + %lo(x)]

		    L1: ...

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 )		sethi  %hi(x), %l0
   {  // Swap x and y	ld     [%l0 + %lo(x)], %l2  // reg l2 = x
      help = x;		sethi  %hi(y), %l1
      x = y;		ld     [%l1 + %lo(y)], %l3  // reg l3 = y
      y = help;		cmp    %l2, %l3             // comp x against y
   }			ble    L1		    // Skip if x <= y
			nop			    // Remember !!!

			st     %l2, [%l1 + %lo(y)]  // y = x value in l0
			st     %l3, [%l0 + %lo(x)]  // x = y value in l1
		    L1: ...

Example Program: (Demo code of Example 2 above)
- Prog file: click here
  Assemble program with: as255s sparc-if