CS255 Syllabus

The while statement

Program control of the while statement:

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

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

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

Example 1: division by repeated subtraction

   int A, B;
   int Q, R;

   (Computes: Q = A/B
	      R = A%B)

   Q = 0;                        MOVE.L #0, Q
   R = A;                        MOVE.L  A, R

   while ( R >= B )       Loop:  MOVE.L R, D0
   {                             CMP.L  B, D0
      Q = Q + 1;                 BLT    LoopExit
      R = R - B;
   }                             MOVE.L Q, D0
				 ADD.L  #1, D0
				 MOVE.L D0, Q

				 MOVE.L R, D0
				 SUB.L  B, D0
				 MOVE.L D0, R

				 BRA    Loop

		       LoopExit:

The flow chart of the program segment:

Here is the assembler program, you can assemble it yourself: click here
The following assembler program does the same thing, just "faster". But since the focus of the course is understanding how the computer works, and not make it run faster, this program will not be discussed. If you're curious, take a look. Basically, don't use the memory if you don't have to: click here

Observation

The while statement:

while ( C ) { s1; s2; ... }

give rise to an assembler program with the following structure:

WhileStart: instructions to perform a compare specified by the condition C branch of FALSE of the condition C to label WhileEnd instructions to perform s1 instructions to perform s2 .... bra WhileStart WhileEnd:

Note:

The statements inside the while body can be "translated" into assembler code independently from the while-statement
Use this property to handle: