README.md

While Loops

Overview

We have already covered the if statement. A while loop is exactly the same with the addition of at least one branch and a label. It really is that simple.

To illustrate this, here is a flow chart of an if statement (on the left) compared to a while loop (on the right).

The closing brace in an if statement is indicated by the red arrow head. This isn't a branch, the code flow simply falls through to the statement beyond the closing brace. In the while loop, the behavior of the closing brace changes to be that of a branch back to just before the evaluation of the boolean condition (the "Decision").

A new label is placed before evaluating the "Decision".

A new unconditional branch is placed after the end of the "Code Block."

For review, consider this C or C++ code:

if (a >= b) {
    // CODE BLOCK
}

here is the assembly language for this if statement:

    // Assume value of a is in x0                                       // 1 
    // Assume value of b is in x1                                       // 2 
    cmp     x0, x1                                                      // 3 
    ble     1f                                                          // 4 
    // CODE BLOCK                                                       // 5 
1:                                                                      // 6 

Now, consider this while loop:

while (a >= b) {
    // CODE BLOCK
}

Here is the code for the while showing the addition of one new label and one new unconditional branch:

    // Assume value of a is in x0                                       // 1 
    // Assume value of b is in x1                                       // 2 
                                                                        // 3 
 1: cmp     x0, x1                                                      // 4 
    bgt     2f                                                          // 5 
    // CODE BLOCK                                                       // 6 
    b       1b                                                          // 7 
                                                                        // 8 
2:                                                                      // 9 

Temporary label 2 on line 9 takes the place of the line after the closing brace in a while loop.

Temporary label 1 on line 4 is the end point of the red arrow in the right hand flow chart above.

Summary

A while loop is an extension of the if statement. A simple if contains one conditional branch and one label.

A while loop contains at least two labels, one conditional branch and one unconditional branch. We acknowledge the possibility that the unconditional branch could be made a conditional one, but this is rarely done in assembly language and impossible in higher level languages like C and C++ since the branch is simply the closing }.

Questions

1

(T | F) This code has a problem:

    // Assume value of a is in x0                                       // 1 
    // Assume value of b is in x1                                       // 2 
                                                                        // 3 
 1: cmp     x0, x1                                                      // 4 
    b       2f                                                          // 5 
    // CODE BLOCK                                                       // 6 
    b       1b                                                          // 7 
                                                                        // 8 
2:                                                                      // 9 

Answer: True - the unconditional branch on line 5 doesn't reference the results of the cmp that comes before it. The CODE BLOCK will never be executed.

2

(T | F) This attempt at a while loop has a problem:

    // Assume value of a is in x0                                       // 1 
    // Assume value of b is in x1                                       // 2 
                                                                        // 3 
 1: cmp     x0, x1                                                      // 4 
    ble     2f                                                          // 5 
    // CODE BLOCK                                                       // 6 
                                                                        // 7 
2:                                                                      // 8 

Answer: True - missing a branch back to label 1, there is no loop.