This work is created by Perry Kivolowitz, Professor and Chair of Computer Science at Carthage College. It is copyright © 2021 and may be freely shared for educational purposes.
This chapter reviews how braces make their way into assembly language.
While the symbols { and } don't carry over per se, the effect of braces is clearly required.
{ can appear as labels (example in a do-while loop).
} can appear as branches. If a branch, a label typically appears on the next line after the branch.
The flowchart on the left depicts an if statement.
The flowchart on the right depicts a while loop.
In C and C++, braces surround code blocks (they also define scopes, but scope is a compiler concept). Yet, braces don't appear in flowcharts. Nor do they appear in assembly language.
In an if statement, the opening brace is not depicted per se, but you can see where it would be, coming just before the code block. The closing brace also is not depicted but you can see where it would be, coming just after the code block. A label in the assembly language would come just after where the closing brace would be so that the decision code has a target to branch to depending on the outcome of the decision.
In a while loop, the opening brace works just like the if statement. The closing brace in a loop shows up explicitly in assembly language in the form of a branch. As in the if statement closing brace, a label is found after the branch so that the decision code has a target to which to branch depending upon the outcome of the decision.
Recall:
// 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 The location of the opening brace "is" immediately before line 5. The location of the closing brace "is" immediately after line 5. In short, where the braces would be is surrounding the code block.
Notice that after the location of where the closing brace would be, a label is found. This is the target of the branch found on line 4.
Recall:
// 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
b 1b // 7
// 8
2: // 9 The opening brace "comes" immediately before line 6. The closing brace shows up as the branch on line 7. Again, there is a label immediately after where the closing brace would be so that the branch on line 5 can skip over the code block implementing the body of the while loop.