Conditional Branches

Conditional Branches

After setting the flags with CMP, you use conditional branch instructions to control program flow. This is how assembly implements if, else, switch, and every other decision-making construct.

B.cond -- Branch if Condition

The syntax is B. followed by a condition code:

Tip: Use the signed variants (GT, GE, LT, LE) for most comparisons. Use unsigned variants (HI, LO) when comparing addresses or values that cannot be negative.

As Spock once said, "There are always possibilities." Conditional branches are how the CPU explores them -- choosing a different code path based on the outcome of each comparison.

Implementing if/else

The pattern is: compare, branch over the "else" code, then skip over the "then" code:

CMP X0, #10
B.GT greater          // if X0 > 10, jump to "greater"
// else branch (X0 <= 10):
// ... code ...
B done                // skip over the "greater" branch
greater:
// then branch (X0 > 10):
// ... code ...
done:

Key insight: Assembly if/else is "inside out" compared to high-level code. You branch to skip code, not to run it. The condition in B.GT is the opposite of what you would write as the else condition.

Unconditional Branch

B label always jumps to the label:

B skip_this           // always jump
// this code is skipped
skip_this:

CBZ and CBNZ

These combine a compare-to-zero and branch in one instruction. They are very common in loops:

CBZ X0, is_zero       // Branch if X0 == 0
CBNZ X0, not_zero     // Branch if X0 != 0

These do not set flags and do not require a prior CMP instruction -- they test the register directly.

Finding Maximum -- The Pattern

A common pattern for finding the maximum of N values:

MOV X3, X0           // max = first value
CMP X1, X3           // compare second with max
B.LE skip1           // if second <= max, skip
MOV X3, X1           // max = second
skip1:
// repeat for each additional value...

Your Task

Write a program that finds the maximum of three numbers: 17, 42, and 29. Store each in a register, compare them using CMP and conditional branches, and print the maximum (42) followed by a newline.