Conditional branch with flags in ARM Architecture - Step-by-Step Execution

Concept Flow - Conditional branch with flags

Execute instruction

↓

Update flags (N,Z,C,V)

↓

Evaluate condition code

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Branch

After an instruction runs, the processor updates special flags. Then it checks these flags against a condition. If the condition is true, it jumps (branches); otherwise, it keeps going.

Execution Sample

ARM Architecture

CMP R0, #0
BEQ label
MOV R1, #1
label: MOV R1, #2

Compare R0 to zero, branch if equal to label, else set R1 to 1; at label set R1 to 2.

Analysis Table

Step	Instruction	Flags Updated (N,Z,C,V)	Condition Checked	Branch Taken?	Action	R1 Value
1	CMP R0, #0 (R0=0)	N=0,Z=1,C=0,V=0	BEQ (Z==1)?	Yes	Jump to label	N/A
2	label: MOV R1, #2	No flags change	N/A	N/A	Set R1=2	2
3	End	N/A	N/A	N/A	Program continues	2

💡 Branch taken at step 1 because zero flag Z=1, condition BEQ is true.

State Tracker

Variable	Start	After Step 1	After Step 2	Final
R0	0	0	0	0
R1	undefined	undefined	2	2
Flags (N,Z,C,V)	N=0,Z=0,C=0,V=0	N=0,Z=1,C=0,V=0	N=0,Z=1,C=0,V=0	N=0,Z=1,C=0,V=0

Key Insights - 2 Insights

Why does the branch happen even though R1 was not set before?

The branch depends on flags set by CMP, not on R1. As shown in execution_table step 1, CMP sets Z=1, so BEQ condition is true and branch occurs before MOV R1, #1.

What if R0 was not zero? Would the branch still happen?

No. If R0 was not zero, CMP would set Z=0, making BEQ false. The branch would not be taken, and MOV R1, #1 would execute instead (not shown here).

Visual Quiz - 3 Questions

Look at the execution_table at step 1, what is the value of the zero flag (Z)?

A Undefined

B 0

C 1

D Not set

💡 Hint

Check the 'Flags Updated' column at step 1 in execution_table.

At which step does the program jump to the label?

A Step 1

B Step 2

C Step 3

D No jump occurs

💡 Hint

Look at the 'Branch Taken?' column in execution_table.

If R0 was 5 instead of 0, what would happen at step 1?

A Branch would be taken

B Branch would not be taken

C Flags would not update

D Program would crash

💡 Hint

Recall CMP sets Z=1 only if values are equal; see key_moments explanation.

Concept Snapshot

Conditional branch uses flags set by instructions like CMP.
Flags: N (negative), Z (zero), C (carry), V (overflow).
Branch instructions check these flags to decide jump.
If condition true, jump; else continue sequentially.
Example: BEQ branches if Z=1 (equal).

Full Transcript

In ARM architecture, after an instruction executes, special flags are updated to reflect the result. These flags include Negative (N), Zero (Z), Carry (C), and Overflow (V). Conditional branch instructions check these flags to decide whether to jump to another part of the program or continue in order. For example, the BEQ instruction checks if the zero flag Z is set to 1, meaning the previous comparison found equality. If true, the program jumps to the specified label; if false, it continues with the next instruction. This visual trace shows a CMP comparing R0 to zero, setting flags accordingly, and then a BEQ that branches if equal. The variable tracker shows how R1 is set only after the branch. Understanding how flags control branching is key to reading ARM assembly flow.