Compare and branch patterns in ARM Architecture - Step-by-Step Execution

Concept Flow - Compare and branch patterns

Start

↓

Compare Registers

↓

Set Condition Flags

↓

Evaluate Condition

Yes | No↓

Branch Taken

↓

Execute Branch Target

↓

End

The CPU compares values, sets flags, then decides to branch or continue based on conditions.

Execution Sample

ARM Architecture

CMP R0, R1
BEQ label
; next instructions
label: NOP

Compare R0 and R1; if equal, branch to label; otherwise continue.

Analysis Table

Step	Instruction	Operation	Condition Flags	Branch Taken	Next PC
1	CMP R0, R1	Compare R0 and R1	Z=0 (not equal)	No	PC+4
2	BEQ label	Check Z flag	Z=0	No	PC+4
3	Next instruction	Execute sequentially	-	No	PC+8
4	label: NOP	No operation	-	N/A	PC+12

💡 Branch not taken because Z flag is 0 (R0 != R1)

State Tracker

Variable	Start	After CMP	After BEQ	Final
R0	5	5	5	5
R1	3	3	3	3
Z flag	?	0	0	0
PC	1000	1004	1008	1012

Key Insights - 3 Insights

Why does the branch not happen even though there is a BEQ instruction?

Because the CMP sets the zero flag (Z) to 0 since R0 and R1 are not equal, so the BEQ condition is false (see execution_table step 2).

What does the CMP instruction actually do?

CMP subtracts the second register from the first and sets condition flags but does not store the result (see execution_table step 1).

How does the CPU decide where to go next after a compare and branch?

It checks the condition flags set by CMP and if the branch condition matches, it jumps to the label; otherwise, it continues sequentially (see execution_table steps 2 and 3).

Visual Quiz - 3 Questions

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

A 1

B 0

C Undefined

D Not set

💡 Hint

Check the 'Condition Flags' column in execution_table row 1.

At which step does the CPU decide not to branch?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look at the 'Branch Taken' column in execution_table row 2.

If R0 and R1 were equal, how would the 'Branch Taken' value change at step 2?

A Undefined

B No

C Yes

D Depends on PC

💡 Hint

BEQ branches if zero flag is 1, see execution_table step 2 logic.

Concept Snapshot

CMP compares two registers and sets flags.
BEQ branches if zero flag is set.
CPU checks flags after CMP to decide branch.
If condition false, execution continues sequentially.
Branch changes program counter to target label.

Full Transcript

In ARM assembly, compare and branch patterns work by first comparing two registers using the CMP instruction. CMP subtracts one register from another and sets condition flags like the zero flag (Z) based on the result. Then, a branch instruction like BEQ checks these flags. If the zero flag is set (meaning the compared values are equal), the branch is taken and the program counter jumps to the label. Otherwise, execution continues to the next instruction. This flow ensures decisions in code based on comparisons.