Branch instruction (B) in ARM Architecture - Step-by-Step Execution

Concept Flow - Branch instruction (B)

Start Execution

↓

Fetch Branch Instruction

↓

Decode Instruction

↓

Calculate Target Address

↓

Update Program Counter (PC)

↓

Jump to Target Address

↓

Continue Execution from New Address

The branch instruction changes the flow by updating the program counter to a new address, causing the CPU to jump to that location.

Execution Sample

ARM Architecture

B label
...
label:
  MOV R0, #1

This code jumps to 'label' skipping intermediate instructions, then sets register R0 to 1.

Analysis Table

Step	Instruction	Program Counter (PC)	Action	Next PC
1	B label	0x1000	Calculate target address for 'label'	0x1008
2	B label	0x1000	Update PC to target address	0x1008
3	MOV R0, #1	0x1008	Execute MOV instruction	0x100C
4	...	0x100C	Continue sequential execution	0x1010

💡 Execution continues from updated PC after branch; branch instruction changes flow to 0x1008

State Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4
PC	0x1000	0x1000	0x1008	0x100C	0x1010
R0	undefined	undefined	undefined	1	1

Key Insights - 2 Insights

Why does the program counter (PC) jump to 0x1008 instead of continuing sequentially?

Because the branch instruction (B) updates the PC to the target address (0x1008), changing the flow as shown in execution_table rows 1 and 2.

Does the branch instruction execute the instructions between 0x1000 and 0x1008?

No, the branch skips intermediate instructions by directly updating PC to 0x1008, so instructions between are not executed (see execution_table step 2).

Visual Quiz - 3 Questions

Look at the execution_table at step 2. What is the value of PC after executing the branch instruction?

A 0x1008

B 0x1000

C 0x100C

D 0x1010

💡 Hint

Check the 'Next PC' column in execution_table row 2.

At which step does register R0 get the value 1?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Action' column where MOV R0, #1 is executed.

If the branch instruction was removed, what would happen to the PC after step 1?

A PC would update to 0x1008

B PC would continue sequentially to 0x1004

C PC would jump to 0x1010

D PC would stay at 0x1000

💡 Hint

Without branch, PC increments sequentially; check variable_tracker for PC values.

Concept Snapshot

Branch instruction (B) changes program flow by updating the PC to a target address.
Syntax: B label
It causes the CPU to jump, skipping instructions in between.
Used for loops, conditionals, and function calls.
PC updates immediately to target address after branch.

Full Transcript

The branch instruction (B) in ARM architecture changes the flow of execution by updating the program counter (PC) to a new address. When the CPU encounters a branch instruction, it calculates the target address and sets the PC to that address, causing the next instruction to be fetched from there. This skips any instructions between the branch and the target. For example, if the PC is at 0x1000 and the branch target is 0x1008, the PC jumps directly to 0x1008. This is shown in the execution table where after executing the branch, the PC updates from 0x1000 to 0x1008. The branch is useful for implementing loops, conditionals, and jumps in code. Registers like R0 remain unchanged until instructions at the target address execute, such as MOV R0, #1 at 0x1008. Understanding how the PC updates helps clarify how the CPU changes execution flow with branch instructions.

Practice

1. What does the ARM B instruction do in a program?

easy

A. It adds two numbers and stores the result.

B. It jumps to a specified label to change the flow of execution.

C. It loads data from memory into a register.

D. It compares two values and sets flags.

Solution

1. Step 1: Understand the purpose of the B instruction

   The B instruction is used to jump to a different part of the program, identified by a label.

2. Step 2: Compare with other instructions

   Other instructions like add, load, or compare do not change program flow by jumping.

3. Final Answer:

   It jumps to a specified label to change the flow of execution. -> Option B

4. Quick Check:

   Branch (B) = Jump to label [OK]

Hint: B means Branch: jump to a label in code [OK]

Common Mistakes:

• Confusing B with arithmetic instructions
• Thinking B loads or stores data
• Assuming B compares values

2. Which of the following is the correct syntax for a branch instruction in ARM assembly?

easy

A. B label

B. branch to label

C. jmp label

D. goto label

Solution

1. Step 1: Recall ARM branch syntax

   ARM uses the instruction B followed by the label name to branch.

2. Step 2: Eliminate incorrect syntax

   Instructions like 'branch to', 'jmp', or 'goto' are not valid ARM syntax.

3. Final Answer:

   B label -> Option A

4. Quick Check:

   Correct syntax = B label [OK]

Hint: Branch uses 'B' followed by label name [OK]

Common Mistakes:

• Using non-ARM keywords like 'jmp' or 'goto'
• Writing full words instead of 'B'
• Omitting the label after B

3. Given the following ARM assembly code snippet, what will be the value in register r0 after execution?

B skip
MOV r0, #1
skip:
MOV r0, #5

medium

A. 5

B. 1

C. 0

D. Undefined

Solution

1. Step 1: Analyze the branch instruction

   The instruction B skip jumps over the MOV r0, #1 line directly to the label skip.

2. Step 2: Determine which MOV executes

   Since the jump skips the first MOV, only MOV r0, #5 runs, setting r0 to 5.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   Branch skips first MOV, r0 = 5 [OK]

Hint: Branch skips code, so last MOV sets r0 [OK]

Common Mistakes:

• Ignoring the branch and thinking both MOV run
• Assuming r0 stays 0 by default
• Confusing label position

4. Identify the error in this ARM assembly code snippet:

B 100
MOV r0, #10
100:
MOV r1, #20

medium

A. MOV instructions cannot use immediate values.

B. The branch instruction must be 'BR' instead of 'B'.

C. There is no error; the code is correct.

D. Label '100' is invalid because it starts with a number.

Solution

1. Step 1: Check label naming rules

   Labels in ARM assembly cannot start with a digit; '100' is invalid.

2. Step 2: Verify branch and MOV syntax

   The branch instruction 'B' is correct, and MOV can use immediate values like #10.

3. Final Answer:

   Label '100' is invalid because it starts with a number. -> Option D

4. Quick Check:

   Labels cannot start with digits [OK]

Hint: Labels must start with a letter, not a number [OK]

Common Mistakes:

• Thinking 'B' is wrong branch syntax
• Believing MOV can't use immediate values
• Assuming numeric labels are allowed

5. You want to create a loop in ARM assembly that repeats 3 times using the branch instruction B. Which approach correctly uses B to implement this loop?

hard

A. Use B with labels named as numbers only.

B. Use multiple unconditional branches without a counter.

C. Use a counter register, decrement it, and branch back if not zero.

D. Use B to jump forward only, no looping possible.

Solution

1. Step 1: Understand loop control with branch

   Loops require a way to repeat code, usually by checking a counter and branching back if needed.

2. Step 2: Evaluate options for looping

   Using a counter register and branching back when not zero is the standard method. Branching forward only or using invalid labels won't create a loop.

3. Final Answer:

   Use a counter register, decrement it, and branch back if not zero. -> Option C

4. Quick Check:

   Loop = counter + branch back [OK]

Hint: Loop needs counter and branch back to label [OK]

Common Mistakes:

• Trying to loop without a counter
• Using only forward branches
• Using invalid label names for loops