[Solved] You wrote this ARM assembly code to call a subroutine:BL my_subroutine MOV R1, #10 BX LRBut the program crashes after returning from my_subroutine. What is the likely error? | ARM Architecture

ARM Architecture - Subroutines and Stack

You wrote this ARM assembly code to call a subroutine:

BL my_subroutine
MOV R1, #10
BX LR

But the program crashes after returning from my_subroutine. What is the likely error?

AThe subroutine name is case-sensitive and must be uppercase

BThe MOV instruction is invalid after a subroutine call

CThe BX LR instruction should be replaced with BL

DThe subroutine did not preserve the link register (LR) before returning

Step-by-Step Solution

Solution:

Step 1: Understand link register usage
ARM uses the link register (LR) to store return address when calling subroutines with BL.
Step 2: Identify common mistake
If the subroutine modifies LR without saving/restoring it, the return address is lost, causing a crash on BX LR.
Final Answer:
The subroutine did not preserve the link register (LR) before returning -> Option D
Quick Check:
LR must be saved/restored in subroutines [OK]

Quick Trick: Always save LR in subroutines to avoid crashes [OK]

Common Mistakes:

Master "Subroutines and Stack" in ARM Architecture

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