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Why does the AAPCS require that the stack pointer (SP) be 8-byte aligned at a subroutine call?

hard📝 Conceptual Q10 of 15
ARM Architecture - Subroutines and Stack
Why does the AAPCS require that the stack pointer (SP) be 8-byte aligned at a subroutine call?
ATo prevent the use of callee-saved registers
BTo ensure data alignment for efficient memory access
CTo allow more arguments to be passed in registers
DTo save space on the stack
Step-by-Step Solution
Solution:

  1. Step 1: Understand stack alignment purpose

    Proper alignment ensures that data accesses are efficient and avoid faults.

  2. Step 2: Identify reason for 8-byte alignment

    8-byte alignment matches typical data size requirements and CPU expectations.

  3. Final Answer:

    To ensure data alignment for efficient memory access -> Option B

  4. Quick Check:

    Stack alignment = efficient memory access [OK]
Quick Trick: Stack aligned for efficient memory access [OK]
Common Mistakes:
  • Thinking alignment saves stack space
  • Confusing alignment with register usage
  • Assuming alignment prevents callee-saved use

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