ARMv7 Assembly: Nested Function Calls

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Questions and Answers

In ARMv7 assembly, what is the primary role of the Link Register (LR) when dealing with nested function calls?

To manage the allocation of memory for local variables within a function.
To store the value of the first argument passed to a function.
To store the size of the stack frame used by a function.
To hold the return address, enabling the program to resume execution after a function call. (correct)

What happens to the Link Register (LR) when a nested function call (using BL instruction) occurs in ARMv7 assembly?

The current value of LR is saved onto the stack before being overwritten with the return address of the nested call.
The current value of LR is overwritten with the return address of the nested call, and the previous return address is lost. (correct)
The current value of LR is automatically saved in a shadow register, preserving it for when the nested function returns.
The nested call uses a separate register for its return address, leaving the original LR unchanged.

Consider the following ARMv7 assembly code:

main:
  MOV R1, #3
  BL outer
  BX LR
outer:
  BL inner
  ADD R0, R1, #2
  BX LR
inner:
  ADD R0, R1, #1
  BX LR

What value will be stored in register R0 after the execution of this code?

3
5 (correct)
4
6

In the context of nested function calls in ARMv7, what is the purpose of the `BX LR` instruction?

To branch to the address stored in register LR, effectively returning from the current function. (A) Signup and view all the answers

What is a potential issue if the Link Register(LR) is not properly managed in nested function calls?

The program might return to the wrong address, leading to unexpected behavior. (D) Signup and view all the answers

Consider the following assembly snippet:

main:
  MOV R1, #10
  BL outer_func
  ; Some code here
outer_func:
  BL inner_func
  ADD R0, R0, #3
  BX LR
inner_func:
  ADD R0, R1, #2
  BX LR

If the some code here section is reached, what value would RO have after outer_func is executed and returns?

15 (B) Signup and view all the answers

What will be the final value of R0 after executing the assembly code?

.data
nums: .word 1, 2, 3
.text
.global main
main:
 LDR R1, =nums
 BL process_array
 BX LR
process_array:
 MOV R0, #0
 MOV R2, #0
 BL sum_loop
 BX LR
sum_loop:
 LDR R3, [R1, R2]
 ADD R0, R0, R3
 ADD R2, R2, #4
 CMP R2, #12
 BLT sum_loop
 BX LR

6 (A) Signup and view all the answers

In the "Nested Loop Function" example, what's the purpose of `ADD R2, R2, #4`?

It increments the loop counter by 4, moving to the next element in the array (assuming each element is a 4-byte word). (B) Signup and view all the answers

In the Nested Add exercise, what is the task?

Outer adds 4, inner adds 2 to R1, return in R0. (B) Signup and view all the answers

Flashcards

Nested Function Call

A function call within another function. Essential for code modularity and reusability in ARMv7 assembly.

Link Register (LR)

The register that stores the return address when a branch instruction (like BL) is executed. It's crucial for returning to the correct location after a function call.

Branch with Link (BL)

An ARMv7 assembly instruction (BL) that branches to a subroutine and simultaneously saves the return address in the Link Register (LR).

Branch Exchange (BX LR)

An ARMv7 assembly instruction (BX LR) used to return from a subroutine. It branches to the address stored in the Link Register (LR).