Nested subroutine calls in ARM Architecture - Step-by-Step Execution

Concept Flow - Nested subroutine calls

Start main

↓

Call subroutine A

↓

subroutine A starts

↓

Call subroutine B

↓

subroutine B starts

↓

subroutine B ends

↓

Return to subroutine A

↓

subroutine A ends

↓

Return to main

↓

Program ends

The main program calls subroutine A, which calls subroutine B. Each subroutine runs and returns control back to its caller in order.

Execution Sample

ARM Architecture

main:
  BL subA
  ; continue main

subA:
  BL subB
  BX LR

subB:
  BX LR

Main calls subA, subA calls subB, then returns back step-by-step.

Analysis Table

Step	Instruction	Action	Call Stack	Return Address
1	BL subA	Call subA, save return to main+4	[main]	main+4
2	BL subB	Call subB, save return to subA+4	[main, subA]	subA+4
3	BX LR (in subB)	Return to subA at subA+4	[main, subA]	subA+4
4	BX LR (in subA)	Return to main at main+4	[main]	main+4
5	Main continues	Program continues after subA call	[]

💡 All subroutines returned; call stack empty; main resumes and program ends.

State Tracker

Call Stack	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
Stack Frames	[]	[main]	[main, subA]	[main, subA]	[main]	[]

Key Insights - 3 Insights

Why does the call stack grow when subA calls subB?

Because each BL instruction saves the return address on the stack before jumping, as shown in execution_table step 2 where subB is called and the stack becomes [main, subA].

How does the program know where to return after subB finishes?

The return address saved by BL in step 2 is used by BX LR in step 3 to jump back to subA at subA+4, as shown in the call stack and return address columns.

What happens if BX LR is missing in a subroutine?

The program would not return to the caller, causing incorrect flow or crash. The execution_table shows BX LR instructions at steps 3 and 4 ensure proper returns.

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what is the call stack after BL subB?

A [subA]

B [main]

C [main, subA]

D []

💡 Hint

Refer to the 'Call Stack' column in execution_table row for step 2.

At which step does the program return from subB to subA?

A Step 1

B Step 3

C Step 4

D Step 5

💡 Hint

Check the 'Action' column for when BX LR in subB returns to subA.

If the BL instruction at step 1 was removed, what would happen?

A subA would not be called; main continues without subA

B subB would be called directly

C Program would crash immediately

D subA would run twice

💡 Hint

Look at step 1 where BL subA causes the call; removing it means no call to subA.

Concept Snapshot

Nested subroutine calls use BL (Branch with Link) to jump and save return address.
Each call adds a return address to the call stack.
Subroutines return using BX LR to jump back.
Calls can be nested; returns unwind the stack step-by-step.
Proper return instructions are essential to maintain flow.

Full Transcript

This visual trace shows how nested subroutine calls work in ARM architecture. The main program calls subroutine A using BL, which saves the return address and jumps to subA. Then subA calls subB similarly. Each call adds a frame to the call stack. When subB finishes, it returns to subA using BX LR, which uses the saved return address. Then subA returns to main the same way. The call stack grows and shrinks as calls and returns happen, ensuring the program continues correctly. Missing return instructions would break this flow.