Branch and link (BL) for subroutines in ARM Architecture - Step-by-Step Execution

Concept Flow - Branch and link (BL) for subroutines

Start Execution

↓

Encounter BL instruction

↓

Save return address in LR

↓

Jump to subroutine address

↓

Execute subroutine instructions

↓

Return using LR

↓

Continue main program

The BL instruction saves the return address, jumps to the subroutine, and later returns to continue the main program.

Execution Sample

ARM Architecture

BL subroutine_address
...
subroutine_address:
  ; subroutine code
  BX LR

This code calls a subroutine using BL, which saves the return address and jumps to the subroutine, then returns using BX LR.

Analysis Table

Step	Instruction	Action	LR (Link Register)	PC (Program Counter)	Notes
1	BL subroutine_address	Save return address (PC+4) to LR, jump to subroutine	PC+4	subroutine_address	Prepare to call subroutine
2	subroutine instructions	Execute subroutine code	PC+4	subroutine_address + offset	Running subroutine
3	BX LR	Branch to address in LR (return)	PC+4	PC+4	Return to instruction after BL
4	Next instruction after BL	Continue main program	PC+4	PC+8	Execution resumes

💡 Return to main program after subroutine finishes using LR

State Tracker

Register	Start	After BL	During Subroutine	After BX LR
LR	undefined	Address of next instruction after BL	unchanged	unchanged
PC	start address	subroutine_address	advances through subroutine	return address (after BL)

Key Insights - 3 Insights

Why does BL save PC-4 in LR instead of PC?

BL saves PC+4 because PC points to the current instruction plus 8 bytes in ARM, so PC+4 points to the next instruction after BL, ensuring correct return (see execution_table step 1).

How does the program know where to return after the subroutine?

The return address is stored in LR by BL, and the subroutine uses BX LR to jump back (see execution_table steps 1 and 3).

What happens if BX LR is missing in the subroutine?

Without BX LR, the program won't return to the main code, causing incorrect execution flow or crash (no return in execution_table).

Visual Quiz - 3 Questions

Look at the execution_table, what value does LR hold immediately after the BL instruction?

A The address of the instruction after BL (PC+4)

B The address of the subroutine

C The current PC value

D Zero

💡 Hint

Check execution_table row 1 under LR column.

At which step does the program jump back to the main code after the subroutine?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look for the BX LR instruction in execution_table.

If the BL instruction did not save the return address in LR, what would happen?

A The subroutine would execute and return normally

B The program would jump to a random address on return

C The program would continue sequentially without jumping

D The subroutine would never be called

💡 Hint

Consider the role of LR in execution_table steps 1 and 3.

Concept Snapshot

BL instruction saves the return address (next instruction) in LR.
It then jumps to the subroutine address.
Subroutine executes and returns using BX LR.
This allows the program to resume after the call.
BL is essential for function calls in ARM.

Full Transcript

The Branch and Link (BL) instruction in ARM saves the address of the instruction following BL into the Link Register (LR). It then jumps to the subroutine address specified. The subroutine executes its instructions and returns control to the main program by branching to the address stored in LR using the BX LR instruction. This mechanism allows the program to call functions or subroutines and return correctly to continue execution. The LR holds the return address, and the PC changes to the subroutine address during the call. Without saving the return address, the program would not know where to return after the subroutine finishes.