Branch and link (BL) for subroutines in ARM Architecture - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the BL instruction do in ARM architecture?

The BL (Branch and Link) instruction jumps to a subroutine address and saves the return address in the link register (LR) so the program can return after the subroutine finishes.

beginner

Why is the link register (LR) important when using BL?

LR stores the address to return to after the subroutine completes, allowing the program to continue from where it left off.

beginner

How does BL differ from a simple branch (B) instruction?

BL saves the return address in LR before jumping, while B just jumps without saving the return point.

intermediate

What happens if you forget to use BL and use B instead for a subroutine call?

The program will jump to the subroutine but won’t save the return address, so it cannot return properly, causing errors or crashes.

beginner

How do you return from a subroutine called with BL in ARM?

You return by branching to the address stored in the link register (LR), usually with the instruction 'BX LR' or 'MOV PC, LR'.

What does the BL instruction do in ARM?

A Performs arithmetic addition

B Only jumps to a new address without saving return

C Jumps to a subroutine and saves return address

D Loads a value into a register

BL jumps to a subroutine and saves the return address in the link register.

Where is the return address stored after a BL instruction?

A Link Register (LR)

B Stack Pointer (SP)

C Program Counter (PC)

D General Purpose Register R0

The return address is saved in the Link Register (LR) after BL.

Which instruction is used to return from a subroutine called by BL?

A B LR

B BX LR

C BL LR

D MOV PC, LR

BX LR branches to the address in LR, returning from the subroutine.

What happens if you use B instead of BL to call a subroutine?

A Subroutine runs and returns correctly

B Program crashes immediately

C Nothing happens

D Subroutine runs but return address is lost

Using B jumps without saving return address, so the program cannot return properly.

BL instruction is mainly used for:

A Calling subroutines

B Looping

C Data transfer

D Arithmetic operations

BL is used to call subroutines by branching and saving return address.

Explain how the BL instruction works and why the link register is important.

Think about how the program remembers where to come back after a subroutine.

Describe what could go wrong if you use a branch (B) instruction instead of BL for calling a subroutine.

Consider what happens when the program tries to return without knowing where to go.