What is Link Register in ARM: Explanation and Usage

The Link Register (LR) in ARM is a special register that stores the return address when a function call is made. It helps the processor know where to continue execution after the function finishes.

How It Works

The Link Register (LR) in ARM acts like a bookmark that remembers where the program should return after finishing a function. When the processor calls a function, it saves the address of the next instruction (the return point) into the LR. This way, after the function completes, the processor can jump back to the saved address and continue running the program.

Think of it like calling a friend and leaving a note with your address so they know where to come back after their visit. The LR holds this "note" temporarily during the function call. This mechanism avoids the need to use memory for storing return addresses, making function calls faster and more efficient.

Example

This simple ARM assembly example shows how the Link Register is used during a function call and return.

armasm

    BL my_function   @ Branch with Link: calls my_function and saves return address in LR
    ...              @ code continues here after my_function returns

my_function:
    @ function code here
    BX LR           @ Branch to address in LR to return

When to Use

The Link Register is used automatically by ARM processors during function calls to keep track of where to return after the function finishes. Programmers and compilers rely on LR to manage function calls efficiently without extra memory overhead.

In low-level programming or writing assembly code, you might directly manipulate the LR to implement custom function calls, interrupts, or context switches. Understanding LR is important for debugging, optimizing code, or working with ARM exception handling.

Key Points

• The Link Register stores the return address during function calls.
• It helps the processor know where to continue after a function ends.
• Using LR avoids extra memory access for return addresses, improving speed.
• In assembly, BL instruction sets LR automatically.
• Returning from a function is done by branching to the address in LR.

Key Takeaways

The Link Register (LR) holds the return address for function calls in ARM.

LR enables fast and efficient function returns without using memory.

The BL instruction sets LR automatically when calling functions.

Branching to LR returns control to the caller after a function finishes.

Understanding LR is essential for ARM assembly programming and debugging.