Why Subroutine call convention (AAPCS) in ARM Architecture? - Purpose & Use Cases

The Big Idea

What if every function call in your program was a mystery box you had to carefully open and close yourself?

The Scenario

Imagine writing a program where every time you call a small task (subroutine), you have to remember exactly which registers to save and restore yourself, and how to pass each piece of data manually.

Without a clear rulebook, your code becomes a confusing mess, and different parts of your program might not work well together.

The Problem

Manually managing which registers to save or restore is slow and error-prone.

You might overwrite important data or cause crashes because one part of the program expects data in a certain place, but another part changed it unexpectedly.

This makes debugging very hard and wastes time.

The Solution

The AAPCS (ARM Architecture Procedure Call Standard) sets clear rules for how subroutines should pass arguments, use registers, and return values.

This standard ensures every part of the program agrees on how to communicate, making code reliable and easier to maintain.

Before vs After

✗ Before

push {r0, r1}
mov r0, #5
bl some_function
pop {r0, r1}

✓ After

bl some_function  ; AAPCS ensures r0-r3 used for args, r0 for return

What It Enables

It enables smooth, predictable communication between different parts of a program and even between different programs or libraries on ARM devices.

Real Life Example

When your smartphone runs apps, the operating system and apps use AAPCS to call functions safely and efficiently without crashing or mixing up data.

Key Takeaways

Manual register management is complex and risky.

AAPCS provides a clear, shared set of rules for subroutine calls.

This makes ARM programs more reliable and easier to build and maintain.

Practice

1. According to the AAPCS, which registers are primarily used to pass the first four arguments to a subroutine?

easy

A. r0 to r3

B. r4 to r7

C. r8 to r11

D. lr and pc

Solution

1. Step 1: Understand argument passing in AAPCS

   The AAPCS specifies that the first four arguments to a subroutine are passed using registers r0, r1, r2, and r3.

2. Step 2: Identify the correct register range

   Registers r4 to r11 are used for other purposes, and lr (link register) stores the return address, not arguments.

3. Final Answer:

   r0 to r3 -> Option A

4. Quick Check:

   Arguments passed in r0-r3 [OK]

Hint: Remember: first 4 args in r0-r3 registers [OK]

Common Mistakes:

• Confusing argument registers with callee-saved registers
• Thinking lr or pc hold arguments
• Assuming stack is always used first

2. Which register is used to store the return address when a subroutine is called under AAPCS?

easy

A. lr

B. r0

C. pc

D. sp

Solution

1. Step 1: Identify the purpose of the link register (lr)

   In AAPCS, the link register (lr) holds the return address when a subroutine is called.

2. Step 2: Differentiate from other registers

   r0 is for return values, pc is the program counter (current instruction), and sp is the stack pointer, not for return addresses.

3. Final Answer:

   lr -> Option A

4. Quick Check:

   Return address stored in lr [OK]

Hint: Return address always saved in lr register [OK]

Common Mistakes:

• Confusing lr with pc (program counter)
• Thinking r0 holds return address
• Assuming sp stores return address

3. Consider a subroutine that takes 5 integer arguments. According to AAPCS, how are these arguments passed?

medium

A. All 5 arguments passed in r0 to r4

B. First 4 in r0 to r3, 5th on the stack

C. All 5 arguments passed on the stack

D. First 3 in r0 to r2, remaining 2 in r3 and r4

Solution

1. Step 1: Recall argument passing rules for more than 4 arguments

   AAPCS passes the first 4 arguments in registers r0 to r3. Any additional arguments are passed on the stack.

2. Step 2: Apply to 5 arguments

   For 5 arguments, the first 4 go into r0-r3, and the 5th argument is placed on the stack.

3. Final Answer:

   First 4 in r0 to r3, 5th on the stack -> Option B

4. Quick Check:

   Extra args beyond 4 go on stack [OK]

Hint: Only first 4 args in registers; rest go on stack [OK]

Common Mistakes:

• Assuming all args fit in registers
• Passing 5th arg in r4 incorrectly
• Ignoring stack usage for extra args

4. A programmer notices that after a subroutine call, the value in register r4 is unexpectedly changed. What is the most likely cause under AAPCS?

medium

A. r4 is a caller-saved register and was not preserved by the caller

B. r4 holds the return address and was corrupted

C. r4 is used to pass arguments and was overwritten

D. r4 is a callee-saved register and the callee did not save it

Solution

1. Step 1: Understand register saving responsibilities

   Under AAPCS, registers r4 to r11 are callee-saved, meaning the called function must save and restore them if it uses them.

2. Step 2: Identify the cause of unexpected change

   If r4 changed unexpectedly, it means the callee did not save and restore it properly, causing corruption.

3. Final Answer:

   r4 is a callee-saved register and the callee did not save it -> Option D

4. Quick Check:

   Callee must save r4-r11 [OK]

Hint: Callee saves r4-r11; caller saves r0-r3 [OK]

Common Mistakes:

• Thinking r4 is caller-saved
• Confusing argument registers with saved registers
• Assuming r4 holds return address

5. You need to write a subroutine that returns a 64-bit integer result. According to AAPCS, how should the return value be passed back to the caller?

hard

A. On the stack, not in registers

B. In r0 only, using 64 bits

C. In r0 and r1 combined

D. In r2 and r3 combined

Solution

1. Step 1: Recall return value passing rules for 64-bit values

   AAPCS specifies that 64-bit return values are passed using two registers: r0 and r1 combined.

2. Step 2: Eliminate incorrect options

   r0 alone holds 32 bits, so it cannot hold 64-bit values. Stack is not used for normal return values. r2 and r3 are not used for return values.

3. Final Answer:

   In r0 and r1 combined -> Option C

4. Quick Check:

   64-bit return uses r0 and r1 [OK]

Hint: 64-bit returns use r0 and r1 registers together [OK]

Common Mistakes:

• Assuming 64-bit fits in r0 alone
• Returning via stack unnecessarily
• Using wrong registers for return