Subroutine call convention (AAPCS) in ARM Architecture

Introduction

The Subroutine Call Convention (AAPCS) defines clear rules for how functions communicate in ARM systems. It helps different parts of a program work together smoothly by agreeing on how to pass data and return results.

When writing functions that need to call other functions in ARM-based programs.

When mixing code compiled by different tools or languages on ARM processors.

When debugging or optimizing ARM assembly code involving function calls.

When designing libraries or APIs for ARM systems to ensure compatibility.

When learning how ARM processors handle function calls and data passing.

Core Concept

ARM Architecture

No single code syntax; it is a set of rules about register usage and stack management during function calls.

AAPCS specifies which registers are used to pass arguments and return values.

It defines which registers must be preserved by the called function and which can be freely used.

Key Points

Registers r0 to r3 carry the first four arguments to a function. The first argument is in r0, the second in r1, and so on. The function returns its result in r0.

ARM Architecture

r0-r3: Used to pass up to four integer or pointer arguments to a function.
r0: Also used to return the result from a function.

Registers r4 to r11 must keep their values after the function call. If the function changes them, it must save their original values and restore them before returning.

ARM Architecture

r4-r11: Callee-saved registers. The called function must save and restore these if it uses them.

The stack pointer manages memory for temporary data during function calls, like local variables or saved registers.

ARM Architecture

sp (stack pointer): Points to the current top of the stack, used for local variables and saving registers.

The link register stores where the program should return after the function finishes.

ARM Architecture

lr (link register): Holds the return address when a function is called.

Detailed Explanation

This example shows a simple function that adds two numbers. The first two numbers are passed in registers r0 and r1. The function adds them and returns the result in r0. The link register (lr) is used to return to the caller.

ARM Architecture

; Example ARM assembly function following AAPCS

; Function: add_two_numbers
; Adds two integers passed in r0 and r1, returns result in r0

add_two_numbers:
    add r0, r0, r1    ; Add r1 to r0, result in r0
    bx lr             ; Return to caller using link register

; Caller code example

    mov r0, #5        ; First argument = 5
    mov r1, #7        ; Second argument = 7
    bl add_two_numbers ; Call function
    ; Result now in r0 (should be 12)

Important Notes

Following AAPCS ensures that functions can call each other without corrupting data.

Not following the convention can cause bugs that are hard to find, like wrong results or crashes.

Stack alignment is important; the stack pointer must be 8-byte aligned before calling a function.

Summary

AAPCS defines how ARM functions pass arguments and return values using registers.

Registers r0-r3 are for arguments and return values; r4-r11 must be preserved by the called function.

The link register (lr) holds the return address, and the stack pointer (sp) manages local data.

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