Subroutine call convention (AAPCS) in ARM Architecture - Mini Project: Build & Apply

Understanding Subroutine Call Convention (AAPCS) in ARM Architecture

📖 Scenario: You are learning how ARM processors manage function calls using the ARM Architecture Procedure Call Standard (AAPCS). This standard defines how data is passed between functions and how the processor's registers are used during subroutine calls.Imagine you are a programmer who needs to understand how to prepare and clean up after calling a function in ARM assembly or low-level programming.

🎯 Goal: Build a clear step-by-step understanding of the AAPCS subroutine call convention by identifying the roles of registers, how arguments are passed, and how the return address is handled.

📋 What You'll Learn

Create a list of the first four argument registers used in AAPCS

Define a variable for the register used to hold the return value

Write a description of how the stack pointer is used during a subroutine call

Add a note about which register holds the return address after a subroutine call

💡 Why This Matters

🌍 Real World

Understanding AAPCS is essential for low-level programming, debugging, and writing efficient ARM assembly code.

💼 Career

Knowledge of calling conventions is important for embedded systems developers, compiler engineers, and anyone working close to hardware on ARM platforms.

1

Define the argument registers

Create a list called argument_registers containing the exact strings 'r0', 'r1', 'r2', and 'r3' which represent the first four registers used to pass arguments in AAPCS.

ARM Architecture

# Create the list argument_registers with 'r0', 'r1', 'r2', 'r3'
# Your code here

Need a hint?

Remember, AAPCS uses registers r0 to r3 to pass the first four arguments to a function.

2

Define the return value register

Create a variable called return_register and set it to the string 'r0', which is the register used to hold the return value in AAPCS.

ARM Architecture

argument_registers = ['r0', 'r1', 'r2', 'r3']
# Define return_register as 'r0'
# Your code here

Need a hint?

The return value is usually placed in register r0 according to AAPCS.

3

Describe the stack pointer usage

Create a string variable called stack_pointer_usage that explains in simple words how the sp (stack pointer) register is used during a subroutine call in AAPCS. For example, mention that it points to the current top of the stack and helps save registers or local data.

ARM Architecture

argument_registers = ['r0', 'r1', 'r2', 'r3']
return_register = 'r0'
# Write a simple explanation about sp usage in AAPCS
# Your code here

Need a hint?

Think about how the stack pointer helps keep track of saved data when a function runs.

4

Note the return address register

Create a string variable called return_address_register and set it to the string 'lr', which is the register that holds the return address after a subroutine call in AAPCS.

ARM Architecture

argument_registers = ['r0', 'r1', 'r2', 'r3']
return_register = 'r0'
stack_pointer_usage = 'The sp register points to the top of the stack and is used to save registers and local variables during a subroutine call.'
# Define return_address_register as 'lr'
# Your code here

Need a hint?

The link register (lr) stores where to return after the function finishes.