Functions

Functions

A stored subroutine that performs a specific task based on the parameters with which it is provided 

• Complex operations can be performed by calling a procedure

• Make code easier to understand and manage

• Program elements that can easily be reused

• Same procedure can be called many times within a program

Is Branching to a Label a Function?

no, you should follow specific rules in order to implement a function

Application Programming Interface (API)

• Defines the interfaces by which one software program communicates with another at the source code level 

Defines the interface only 

• The user of it can ignore the implementation 

• Many implementations of the same one

• C standard library hides many low-level details of the system

Functions as Detectives

• Assigned a secret mission (function call)

• Acquires necessary resources (acquire parameters and memory - stack) 

• Perform the mission (execute instructions) 

• Leaves no trace (clean up memory) 

• Returns safely to the point of origin (function return)

Breaking Down Function Execution

1. Caller stores arguments in registers or memory

2. Function call: Caller transfers flow control to the callee 

3. Callee acquires/allocates memory for doing work 

4. Callee executes the function body 

5. Callee stores the result in “some” register 

6. Callee deallocates memory 

7. Function return: Callee returns control the caller

Instructions for Procedure Calls

Calling Convention

A function implementation should follow these to ensure interoperability

Many times, they make your code inefficient

Leave no trace

With one in place: 

• Functions written by different programmers can interoperate

• Functions compiled by two different compilers can interoperate

• A library function by a third party can be used without corrupting state

Convention 1 - Registers for Procedure Calls

• r0-r3: “argument” registers in which to pass parameters 

• r0: return value registers 

• lr: return address register (link register)

Convention 2 - Preserving Registers

Assembly convention 

• Registers must be restored after procedure call

• If usage of these registers is avoided no spilling of registers on the stack is required

Convention 3 - Preserving Registers

• Sometimes a procedure needs to use more registers than just four arguments and two return values

• Register content must be preserved during a procedure call

• Moving the contents of registers to the main memory is called spilling registers

• Registers are stored to memory using a conceptual data structure known as a stack

• The stack pointer register SP points to the contents of the register most recently pushed onto the stack

Procedure Nesting

• bl save pc to lr

• We must always save the lr register, if a nested procedure is called

• Lr can be saved in stack

• If usage of these registers is avoided, no spilling of registers on the stack is required

Stack Memory

Stack is a memory region used to store local state for your functions

It is a dynamic memory region

• The stack pointer (SP) starts at address 0x20020000

• The current stack pointer bottom is pointed by register SP

• A stack is like a Last In First Out (LIFO) Queue

• You can use register sp to grow (decrease register sp) and shrink (increase register sp)

What is the Stack used to store?

• Preserved registers

• Local function variables

• input arguments to the function

Memory Layout

The Stack

Growing the Stack

Instructions for Stack

Simple Procedure in C

Complex Calculations

must be broken down into smaller steps → compiler