x86-64 (AT&T Syntax)

ABI

ABI - Application Binary Interface is a type of convention for function calls, similar to how there is for syscalls. The ABI that linux uses is called - System V ABI

Question

Why do I care? Why should I follow their convention?

Answer

It allows you to call functions written by other people, even in other programming languages and allows you to estimate the behavior and safety of your registers/stack.

Stack

Question

What is the stack and how does asm use it?

Answer

• The stack is an area of memory that is reserved for stacking temporary items.
• The operating system preallocates space for the stack and then puts the pointer to this memory in the stack pointer , %rsp.
• While pointers generally refer to the beginning of a memory region, at the beginning of a program %rsp points to the end of the memory region containing the stack. (Bartlett, ch. 11.2)

Question

Why must I keep the stack 16byte aligned before a function call?

Answer

This comes from the System V ABI rule, there are instructions that use vectors (a single register that holds values of the same type processed in parallel). These instructions expect 16 contiguous bytes.

Question

What does it mean to align the stack to 16 bytes?

Answer

%rsp holds the pointer (address) to the top of the stack

rsp = 0x1000   → aligned (0x1000 % 16 == 0)
rsp = 0x1008   → NOT aligned (0x1008 % 16 == 8)
rsp = 0x1010   → aligned