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

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

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#

Q: What is the stack and how does asm use it?

  • 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)

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

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.

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

%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
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