Demo of function call, parameters, and local variable usage in NASM

section .text
  global _start

_start:
  push 1
  push 2
  call add2 ; rax = add2(1, 2)

  push 3
  push 4
  call add2 ; rax = add2(3, 4)

  jmp .exit

.exit:
  mov rax, 60 ; syscall 60 is exit
  mov rdi, 1 ; exit code is 1
  syscall

# rax = add2(i1, i2), the function demonstrates local variable usage as well.
add2:
  push rbp
  mov rbp, rsp
    mov rax, [rbp+16] ; int64 i1
    mov rdx, [rbp+16+8] ; int64 i2
    sub rsp, 8 ; int64 localA
    sub rsp, 8 ; int64 localB
    push rdx ; localB = i2
    push rax ; localA = i1
    add rax, rdx ; i1 += i2
    ; return rax(i.e. i1)

  mov rsp, rbp
  pop rbp
  ret