Function Calls Part 1

Notes

• This is Jorgensen chapter 12
• He tells us that there are two main issues with functions
  • Linkage: ie returning to the correct place after a function is called
  • Argument Transmission: passing parameters.
• I would add a third
  • Dealing with the local environment
    • Saving/restoring temporary registers in the calling function
    • saving/restoring preserved registers in the called function
    • Local variables (memory) in the called function
• A discussion of variables
  • Variables declared in the .data and .bss sections are global
  • A recursive function at least will need local copies of these.
  • In addition, we probably don't want global variables for everything anyways.
  • So local variables are declared on the stack.
  • He makes an argument for global as well, but...
• Call and ret
  • call
    • pushes the current program counter onto the stack
    • Jumps the the function beginning.
  • ret
    • pops the stack into the pc
  • This accomplishes the return to an arbitrary point in code.
  • Jorgensen notes that it is important to keep the stack right, or do not corrupt the stack
  • By the way, this mechanism is the root of the stack overflow security attack.
• Parameters
  • rdi, rsi, rdx, rcx, r8, r9
  • Then in reverse order on the stack.
• rax is used for return values
• Registers
  • rbx, rbp, r12-r15 are all preserved
    • IE if you use them in a function, you should
      • Save the value before the function you use it
      • Restore the value before the function returns.
  • all others are temporary
    • That means if you are using them and make a function call
      • You should save them before you make the call
      • You should restore them after the call returns.