Computing base^power for Integer power

• Our target, pow.asm.
• I would like to write a fully recursive function.

• int pow(int base, int exponent) {
      int rv;
  
      if (exponent ==  0) {
          rv = 1
      } else {
          rv = base * pow(base, exponent-1)
      }
      return rv;
  }
  
  int main() {
     cout << pow(2, 10) << endl;
     return 0;
  }

• The power function
  • Parameters and return
    • I will pass the base in rdi, this is scratch so no problem.
      • The base will not change, so I will not mess with this.
    • I will pass the power in esi, this is scratch too
      • The exponent will decrease as we compute
      • So I will save it on the stack.
      • This is not required, but it will help with understanding.
  • Setup
    • the return address is saved by the function call.
    • So our first step is to save the base pointer (rbp)
    • And change the base pointer to point to the stack pointer (rsp)

    • push rbp      ; save the current base pointer
      mov rbp, rsp  ; set the base pointer to point to the current stack pointer
      push rsi      ; not needed but we want to look at it.

    • The Stack
      Register	Value
      sp on enter	return address
      bp	old bp
      sp	rsi

  • The function is reasonably straight forward.
  • Exit code
    • We need to get things off of the stack.
    • We don't actually need to pop things, but we will discuss that later.
    • So we pop
      • the exponent, put it back in esi,
      • The base pointer, put it back into the base pointer.
      • The impact of doing this is to restore the stack pointer.
• Adding an I/O Library
  • We can link multiple files together.
  • This makes it ideal to build a library to contain the I/O routines
  • In MY_IO.asm I have a print integer.
    • Note that PRINT_INT is marked as global here
  • And that it is marked as extern in the main file.
    • Well, sort of
    • I made a MY_IO.h to hold these headers and other system calls.
    • I am not happy with this, we will clean it up in class.
• Assembling and linking become an more of a problem.
  • We must assemble both
  • Then link.

  • nasm -f elf64 -g MY_IO.asm -o MY_IO.o
    nasm -f elf64 -g pow2.asm -o pow2.o
    ld -g  pow2.o MY_IO.o -o pow2 

  • I added a line in the makefile to do this.
  • There is one part I am not happy with, listing pow2.o as a dependancy.
  • I will work on this as well.