Assembly - Program does not return anything

Question

I have been tearing my hairs out learning assembly haha. I am trying to get the program to do the following computation: 2^3 + 5^2.

I tried to debug with gdb, but I don't understand much at all and internet searches don't reveal much. This program is for x86-64. The program compiles fine , but after typing echo $?, I expected 33 but got 0 instead.

Here is the code:

  1 .section .data
  2 .section .text
  3 .globl _start
  4 _start:
  5         pushq $3 #push the 2nd arg
  6         pushq $2 #push the 1st arg
  7 
  8         call power #call function
  9         addq $16, %rsp #move stack back 16 bytes to get rid of first two params
 10 
 11         pushq %rax #save the first answer before
 12 
 13         pushq $2 #push the 2nd arg
 14         pushq $5 #push the 1st arg
 15 
 16         call power #call function
 17         addq $16, %rsp #move stack back 16 bytes to get rid of first two params
 18         popq %rbx #2nd answer already in rax. We saved saved first answer onto stack and now pop it into rbx.
 19 
 20         addq %rax, %rbx #add together
 21 
 22         movq $60, %rax #exit
 23         syscall
 24 
 25 .type power, @function
 26 power:
 27         pushq %rbp #save old base pointer
 28         movq %rsp, %rbp #make stack pointer base pointer
 29         subq $8, %rsp #get room for local storage
 30 
 31         movq 16(%rbp), %rbx #put 1st arg in rbx
 32         movq 24(%rbp), %rcx # put 2nd arg in rbx
 33 
 34         movq %rbx, -8(%rbp) #store current result
 35 
 36 power_loop_start:
 37         cmpq $1, %rcx #if power is one, we are done
 38         je end_power #jump to end_power
 39 
 40         movq -8(%rbp), %rax #move current result into rax
 41         imulq %rbx, %rax #multiply current result by base number
 42 
 43         movq %rax, -8(%rbp) #store the current result
 44         decq %rcx #decrease the power
 45 
 46         jmp power_loop_start
 47 
 48 end_power:
 49         movq -8(%rbp), %rax #return value back to rax
 50         movq %rbp, %rsp #restore stack pointer
 51         popq %rbp #restore base pointer
 52         retq

Thank you in advance.

Answer 1

since you wish to return the resulted value as exit code, you'd store the result into register rdi as the argument for system call exit. In short, please add below line at line#21 :
movq %rbx, %rdi