CODEWITHSUNDEEP
assemblyx86-64gnu-assemblerattexponentiation
user16713791
user16713791

Reputation: 129

x86 Assembly: handling exponent 1 in power calculation

I have this code I wrote in x86. The code does a power of the first number I'm pushing to the queue by the second one.

Now all cases work as expected except one case that's when the second number equals $1 and then for some reason the output is 0, however I didn't managed to determine why when I debugged it...

(BTW I'm using mul instruction that one time put the output in %rax and one time in %rdx, so I did a logic that determines from which one to take the output each time (by counting the iterations). It seems to me that this implementation is quite bad so I would like to hear how you would deal with that).

Here's the code:

.section .data
.section .text
.globl _start
_start:
  pushq $3
  pushq $1
  call power_load_values
  jmp exit_with_solution
 
power_load_values:
  mov %rsp,%rbp
  mov 4(%rsp),%rcx
  mov 4(%rsp),%rax
  mov 8(%rsp),%rbx
  jmp power_loop
 
return_to_main:
  mov %rbp,%rsp
  add $8,%rbp
  ret
 
power_loop:
  inc %rdx
  cmp $1,%rbx
  je return_to_main ;if the second number is 1 so the output is just the first one (no need to do power)
 
  cmp $0,%rbx
  je set_as_one ;power zero is one
 
  mul %rcx
  dec %rbx
 
  test $1,%rdx ;checking if its odd or even. event - take from rax, odd - take from rdx 
  jnz from_rdx
  jz from_rax
 
from_rdx:
  mov %rdx,%rcx
  mov 8(%rsp),%rax ;because rax is being used for 'mul' operator and it resets after calling it
  jmp power_loop
 
from_rax:
  mov %rax,%rcx
  mov 8(%rsp),%rax ;because rax is being used for 'mul' operator and it resets after calling it
  jmp power_loop
 
set_as_one:
  mov $1,%rcx
  jmp return_to_main
 
exit_with_solution:
  movl $1,%eax
  mov %rcx,%rbx
  int $0x80

I would like to hear some advice from you guys.

Upvotes: 1

Views: 57

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