How to resolve the algorithm Factorial step by step in the x86 Assembly programming language

Published on 12 May 2024 09:40 PM
#X86 assembly

How to resolve the algorithm Factorial step by step in the x86 Assembly programming language

Table of Contents

Problem Statement

Write a function to return the factorial of a number. Solutions can be iterative or recursive. Support for trapping negative   n   errors is optional.

Let's start with the solution:

Step by Step solution about How to resolve the algorithm Factorial step by step in the x86 Assembly programming language

Source code in the x86 programming language

global factorial
section .text

; Input in ECX register (greater than 0!)
; Output in EAX register
factorial:
  mov   eax, 1
.factor:
  mul   ecx
  loop  .factor
  ret

global fact
section .text

; Input and output in EAX register
fact:
  cmp    eax, 1
  je    .done   ; if eax == 1 goto done

  ; inductive case
  push  eax  ; save n (ie. what EAX is)
  dec   eax  ; n - 1
  call  fact ; fact(n - 1)
  pop   ebx  ; fetch old n
  mul   ebx  ; multiplies EAX with EBX, ie. n * fac(n - 1)
  ret

.done:
  ; base case: return 1
  mov   eax, 1
  ret

global factorial
section .text

; Input in ECX register
; Output in EAX register
factorial:
  mov   eax, 1  ; default argument, store 1 in accumulator

.base_case:
  test  ecx, ecx
  jnz   .inductive_case  ; return accumulator if n == 0
  ret

.inductive_case:
  mul   ecx         ; accumulator *= n
  dec   ecx         ; n -= 1
  jmp   .base_case  ; tail call

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