10. Write x86 ALP to find the factorial of a given integer number on a command line by using recursion. Explicit stack manipulation is expected in the code.

;Problem Statement: Write x86 ALP to find the factorial of a given integer ;number on a command line by using
;recursion. Explicit stack manipulation is expected in the code.


%macro scall 4   ;common macro for input/output
 mov rax,%1
 mov rdi,%2
 mov rsi,%3
 mov rdx,%4
 syscall
%endmacro

section .data
 num db 00h
 msg db "Factorial is : "
 msglen equ $-msg
 msg1 db "*****Program to find Factorial of a number***** ",0Ah
   db "Enter the number : ",
 msg1len equ $-msg1

 zerofact db " 00000001 "
 zerofactlen equ $-zerofact

section .bss
 dispnum resb 16
 result resb 4
 temp resb 3


section .text
global _start
_start:

 scall 1,1,msg1,msg1len
 scall 0,0,temp,3   ;accept number from user
 call convert       ;convert number from ascii to hex
 mov [num],dl

 scall 1,1,msg,msglen

 mov rdx,0        ;RMD
 mov rax,0        ;RMD
 mov al,[num]   ;store number in accumulator
 cmp al,01h 
 jbe endfact
 mov rbx,0        ;RMD
 mov bl,01h
 call factr    ;call factorial procedure
 call display

 call exit
endfact:
 scall 1,1,zerofact,zerofactlen
 call exit

 factr:    ;recursive procedure

   cmp rax,01h
   je retcon1
   push rax 
   dec rax
 
   call factr

  retcon:
   pop rbx
   mul ebx
   jmp endpr

  retcon1:   ;if rax=1 return
   pop rbx
   jmp retcon
  endpr:

 ret

 display:   ; procedure to convert hex to ascii

   mov rsi,dispnum+15
   mov rcx,0        ;RMD
   mov cl,16

  cont:
   mov rdx,0        ;RMD
   mov rbx,0        ;RMD
   mov bl,10h
   div ebx
   cmp dl,09h
   jbe skip
   add dl,07h
  skip:
   add dl,30h
   mov [rsi],dl
   dec rsi
   loop cont

   scall 1,1,dispnum,16

 ret

 convert:   ;procedure to convert ascii to hex
   mov rsi,temp
   mov cl,02h
   MOV rax,0        ;RMD
   mov rdx,0        ;RMD
  contc:
   rol dl,04h
   mov al,[rsi]
   cmp al,39h
   jbe skipc
   sub al,07h
  skipc:
   sub al,30h
   add dl,al
   inc rsi
   dec cl
   jnz contc

 ret

 exit:    ;exit system call
  
   mov rax,60
   mov rdi,0
   syscall

ret

OUTPUT:
swlab@swlab-Veriton-M200-H81:~/Desktop/dhokane$ nasm -f elf64 fact.asm
swlab@swlab-Veriton-M200-H81:~/Desktop/dhokane$ ld -o fact fact.o
swlab@swlab-Veriton-M200-H81:~/Desktop/dhokane$ ./fact
*****Program to find Factorial of a number*****
Enter the number : 03
Factorial is : 0000000000000006
swlab@swlab-Veriton-M200-H81:~/Desktop/dhokane$