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.

                                            Savitribai Phule Pune University
                            Second Year of Computer Engineering (2015 Course)
                                                     210257: Microprocessor Lab
                                                                Assignment No.09

Problem Statement: 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.

;**************************************************************
section .data

    nummsg db "***Program to find Factorial of a number*** ",10
           db "Enter the number : ",
    nummsg_len equ $-nummsg

    resmsg db "Factorial is : "
    resmsg_len equ $-resmsg

    thankmsg db 10,"Thank you ",10
    thankmsg_len equ $-thankmsg

    zerofact db " 00000001 "
    zerofactlen equ $-zerofact

;************************************************************
section .bss

    dispbuff resb 16
    result resb 4
    num resb 1
    num1 resb 1
    numascii resb 3

    %macro display 2
       mov rax,01
       mov rdi,01
       mov rsi,%1
       mov rdx,%2
       syscall
    %endmacro

       %macro accept 2
      mov rax,0
      mov rdi,0
      mov rsi,%1
      mov rdx,%2
      syscall
       %endmacro    

;************************************************************
section .text
global _start
_start:
  
    display nummsg,nummsg_len
    accept numascii,3            ;accept number from user
    call packnum8_proc            ;convert number from ascii to hex
    mov [num],bl
  
    display resmsg,resmsg_len

    mov al,[num]            ;store number in accumulator
    cmp al,01h          
    jbe endfact

    mov bl,[num]
    call proc_fact
    mov rbx,rax
    call disp64_proc
    jmp exit
  

endfact:
    display zerofact,zerofactlen
  
exit:    display thankmsg,thankmsg_len

    mov rax,60
    mov rdi,0
    syscall
    ret

;************************************************************
disp64_proc:
    mov rdi,dispbuff    ;point esi to buffer
    mov rcx,16        ;load number of digits to display
dispup1:
    rol rbx,4        ;rotate number left by four bits
    mov dl,bl        ;move lower byte in dl
    and dl,0fh        ;mask upper digit of byte in dl
    add dl,30h        ;add 30h to calculate ASCII code
    cmp dl,39h        ;compare with 39h
    jbe dispskip1        ;if less than 39h akip adding 07 more
    add dl,07h        ;else add 07

dispskip1:
    mov [rdi],dl        ;store ASCII code in buffer
    inc rdi            ;point to next byte
    loop dispup1        ;decrement the count of digits to display
                ;if not zero jump to repeat

    display dispbuff,16    ;
  
    ret
;************************************************************
packnum8_proc:
    mov bx,0
    mov ecx,02
    mov esi,numascii
up1:
    rol bl,04
    mov al,[esi]
    cmp al,39h
    jbe skip1
    sub al,07h
skip1:    sub al,30h
    add bl,al
    inc esi
    loop up1
    ret
;***********************Recursion*************************************
;There are two kinds of recursion: direct and indirect. 
;In direct recursion, the procedure calls itself and
;in indirect recursion, the first procedure calls a second ;procedure,which in turn, calls the first procedure.

proc_fact:
    cmp bl, 1
    jne do_calculation
    mov ax, 1
    ret
  
do_calculation:
    push rbx  
    dec bl
    call proc_fact
    pop rbx
    mul bl
    ret
;****************************************8
;[root@localhost vikas]# cd MIT2016/
;[root@localhost MIT2016]# nasm -f elf64 factorial.asm
;[root@localhost MIT2016]# ld -o factorial factorial.o
;[root@localhost MIT2016]# ./factorial
;***Program to find Factorial of a number***
;Enter the number : 04
;Factorial is : 0000000000000018
;Thank you
;[root@localhost MIT2016]#