; THIS CODE HAS NOT BEEN TESTED AND IS NOT FULLY OPERATIONAL.

; 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.

; Code from Microprocessor (SPPU - Second Year - Computer Engineering - Content) repository on KSKA Git: https://git.kska.io/sppu-se-comp-content/Microprocessor/


; BEGINNING OF CODE
%macro print 2
mov rax,1
mov rdi,1
mov rsi,%1
mov rdx,%2
syscall
%endmacro

%macro exitprog 0
mov rax,60
xor rdi,rdi
syscall
%endmacro

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

section .data
nwline db 10
m0 db 10,13,"----- Program to calculate factorial of a given number -----",10,10
l0 equ $-m0
m2 db 10,"Enter a number (2 digit HEX no.): "
l2 equ $-m2
m4 db 10,"The factorial is: "
l4 equ $-m4
factorial dq 1

section .bss
no1 resq 1
input resb 1
output resb 1

section .text
global _start
_start:

print m0,l0
print m2,l2
call getnum

mov [no1],rax								;accept number
gtch input									;to read and discard ENTER key  ;pressed

mov rcx,[no1]

call facto
mov rax,00

print m4,l4
mov rax,qword[factorial]
call disphx16								;displays a 8 digit hex number ;in rax

exitprog

facto:
push rcx
cmp rcx,01
jne ahead
jmp exit2

ahead: dec rcx

call facto

exit2:
pop rcx
mov rax,rcx
mul qword[factorial]
mov qword[factorial],rax
ret
;Procedure to get a 2 digit hex no from user
;number returned in rax

getnum:
mov cx,0204h
mov rbx,0
ll2:
push rcx
gtch input
pop rcx
mov rax,0
mov al,byte[input]
sub rax,30h
cmp rax,09h
jbe skip1
sub rax,7

skip1:
shl rbx,cl
add rbx,rax
dec ch
jnz ll2
mov rax,rbx
ret

disphx16:  									;displays a 16 digit hex number
mov rbx,rax
mov cx,1004h								;16 digits to display and 04 count  ; to rotate

ll6:
rol rbx,cl
mov rdx,rbx
and rdx,0fh
add rdx,30h
cmp rdx,039h
jbe skip4
add rdx,7

skip4:
mov byte[output],dl
push rcx
print output,1
pop rcx
dec ch
jnz ll6
ret
; END OF CODE