section .data
	m1 db 	10,"Enter 4-digit Hex number: "
	l1 equ 	$- m1
	m2  db 10,"Equivalent BCD number is: "
	l2 equ 	$- m2
	emsg db 10,"You entered Invalid Data!!!",
	l3 equ 	$-emsg
;---------------------------------------------------------------------
section .bss
	buf 		resb 	6
	buf_len:	equ	$-buf
	digitcount	resb	1
	ans		resw	1
	char_ans	resb	4
;---------------------------------------------------------------------
;macros as per 64-bit convensions

%macro  print   2
	mov 	rax,1		; Function 1 - write
  	mov 	rdi,1		; To stdout
   	mov 	rsi,%1		; String address
   	mov 	rdx,%2		; String size
   	syscall			; invoke operating system to WRITE
%endmacro

%macro  read   2
	mov 	rax,0		; Function 0 - Read
  	mov 	rdi,0		; from stdin
   	mov 	rsi,%1		; buffer address
   	mov 	rdx,%2		; buffer size
   	syscall			; invoke operating system to READ
%endmacro

%macro exit 0
	mov rax, 60		; system call 60 is exit
	mov rdi,0 		; we want return code 0
	syscall 		; invoke operating system to exit
%endmacro
;--------------------------------------------------------------- 
section .text
	global _start
_start:
	print	m1,l1
	call 	accept_num
	mov 	ax,bx

	mov 	rbx,10 
back:
	xor 	rdx,rdx
	div 	rbx

	push 	dx                        ;Remainder
	inc 	byte[digitcount]

	cmp 	rax,0h              ;quotient
	jne 	back

	print	m2, l2

print_bcd:
	pop 	dx
	add 	dl,30h		; possible digits are 0-9 so add 30H only
	mov	[char_ans],dl	; store character in char_ans
	
	print	char_ans,1	; print	on screen in reverse order

	dec 	byte[digitcount]
	jnz 	print_bcd

	exit
;------------------------------------------------------------------
accept_num:
	read	buf,5		; buflen = 4 + 1

	xor 	bx,bx
	mov 	rcx,4
	mov 	rsi,buf
next_digit:
	shl	bx,04        
	mov	al,[rsi]
        
       cmp    	al,"0"     	; "0" = 30h or 48d
        jb     	error      	; jump if below "0" to error
        cmp    	al,"9"   
        jbe    	sub30     	; subtract 30h if no is in the range "0"-"9"

        cmp    	al,"A"    	; "A" = 41h or 65d
        jb     	error       	; jump if below "A" to error
        cmp    	al,"F"
        jbe    	sub37       	; subtract 37h if no is in the range "A"-"F"

        cmp    	al,"a"      	; "a" = 61h or 97d
        jb     	error       	; jump if below "a" to error
        cmp    	al,"f"
        jbe     sub57       	; subtract 57h if no is in the range "a"-"f"

error:  print 	emsg, l3	; "You entered Invalid Data!!!"  
        exit

sub57:	sub    	al,57h      	; subtract 57h if no is in the range "a"-"f"
        jmp next
sub37:  sub    	al,37h      	; subtract 37h if no is in the range "a"-"f"
        jmp next
sub30:  sub    	al,30h      	; subtract 30h if no is in the range "0"-"9"
    	
next:	add	bx,ax		; prepare number
	inc	rsi		; point to next digit
	loop	next_digit
ret