cpu 8086
org 0x3000

xor dx, dx

cmp byte [si], 0
jne atoi

mov ah, 2
mov si, overflow_msg
int 0x21
int 0x20

atoi:
	lodsb

	cmp al, 0
	je .end

	cmp al, '0'
	jl .notdigit

	cmp al, '9'
	jg .notdigit

	xor ah, ah
	sub al, '0'
	mov cx, ax

	mov ax, dx
	mul word [ten]
	test dx, dx
	jnz .overflow

	mov dx, ax
	add dx, cx
	jc .overflow

	jmp atoi

	.notdigit:
		mov byte [char_slot], al
		mov si, notdigit_msg
		mov ah, 2
		int 0x21
		int 0x20

	.overflow:
		mov si, overflow_msg
		mov ah, 2
		int 0x21
		int 0x20

	.end:

std
mov di, hexstring+3
xor bh, bh
mov cx, 4
itoh:
	mov bl, 0xf
	and bl, dl
	mov al, [hexdigits + bx]
	stosb

	shr dx, 1
	shr dx, 1
	shr dx, 1
	shr dx, 1

	loop itoh

cld

mov si, hexstring
mov ah, 2
int 0x21
int 0x20

notdigit_msg db "Not a digit '"
char_slot db 0, "'", 0
overflow_msg db "Value must be in range 0 to 65535", 0

hexstring db "xxxx", 0

hexdigits db "0123456789abcdef"
ten dw 10