cpu 8086
org 0x3000

xor dx, dx

cmp byte [si], 0
jne htoi

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

htoi:
	lodsb

	cmp al, 0
	je .end

	.digit09:
		cmp al, '0'
		jl .notdigit

		cmp al, '9'
		jg .digitcapital

		sub al, '0'
		jmp .digit

	.digitcapital:
		cmp al, 'A'
		jl .notdigit

		cmp al, 'F'
		jg .digitminuscule

		sub al, 'A' - 10
		jmp .digit

	.digitminuscule:
		cmp al, 'a'
		jl .notdigit

		cmp al, 'f'
		jg .notdigit

		sub al, 'a' - 10
		jmp .digit

	.digit:
		xor ah, ah

		test dx, 0xf000
		jnz .overflow

		shl dx, 1
		shl dx, 1
		shl dx, 1
		shl dx, 1
		or dx, ax

		jmp htoi

	.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:

mov si, decimalstring + 5
mov ax, dx
itoa:
	dec si
	xor dx, dx
	div word [ten]
	add dl, '0'
	mov byte [si], dl

	test ax, ax
	jnz itoa

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 ffff", 0

decimalstring db "xxxxx", 0

ten dw 10