NASM: Convert multi character input to decimal - loops

Question

I am trying to write a program that gets a number with one or two digits and write Hello! as many times as that number.
I used this posts to write my code:
NASM: The loop turns into an infinite loop
Check null character in assembly language
Multi-Digit Input from NASM I did not understand this :)
But my code only works for two digit numbers and the result for single digit numbers are wrong.
My code:
section .data
msg db 'Hello!',0xA
len equ $-msg
section .bss
n resb 2
section .text
global _start
_start:
;get n
mov edx, 2
mov ecx, n
mov ebx, 0
mov eax, 3
int 0x80
lea esi,[n] ;address of our text
call toint
;loop for print 'Hello!\n'
print_loop:
push ecx
mov edx, len
mov ecx, msg
mov ebx, 1
mov eax, 4
int 0x80
pop ecx
loop print_loop
mov eax, 1
int 0x80
toint:
push eax
push ebx
xor ebx, ebx
next_digit:
movzx eax, byte[esi]
sub al , '0'
imul ebx, 10
add ebx, eax
inc esi
cmp byte [esi] , 0x0 ;check next character is null or not
jne next_digit
; end_loop:
mov ecx, ebx
pop eax
pop ebx
ret

Answer 1

The sys_read call returns in EAX the count of characters that were sent to your inputbuffer. Because you allowed for an input of max. 2 characters, this count will be either 0, 1, or 2. You could use this info in your toint routine.
; IN (eax,esi) OUT (ecx)
toint:
mov ecx, eax ; Number of digits
jecxz done
push eax ; (1)
push ebx ; (2)
push esi ; (3)
xor ebx, ebx
next:
movzx eax, byte [esi]
sub al , '0'
imul ebx, 10
add ebx, eax
inc esi
dec ecx
jnz next
mov ecx, ebx
pop esi ; (3)
pop ebx ; (2)
pop eax ; (1)
done:
ret
Please notice that there's a reversed ordering to be observed when preserving/restoring registers on the stack! (Your code missed this...)
4 tips
a. Prefer the MOV variant to load an address. It's always a shorter instruction.
b. Guard yourself against an input of zero.
c. Don't use LOOP. It's a slow instruction.
d. Provide the exit code to terminate the program.
mov esi, n ; (a)
call toint ; -> ECX
jecxz Exit ; (b)
print_loop:
...
dec ecx ; (c)
jnz print_loop
Exit:
xor ebx, ebx ; (d)
mov eax, 1
int 0x80