emu8086 calculation with input value, can't get my code to work

Question

I am trying to get this code to work, however I keep experiencing problems in the conversion, I tried a bunch of examples that were answers to similar questions, but none of them work, but from it I sort of patched my own version of the code, I think, however it still doesn't work.

org  100h

jmp calc
message db 0dh, 0ah, "Input width. $"                    
width dw 10,?, 10 dup('?')
w dw 0

 calc: 
    mov dx, offset message
    mov ah, 9                ;output message
    int 21h  

    mov dx, offset width
    mov ah, 0ah            ;read input for width
    int 21h
    mov dx, width 


    .next_digit1:
    mov cx, width[bx+2]
    inc bx
    sub cx, '0'
    mul w, 10               ;convert width string into a number
    add w, cx
    loop .next_digit1

    mov cx, w
    add cx, 100        ; do calculation
    mov w, cx  


    mov ah, 2         ;output the result
    int 21h   

my biggest problem is that the string to number conversion isn't working, and the other conversion method I tried that was suggested to me here previously:

; Input:
; ESI = pointer to the string to convert
; Output:
; EAX = integer value
string_to_int:
  xor ebx,ebx    ; clear ebx
.next_digit:
  movzx eax,byte[esi]
  inc esi
  sub al,'0'    ; convert from ASCII to number
  imul ebx,10
  add ebx,eax   ; ebx = ebx*10 + eax
  loop .next_digit  ; while (--ecx)
  mov eax,ebx
  ret

does not seem to work, mainly because the line

movzx eax,byte[esi]

gives an illegal instructor error, and if i change it to just mov, then byte[esi] gives a wrong parameter error and undefined var error.

Answer 1

byte is a keyword, but you need to use it in conjuction with ptr:

movzx eax, byte ptr [esi]  # clear EAX and load the byte into its lowest eight bits

Your emulator need not support movzx. Then, alternatively, you can do what movzx does in two steps:

xor eax,eax                # clear EAX
mov al, byte ptr [esi]     # load the byte into EAX's lowest eight bits