macro nop n {
  if n == 0
  else if n == 1
    nop
  else if n==2
    mov edi,edi
  else if n==3
    mov di,di
  else if n==4
    ;lea esp,[esp+0]
    dd $0024648d
  else if n==5
    lea bp,[bp]
  else if n==6
    ;lea ebx,[ebx+0L]
    DB 8DH, 9BH, 00H, 00H, 00H, 00H
  else if n==7
    ;lea esp,[esp+0L]
    DB 8DH,0A4H, 24H, 00H, 00H, 00H, 00H
  else if n==8
    ;lea sp,[esp+0L]
    DB 66H, 8DH,0A4H, 24H, 00H, 00H, 00H, 00H
  else if n==9
    mov [ss:esp-4],dword 0
  else
    jmp near $+n
    db n-5 dup 0
  end if
}    

virtual at 0
 xchg eax,eax
 detected_16bit = $-1
end virtual

if detected_16bit
 ;...
end if
    

Your code for n=9 is not a nop "mov [ss:esp-4],dword 0" alters the stack.

To detect 16/32 bit mode you can use something like this:

Code:

virtual at 0
 xchg eax,eax
 detected_16bit = $-1
end virtual

if detected_16bit
 ;...
end if
    

That code is from 'WIN32AX.INC'

Yes it did modified [esp-4] but that's out of the stack. Anytime it gets changed.