访问违规MASM x86程序集

Question

I am working on a project and I am currently getting an Access Violation in one of my lines. I was wondering if I could get a second opinion on what is wrong. Here is my code (Note, I get the error during runtime, but it does build):

.data
BlueTextOnGray = blue + (lightGray * 16)
DefaultColor = lightGray + (black * 16)
arrayD SDWORD 12345678h,1A4B2000h,3434h,7AB9h

fib BYTE 1,2
  BYTE NUMBER_FIBS_TO_COMPUTE dup(0)

prompt  BYTE    "Enter an ending integer: ",0
error   BYTE    "Invalid stopping point! 



.code

main PROC

    mov eax,BlueTextOnGray
    call    SetTextColor
    call    Clrscr          ; Clear the screen
    call    Crlf            ; New line

    mov edx,OFFSET prompt
    call    WriteString
    call    ReadInt         ; Input integer into EAX
    call    Crlf            ; New line

  lea esi, [fib+2]
  mov cl, NUMBER_FIBS_TO_COMPUTE
@@:
  mov al, [esi-2]
  add al, [esi-1]
  mov [esi], al   ;<------------This is where the error occurs
  inc esi
  loop @B

; here print out the results or examine them with debugger

E1: call    Crlf            ; New line
    call    WaitMsg         ; "Press any key..."
    mov eax,DefaultColor
    call    SetTextColor
    call    Clrscr



exit
main ENDP
END main

Is there a rule that I am missing. I have done my research but I cannot seem to find the answer that fits my situation.

Any help would be great! (Also note that I am not done with it so there might be other mistakes.)

Thanks!

Answer 1

Your problem is that, wherever fib points to, which is loaded into esi , that memory page is marked as read-only.

Generally, an access violation is caused by attempting to write to a memory location that is marked read-only in the GDT. A segmentation fault occurs when you attempt to read from a memory location that your process does not have access to at all.

As @Jester points out, you're not paying attention to the high order bits in ECX . While you set your loop control value in CL , your loop may run far higher than you intend since ECX is unknown. This would quickly put you into read-only territory in your memory.