Q80379: Stepping Through Self-Modifying Code in CodeView

Article: Q80379
Product(s): Microsoft Programming Utilities
Version(s): 2.2,3.0,3.11,3.14,4.0,4.01,4.02,4.05,4.1
Operating System(s): 
Keyword(s): kb16bitonly
Last Modified: 26-JUN-2001

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The information in this article applies to:

- Microsoft CodeView for MS-DOS, versions 2.2, 3.0, 3.11, 3.14, 4.0, 4.01, 4.02, 4.05, 4.1 
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SUMMARY
=======

Code that is written to modify its own contents may run differently when run one
line at a time in the Microsoft CodeView Debugger (CV). This occurs because of
the the way the 80x86 processors fetch instructions. Because this is a
limitation of how CodeView works, care should be used when debugging
self-modifying code.

The code may run one way outside of CodeView, and another way from within
CodeView. Also, running the program by using the go (F5) command instead of the
step (F10) or trace (F8) command may affect the program's execution. Different
processors can also affect this behavior.

MORE INFORMATION
================

The 80x86 processor will try to pre-fetch the next instruction from memory while
processing the current instruction. If the code being modified is already
waiting in the instruction queue, then the processor will run the unmodified
code. However, a jump instruction will invalidate the queue, causing the next
instruction to be fetched before execution. In self-modifying code, this will
cause the modified instruction to be run. When stepping through a program in
CodeView, the instruction queue is always marked invalid, so all modified
instructions are run. The code below demonstrates this behavior.

If you step through the sample code in CodeView it will output "Num = 1". If you
run straight though the code, allowing the processor to pre-fetch instructions,
it will output "Num = 0".

Sample Code
-----------

  /* Compile options needed: /Zi
  */ 

  #include<stdio.h>

  void main(void)
  {
  unsigned int Num;

  _asm {
       mov ax, cs
       push ds
       mov ds, ax
       inc byte ptr [cnt + 1]    /* Adding a "jmp cnt" between the  */ 
  cnt: mov ax, 0                 /* INC and the MOV will cause the  */ 
       mov Num, ax               /* modified code to be run         */ 
       pop ds
       }
     printf("Num = %d",Num);
  }

Additional query words: kbinf 2.20 3.00 4.00 4.10

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Keywords          : kb16bitonly 
Technology        : kbAudDeveloper kbCodeView kbZNotKeyword3 kbCodeView220DOS kbCodeView300DOS kbCodeView311DOS kbCodeView314DOS kbCodeView400DOS kbCodeView401DOS kbCodeView402DOS kbCodeView405DOS kbCodeView410DOS
Version           : :2.2,3.0,3.11,3.14,4.0,4.01,4.02,4.05,4.1

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