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Common Errors

Got a bug? This is a list of common errors that might help solve your problem

  1. Pushes and Pops don't match or are in the incorrect sequence.
    I cannot even begin to count the number of times I have seen this
          push ax
          push bx
          ...
    
    then the programmer does a cut and paste, then rename push to pop resulting in
     
           pop  ax
           pop  bx
           ...
    
    You need to reverse the order as well!
    of course a simple error is easy to miss like the following
     
           ...
           push ax
           push bx
           cmp  ax, bx
           je   next
           pop  ax
           jmp  done
    
      next:
           pop  bx
           pop  ax
    
      done:
           ret
    

    Notice that if we branch we will pop both ax and bx, but if we don't branch then one pop is missed. When we return, think about what is on the stack. If you need to, draw a picture showing the stack as you enter this procedure and as you execute through it. I will be doing an example like this on the board in class.

  2. A fairly common error that occurs more often the night before a project is due is the following.
           mov  ah, 09
           mov  bx, SEG myString
           mov  dx, bx
           mov  dx, OFFSET myString
           int  21h
    

    What's wrong with this block of code to print a string? Well, the use of DX twice in a row should look a little suspicious. What we meant to type was mov ds, bx NOT DX! Reading though the code is usually the fastest way to catch errors like this. Take a break if you have been working for a while and come back to your code - usually you will catch it quickly. Sometimes an extra pair of eyes will be necessary. Drop by my office hours or ask a friend to look over your code if you are really stuck.


If you have a common error you think might benefit the class, please let me know - write up the description and email it to me. I will post it for everyone else.



Last Modified: January 22nd, 1999 - Barry E. Mapen Last Modified: January 26, 1999 - Barry E. Mapen