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<h2>17.3 Converted code GP Faults!</h2>

<p><em><strong>Q</strong>: OK, I've succeeded in converting and compiling my assembly-language
program, but when I run it, I get "Segmentation Violation" and "General
Protection Fault".  This program works when compiled with <small>MASM</small>, so
how can this be?</em>

<br><p>
<p><strong>A</strong>: In DJGPP, your program runs in <strong>protected mode</strong>.  There are
certain things you can't do in protected-mode programs (that's why it is
called protected mode).  This issue is too complex to describe here, so
only a few of the more important aspects will be briefly mentioned.  If
you are serious about writing assembly language protected-mode code, or
have a large body of existing code to convert to protected mode, you
should read any of the available books about protected-mode programming
with 80x86 processors.

<p>Here is a short list of some of the techniques found in many real-mode
programs, which will trigger protection violation or erratic behavior in
protected mode:

<ul>
<li>Loading arbitrary values into segment registers, then using them to
reference code or data.

<li>Referencing code with data segment register, or vice versa.

<li>Assuming certain locations (like BIOS area or video memory) will be found
at certain absolute addresses.

<li>Calling DOS or BIOS services with <code>INT NN</code> instruction.

<li>Hooking interrupts by poking absolute addresses. 
</ul>

<p>If your code uses one or more of these techniques, refer to
<a href="faq18.html">low-level programming chapter</a>, which describes the DJGPP facilities
that will allow you to rewrite your code.

<p><hr>
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