 access CR1, CR5, CR6, or CR7.</p></td></tr></table>
<h2>Real-Address Mode Exceptions</h2>
<table class="exception-table">
<tr>
<td>#GP</td>
<td>
<p>If an attempt is made to write a 1 to any reserved bit in CR4.</p>
<p>If an attempt is made to write 1 to CR4.PCIDE.</p>
<p>If an attempt is made to write invalid bit combinations in CR0 (such as setting the PG flag to 1 when the PE flag is set to 0).</p></td></tr>
<tr>
<td>#UD</td>
<td>
<p>If the LOCK prefix is used.</p>
<p>If an attempt is made to access CR1,