&gt;  [Using 64-bit registers in 32-bit mode] is not possible. If the system is in 32-bit mode, it will act as a 32-bit system, the extra 32-bits of the registers is completely invisible, just as it would be if the system was actually a &quot;true 32-bit system&quot;.<p>That&#x27;s not entirely true. Under Linux, with the x32 ABI (see <a href="https:&#x2F;&#x2F;lwn.net&#x2F;Articles&#x2F;456731&#x2F;" rel="nofollow">https:&#x2F;&#x2F;lwn.net&#x2F;Articles&#x2F;456731&#x2F;</a>) you have access to the entire register file, but still use 32-bit pointers.<p>In many ways, it combines the advantages of 32- and 64-bit mode code.