I also wondered about that when AMD64 first came out, and my feeling was &quot;because they wanted to make it harder for Intel too&quot;. At the time, Intel was likely already developing a 64-bit extension of x86.<p>On the other hand, I think the 16 to 32-bit extension (with the 386) was done quite well. 16 and 32-bit code can coexist, and it&#x27;s even possible to use the 32-bit registers in 16-bit mode; not so with AMD64. It&#x27;s not too difficult to figure out how to add 64-bit support in a more non-disruptive way, without having to do silly things like removing instructions and features that they had to reintroduce later [1][2].<p>[1] <a href="http:&#x2F;&#x2F;www.pagetable.com&#x2F;?p=25" rel="nofollow">http:&#x2F;&#x2F;www.pagetable.com&#x2F;?p=25</a>
[2] <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;X86-64#Differences_between_AMD64_and_Intel_64" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;X86-64#Differences_between_AMD...</a>