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Why do most x64 instructions zero the upper part of a 32 register

If I do a movl something, %eax and rax contained something like 0x1234567812345678, will it automatically zero out the top half of rax?

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tangrs
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  • Thanks, if you post it as an answer, I'll be able to accept it – tangrs Nov 14 '12 at 09:58
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    I could do that, but there's a more interesting answer (including likely reasons) here: http://stackoverflow.com/questions/11177137/why-do-most-x64-instructions-zero-the-upper-part-of-a-32-register – harold Nov 14 '12 at 10:07

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