Cross-license deal came 2009 after long going legal battle. Intel released first 64-bit CPU's 2004.
AMD was able to make x86 CPU's because IBM wanted another supplier for CPU's and AMD was handed x86 license. Then AMD reverse engineered multiple Intel CPU's. Intel tried to block AMD from making 386 "because of 32-bits" but AMD reverse engineered it too and court decided it was OK. Intel did exactly same for x86-64. Like with 386, no license was needed.
Fact that Intel64 lacked two instructions AMD CPU's had but were removed from certain documents (but later added them) proves Intel reverse-engineered x86-64. Otherwise AMD would have handed information of all instructions for Intel. That's also reason why Intel64 was not exact replica of x86-64.
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