"wouldn't it be more likely that they would just force one of the legacy modes or disable the long mode?"
It would be pretty easy to disable the ability to enter long mode either by a laser fuse directly on the die or on the package. I am dubious that it would be worth while taking 64 bit mode out because the implications of it run through out the whole pipeline.
And yes, it is pretty bizarre when you think about it. Seems like it would be easier to just not support it in the BIOS.
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