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> Historically, almost all attempts to replace an enormously successful product line with something incompatible have failed.

Except AArch64, which has a legacy 32 bit mode for compatibility but is otherwise a totally separate ISA, unlike x86-64 and i386.

Aarch64 looks at first glance to be pretty dissimilar, but if you consider that its really a follow-on to THUMB2 it doesn't look as foreign. Also a lot of things appear different at first glance but aren't because they have basically been renamed.

one example CPUID->MIDR

http://infocenter.arm.com/help/topic/com.arm.doc.ddi0432c/Bh...

https://developer.arm.com/docs/ddi0500/f/4-system-control/45...

The memory model is the same.. etc, etc, etc.

If AMD hadn't come up with x64, they would have succeeded it.

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