They weren't horrible either, AArch64 is incompatible with AArch32 but you can still implement both on the same chip with shared internals.
AMD didn't have to extend x86 the way they did, but without buy in from intel there was no way forward unless they went the route they did. Because unless both had agreed to shift to UEFI at the same time and agreed on an ISA it wasn't going to happen. This is why even a modern x86-64 processor has to boot up in real mode... because there was no guarantee that the x64 extensions were going to take off, so AMD had to maintain that strict compatibility to be competitive.
AArch64 had no prohibition, because there is no universal boot protocol for ARM. Insofar as the UEFI or loader sets the CPU in a state the OS can use then it's fine. The fact that there is one IP holder helped as well.
That said could AMD make a x86-64 processor without real mode or compatibility mode support? Yes they can. In fact I would hope that the processors they ship to console manufacturers fit that bill. There is a lot they could strip out if they only intend to support x86-64.
AMD didn't have to extend x86 the way they did, but without buy in from intel there was no way forward unless they went the route they did. Because unless both had agreed to shift to UEFI at the same time and agreed on an ISA it wasn't going to happen. This is why even a modern x86-64 processor has to boot up in real mode... because there was no guarantee that the x64 extensions were going to take off, so AMD had to maintain that strict compatibility to be competitive.
AArch64 had no prohibition, because there is no universal boot protocol for ARM. Insofar as the UEFI or loader sets the CPU in a state the OS can use then it's fine. The fact that there is one IP holder helped as well.
That said could AMD make a x86-64 processor without real mode or compatibility mode support? Yes they can. In fact I would hope that the processors they ship to console manufacturers fit that bill. There is a lot they could strip out if they only intend to support x86-64.