Back in 1985-6, Acorn computers had a problem - improving performance versus backward compatibility. They had to move on from their existing 6502 8-bit processor based computer (the BBC Micro) but they weren't willing to compromise on ANY of the properties of the 6502 when moving to a 16/32-bit processor. Both the Intel 8086 and the Motorola 68000 had inferior interrupt latency to the 6502 and neither was software compatible. Acorn made a brilliant decision - design their own processor and so ARM was born. It was totally non-backward-compatible with anything BUT it was so fast that it could run software emulations of processors such as the 6502 so fast that 6502 software ran at least as fast on the ARM emulation as on an original 6502.
This is the approach that any computer designer should take - forget hardware backward compatibilty and all the legacy overheads that it entails; go for speed and software emulation if backward compatibility is needed - it works, and you only have to look at the success of the ARM to see that such decisions can bring major success.
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