> The i486 added a few more control bits, and some of them
> went into CR0. But instead of overflowing the new bits
> into CR1, Intel decided to skip it and open up CR4
> instead – for unknown reasons.
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My suspicion is that some application in the 286 - OS/2 days was found to expect an 'invalid opcode' fault from CR1, so that's why it had to stay reserved.