FTA: “For design reasons that are a complete mystery to me, the MMX registers are actually sub-registers of the x87 STn registers”

I think the main argument for doing that was that it meant that existing OSes didn’t need changes for the new CPU. Because they already saved the x87 registers on context switch, they automatically saved the MMX registers, and context switches didn’t slow down.

It also may have decreased the amount of space needed, but that difference can’t have been very large, I think