Advanced Micro Devices Inc (AMD)

[CombJelly]

"How long does that take?"

It depends on how major the changes are. A complete, clean sheet? 5-7 years. Maybe more. Incremental changes? 2-3 years. Again, sometimes more. As noted by someone else, adding 64 bit mode isn't that big of a deal. Some work on the decoder and making the datapath 64 bits. The decoder would decompose the instructions to uops, it doesn't really care what the instruction stream is. Ok, that might not be totally true. It may be the case that a particular implementation might be optimized more for a specific architecture. But I would be suprised if it were a big job...