Yes, exactly. AMD made it a mandatory part of the AMD64 spec. Before that, SSE2 was an optional part of the ISA. Now, it's required if you support AMD64. Not a problem for AMD, since it had a cross license agreement with Intel, but an issue for anyone else. It's not a coincidence that AMD64 support is being proposed when the AMD64 and SSE2 parents are expiring (likely Microsoft has a license agreement with AMD, given how short of cash AMD was until recently, and AMD's need for MS to support AMD64 in the first place.). So, quite possibly the main holdup for Microsoft to emulate AMD64 has been to wait until the SSE2 patents expired.