Paul, microcode, I wouldn't be at all surprised. Intel can move fast to put together an inferior product which they subsequently throw against their famous marketing department to 'fix'. I'm not expert on microprocessors internals but there is something here that bothers me: Internal 64-bit registers. Now, you can emulate the 64-bit modes and instructions in microcode, but there have to be 64-bit registers available for them to work.
Let's assume Intel did need to make a microcode fix. This leads to two possibilities (maybe more?):
1. Yamhill was incompatible overall, but had a similar register set (maybe bigger register set) to AMD64. Thus the microcode is changed to implement AMD64 on Yamhill registers.
2. No 64-bit registers are available so the processor has to grab memory - from cache, presumably - to implement memory registers.
I favor the first explanation. The second possibility is just too ugly to imagine!
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