Well... Compaq told MS they wouldn't fund this work, or at least not all of it, any more (since NT sales were a tiny fraction of […]
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Well... Compaq told MS they wouldn't fund this work, or at least not all of it, any more (since NT sales were a tiny fraction of Alpha sales) -- they wanted MS to bring the HAL development in-house. MS said that they wouldn't pick it up either (since Alpha sales were a tiny fraction of NT sales). So Compaq pulled the plug on that staff.
Alpha NT HAL support WAS fairly expensive. It was a more complex HAL in some ways -- for example, the platforms included real hardware mapping registers between PCI space and physical address space, allowing DMA scatter/gather even for devices without s/g abillity. But the main reason was that Alphas needed a lot more HALs, and more differences among HALs, than x86.
The 64-bit NT project (codename "Sundown") started shortly after NT4 RTM, long before CPQ dropped their ball, and was originally targeting both Alpha and ia64. Even after Alpha NT was dropped as a product they continued work on the 64-bit Alpha port, because they had a LOT of Alpha hardware around in the labs to test it on, while ia64 silicon was still years away.
And, they always had the idea that maintaining two different ports would help prevent ia64-specific stuff from getting into places where it wasn't supposed to be. But instead of supporting all the various HALs they just picked one Alpha reference platform and did the HAL for that.
Last I heard, davec (Dave Cutler) still had an Alpha box in his office. Not sure what version it's running. But I'm sure it's running very fast.