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To avoid x86-isms slipping into the OS, Windows NT was developed originally on an i860-based platform, then one based on MIPS.

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NT was designed in part by a former Digital Equipment Corp architect who did a lot of work on Vax/VMS.

Windows NT was the subject of the appendix of my Operating system book. Replaced now by "Windows 2000" (NT5.0), which was the next OS.

https://www.os-book.com/OS8/os8c/appendices-dir/c.pdf

That architect is Dave Cutler: https://news.microsoft.com/features/the-engineers-engineer-c...

They also avoided the DWORD x86ism and peppered LONG everywhere, preventing a clean upgrade to LP64 since long reasonably had to match as well.

Why was SetWindowLong (taking LONG) superseded by SetWindowLongPtr (taking LONG_PTR) on 64-bit versions of Windows?

Because SetWindowLong can be used to set a callback function, i.e. a 64-bit pointer - but LONG remains 32 bit on 64-bit Windows. Backward compatibility stipulated the need for a new function name

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