> I heard several rumours about why ACPI. Someone said that ACPI was forced by Microsoft. In reality it was decision taken by all major distros and Microsoft.

Microsoft went out of their way looking for methods to make ACPI not work well with Linux. And Microsoft viewed ACPI as their spec that they didn't want Linux "getting for free".

http://antitrust.slated.org/www.iowaconsumercase.org/011607/...

And it's not like the distros had a choice. ACPI is the only way to, for instance, enumerate other cores than the boot processor on x86. Features as simple as SMP requires ACPI on x86.