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Hello, how can I meassure RPM of the CPU (or some aux fan..) fan? There are three wires connected to board, two of them must be DC power and via the last wire should fan tell when it's reversing it's motor's polarity to spin up for another 180 degree, right? So by calculating these pulses I could be able to get the actual RPM.
But how can I get to that wire? I mean, how can i comunicate with it, read it or whatever? Can APM do it? Is there any IO register assigned to it? Seriously, no idea.
What do you think? How can I read the fan's RPM?
Thanks.

ACPI. If you're lucky.

If you're not lucky, the fan controller will be attached to some random ISA/LPC device or SMBus device (Said SMBus device will probably be attached to some random ISA/LPC SMBus host). ACPI may provide you access to this SMBus.

(hmm it seems that i remember seeing this in a BIOS setup.)

Oh yeah i was using an old hp computer and i went into the BIOS and saw fan config (or something like that)

it had a readout of RPMs, speed, current temp and voltage

tell me if u want to know wat type of hp it was

I have discovered, that implementing ACPI is difficult
I dont have to really implement the whole ACPI interface to my system right?
It should be enough just to enable it and read my RPM info right?

Given the grand size of ACPI, implementing the needed parts is still a big task.

I wonder what the reason is why you want a readout of the CPU fan above all other functionality?

What chipset does your system use? see if the manufacture released any datasheets.