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Hello,

I've finally found out how to switch CPU speed on G4 Macs with Intrepid
chipset ( like iBook G4, some PowerMacs and PowerBooks and probably
others )

The problem:
cpu0 at mainbus0: 7455 (Revision 3.3), ID 0 (primary)
cpu0: HID0 8450c0bc<EMCP,TBEN,NAP,DPM,ICE,DCE,SGE,BTIC,LRSTK,FOLD,BHT>,
PS=3D1=20
cpu0: 614.39 MHz, 256KB L2 cache

It's supposed to run at 800 MHz but that's what the firmware sets up.
With the attached patch you'll get:

obio0 at pci1 dev 23 function 0: addr 0x80000000
enabling Intrepid CPU speed control
CPU speed is low

and this:
~# sysctl machdep
...
machdep.cpu_speed =3D 0

~# envstat -r
case temperature:     49.000 degC
 CPU temperature:     54.000 degC
 GPU temperature:     51.000 degC
        voltage0:      1.589 V
            fan0:          0 RPM


Now we can do
~# sysctl -w machdep.cpu_speed=3D1
machdep.cpu_speed: 0 -> 1

That's it, now the CPU runs at full speed.

We can check core voltage to see that something changed:
~# envstat -r
case temperature:     49.000 degC
 CPU temperature:     54.000 degC
 GPU temperature:     51.000 degC
        voltage0:      1.810 V
            fan0:          0 RPM

~# flops

   FLOPS C Program (Double Precision), V2.0 18 Dec 1992

   Module     Error        RunTime      MFLOPS
                            (usec)
     1     -7.6739e-13      0.0816    171.6341

with speed back to low we get:

~# flops

   FLOPS C Program (Double Precision), V2.0 18 Dec 1992

   Module     Error        RunTime      MFLOPS
                            (usec)
     1     -7.6739e-13      0.0966    144.9620

So far this in only known to work on my iBook. It may or may not work on
different hardware.

have fun
Michael

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