Pentium M undervolting and underclocking

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Intel Pentium M processors from the Dothan and Banias families can be instructed to operate at voltage and clock frequencies lower than the nominal ones recommended by Intel and used by ThinkPads by default. Experience shows that the processor may continue working correctly at lower-than-nominal voltages and frequencies, thereby reducing power consumption, heat and fan noise.

In the Pentium M, speed and voltage are controlled by software (through the MSR registers). It is up to the operating system to choose the right voltage for each frequency. Normally this is done according to tables published by Intel or according to ACPI tables. However, this can be overriden - in the case of Linux, by a kernel patch.

Following this instructions will operate your CPU under conditions it was not designed for. Even if your system seems stable, it may still suffer transient faults leading to arbitrary data corruption. In addition, errors in following these instructions (or changes between processor models) may operate the CPU above its nominal parameters, which, if taken too far, can cause kernel panics or even possibly hardware damage

For an example of what may be achieved, consider these examples: when voltages on one ThinkPad T43 were reduced by 20-30%, stable CPU temperature dropped by 7-10deg under both idle and burn-in conditions. Combined with fan speed control, this greatly reduced the problem with fan noise. On one user's ThinkPad R51, an undervolt brought the full-load processor temperature down from 87 degrees to 63 degrees while maintaining full stability. The effect was, however, negligible at idle.

While undervolting has a clear measurable effect, it's not clear if underclocking really works. /proc/cpuinfo reflects the underclocked frequency, but enabling debug output on cpufreq causes it to say things like "CPU frequency out of sync: cpufreq and timing core thinks of 533000, is 800000 kHz." where the former is the chosen underclocked frequency and the latter is the documented minimum frequency. This discrepancy also causes Software Suspend 2 to oops during suspend.

There are several methods to control CPU voltage in Linux, and currently all of them require a patched kernel.

Alternative 0: linux-phc

Use the linux-phc kernel patch. This is the recommended method, as it is most generic and best supported.

The sysfs interface of linux-phc (version 0.3.0 or newer) requires you to specify VID (Voltage ID) numbers; see the documentation in the source package. For pre-Core Intel CPUs, VID is related to voltage (in mV) as follows: VID=(voltage-700)/16.

A typical configuration command would look something like this:

# echo '27 22 16 10 3' > /sys/devices/system/cpu/cpu0/cpufreq/phc_vids

Note :
PHC plus patches for >=2.6.35 kernel is in the Gentoo tree as sys-power/phc-intel. sys-power/phc-k8 and the GUI sys-power/phctool.

Alternative 1: patch with hard-coded voltages


  1. Determine the stable voltages for each frequency, using some overclocking/undervolting utility that has specific support for Pentium M ("Centrino") processors. It seems that no such utility exists for Linux. For windows, one good choice is Notebook Hardware Control (NHC), which conveniently also includes a table of nominal frequencies and voltages (in its help file).
    Ascertain that at the undervolted settings the CPU actually performs correct computation (Prime95 in "Tortute Test" mode seems to be a good partial test emphasizing FPU and memory access).
    Note that beside reducing voltages (undervolting), you can also try to add lower frequencies (underclocking). If you undervolt a particular frequency too much your machine is likely to crash, so try not to have any other programs open.
  2. Determine the model name string reported by the CPU, e.g., via the "Model name" line in /proc/cpuinfo, and likewise the cpu_family, model and stepping.
  3. Grab a copy of the example patch below and update it to reflect the parameters, frequencies and voltages you found.
  4. Apply the patch to your kernel (tested with and 2.6.14-rc2).
  5. To prevent the ACPI table from overriding your table, disable the CONFIG_X86_SPEEDSTEP_CENTRINO_ACPI kernel option.
  6. Compile and install the new kernel.

The changes should be active now. If you want to see debug information attesting to thus, do the following:

  1. Reboot in single mode.
  2. # echo 2 > /sys/module/cpufreq/parameters/debug
  3. # modprobe speedstep-centrino
  4. # dmesg and check for the message "speedstep-centrino: found "Intel(R) Pentium(R) M processor ..."

If you don't see that message, you got the cpu_id or model_name parmaeters wrong. If your system crashes, re-test the voltages and adjust accordingly.

Please report your results (including voltages)!

Linux kernel patch (example)

The following example shows how to undervolt a Pentium M 750 (1.86GHz) on a ThinkPad T43. As discussed above, the parameters are specific to this one CPU. You will need to experimentally find the correct settings for your own CPU and adjust the patch accordingly.

Note that the Pentium M 750 has a 533MHz FSB (quad-pumped 133MHz), hence the use of OP133. If you have a Pentium M with 400MHz FSB (i.e., quad-pumped 100MHz, found in Banias and older Dothan) then in the voltage table change OP133 to OP.

undervolt-pentium-m- (download)

Don't forget to disable CONFIG_X86_SPEEDSTEP_CENTRINO_ACPI.

Alternative 2: patch with user-space control

If you want to use the Ubuntu 2.6.15 Kernel, please have a look at this thread:

The bdz.undervolt.2005.10.22.a.patch patch, written by Gentoo-Wiki user "Bdz", allows voltages to be changed without reboot.

It presents a userspace interface, as in the following example:

# cat /sys/devices/system/cpu/cpu0/cpufreq/voltage_table
# echo "1084,1084,1084,1084,1084,1084,1084,988,908,860" >/sys/devices/system/cpu/cpu0/cpufreq/voltage_table

The relevant page on Gentoo-Wiki providues further information and some helpful hints and scripts for voltage adjustment.

When applying this patch be sure to keep the "ACPI tables for decoding frequency pairs" option -- this patch only allows adjustment of the voltages for the normal clock speeds as reported by the ACPI table in the BIOS. On some models (e.g., ThinkPad T43) this does not include all clock speeds supported by the processor.


This is a Gentoo initscript for the patch from Gentoo-Wiki user "Bdz".

Configuration file


  1. /etc/conf.d/undervoltage
  1. voltage table sysfs interface


  1. switch back to DEFAULT_VTABLE if undervoltage is stopped? [yes/no]


  1. pentium-m banias 1,7GHz default voltages


  1. pentium-m banias 1,7GHz lowered voltages [ -208mV ]

MOD_VTABLE="1276,1100,1020,908,796,748" </bash>



  1. !/sbin/runscript
  2. Copyright 1999-2005 Gentoo Foundation
  3. Distributed under the terms of the GNU General Public License v2
  4. $Header: $

sysfs_check() {

       if ! [ -e ${VTABLE_SYSFS} ]; then
               logger "No sysfs voltage_table present. Modifying vcore voltage failed."
               eerror "It seems that the undervolting patch has not been applied to the kernel."
               eerror "see for further information"
               return 1


start() {

       sysfs_check || return 1
       ebegin "Switching to modified voltage table [${MOD_VTABLE}]"
               echo ${MOD_VTABLE} > ${VTABLE_SYSFS}
       eend $?


  1. I think it is not necessary to switch to the default voltage table on shutdown

stop() {

       if [ $SWITCH_BACK = "yes" ]; then
               ebegin "Switching back to default voltage table [${DEFAULT_VTABLE}]"
                       echo ${DEFAULT_VTABLE} > ${VTABLE_SYSFS}
               eend $?

} </bash>

Alternative 3: another patch with user-space control

The centrino-voltages.diff patch, written by Rickard Holmberg, also provides user-space control. See here and here for usage.

Stress Testing

This script illustrates a very conservative method to stress test your lowered voltage settings.

Tested frequencies/voltages

If you have done excessive testing with user defined frequencies (stable system for over a month). and no lookups occur when computing primes with mprime (gentoo: 'emerge gimps') you can post your frequencies here. But note that this is just a rough indication, since the stable values will differ between individual processors, even in the same model.

CPU Type GHz Frequency Steps
Pentium-M Banias 1.6 1212,1084,956,860,796,748
Pentium-M Banias 1.7 1276,1100,1020,908,796,748
Pentium-M Dothan 2.0 1084,940,844,764,748
Pentium-M Dothan 1.86 1068,972,876,780,700
Pentium-M SL6F7 1.6 1196,1052,956,860,780,732
Pentium-M Dothan 1.6 940,876,812,748,716,716
Pentium-M Dothan 1.5 924,892,860,828,812,796,764,732,700
Pentium-M Dothan 1.4 924,892,860,828,796,764,732,700

Following values were obtained with: (running at the same time) <bash> nice -n19 glxgears & mplayer somedivx.avi & cd /usr/src/linux && make -j2 </bash>

  • note1: for pre-Core Intel CPUs, VID is related to voltage (in mV) as follows: VID=(voltage-700)/16 (or: voltage=(VID*16+700) mV)
  • note2: when testing for stability you should pay attention to sound too, occasional distortions which disappear after bumping voltage a notch or two
  • note4: if you use phc_vids for your script, then take only second number in each pair
Machine CPU Type GHz phc_controls (fid:vid)
T42 Pentium-M Dothan (735) 1.7 17:19 14:12 12:8 10:4 8:1 6:1
X31 Pentium-M Banias (1.4) 1.4 14:35 12:32 10:25 8:20 6:10 (fairly conservative)
X32 Pentium-M Dothan (745) 1.8 18:26 16:20 14:15 12:11 10:6 8:2 6:1
X40 Pentium-M Banias LV (718) 1.2 12:18 11:15 10:12 9:9 8:6 6:3
X41 Tablet Pentium-M Dothan LV (758) 1.5 15:15 14:13 13:11 12:9 11:7 10:5 9:3 8:2 6:2