case temperatures. Fan speed controllers Another method, popular with PC hardware enthusiasts, is the manual fan speed controller. They can be mounted in an expansion slot or a 5.25" or 3.5" drive bay or come built into a computer's case. Using switches or knobs, attached fans can have their speeds adjusted by one of the above methods. Hardware Most modern motherboards feature hardware monitoring chips, which are capable of performing fan control, usually through the PWM method as described above. These chips can be configured through BIOS, or by using specialised software once the operating system has booted. Processors produce varying levels of heat depending on system load, thus it makes sense to reduce the speed of the fans during idle to decrease the noise produced by fans running full speed, until the load does goes up, at which point fan speed must be adjusted promptly to avoid overheating. Modern hardware monitor chips, once configured, are capable of independently running this monitoring loop without any need for a functioning BIOS or an operating system. This automatic control offered by some chips may be called Thermal Cruise mode for maintaining a thermal envelope, as well as Fan Speed Cruise mode for maintaining a specific fan speed automatically. However, not all software is capable of accessing these advanced configuration parameters provided by some chips, and it is very common that the generic software implements only the most basic interfacing with the chips, namely, an explicit setting for the duty cycle for each fan control setting, subsequently performing the duty cycle adjustments itself in software, and thus requiring that both the operating system, as well as this third-party software itself to continue running on the main CPU to perform the monitoring loop. This may not be a problem until the system or the utility crashes, at which point the system may overheat due to the failure of the fans to maintain adequate cooling whilst running at reduced voltage and speed. Software Many companies now provide software to control fan speeds on their motherboards under Microsoft Windows or Mac OS X/MacOS. Different software is used by different motherboards. There are also third-party programs that work on a variety of motherboards and allow wide customization of fan behavior depending on temperature readings from the motherboard, CPU, and GPU sensors, as well as allowing manual control. Two such programs are SpeedFan and Argus Monitor. See also