A Windows driver that adds value to peripheral devices or supports a specialized device in the PC. Either written by Microsoft or the vendor of the hardware, any number of filter drivers can be added to Windows. Upper level filter drivers sit above the primary driver for the device (the function driver), while lower level filter drivers sit below the function driver and above the bus driver. Filters may work on a certain brand of device such as a mouse or keyboard, or they may perform some operation on a class of devices, such as any mouse or any keyboard. Another type of filter driver is the bus filter driver, which may be added on top of the bus driver. For example, an ACPI bus filter is added to support power management for each device (see ACPI). See driver.