This article was previously published under Q321779
When you start your computer, the basic input/output system (BIOS) checks (among other things) the system-specific settings that are stored in the complementary metal-oxide semiconductor (CMOS) chip. You can modify these settings as the system changes.
To change the CMOS settings, you must enter CMOS Setup by pressing a specific key or a combination of keys during the initial startup sequence. For example, press DEL or CTRL+ALT+ESC during the startup. (The specific key combination that you press is typically indicated during startup as "Press <keyname> to enter Setup".)
After you have entered Setup, windows that display various options and settings appear. Some of these options are standard, while others are specific to the BIOS manufacturer.
One of the CMOS settings is the PNP OS option. This setting tells BIOS how many devices to configure at startup. The table in "More Information" shows the effect of this option on the configuration of the motherboard devices.
The original intent for designing this option was to give Microsoft Windows versions 95 and 98 more freedom to adjust hardware configurations. By default, these operating systems would never move a device configured at startup for fear of breaking a DOS driver. Later versions of Windows also typically leave BIOS-configured hardware alone, even if the BIOS placed the hardware in a less than optimal configuration. This is because moving such hardware frequently exposes latent bugs in the BIOS.
PNP OS set to Yes
The BIOS configures only critical devices (for example, video, hard-disk, and key- Board). NOTE: In this mode, neither the BIOS nor Windows configures the motherboard devices at startup. Therefore, for these earlier computers, you must set PNP OS to No.
PNP OS set to No
The BIOS configures critical devices and all motherboard devices under the assumption that Windows cannot.
Former (legacy) ACPI BIOS PNP BIOS or APM BIOS
The PNP OS option is irrelevant. Windows (only Windows versions 98, MillenniumEdition, 2000 and XP) uses the Advanced Configuration and Power Interface (ACPI) BIOS interface to configure motherboard devices and retrieve system information. NOTE: Although a system does have an ACPI BIOS, ACPI might have been disabled by Windows because of a bug in the ACPI BIOS. In this case, Windows will fall back to PNP BIOS and APM BIOS in read-only mode. Therefore, for any computer with a buggy ACPI BIOS, set PNP OS to No.