PNP OS Installed

Common Options : Yes, No

 

Quick Review

What this BIOS feature actually does is determine what devices are configured by the BIOS when the computer boots up and what are left to the operating system.

Non-ACPI BIOSes are found in older motherboards that do not support the new ACPI (Advanced Configuration and Power Interface) initiative. With such a BIOS, setting the PNP OS Installed feature to No allows the BIOS to configure all devices under the assumption that the operating system cannot do so. Therefore, all hardware settings are fixed by the BIOS at boot up and will not be changed by the operating system.

On the other hand, if you set the feature to Yes, the BIOS will only configure critical devices that are required to boot up the system. The other devices are then configured by the operating system. This allows the operating system some flexibility in shuffling system resources like IRQs and IO ports to avoid conflicts. It also gives you some degree of freedom when you want to manually assign system resources.

Of course, all current motherboards now ship with the new ACPI BIOS. If you are using an ACPI-compliant operating system (i.e. Windows 98 and above) with an ACPI BIOS, then this PNP OS Installed feature is no longer relevant. This is because the operating system will use the ACPI BIOS interface to configure all devices as well as retrieve system information.

But if your operating system does not support ACPI, then the BIOS will fall back to PNP mode. In this situation, consider the BIOS as you would a Non-ACPI BIOS. If there is no need to configure any hardware manually, it is again recommended that you set this feature to No.

If you are using an old Linux kernel (prior to 2.6.0), Jonathan has the following advice –

Although Linux (prior to kernel 2.6) is not really PnP-compatible, most distributions use a piece of software called ISAPNPTOOLS to setup ISA cards. If you have PnP OS set to No, the BIOS will attempt to configure ISA cards itself. This does not make them work with Linux, though, you still need to use something like ISAPNPTOOLS. However, having both the BIOS and ISAPNPTOOLS attempting to configure ISA cards can lead to problems where the two don’t agree.

The solution? Set PnP OS to Yes, and let ISAPNPTOOLS take care of ISA cards in Linux, as BIOS configuration of ISA cards doesn’t work for Linux anyway (with the current stable and development kernels). Most times, it probably won’t make a difference, but someone somewhere will have problems, and Linux will always work with PnP OS set to Yes.

Britt Turnbull recommends disabling this feature if you are running the OS/2 operating system, especially in a multi-boot system. This is because booting another operating system can update the BIOS which may later cause problems when you boot up OS/2.

To sum it all up, except for certain cases, it is highly recommended that you to set this BIOS feature to No, irrespective of the operating system you actually use. Exceptions to this would be the inability of the BIOS to configure the devices properly in PnP mode and a specific need to manually configure one or more of the devices.

 

Details

This BIOS feature is quite misleading because its name alludes that you should set it to Yes if you have an operating system that supports Plug and Play (PnP). Unfortunately, it isn’t quite so simple.

What this BIOS feature actually does is determine what devices are configured by the BIOS when the computer boots up and what are left to the operating system. This is rather different from what the name implies, right?

Before you can determine the appropriate setting for this feature, you should first determine the kind of BIOS that came with your motherboard. For the purpose of this discussion, the BIOS can be divided into two types – ACPI BIOS and Non-ACPI BIOS.

You will also need to find out if your operating system supports and is currently running in ACPI mode. Please note that while an operating system may tout ACPI support, it is possible to force the operating system to use the older PnP mode. So, find out if your operating system is actually running in ACPI mode. Of course, this is only possible if your motherboard comes with an ACPI BIOS. With a Non-ACPI BIOS, all ACPI-compliant operating systems automatically revert to PnP mode.

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Non-ACPI BIOSes are found in older motherboards that do not support the new ACPI (Advanced Configuration and Power Interface) initiative. This can be either the ancient non-PnP BIOS (or Legacy BIOS) or the newer PnP BIOS. With such a BIOS, setting the PNP OS Installed feature to No allows the BIOS to configure all devices under the assumption that the operating system cannot do so. Therefore, all hardware settings are fixed by the BIOS at boot up and will not be changed by the operating system.

On the other hand, if you set the feature to Yes, the BIOS will only configure critical devices that are required to boot up the system. For example, the graphics card and the hard disk. The other devices are then configured by the operating system. This allows the operating system some flexibility in shuffling system resources like IRQs and IO ports to avoid conflicts. It also gives you some degree of freedom when you want to manually assign system resources.

While all this flexibility in hardware configuration sounds like a good idea, shuffling resources can sometimes cause problems, especially with a buggy BIOS. Therefore, it is recommended that you set this feature to No, to allow the BIOS to configure all devices. You should only set this feature to Yes if the Non-ACPI BIOS cannot configure the devices properly or if you want to manually reallocate hardware resources in the operating system.

Of course, all current motherboards now ship with the new ACPI BIOS. If you are using an ACPI-compliant operating system (i.e. Windows 98 and above) with an ACPI BIOS, then this PNP OS Installed feature is no longer relevant. It actually does not matter what setting you select. This is because the operating system will use the ACPI BIOS interface to configure all devices as well as retrieve system information. There is no longer a need to specifically split the job up between the BIOS and the operating system.

But if your operating system does not support ACPI, then the BIOS will fall back to PNP mode. In this situation, consider the BIOS as you would a Non-ACPI BIOS. If there is no need to configure any hardware manually, it is again recommended that you set this feature to No.

Please note that bugs in some ACPI BIOS can cause even an ACPI-compliant operating system to disable ACPI. This reverts the BIOS to PnP mode. However, there is an additional catch to it. Certain operating systems (i.e. Windows 98 and above) will only access the buggy BIOS in read-only mode. This means the operating system will rely entirely on the BIOS to configure all devices and provide it with the hardware configuration. As such, you must set the feature to No if you have a buggy ACPI BIOS.

If you are using an old Linux kernel (prior to 2.6.0), Jonathan has the following advice –

Although Linux (prior to kernel 2.6) is not really PnP-compatible, most distributions use a piece of software called ISAPNPTOOLS to setup ISA cards. If you have PnP OS set to No, the BIOS will attempt to configure ISA cards itself. This does not make them work with Linux, though, you still need to use something like ISAPNPTOOLS. However, having both the BIOS and ISAPNPTOOLS attempting to configure ISA cards can lead to problems where the two don’t agree.

The solution? Set PnP OS to Yes, and let ISAPNPTOOLS take care of ISA cards in Linux, as BIOS configuration of ISA cards doesn’t work for Linux anyway (with the current stable and development kernels). Most times, it probably won’t make a difference, but someone somewhere will have problems, and Linux will always work with PnP OS set to Yes.

Britt Turnbull recommends disabling this feature if you are running the OS/2 operating system, especially in a multi-boot system. This is because booting another operating system can update the BIOS which may later cause problems when you boot up OS/2. In addition, if you add or change hardware, you should enable full hardware detection during the initial boot sequence of OS/2 (ALT-F1 at boot screen -> F5) so that the new hardware can be registered correctly.

Thomas McGuire of 3D Spotlight sent me this e-mail from Robert Kirk at IBM :-

“Actually, the setting “PnP OS” is really misnamed. A better thing would be to say “do you want the system to attempt to resolve resource conflicts, or do you want the OS to resolve system conflict?”. Setting the system to PnP OS says that even if the machine determines some kind of resource problem, it should not attempt to handle it… Rather, it should pass it on to the OS to resolve the issue. Unfortunately, the OS can’t resolve some issues…. which sometimes results in a lock or other problems.

For stability reasons, it is better to set EVERY motherboard’s PnP OS option to No, regardless of manufacturer but still allow the BIOS to auto configure PnP devices. Just leave the PnP OS to No. It won’t hurt a thing, you lose nothing, your machine will still autoconfigure PnP devices and it will make your system more stable.”

Thanks, Thomas! That was really useful info.

To sum it all up, except for certain cases, it is highly recommended that you to set this BIOS feature to No, irrespective of the operating system you actually use. Exceptions to this would be the inability of the BIOS to configure the devices properly in PnP mode and a specific need to manually configure one or more of the devices.

 

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