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PCI-to-PCI Bridges

Motherboards with PCI-to-PCI bridges may give some problems when used with a PCMCIA card reader. The issue is due to the various types of PCI-to-PCI bridge and patchy operating system support for enumeration of PCI bridges.

The PCI bus specification allows for approximatly four to six devices on the PCI bus, the actual number generally depends on the bus loading of the devices, so can be more. Motherboard manufacturers generally want to sell motherboards with more integrated features etc, so a PCI-to-PCI bridge is sometimes used to extend the capability to support more devices.

To reduce costs, chipset manufacturers integrate devices into their chipsets, for instance modern Intel chipsets (i810e to i850 etc) incorporate devices like integrated VGA and sound, then a built-in PCI-to-PCI bridge is provided for the PCI expansion bus slots. On these motherboards the PCI bus expansion slots generally appear as PCI bus number 2 and no primary PCI bus slots are available.

Generally, the Windows PCI drivers will support modern Intel chipset based motherboards with integrated PCI bridges. These PCI chipsets are usually setup as 'subtractive decode' bridges, and do not normally present a problem in this situation.

The problems occur when the motherboard manufacturer has added a PCI-to-PCI bridge to extend the PCI bus further, or a daughter board or backplane with PCI bus slots has a additional PCI bridge incorporated. These are usually 'positive decode' PCI-to-PCI bridges. The standard Windows PCI drivers do not support enumeration of 'positive decode' PCI bridges. This means that on these systems you are likely to have problems with allocation of resources. These problems usually show up as failures of the PCMCIA controller in Windows 9x/Me/2000 and XP as 'Code 10' or 'Code 12' failures and a 'No PCMCIA controller found' message in Windows NT4.

When a PCI-to-PCI bridge is present, the PCI bus number that the PCMCIA controller appears on is greater than 0 (zero). The PSeries diagnostics program may be used to identify this situation.

Work arrounds...

In Windows NT4.0 Systemsoft CardWizard supports enumeration of positive decode PCI bridges. 

Known problems:

Microstar MS-6373 motherboard (http://www.msi.com.tw/products/mainboard/mainboard.php?model=MS-6373) is not supported in Windows. This motherboard contains the nVIDIAŽ nForce 420D Chipset, and the offending PCI-to-PCI bridge is the nVIDIA-nForce PCI Bridge made by nVIDIA Corporation. Other motherboards with this chipset (like the Asus A7N266-E) may also not be supported in Windows, but we have no test results for these.