Microsoft KB Archive/169576

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PRB: OSR2 Sets Bridge Control Register ISA Enable Bit

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Q169576

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The information in this article applies to:


 * Microsoft Win32 Device Driver Kit (DDK) Windows 95

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SYMPTOMS
PCI devices that are assigned I/O resources by the BIOS with the lower 10 bits in the range of 100-3FFh may not work properly when installed behind a PCI bridge on Windows 95 OSR2.

CAUSE
OSR2 sets the PCI Bridge Control Register ISA Enable bit. Setting the ISA Enable bit causes the bridge to block I/O transactions to the device when the lower 10 bits of the address are in the range 100h-3FFh. This results in the following behavior:

I/O Range: (repeating pattern up to FFFF, only the first 10FFh bytes are shown.)

  1000 - 10FF Forward D00 -  FFF    Block Forwarding C00 -  CFF    Forward 900 -  BFF    Block Forwarding 800 -  8FF Forward 500 -  7FF Block Forwarding 400 -  4FF Forward 100 -  3FF    Block Forwarding 0   -   FF Forward

This can be a problem if the BIOS assigns a PCI device I/O resources in a range that will be blocked by the ISA Enable Mode.

STATUS
This problem has been resolved in the next version of the Windows operating system.

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Issue type :

Technology : kbAudDeveloper kbWin95search kbWinDDKSearch kbWin32sSearch kbWin32DDK95 kbWin32DDKSearch