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Microsoft KB Archive/169576

From BetaArchive Wiki

PRB: OSR2 Sets Bridge Control Register ISA Enable Bit


The information in this article applies to:

  • Microsoft Win32 Device Driver Kit (DDK) Windows 95


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.


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.


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

Keywords :
Issue type :
Technology : kbAudDeveloper kbWin95search kbWinDDKSearch kbWin32sSearch kbWin32DDK95 kbWin32DDKSearch

Last Reviewed: March 5, 1999
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