Xilinx is committed to fixing all XST fatal errors and will analyze any problems that you are experiencing in order to improve future versions of XST. Consequently, even if this Answer Record allows you to work around the fatal error, please open a WebCase with Xilinx Customer Support at:
This fatal error is issued for a variety of reasons. Read the last several items in the synthesis report to see what XST was performing last; if this corresponds to one of the synthesis options, turn that option Off and re-run synthesis. XST might bypass the portion in the synthesis engine that is causing the error. First, be sure that the Advanced Options are turned On in ISE:
1. Select the "Edit" pull-down menu in Project Navigator.
2. Select "Preferences."
3. Select the "Processes" tab.
4. Change the Property Display Level from "Standard" to "Advanced."
5. Click "OK."
To select the synthesis options:
1. Highlight the HDL file that you want to synthesize.
2. Right-click the "Synthesize - XST" process.
3. Select "Properties."
The window that appears contains all synthesis properties that are available from the GUI.
NOTE: The most common switch that seems to resolve some fatal errors is the "shift Register Extraction" in the "HDL Options" tab.
XST might generate the fatal error when you infer block RAM/ROM.
This problem has been fixed in the latest 6.1i Service Pack, available at: