Error 10327: Can't determine definition of operator ""+""

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In a combinational design you need to care that output signals are set in all branches of a conditional statement. If you review the decoder process, you'll realize that it's not the case for sign and ofv. The error message is clearly explaining the issue, by the way.

Simply add a default assignment as first process statement, e.g. sign <= '0'; ovf <= '0';

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Thank you for helping me out! :)